A* search algorithm
The A* search algorithm is an extension of Dijkstra's algorithm useful for finding the lowest cost path between two nodes (aka vertices) of a graph. The path may traverse any number of nodes connected by edges (aka arcs) with each edge having an associated cost. The algorithm uses a heuristic which associates an estimate of the lowest cost path from this node to the goal node, such that this estimate is never greater than the actual cost.
The algorithm should not assume that all edge costs are the same. It should be possible to start and finish on any node, including ones identified as a barrier in the task.
- Task
Consider the problem of finding a route across the diagonal of a chess board-like 8x8 grid. The rows are numbered from 0 to 7. The columns are also numbered 0 to 7. The start position is (0, 0) and the end position is (7, 7). Movement is allow by one square in any direction including diagonals, similar to a king in chess. The standard movement cost is 1. To make things slightly harder, there is a barrier that occupy certain positions of the grid. Moving into any of the barrier positions has a cost of 100.
The barrier occupies the positions (2,4), (2,5), (2,6), (3,6), (4,6), (5,6), (5,5), (5,4), (5,3), (5,2), (4,2) and (3,2).
A route with the lowest cost should be found using the A* search algorithm (there are multiple optimal solutions with the same total cost).
Print the optimal route in text format, as well as the total cost of the route.
Optionally, draw the optimal route and the barrier positions.
Note: using a heuristic score of zero is equivalent to Dijkstra's algorithm and that's kind of cheating/not really A*!
- Extra Credit
Use this algorithm to solve an 8 puzzle. Each node of the input graph will represent an arrangement of the tiles. The nodes will be connected by 4 edges representing swapping the blank tile up, down, left, or right. The cost of each edge is 1. The heuristic will be the sum of the manhatten distance of each numbered tile from its goal position. An 8 puzzle graph will have 9!/2 (181,440) nodes. The 15 puzzle has over 10 trillion nodes. This algorithm may solve simple 15 puzzles (but there are not many of those).
- See also
- Wikipedia webpage: A* search algorithm.
- An introduction to: Breadth First Search |> Dijkstra’s Algorithm |> A*
- Related tasks
C
<lang c>
- include <stdlib.h>
- include <stdio.h>
- include <string.h>
- include <float.h>
/* and not not_eq */
- include <iso646.h>
/* add -lm to command line to compile with this header */
- include <math.h>
- define map_size_rows 10
- define map_size_cols 10
char map[map_size_rows][map_size_cols] = {
{1, 1, 1, 1, 1, 1, 1, 1, 1, 1}, {1, 0, 0, 0, 0, 0, 0, 0, 0, 1}, {1, 0, 0, 0, 0, 0, 0, 0, 0, 1}, {1, 0, 0, 0, 0, 1, 1, 1, 0, 1}, {1, 0, 0, 1, 0, 0, 0, 1, 0, 1}, {1, 0, 0, 1, 0, 0, 0, 1, 0, 1}, {1, 0, 0, 1, 1, 1, 1, 1, 0, 1}, {1, 0, 0, 0, 0, 0, 0, 0, 0, 1}, {1, 0, 0, 0, 0, 0, 0, 0, 0, 1}, {1, 1, 1, 1, 1, 1, 1, 1, 1, 1}
};
/* description of graph node */ struct stop {
double col, row; /* array of indexes of routes from this stop to neighbours in array of all routes */ int * n; int n_len; double f, g, h; int from;
};
int ind[map_size_rows][map_size_cols] = {
{-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}, {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1}
};
/* description of route between two nodes */ struct route {
/* route has only one direction! */ int x; /* index of stop in array of all stops of src of this route */ int y; /* intex of stop in array of all stops od dst of this route */ double d;
};
int main() {
int i, j, k, l, b, found; int p_len = 0; int * path = NULL; int c_len = 0; int * closed = NULL; int o_len = 1; int * open = (int*)calloc(o_len, sizeof(int)); double min, tempg; int s; int e; int current; int s_len = 0; struct stop * stops = NULL; int r_len = 0; struct route * routes = NULL;
for (i = 1; i < map_size_rows - 1; i++) { for (j = 1; j < map_size_cols - 1; j++) { if (!map[i][j]) { ++s_len; stops = (struct stop *)realloc(stops, s_len * sizeof(struct stop)); int t = s_len - 1; stops[t].col = j; stops[t].row = i; stops[t].from = -1; stops[t].g = DBL_MAX; stops[t].n_len = 0; stops[t].n = NULL; ind[i][j] = t; } } }
/* index of start stop */ s = 0; /* index of finish stop */ e = s_len - 1;
for (i = 0; i < s_len; i++) { stops[i].h = sqrt(pow(stops[e].row - stops[i].row, 2) + pow(stops[e].col - stops[i].col, 2)); }
for (i = 1; i < map_size_rows - 1; i++) { for (j = 1; j < map_size_cols - 1; j++) { if (ind[i][j] >= 0) { for (k = i - 1; k <= i + 1; k++) { for (l = j - 1; l <= j + 1; l++) { if ((k == i) and (l == j)) { continue; } if (ind[k][l] >= 0) { ++r_len; routes = (struct route *)realloc(routes, r_len * sizeof(struct route)); int t = r_len - 1; routes[t].x = ind[i][j]; routes[t].y = ind[k][l]; routes[t].d = sqrt(pow(stops[routes[t].y].row - stops[routes[t].x].row, 2) + pow(stops[routes[t].y].col - stops[routes[t].x].col, 2)); ++stops[routes[t].x].n_len; stops[routes[t].x].n = (int*)realloc(stops[routes[t].x].n, stops[routes[t].x].n_len * sizeof(int)); stops[routes[t].x].n[stops[routes[t].x].n_len - 1] = t; } } } } } }
open[0] = s; stops[s].g = 0; stops[s].f = stops[s].g + stops[s].h; found = 0;
while (o_len and not found) { min = DBL_MAX;
for (i = 0; i < o_len; i++) { if (stops[open[i]].f < min) { current = open[i]; min = stops[open[i]].f; } }
if (current == e) { found = 1;
++p_len; path = (int*)realloc(path, p_len * sizeof(int)); path[p_len - 1] = current; while (stops[current].from >= 0) { current = stops[current].from; ++p_len; path = (int*)realloc(path, p_len * sizeof(int)); path[p_len - 1] = current; } }
for (i = 0; i < o_len; i++) { if (open[i] == current) { if (i not_eq (o_len - 1)) { for (j = i; j < (o_len - 1); j++) { open[j] = open[j + 1]; } } --o_len; open = (int*)realloc(open, o_len * sizeof(int)); break; } }
++c_len; closed = (int*)realloc(closed, c_len * sizeof(int)); closed[c_len - 1] = current;
for (i = 0; i < stops[current].n_len; i++) { b = 0;
for (j = 0; j < c_len; j++) { if (routes[stops[current].n[i]].y == closed[j]) { b = 1; } }
if (b) { continue; }
tempg = stops[current].g + routes[stops[current].n[i]].d;
b = 1;
if (o_len > 0) { for (j = 0; j < o_len; j++) { if (routes[stops[current].n[i]].y == open[j]) { b = 0; } } }
if (b or (tempg < stops[routes[stops[current].n[i]].y].g)) { stops[routes[stops[current].n[i]].y].from = current; stops[routes[stops[current].n[i]].y].g = tempg; stops[routes[stops[current].n[i]].y].f = stops[routes[stops[current].n[i]].y].g + stops[routes[stops[current].n[i]].y].h;
if (b) { ++o_len; open = (int*)realloc(open, o_len * sizeof(int)); open[o_len - 1] = routes[stops[current].n[i]].y; } } } }
for (i = 0; i < map_size_rows; i++) { for (j = 0; j < map_size_cols; j++) { if (map[i][j]) { putchar(0xdb); } else { b = 0; for (k = 0; k < p_len; k++) { if (ind[i][j] == path[k]) { ++b; } } if (b) { putchar('x'); } else { putchar('.'); } } } putchar('\n'); }
if (not found) { puts("IMPOSSIBLE"); } else { printf("path cost is %d:\n", p_len); for (i = p_len - 1; i >= 0; i--) { printf("(%1.0f, %1.0f)\n", stops[path[i]].col, stops[path[i]].row); } }
for (i = 0; i < s_len; ++i) { free(stops[i].n); } free(stops); free(routes); free(path); free(open); free(closed);
return 0;
} </lang>
- Output:
▒▒▒▒▒▒▒▒▒▒ ▒x.......▒ ▒.x......▒ ▒.x..▒▒▒.▒ ▒.x▒...▒.▒ ▒.x▒...▒.▒ ▒.x▒▒▒▒▒.▒ ▒..xxxxx.▒ ▒.......x▒ ▒▒▒▒▒▒▒▒▒▒ path cost is 12: (1, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6) (3, 7) (4, 7) (5, 7) (6, 7) (7, 7) (8, 8)
C++
<lang cpp>
- include <list>
- include <algorithm>
- include <iostream>
class point { public:
point( int a = 0, int b = 0 ) { x = a; y = b; } bool operator ==( const point& o ) { return o.x == x && o.y == y; } point operator +( const point& o ) { return point( o.x + x, o.y + y ); } int x, y;
};
class map { public:
map() { char t[8][8] = { {0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 1, 1, 1, 0}, {0, 0, 1, 0, 0, 0, 1, 0}, {0, 0, 1, 0, 0, 0, 1, 0}, {0, 0, 1, 1, 1, 1, 1, 0}, {0, 0, 0, 0, 0, 0, 0, 0}, {0, 0, 0, 0, 0, 0, 0, 0} }; w = h = 8; for( int r = 0; r < h; r++ ) for( int s = 0; s < w; s++ ) m[s][r] = t[r][s]; } int operator() ( int x, int y ) { return m[x][y]; } char m[8][8]; int w, h;
};
class node { public:
bool operator == (const node& o ) { return pos == o.pos; } bool operator == (const point& o ) { return pos == o; } bool operator < (const node& o ) { return dist + cost < o.dist + o.cost; } point pos, parent; int dist, cost;
};
class aStar { public:
aStar() { neighbours[0] = point( -1, -1 ); neighbours[1] = point( 1, -1 ); neighbours[2] = point( -1, 1 ); neighbours[3] = point( 1, 1 ); neighbours[4] = point( 0, -1 ); neighbours[5] = point( -1, 0 ); neighbours[6] = point( 0, 1 ); neighbours[7] = point( 1, 0 ); } int calcDist( point& p ){ // need a better heuristic int x = end.x - p.x, y = end.y - p.y; return( x * x + y * y ); } bool isValid( point& p ) { return ( p.x >-1 && p.y > -1 && p.x < m.w && p.y < m.h ); } bool existPoint( point& p, int cost ) { std::list<node>::iterator i; i = std::find( closed.begin(), closed.end(), p ); if( i != closed.end() ) { if( ( *i ).cost + ( *i ).dist < cost ) return true; else { closed.erase( i ); return false; } } i = std::find( open.begin(), open.end(), p ); if( i != open.end() ) { if( ( *i ).cost + ( *i ).dist < cost ) return true; else { open.erase( i ); return false; } } return false; } bool fillOpen( node& n ) { int stepCost, nc, dist; point neighbour;
for( int x = 0; x < 8; x++ ) { // one can make diagonals have different cost stepCost = x < 4 ? 1 : 1; neighbour = n.pos + neighbours[x]; if( neighbour == end ) return true; if( isValid( neighbour ) && m( neighbour.x, neighbour.y ) != 1 ) { nc = stepCost + n.cost; dist = calcDist( neighbour ); if( !existPoint( neighbour, nc + dist ) ) { node m; m.cost = nc; m.dist = dist; m.pos = neighbour; m.parent = n.pos; open.push_back( m ); } } } return false; } bool search( point& s, point& e, map& mp ) { node n; end = e; start = s; m = mp; n.cost = 0; n.pos = s; n.parent = 0; n.dist = calcDist( s ); open.push_back( n ); while( !open.empty() ) { //open.sort(); node n = open.front(); open.pop_front(); closed.push_back( n ); if( fillOpen( n ) ) return true; } return false; } int path( std::list<point>& path ) { path.push_front( end ); int cost = 1 + closed.back().cost; path.push_front( closed.back().pos ); point parent = closed.back().parent; for( std::list<node>::reverse_iterator i = closed.rbegin(); i != closed.rend(); i++ ) { if( ( *i ).pos == parent && !( ( *i ).pos == start ) ) { path.push_front( ( *i ).pos ); parent = ( *i ).parent; } } path.push_front( start ); return cost; } map m; point end, start; point neighbours[8]; std::list<node> open; std::list<node> closed;
};
int main( int argc, char* argv[] ) {
map m; point s, e( 7, 7 ); aStar as; if( as.search( s, e, m ) ) { std::list<point> path; int c = as.path( path ); for( int y = -1; y < 9; y++ ) { for( int x = -1; x < 9; x++ ) { if( x < 0 || y < 0 || x > 7 || y > 7 || m( x, y ) == 1 ) std::cout << char(0xdb); else { if( std::find( path.begin(), path.end(), point( x, y ) )!= path.end() ) std::cout << "x"; else std::cout << "."; } } std::cout << "\n"; } std::cout << "\nPath cost " << c << ": "; for( std::list<point>::iterator i = path.begin(); i != path.end(); i++ ) { std::cout<< "(" << ( *i ).x << ", " << ( *i ).y << ") "; } } std::cout << "\n\n"; return 0;
} </lang>
- Output:
██████████ █x.......█ █x.......█ █x...███.█ █x.█...█.█ █x.█...█.█ █.x█████.█ █..xxxx..█ █......xx█ ██████████ Path cost 11: (0, 0) (0, 1) (0, 2) (0, 3) (0, 4) (1, 5) (2, 6) (3, 6) (4, 6) (5, 6) (6, 7) (7, 7)
Common Lisp
<lang lisp>;; * Using external libraries with quicklisp (eval-when (:load-toplevel :compile-toplevel :execute)
(ql:quickload '("pileup" "iterate")))
- * The package definition
(defpackage :a*-search
(:use :common-lisp :pileup :iterate))
(in-package :a*-search)
- * The data
(defvar *size* 8
"The size of the area.")
- I will use simple conses for the positions and directions.
(defvar *barriers*
'((2 . 4) (2 . 5) (2 . 6) (3 . 6) (4 . 6) (5 . 6) (5 . 5) (5 . 4) (5 . 3) (5 . 2) (4 . 2) (3 . 2)) "The position of the barriers in (X Y) pairs, starting with (0 0) at the lower left corner.")
(defvar *barrier-cost* 100 "The costs of a barrier field.")
(defvar *directions* '((0 . -1) (0 . 1) (1 . 0) (-1 . 0) (-1 . -1) (1 . 1))
"The possible directions left, right, up, down and diagonally.")
- * Tha data structure for the node in the search graph
(defstruct (node (:constructor node))
(pos (cons 0 0) :type cons) (path nil) (cost 0 :type fixnum) ; The costs so far (f-value 0 :type fixnum) ; The value for the heuristics )
- * The functions
- ** Printing the final path
(defun print-path (path start end &optional (barriers *barriers*)
&aux (size (+ 2 *size*))) "Prints the area with the BARRIERS." ;; The upper boarder (format t "~v@{~A~:*~}~%" size "█") ;; The actual area ;; The lines (iter (for y from (1- *size*) downto 0) (format t "█") ;; The columns (iter (for x from 0 below *size*) (format t "~A" (cond ((member (cons y x) barriers :test #'equal) "█") ((equal (cons y x) start) "●") ((equal (cons y x) end) "◆") ((Member (cons y x) path :test #'equal) "▪") (t " ")))) ;; The last column and jump to the next line (format t "█~%")) ;; The lower boarder (format t "~v@{~A~:*~}~%" size "█") (iter (for position in path) (format t "(~D,~D)" (car position) (cdr position)) (finally (terpri))))
- ** Generating the next positions
- *** Check if a position is possible
(defun valid-position-p (position)
"Returns T if POSITION is a valid position." (let ((x (car position)) (y (cdr position)) (max (1- *size*))) (and (<= 0 x max) (<= 0 y max))))
- *** Move from the current position in direction
(defun move (position direction)
"Returns a new position after moving from POSITION in DIRECTION assuming only
valid positions."
(let ((x (car position)) (y (cdr position)) (dx (car direction)) (dy (cdr direction))) (cons (+ x dx) (+ y dy))))
- *** Generate the possible next positions
(defun next-positions (current-position)
"Returns a list of conses with possible next positions." (remove-if-not #'valid-position-p (mapcar (lambda (d) (move current-position d)) *directions*)))
- ** The heuristics
(defun distance (current-position goal)
"Returns the Manhattan distance from CURRENT-POSITION to GOAL." (+ (abs (- (car goal) (car current-position))) (abs (- (cdr goal) (cdr current-position)))))
- ** The A+ search
(defun a* (start goal heuristics next &optional (information 0))
"Returns the shortest path from START to GOAL using HEURISTICS, generating the next nodes using NEXT." (let ((visited (make-hash-table :test #'equalp))) (flet ((pick-next-node (queue) ;; Get the next node from the queue (heap-pop queue)) (expand-node (node queue) ;; Expand the next possible nodes from node and add them to the ;; queue if not already visited. (iter (with costs = (node-cost node)) (for position in (funcall next (node-pos node))) (for cost = (1+ costs)) (for f-value = (+ cost (funcall heuristics position goal) (if (member position *barriers* :test #'equal) 100 0))) ;; Check if this state was already looked at (unless (gethash position visited)) ;; Insert the next node into the queue (heap-insert (node :pos position :path (cons position (node-path node)) :cost cost :f-value f-value) queue))))
;; The actual A* search (iter ;; The priority queue (with queue = (make-heap #'<= :name "queue" :size 1000 :key #'node-f-value)) (with initial-cost = (funcall heuristics start goal)) (initially (heap-insert (node :pos start :path (list start) :cost 0 :f-value initial-cost) queue)) (for counter from 1) (for current-node = (pick-next-node queue)) (for current-position = (node-pos current-node)) ;; Output some information each counter or nothing if information ;; equals 0. (when (and (not (zerop information)) (zerop (mod counter information))) (format t "~Dth Node, heap size: ~D, current costs: ~D~%" counter (heap-count queue) (node-cost current-node))) ;; If the target is not reached continue (until (equalp current-position goal)) ;; Add the current state to the hash of visited states (setf (gethash current-node visited) t) ;; Expand the current node and continue (expand-node current-node queue) (finally (return (values (nreverse (node-path current-node)) (node-cost current-node) counter)))))))
- ** The main function
(defun search-path (&key (start '(0 . 0)) (goal '(7 . 7)) (heuristics #'distance))
"Searches the shortest path from START to GOAL using HEURISTICS." (multiple-value-bind (path cost steps) (a* start goal heuristics #'next-positions 0) (format t "Found the shortest path from Start (●) to Goal (◆) in ~D steps with cost: ~D~%" steps cost) (print-path path start goal)))</lang>
- Output:
A*-SEARCH> (search-path) Found the shortest path from Start (●) to Goal (◆) in 323 steps with cost: 11 ██████████ █ ▪▪▪▪◆█ █ ▪ █ █ ▪█████ █ █ ▪█ █ █ █ ▪█ █ █ █ ▪ ███ █ █ ▪ █ █● █ ██████████ (0,0)(1,1)(2,1)(3,1)(4,1)(5,1)(6,2)(7,3)(7,4)(7,5)(7,6)(7,7)
D
ported from c++ code <lang D>
import std.stdio; import std.algorithm; import std.range; import std.array;
struct Point {
int x; int y; Point opBinary(string op = "+")(Point o) { return Point( o.x + x, o.y + y ); }
}
struct Map {
int w = 8; int h = 8; bool[][] m = [ [0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 1, 1, 1, 0], [0, 0, 1, 0, 0, 0, 1, 0], [0, 0, 1, 0, 0, 0, 1, 0], [0, 0, 1, 1, 1, 1, 1, 0], [0, 0, 0, 0, 0, 0, 0, 0], [0, 0, 0, 0, 0, 0, 0, 0] ];
}
struct Node {
Point pos; Point parent; int dist; int cost; bool opEquals(const Node n) { return pos == n.pos; } bool opEquals(const Point p) { return pos == p; } int opCmp(ref const Node n) const { return (n.dist + n.cost) - (dist + cost); }
};
struct AStar {
Map m; Point end; Point start; Point[8] neighbours = [Point(-1,-1), Point(1,-1), Point(-1,1), Point(1,1), Point(0,-1), Point(-1,0), Point(0,1), Point(1,0)]; Node[] open; Node[] closed;
int calcDist(Point b) { // need a better heuristic int x = end.x - b.x, y = end.y - b.y; return( x * x + y * y ); } bool isValid(Point b) { return ( b.x >-1 && b.y > -1 && b.x < m.w && b.y < m.h ); } bool existPoint(Point b, int cost) { auto i = closed.countUntil(b); if( i != -1 ) { if( closed[i].cost + closed[i].dist < cost ) return true; else { closed = closed.remove!(SwapStrategy.stable)(i); return false; } } i = open.countUntil(b); if( i != -1 ) { if( open[i].cost + open[i].dist < cost ) return true; else { open = open.remove!(SwapStrategy.stable)(i); return false; } } return false; } bool fillOpen( ref Node n ) { int stepCost; int nc; int dist; Point neighbour; for( int x = 0; x < 8; ++x ) { // one can make diagonals have different cost stepCost = x < 4 ? 1 : 1; neighbour = n.pos + neighbours[x]; if( neighbour == end ) return true; if( isValid( neighbour ) && m.m[neighbour.y][neighbour.x] != 1 ) { nc = stepCost + n.cost; dist = calcDist( neighbour ); if( !existPoint( neighbour, nc + dist ) ) { Node m; m.cost = nc; m.dist = dist; m.pos = neighbour; m.parent = n.pos; open ~= m; } } } return false; } bool search( ref Point s, ref Point e, ref Map mp ) { Node n; end = e; start = s; m = mp; n.cost = 0; n.pos = s; n.parent = Point(); n.dist = calcDist( s ); open ~= n ; while( !open.empty() ) { //open.sort(); Node nx = open.front(); open = open.drop(1).array; closed ~= nx ; if( fillOpen( nx ) ) return true; } return false; } int path( ref Point[] path ) { path = end ~ path; int cost = 1 + closed.back().cost; path = closed.back().pos ~ path; Point parent = closed.back().parent; foreach(ref i ; closed.retro) { if( i.pos == parent && !( i.pos == start ) ) { path = i.pos ~ path; parent = i.parent; } } path = start ~ path; return cost; }
};
int main(string[] argv) {
Map m; Point s; Point e = Point( 7, 7 ); AStar as; if( as.search( s, e, m ) ) { Point[] path; int c = as.path( path ); for( int y = -1; y < 9; y++ ) { for( int x = -1; x < 9; x++ ) { if( x < 0 || y < 0 || x > 7 || y > 7 || m.m[y][x] == 1 ) write(cast(char)0xdb); else { if( path.canFind(Point(x,y))) write("x"); else write("."); } } writeln(); } write("\nPath cost ", c, ": "); foreach( i; path ) { write("(", i.x, ", ", i.y, ") "); } }
write("\n\n");
return 0;
} </lang>
- Output:
██████████ █x.......█ █x.......█ █x...███.█ █x.█...█.█ █x.█...█.█ █.x█████.█ █..xxxx..█ █......xx█ ██████████ Path cost 11: (0, 0) (0, 1) (0, 2) (0, 3) (0, 4) (1, 5) (2, 6) (3, 6) (4, 6) (5, 6) (6, 7) (7, 7)
Go
<lang go>// Package astar implements the A* search algorithm with minimal constraints // on the graph representation. package astar
import "container/heap"
// Exported node type. type Node interface {
To() []Arc // return list of arcs from this node to another Heuristic(from Node) int // heuristic cost from another node to this one
}
// An Arc, actually a "half arc", leads to another node with integer cost. type Arc struct {
To Node Cost int
}
// rNode holds data for a "reached" node type rNode struct {
n Node from Node l int // route len g int // route cost f int // "g+h", route cost + heuristic estimate fx int // heap.Fix index
}
type openHeap []*rNode // priority queue
// Route computes a route from start to end nodes using the A* algorithm. // // The algorithm is general A*, where the heuristic is not required to be // monotonic. If a route exists, the function will find a route regardless // of the quality of the Heuristic. For an admissiable heuristic, the route // will be optimal. func Route(start, end Node) (route []Node, cost int) {
// start node initialized with heuristic cr := &rNode{n: start, l: 1, f: end.Heuristic(start)} // maintain a set of reached nodes. start is reached initially r := map[Node]*rNode{start: cr} // oh is a heap of nodes "open" for exploration. nodes go on the heap // when they get an initial or new "g" route distance, and therefore a // new "f" which serves as priority for exploration. oh := openHeap{cr} for len(oh) > 0 { bestRoute := heap.Pop(&oh).(*rNode) bestNode := bestRoute.n if bestNode == end { // done. prepare return values cost = bestRoute.g route = make([]Node, bestRoute.l) for i := len(route) - 1; i >= 0; i-- { route[i] = bestRoute.n bestRoute = r[bestRoute.from] } return } l := bestRoute.l + 1 for _, to := range bestNode.To() { // "g" route distance from start g := bestRoute.g + to.Cost if alt, ok := r[to.To]; !ok { // alt being reached for the first time alt = &rNode{n: to.To, from: bestNode, l: l, g: g, f: g + end.Heuristic(to.To)} r[to.To] = alt heap.Push(&oh, alt) } else { if g >= alt.g { continue // candidate route no better than existing route } // it's a better route // update data and make sure it's on the heap alt.from = bestNode alt.l = l alt.g = g alt.f = end.Heuristic(alt.n) if alt.fx < 0 { heap.Push(&oh, alt) } else { heap.Fix(&oh, alt.fx) } } } } return nil, 0
}
// implement container/heap func (h openHeap) Len() int { return len(h) } func (h openHeap) Less(i, j int) bool { return h[i].f < h[j].f } func (h openHeap) Swap(i, j int) {
h[i], h[j] = h[j], h[i] h[i].fx = i h[j].fx = j
}
func (p *openHeap) Push(x interface{}) {
h := *p fx := len(h) h = append(h, x.(*rNode)) h[fx].fx = fx *p = h
}
func (p *openHeap) Pop() interface{} {
h := *p last := len(h) - 1 *p = h[:last] h[last].fx = -1 return h[last]
}</lang> <lang go>package main
import (
"fmt"
"astar"
)
// rcNode implements the astar.Node interface type rcNode struct{ r, c int }
var barrier = map[rcNode]bool{{2, 4}: true, {2, 5}: true,
{2, 6}: true, {3, 6}: true, {4, 6}: true, {5, 6}: true, {5, 5}: true, {5, 4}: true, {5, 3}: true, {5, 2}: true, {4, 2}: true, {3, 2}: true}
// graph representation is virtual. Arcs from a node are generated when // requested, but there is no static graph representation. func (fr rcNode) To() (a []astar.Arc) {
for r := fr.r - 1; r <= fr.r+1; r++ { for c := fr.c - 1; c <= fr.c+1; c++ { if (r == fr.r && c == fr.c) || r < 0 || r > 7 || c < 0 || c > 7 { continue } n := rcNode{r, c} cost := 1 if barrier[n] { cost = 100 } a = append(a, astar.Arc{n, cost}) } } return a
}
// The heuristic computed is max of row distance and column distance. // This is effectively the cost if there were no barriers. func (n rcNode) Heuristic(fr astar.Node) int {
dr := n.r - fr.(rcNode).r if dr < 0 { dr = -dr } dc := n.c - fr.(rcNode).c if dc < 0 { dc = -dc } if dr > dc { return dr } return dc
}
func main() {
route, cost := astar.Route(rcNode{0, 0}, rcNode{7, 7}) fmt.Println("Route:", route) fmt.Println("Cost:", cost)
}</lang>
- Output:
Route: [{0 0} {1 1} {2 2} {3 1} {4 1} {5 1} {6 2} {6 3} {6 4} {6 5} {6 6} {7 7}] Cost: 11
Java
<lang java> package astar;
import java.util.List; import java.util.ArrayList; import java.util.Collections;
class AStar {
private final List<Node> open; private final List<Node> closed; private final List<Node> path; private final int[][] maze; private Node now; private final int xstart; private final int ystart; private int xend, yend; private final boolean diag;
// Node class for convienience static class Node implements Comparable { public Node parent; public int x, y; public double g; public double h; Node(Node parent, int xpos, int ypos, double g, double h) { this.parent = parent; this.x = xpos; this.y = ypos; this.g = g; this.h = h; } // Compare by f value (g + h) @Override public int compareTo(Object o) { Node that = (Node) o; return (int)((this.g + this.h) - (that.g + that.h)); } }
AStar(int[][] maze, int xstart, int ystart, boolean diag) { this.open = new ArrayList<>(); this.closed = new ArrayList<>(); this.path = new ArrayList<>(); this.maze = maze; this.now = new Node(null, xstart, ystart, 0, 0); this.xstart = xstart; this.ystart = ystart; this.diag = diag; } /* ** Finds path to xend/yend or returns null ** ** @param (int) xend coordinates of the target position ** @param (int) yend ** @return (List<Node> | null) the path */ public List<Node> findPathTo(int xend, int yend) { this.xend = xend; this.yend = yend; this.closed.add(this.now); addNeigborsToOpenList(); while (this.now.x != this.xend || this.now.y != this.yend) { if (this.open.isEmpty()) { // Nothing to examine return null; } this.now = this.open.get(0); // get first node (lowest f score) this.open.remove(0); // remove it this.closed.add(this.now); // and add to the closed addNeigborsToOpenList(); } this.path.add(0, this.now); while (this.now.x != this.xstart || this.now.y != this.ystart) { this.now = this.now.parent; this.path.add(0, this.now); } return this.path; } /* ** Looks in a given List<> for a node ** ** @return (bool) NeightborInListFound */ private static boolean findNeighborInList(List<Node> array, Node node) { return array.stream().anyMatch((n) -> (n.x == node.x && n.y == node.y)); } /* ** Calulate distance between this.now and xend/yend ** ** @return (int) distance */ private double distance(int dx, int dy) { if (this.diag) { // if diagonal movement is alloweed return Math.hypot(this.now.x + dx - this.xend, this.now.y + dy - this.yend); // return hypothenuse } else { return Math.abs(this.now.x + dx - this.xend) + Math.abs(this.now.y + dy - this.yend); // else return "Manhattan distance" } } private void addNeigborsToOpenList() { Node node; for (int x = -1; x <= 1; x++) { for (int y = -1; y <= 1; y++) { if (!this.diag && x != 0 && y != 0) { continue; // skip if diagonal movement is not allowed } node = new Node(this.now, this.now.x + x, this.now.y + y, this.now.g, this.distance(x, y)); if ((x != 0 || y != 0) // not this.now && this.now.x + x >= 0 && this.now.x + x < this.maze[0].length // check maze boundaries && this.now.y + y >= 0 && this.now.y + y < this.maze.length && this.maze[this.now.y + y][this.now.x + x] != -1 // check if square is walkable && !findNeighborInList(this.open, node) && !findNeighborInList(this.closed, node)) { // if not already done node.g = node.parent.g + 1.; // Horizontal/vertical cost = 1.0 node.g += maze[this.now.y + y][this.now.x + x]; // add movement cost for this square
// diagonal cost = sqrt(hor_cost² + vert_cost²) // in this example the cost would be 12.2 instead of 11 /* if (diag && x != 0 && y != 0) { node.g += .4; // Diagonal movement cost = 1.4 } */ this.open.add(node); } } } Collections.sort(this.open); }
public static void main(String[] args) { // -1 = blocked // 0+ = additional movement cost int[][] maze = { { 0, 0, 0, 0, 0, 0, 0, 0}, { 0, 0, 0, 0, 0, 0, 0, 0}, { 0, 0, 0,100,100,100, 0, 0}, { 0, 0, 0, 0, 0,100, 0, 0}, { 0, 0,100, 0, 0,100, 0, 0}, { 0, 0,100, 0, 0,100, 0, 0}, { 0, 0,100,100,100,100, 0, 0}, { 0, 0, 0, 0, 0, 0, 0, 0}, }; AStar as = new AStar(maze, 0, 0, true); List<Node> path = as.findPathTo(7, 7); if (path != null) { path.forEach((n) -> { System.out.print("[" + n.x + ", " + n.y + "] "); maze[n.y][n.x] = -1; }); System.out.printf("\nTotal cost: %.02f\n", path.get(path.size() - 1).g);
for (int[] maze_row : maze) { for (int maze_entry : maze_row) { switch (maze_entry) { case 0: System.out.print("_"); break; case -1: System.out.print("*"); break; default: System.out.print("#"); } } System.out.println(); } } }
} </lang>
- Output:
[0, 0] [1, 0] [2, 0] [3, 0] [4, 0] [5, 1] [6, 2] [7, 3] [6, 4] [6, 5] [6, 6] [7, 7] Total cost: 11,00 *****___ _____*__ ___###*_ _____#_* __#__#*_ __#__#*_ __####*_ _______*
JavaScript
Animated.
To see how it works on a random map go here
<lang javascript>
var ctx, map, opn = [], clsd = [], start = {x:1, y:1, f:0, g:0},
goal = {x:8, y:8, f:0, g:0}, mw = 10, mh = 10, neighbours, path;
function findNeighbour( arr, n ) {
var a; for( var i = 0; i < arr.length; i++ ) { a = arr[i]; if( n.x === a.x && n.y === a.y ) return i; } return -1;
} function addNeighbours( cur ) {
var p; for( var i = 0; i < neighbours.length; i++ ) { var n = {x: cur.x + neighbours[i].x, y: cur.y + neighbours[i].y, g: 0, h: 0, prt: {x:cur.x, y:cur.y}}; if( map[n.x][n.y] == 1 || findNeighbour( clsd, n ) > -1 ) continue; n.g = cur.g + neighbours[i].c; n.h = Math.abs( goal.x - n.x ) + Math.abs( goal.y - n.y ); p = findNeighbour( opn, n ); if( p > -1 && opn[p].g + opn[p].h <= n.g + n.h ) continue; opn.push( n ); } opn.sort( function( a, b ) { return ( a.g + a.h ) - ( b.g + b.h ); } );
} function createPath() {
path = []; var a, b; a = clsd.pop(); path.push( a ); while( clsd.length ) { b = clsd.pop(); if( b.x != a.prt.x || b.y != a.prt.y ) continue; a = b; path.push( a ); } }
function solveMap() {
drawMap(); if( opn.length < 1 ) { document.body.appendChild( document.createElement( "p" ) ).innerHTML = "Impossible!"; return; } var cur = opn.splice( 0, 1 )[0]; clsd.push( cur ); if( cur.x == goal.x && cur.y == goal.y ) { createPath(); drawMap(); return; } addNeighbours( cur ); requestAnimationFrame( solveMap );
} function drawMap() {
ctx.fillStyle = "#ee6"; ctx.fillRect( 0, 0, 200, 200 ); for( var j = 0; j < mh; j++ ) { for( var i = 0; i < mw; i++ ) { switch( map[i][j] ) { case 0: continue; case 1: ctx.fillStyle = "#990"; break; case 2: ctx.fillStyle = "#090"; break; case 3: ctx.fillStyle = "#900"; break; } ctx.fillRect( i, j, 1, 1 ); } } var a; if( path.length ) { var txt = "Path: " + ( path.length - 1 ) + "
["; for( var i = path.length - 1; i > -1; i-- ) { a = path[i]; ctx.fillStyle = "#999"; ctx.fillRect( a.x, a.y, 1, 1 ); txt += "(" + a.x + ", " + a.y + ") "; } document.body.appendChild( document.createElement( "p" ) ).innerHTML = txt + "]"; return; } for( var i = 0; i < opn.length; i++ ) { a = opn[i]; ctx.fillStyle = "#909"; ctx.fillRect( a.x, a.y, 1, 1 ); } for( var i = 0; i < clsd.length; i++ ) { a = clsd[i]; ctx.fillStyle = "#009"; ctx.fillRect( a.x, a.y, 1, 1 ); }
} function createMap() {
map = new Array( mw ); for( var i = 0; i < mw; i++ ) { map[i] = new Array( mh ); for( var j = 0; j < mh; j++ ) { if( !i || !j || i == mw - 1 || j == mh - 1 ) map[i][j] = 1; else map[i][j] = 0; } } map[5][3] = map[6][3] = map[7][3] = map[3][4] = map[7][4] = map[3][5] = map[7][5] = map[3][6] = map[4][6] = map[5][6] = map[6][6] = map[7][6] = 1; //map[start.x][start.y] = 2; map[goal.x][goal.y] = 3;
} function init() {
var canvas = document.createElement( "canvas" ); canvas.width = canvas.height = 200; ctx = canvas.getContext( "2d" ); ctx.scale( 20, 20 ); document.body.appendChild( canvas ); neighbours = [ {x:1, y:0, c:1}, {x:-1, y:0, c:1}, {x:0, y:1, c:1}, {x:0, y:-1, c:1}, {x:1, y:1, c:1.4}, {x:1, y:-1, c:1.4}, {x:-1, y:1, c:1.4}, {x:-1, y:-1, c:1.4} ]; path = []; createMap(); opn.push( start ); solveMap();
} </lang>
- Output:
Path: 11 [(1, 1) (2, 2) (2, 3) (2, 4) (2, 5) (2, 6) (3, 7) (4, 8) (5, 8) (6, 8) (7, 8) (8, 8) ]
Julia
The graphic in this solution is displayed in the more standard orientation of origin at bottom left and goal at top right. <lang Julia>using LightGraphs, SimpleWeightedGraphs
const chessboardsize = 8 const givenobstacles = [(2,4), (2,5), (2,6), (3,6), (4,6), (5,6), (5,5), (5,4), (5,3), (5,2), (4,2), (3,2)] vfromcart(p, n) = (p[1] - 1) * n + p[2] const obstacles = [vfromcart(o .+ 1, chessboardsize) for o in givenobstacles] zbasedpath(path, n) = [(div(v - 1, n), (v - 1) % n) for v in path] pathcost(path) = sum(map(x -> x in obstacles ? 100 : 1, path[2:end]))
function surround(x, y, n)
bottomx = x > 1 ? x -1 : x topx = x < n ? x + 1 : x bottomy = y > 1 ? y - 1 : y topy = y < n ? y + 1 : y [CartesianIndex(x,y) for x in bottomx:topx for y in bottomy:topy]
end
function kinggraph(N)
graph = SimpleWeightedGraph(N*N) for row in 1:N, col in 1:N, p in surround(row, col, N) origin = vfromcart(CartesianIndex(row, col), N) targ = vfromcart(p, N) hcost = (targ in obstacles || origin in obstacles) ? 100 : 1 add_edge!(graph, origin, targ, hcost) end graph
end
kgraph = kinggraph(chessboardsize) path = enumerate_paths(dijkstra_shortest_paths(kgraph, 1), 64) println("Solution has cost $(pathcost(path)):\n", zbasedpath(path, chessboardsize))
path2graphic(x, path) = (x in obstacles ? '█' : x in path ? 'x' : '.') for row in 8:-1:1, col in 7:-1:0
print(path2graphic(row*8 - col, path)) if col == 0 println() end
end</lang>
- Output:
Solution has cost 11: Tuple{Int64,Int64}[(0, 0), (1, 1), (2, 2), (3, 1), (4, 1), (5, 1), (6, 2), (7, 3), (7, 4), (6, 5), (6, 6), (7, 7)] ...xx..x ..x..xx. .x█████. .x█...█. .x█...█. ..x.███. .x...... x.......
Kotlin
<lang kotlin> import java.lang.Math.abs
typealias GridPosition = Pair<Int, Int> typealias Barrier = Set<GridPosition>
const val MAX_SCORE = 99999999
abstract class Grid(private val barriers: List<Barrier>) {
open fun heuristicDistance(start: GridPosition, finish: GridPosition): Int { val dx = abs(start.first - finish.first) val dy = abs(start.second - finish.second) return (dx + dy) + (-2) * minOf(dx, dy) }
fun inBarrier(position: GridPosition) = barriers.any { it.contains(position) }
abstract fun getNeighbours(position: GridPosition): List<GridPosition>
open fun moveCost(from: GridPosition, to: GridPosition) = if (inBarrier(to)) MAX_SCORE else 1
}
class SquareGrid(width: Int, height: Int, barriers: List<Barrier>) : Grid(barriers) {
private val heightRange: IntRange = (0 until height) private val widthRange: IntRange = (0 until width)
private val validMoves = listOf(Pair(1, 0), Pair(-1, 0), Pair(0, 1), Pair(0, -1), Pair(1, 1), Pair(-1, 1), Pair(1, -1), Pair(-1, -1))
override fun getNeighbours(position: GridPosition): List<GridPosition> = validMoves .map { GridPosition(position.first + it.first, position.second + it.second) } .filter { inGrid(it) }
private fun inGrid(it: GridPosition) = (it.first in widthRange) && (it.second in heightRange)
}
/**
* Implementation of the A* Search Algorithm to find the optimum path between 2 points on a grid. * * The Grid contains the details of the barriers and methods which supply the neighboring vertices and the * cost of movement between 2 cells. Examples use a standard Grid which allows movement in 8 directions * (i.e. includes diagonals) but alternative implementation of Grid can be supplied. * */
fun aStarSearch(start: GridPosition, finish: GridPosition, grid: Grid): Pair<List<GridPosition>, Int> {
/** * Use the cameFrom values to Backtrack to the start position to generate the path */ fun generatePath(currentPos: GridPosition, cameFrom: Map<GridPosition, GridPosition>): List<GridPosition> { val path = mutableListOf(currentPos) var current = currentPos while (cameFrom.containsKey(current)) { current = cameFrom.getValue(current) path.add(0, current) } return path.toList() }
val openVertices = mutableSetOf(start) val closedVertices = mutableSetOf<GridPosition>() val costFromStart = mutableMapOf(start to 0) val estimatedTotalCost = mutableMapOf(start to grid.heuristicDistance(start, finish))
val cameFrom = mutableMapOf<GridPosition, GridPosition>() // Used to generate path by back tracking
while (openVertices.size > 0) {
val currentPos = openVertices.minBy { estimatedTotalCost.getValue(it) }!!
// Check if we have reached the finish if (currentPos == finish) { // Backtrack to generate the most efficient path val path = generatePath(currentPos, cameFrom) return Pair(path, estimatedTotalCost.getValue(finish)) // First Route to finish will be optimum route }
// Mark the current vertex as closed openVertices.remove(currentPos) closedVertices.add(currentPos)
grid.getNeighbours(currentPos) .filterNot { closedVertices.contains(it) } // Exclude previous visited vertices .forEach { neighbour -> val score = costFromStart.getValue(currentPos) + grid.moveCost(currentPos, neighbour) if (score < costFromStart.getOrDefault(neighbour, MAX_SCORE)) { if (!openVertices.contains(neighbour)) { openVertices.add(neighbour) } cameFrom.put(neighbour, currentPos) costFromStart.put(neighbour, score) estimatedTotalCost.put(neighbour, score + grid.heuristicDistance(neighbour, finish)) } }
}
throw IllegalArgumentException("No Path from Start $start to Finish $finish")
}
fun main(args: Array<String>) {
val barriers = listOf(setOf( Pair(2,4), Pair(2,5), Pair(2,6), Pair(3,6), Pair(4,6), Pair(5,6), Pair(5,5), Pair(5,4), Pair(5,3), Pair(5,2), Pair(4,2), Pair(3,2)))
val (path, cost) = aStarSearch(GridPosition(0,0), GridPosition(7,7), SquareGrid(8,8, barriers))
println("Cost: $cost Path: $path")
} </lang>
- Output:
Cost: 11 Path: [(0, 0), (1, 1), (2, 2), (3, 1), (4, 1), (5, 1), (6, 2), (6, 3), (6, 4), (6, 5), (6, 6), (7, 7)]
Lua
<lang lua> -- QUEUE ----------------------------------------------------------------------- Queue = {} function Queue:new()
local q = {} self.__index = self return setmetatable( q, self )
end function Queue:push( v )
table.insert( self, v )
end function Queue:pop()
return table.remove( self, 1 )
end function Queue:getSmallestF()
local s, i = nil, 2 while( self[i] ~= nil and self[1] ~= nil ) do if self[i]:F() < self[1]:F() then s = self[1] self[1] = self[i] self[i] = s end i = i + 1 end return self:pop()
end
-- LIST ------------------------------------------------------------------------ List = {} function List:new()
local l = {} self.__index = self return setmetatable( l, self )
end function List:push( v )
table.insert( self, v )
end function List:pop()
return table.remove( self )
end
-- POINT ----------------------------------------------------------------------- Point = {} function Point:new()
local p = { y = 0, x = 0 } self.__index = self return setmetatable( p, self )
end function Point:set( x, y )
self.x, self.y = x, y
end function Point:equals( o )
return (o.x == self.x and o.y == self.y)
end function Point:print()
print( self.x, self.y )
end
-- NODE ------------------------------------------------------------------------ Node = {} function Node:new()
local n = { pos = Point:new(), parent = Point:new(), dist = 0, cost = 0 } self.__index = self return setmetatable( n, self )
end function Node:set( pt, parent, dist, cost )
self.pos = pt self.parent = parent self.dist = dist self.cost = cost
end function Node:F()
return ( self.dist + self.cost )
end
-- A-STAR ---------------------------------------------------------------------- local nbours = {
{ 1, 0, 1 }, { 0, 1, 1 }, { 1, 1, 1.4 }, { 1, -1, 1.4 }, { -1, -1, 1.4 }, { -1, 1, 1.4 }, { 0, -1, 1 }, { -1, 0, 1 }
} local map = {
1,1,1,1,1,1,1,1,1,1, 1,0,0,0,0,0,0,0,0,1, 1,0,0,0,0,0,0,0,0,1, 1,0,0,0,0,1,1,1,0,1, 1,0,0,1,0,0,0,1,0,1, 1,0,0,1,0,0,0,1,0,1, 1,0,0,1,1,1,1,1,0,1, 1,0,0,0,0,0,0,0,0,1, 1,0,0,0,0,0,0,0,0,1, 1,1,1,1,1,1,1,1,1,1
} local open, closed, start, goal,
mapW, mapH = Queue:new(), List:new(), Point:new(), Point:new(), 10, 10
start:set( 2, 2 ); goal:set( 9, 9 )
function hasNode( arr, pos )
for nx, val in ipairs( arr ) do if val.pos:equals( pos ) then return nx end end return -1
end function isValid( pos )
return pos.x > 0 and pos.x <= mapW and pos.y > 0 and pos.y <= mapH and map[pos.x + mapW * pos.y - mapW] == 0
end function calcDist( p1 )
local x, y = goal.x - p1.x, goal.y - p1.y return math.abs( x ) + math.abs( y )
end function addToOpen( node )
local nx for n = 1, 8 do nNode = Node:new() nNode.parent:set( node.pos.x, node.pos.y ) nNode.pos:set( node.pos.x + nbours[n][1], node.pos.y + nbours[n][2] ) nNode.cost = node.cost + nbours[n][3] nNode.dist = calcDist( nNode.pos ) if isValid( nNode.pos ) then if nNode.pos:equals( goal ) then closed:push( nNode ) return true end nx = hasNode( closed, nNode.pos ) if nx < 0 then nx = hasNode( open, nNode.pos ) if( nx < 0 ) or ( nx > 0 and nNode:F() < open[nx]:F() ) then if( nx > 0 ) then table.remove( open, nx ) end open:push( nNode ) else nNode = nil end end end end return false
end function makePath()
local i, l = #closed, List:new() local node, parent = closed[i], nil
l:push( node.pos ) parent = node.parent while( i > 0 ) do i = i - 1 node = closed[i] if node ~= nil and node.pos:equals( parent ) then l:push( node.pos ) parent = node.parent end end print( string.format( "Cost: %d", #l - 1 ) ) io.write( "Path: " ) for i = #l, 1, -1 do map[l[i].x + mapW * l[i].y - mapW] = 2 io.write( string.format( "(%d, %d) ", l[i].x, l[i].y ) ) end print( "" )
end function aStar()
local n = Node:new() n.dist = calcDist( start ) n.pos:set( start.x, start.y ) open:push( n ) while( true ) do local node = open:getSmallestF() if node == nil then break end closed:push( node ) if addToOpen( node ) == true then makePath() return true end end return false
end -- ENTRY POINT ----------------------------------------------------------------- if true == aStar() then
local m for j = 1, mapH do for i = 1, mapW do m = map[i + mapW * j - mapW] if m == 0 then io.write( "." ) elseif m == 1 then io.write( string.char(0xdb) ) else io.write( "x" ) end end io.write( "\n" ) end
else
print( "can not find a path!" )
end </lang>
- Output:
Cost: 11 Path: (2, 2) (3, 3) (3, 4) (3, 5) (3, 6) (3, 7) (4, 8) (5, 9) (6, 9) (7, 9) (8, 9) (9, 9) ██████████ █x.......█ █.x......█ █.x..███.█ █.x█...█.█ █.x█...█.█ █.x█████.█ █..x.....█ █...xxxxx█ ██████████
Nim
<lang Nim>
- A* search algorithm.
from algorithm import reverse import sets import strformat import tables
const Infinity = 1_000_000_000
type Cell = tuple[row, col: int]
const
Barriers = [(2, 4), (2, 5), (2, 6), (3, 6), (4, 6), (5, 6), (5, 5), (5, 4), (5, 3), (5, 2), (4, 2), (3, 2)].toHashSet Moves = [(-1, -1), (-1, 0), (-1, 1), (0, 1), (1, 1), (1, 0), (1, -1), (0, -1)]
- ---------------------------------------------------------------------------------------------------
iterator neighbors(cell: Cell): Cell =
## Yield the neighbors of "cell". for move in Moves: let next = (row: cell.row + move[0], col: cell.col + move[1]) if next.row in 0..7 and next.col in 0..7: yield next
- ---------------------------------------------------------------------------------------------------
func d(current, neighbor: Cell): int =
## Return the cost for the move from "current" to "neighbor". if neighbor in Barriers: 100 else: 1
- ---------------------------------------------------------------------------------------------------
func h(cell, goal: Cell): int =
## Compute the heuristic cost for a move form the cell to the goal. ## We use the Chebychev distance as appropriate for this kind of move. max(abs(goal.row - cell.row), abs(goal.col - cell.col))
- ---------------------------------------------------------------------------------------------------
func reconstructedPath(cameFrom: Table[Cell, Cell]; current: Cell): seq[Cell] =
## Return the shortest path from the start to the goal. var cell = current result = @[cell] while cell in cameFrom: cell = cameFrom[cell] result.add(cell) result.reverse()
- ---------------------------------------------------------------------------------------------------
func search(start, goal: Cell): tuple[path: seq[Cell], cost: int] =
## Search the shortest path from "start" to "goal" using A* algorithm. ## Return the path and the cost.
var openSet = [start].toHashSet() var visited: HashSet[Cell] var cameFrom: Table[Cell, Cell] var gScore, fScore: Table[Cell, int] gscore[start] = 0 fScore[start] = h(start, goal)
while openSet.len != 0:
# Find cell in "openset" with minimal "fScore". var current: Cell var minScore = Infinity for cell in openSet: let score = fScore.getOrDefault(cell, Infinity) if score < minScore: current = cell minScore = score
if current == goal: # Return the path and cost. return (reconstructedPath(cameFrom, current), fScore[goal])
openSet.excl(current) visited.incl(current)
# Update scores for neighbors. for neighbor in current.neighbors(): if neighbor in visited: # Already processed. continue let tentative = gScore[current] + d(current, neighbor) if tentative < gScore.getOrDefault(neighbor, Infinity): cameFrom[neighbor]= current gScore[neighbor] = tentative fScore[neighbor] = tentative + h(neighbor, goal) if neighbor notin openSet: openSet.incl(neighbor)
- ---------------------------------------------------------------------------------------------------
proc drawBoard(path: seq[Cell]) =
## Draw the board and the path.
func `$`(cell: Cell): string = ## Return the Unicode string to use for a cell. if cell in Barriers: "██" else: (if cell in path: " #" else: " ·")
echo "████████████████████" for row in 0..7: stdout.write("██") for col in 0..7: stdout.write((row, col)) stdout.write("██\n") echo "████████████████████" echo '\n'
- ---------------------------------------------------------------------------------------------------
proc printSolution(path: seq[Cell]; cost: int) =
## Print the solution. stdout.write(&"Path: ({path[0].row}, {path[0].col})") for idx in 1..path.high: let cell = path[idx] stdout.write(&" → ({cell.row}, {cell.col})") stdout.write(&"\nCost: {cost}\n") echo '\n' drawBoard(path)
- ---------------------------------------------------------------------------------------------------
let (path, cost) = search((0, 0), (7, 7)) if cost == 0:
echo "No solution found"
else:
printSolution(path, cost)
</lang>
- Output:
Path: (0, 0) → (1, 1) → (2, 2) → (3, 1) → (4, 1) → (5, 1) → (6, 2) → (7, 3) → (7, 4) → (6, 5) → (7, 6) → (7, 7) Cost: 11 ████████████████████ ██ # · · · · · · ·██ ██ · # · · · · · ·██ ██ · · # ·██████ ·██ ██ · #██ · · ·██ ·██ ██ · #██ · · ·██ ·██ ██ · #██████████ ·██ ██ · · # · · # · ·██ ██ · · · # # · # #██ ████████████████████
OCaml
A very close/straightforward implementation of the Wikipedia pseudocode.
<lang ocaml> module IntPairs =
struct type t = int * int let compare (x0,y0) (x1,y1) = match Stdlib.compare x0 x1 with | 0 -> Stdlib.compare y0 y1 | c -> c end
module PairsMap = Map.Make(IntPairs) module PairsSet = Set.Make(IntPairs)
let find_path start goal board =
let max_y = Array.length board in let max_x = Array.length board.(0) in
let get_neighbors (x, y) = let moves = [(0, 1); (0, -1); (1, 0); (-1, 0); (1, 1); (1, -1); (-1, 1); (-1, -1)] in let ms = List.map (fun (_x, _y) -> x+_x, y+_y) moves in let ms = List.filter (fun (x, y) -> x >= 0 && x < max_x && y >= 0 && y < max_y && board.(y).(x) <> 0 ) ms in (ms) in let h (x0, y0) (x1, y1) = abs (x0 - x1) + abs (y0 - y1) in let openSet = PairsSet.add start PairsSet.empty in let closedSet = PairsSet.empty in
let fScore = PairsMap.add start (h goal start) PairsMap.empty in let gScore = PairsMap.add start 0 PairsMap.empty in
let cameFrom = PairsMap.empty in
let reconstruct_path cameFrom current = let rec aux acc current = let from = PairsMap.find current cameFrom in if from = start then (from::acc) else aux (from::acc) from in aux [current] current in let d current neighbor = let x, y = neighbor in board.(y).(x) in let g gScore cell = match PairsMap.find_opt cell gScore with | Some v -> v | None -> max_int in
let rec _find_path (openSet, closedSet, fScore, gScore, cameFrom) = if PairsSet.is_empty openSet then None else let current = PairsSet.fold (fun p1 p2 -> if p2 = (-1, -1) then p1 else let s1 = PairsMap.find p1 fScore and s2 = PairsMap.find p2 fScore in if s1 < s2 then p1 else p2 ) openSet (-1, -1) in if current = goal then Some (reconstruct_path cameFrom current) else let openSet = PairsSet.remove current openSet in let closedSet = PairsSet.add current closedSet in let neighbors = get_neighbors current in neighbors |> List.fold_left (fun ((openSet, closedSet, fScore, gScore, cameFrom) as v) neighbor -> if PairsSet.mem neighbor closedSet then (v) else let tentative_gScore = (g gScore current) + (d current neighbor) in if tentative_gScore < (g gScore neighbor) then let cameFrom = PairsMap.add neighbor current cameFrom in let gScore = PairsMap.add neighbor tentative_gScore gScore in let f = (g gScore neighbor) + (h neighbor goal) in let fScore = PairsMap.add neighbor f fScore in let openSet = if not (PairsSet.mem neighbor openSet) then PairsSet.add neighbor openSet else openSet in (openSet, closedSet, fScore, gScore, cameFrom) else (v) ) (openSet, closedSet, fScore, gScore, cameFrom) |> _find_path in _find_path (openSet, closedSet, fScore, gScore, cameFrom)
let () =
let board = [| [| 1; 1; 1; 1; 1; 1; 1; 1; |]; [| 1; 1; 1; 1; 1; 1; 1; 1; |]; [| 1; 1; 1; 0; 0; 0; 1; 1; |]; [| 1; 1; 1; 1; 1; 0; 1; 1; |]; [| 1; 1; 0; 1; 1; 0; 1; 1; |]; [| 1; 1; 0; 1; 1; 0; 1; 1; |]; [| 1; 1; 0; 0; 0; 0; 1; 1; |]; [| 1; 1; 1; 1; 1; 1; 1; 1; |]; |] in let start = (0, 0) in let goal = (7, 7) in
let dim_x = Array.length board.(0) in let dim_y = Array.length board in
let r = find_path start goal board in
match r with | None -> failwith "path not found" | Some p -> List.iter (fun (x, y) -> Printf.printf " (%d, %d)\n" x y) p; print_newline (); let _board = Array.init dim_y (fun y -> Array.init dim_x (fun x -> if board.(y).(x) = 0 then '#' else '.')) in List.iter (fun (x, y) -> _board.(y).(x) <- '*') p;
Array.iter (fun line -> Array.iter (fun c -> Printf.printf " %c" c; ) line; print_newline () ) _board; print_newline ()</lang>
- Output:
(0, 0) (1, 1) (2, 2) (2, 3) (1, 4) (1, 5) (1, 6) (2, 7) (3, 7) (4, 7) (5, 7) (6, 7) (7, 7) * . . . . . . . . * . . . . . . . . * # # # . . . . * . . # . . . * # . . # . . . * # . . # . . . * # # # # . . . . * * * * * *
Ol
<lang scheme>
- level
- list of lists, any except 1 means the cell is empty
- from
- start cell in (x . y) mean
- to
- destination cell in (x . y) mean
(define (A* level from to)
(define (hash xy) ; internal hash (+ (<< (car xy) 16) (cdr xy)))
; "is the cell is empty?" (define (floor? x y) (let ((line (list-ref level y))) (if line (not (eq? (list-ref line x) 1)))))
(unless (equal? from to) ; search not finished yet (let step1 ((n 999) ; maximal count of search steps (c-list-set #empty) (o-list-set (put #empty (hash from) [from #f 0 0 0]))) (unless (empty? o-list-set) ; do we have a space to move? ; no. let's find cell with minimal const (let*((f (ff-fold (lambda (s key value) (if (< (ref value 5) (car s)) (cons (ref value 5) value) s)) (cons 9999 #f) o-list-set)) (xy (ref (cdr f) 1)) ; move the cell from "open" to "closed" list (o-list-set (del o-list-set (hash xy))) (c-list-set (put c-list-set (hash xy) (cdr f))))
; (if (or (eq? n 0) (equal? xy to)) (let rev ((xy xy)) ; let's unroll the math and return only first step (let*((parent (ref (get c-list-set (hash xy) #f) 2)) (parent-of-parent (ref (get c-list-set (hash parent) #f) 2))) (if parent-of-parent (rev parent) (cons (- (car xy) (car parent)) (- (cdr xy) (cdr parent))))))
(let*((x (car xy)) (y (cdr xy)) (o-list-set (fold (lambda (n v) (if (and (floor? (car v) (cdr v)) (eq? #f (get c-list-set (hash v) #f))) (let ((G (+ (ref (get c-list-set (hash xy) #f) 3) 1)); G of parent + 1 ; H calculated by "Manhattan method" (H (* (+ (abs (- (car v) (car to))) (abs (- (cdr v) (cdr to)))) 2)) (got (get o-list-set (hash v) #f)))
(if got (if (< G (ref got 3)) (put n (hash v) [v xy G H (+ G H)]) n) (put n (hash v) [v xy G H (+ G H)]))) n)) o-list-set (list (cons x (- y 1)) (cons x (+ y 1)) (cons (- x 1) y) (cons (+ x 1) y))))) (step1 (- n 1) c-list-set o-list-set))))))))
</lang>
- Output:
<lang scheme> (define level '(
(1 1 1 1 1 1 1 1 1 1) (1 A 0 0 0 0 0 0 0 1) (1 0 0 0 0 0 0 0 0 1) (1 0 0 0 0 1 1 1 0 1) (1 1 0 0 0 0 0 1 0 1) (1 0 0 1 0 0 0 1 0 1) (1 0 0 1 1 1 1 1 0 1) (1 0 0 0 0 0 0 0 0 1) (1 0 0 0 1 0 0 0 B 1) (1 1 1 1 1 1 1 1 1 1)
)) (for-each print level)
- let's check that we can't move to (into wall)
(print (A* level '(1 . 1) '(9 . 9)))
(define to '(8 . 8)) (define (plus a b) (cons (+ (car a) (car b)) (+ (cdr a) (cdr b)))) ; helper
(define path (let loop ((me '(1 . 1)) (path '()))
(if (equal? me to) (begin (print "here I am!") (cons to path)) (let ((move (A* level me to))) (unless move (begin (print "no way, sorry :(") #false) (let ((step (plus me move))) (print me " + " move " -> " step) (loop step (cons me path))))))))
- let's draw the path?
(define (has? lst x) ; helper
(cond ((null? lst) #false) ((equal? (car lst) x) lst) (else (has? (cdr lst) x))))
(define solved
(map (lambda (row y) (map (lambda (cell x) (cond ((equal? (cons x y) '(1 . 1)) "A") ((equal? (cons x y) '(8 . 8)) "B") ((has? path (cons x y)) "*") (else cell))) row (iota 10))) level (iota 10)))
(for-each print solved) </lang>
the map: (1 1 1 1 1 1 1 1 1 1) (1 A 0 0 0 0 0 0 0 1) (1 0 0 0 0 0 0 0 0 1) (1 0 0 0 0 1 1 1 0 1) (1 1 0 0 0 0 0 1 0 1) (1 0 0 1 0 0 0 1 0 1) (1 0 0 1 1 1 1 1 0 1) (1 0 0 0 0 0 0 0 0 1) (1 0 0 0 1 0 0 0 B 1) (1 1 1 1 1 1 1 1 1 1) we should not reach the '(9 . 9) cell: #false ok, we got #false, so really can't. now try to reach cell '(8 . 8) - the 'B' point: (1 . 1) + (0 . 1) -> (1 . 2) (1 . 2) + (0 . 1) -> (1 . 3) (1 . 3) + (1 . 0) -> (2 . 3) (2 . 3) + (0 . 1) -> (2 . 4) (2 . 4) + (0 . 1) -> (2 . 5) (2 . 5) + (0 . 1) -> (2 . 6) (2 . 6) + (0 . 1) -> (2 . 7) (2 . 7) + (1 . 0) -> (3 . 7) (3 . 7) + (1 . 0) -> (4 . 7) (4 . 7) + (1 . 0) -> (5 . 7) (5 . 7) + (0 . 1) -> (5 . 8) (5 . 8) + (1 . 0) -> (6 . 8) (6 . 8) + (1 . 0) -> (7 . 8) (7 . 8) + (1 . 0) -> (8 . 8) here I am! (1 1 1 1 1 1 1 1 1 1) (1 A 0 0 0 0 0 0 0 1) (1 * 0 0 0 0 0 0 0 1) (1 * * 0 0 1 1 1 0 1) (1 1 * 0 0 0 0 1 0 1) (1 0 * 1 0 0 0 1 0 1) (1 0 * 1 1 1 1 1 0 1) (1 0 * * * * 0 0 0 1) (1 0 0 0 1 * * * B 1) (1 1 1 1 1 1 1 1 1 1)
Phix
rows and columns are numbered 1 to 8. start position is {1,1} and end position is {8,8}. barriers are simply avoided, rather than costed at 100. Note that the 23 visited nodes does not count walls, but with them this algorithm exactly matches the 35 of Racket. <lang Phix>sequence grid = split(""" x:::::::
""",'\n')
constant permitted = {{-1,-1},{0,-1},{1,-1},
{-1, 0}, {1, 0}, {-1, 1},{0,+1},{1,+1}}
sequence key = {7,0}, -- chebyshev, cost
moves = Template:1,1, data = {moves}, acta = {} -- actually analysed set
setd(key,data) bool found = false integer count = 0 while not found do
if dict_size()=0 then ?"impossible" exit end if key = getd_partial_key(0) data = getd(key) moves = data[$] if length(data)=1 then deld(key) else data = data[1..$-1] putd(key,data) end if count += 1 acta = append(acta,moves[$]) for i=1 to length(permitted) do sequence newpos = sq_add(moves[$],permitted[i]) integer {nx,ny} = newpos if nx>=1 and nx<=8 and ny>=1 and ny<=8 and grid[nx,ny] = ':' then -- (unvisited) grid[nx,ny] = '.' sequence newkey = {max(8-nx,8-ny),key[2]+1}, newmoves = append(moves,newpos) if newpos = {8,8} then moves = newmoves found = true exit end if integer k = getd_index(newkey) if k=0 then data = {newmoves} else data = append(getd_by_index(k),newmoves) end if putd(newkey,data) end if end for
end while if found then
printf(1,"visited %d nodes\ncost:%d\npath:%v\n",{count,length(moves)-1,moves}) for i=1 to length(acta) do integer {x,y} = acta[i] grid[x,y] = '_' end for for i=1 to length(moves) do integer {x,y} = moves[i] grid[x,y] = 'x' end for puts(1,join(grid,'\n'))
end if</lang>
- Output:
visited 23 nodes cost:11 path:{{1,1},{2,2},{3,3},{4,2},{5,2},{6,2},{7,3},{8,4},{8,5},{8,6},{8,7},{8,8}} x......: .x____.: ._x_###: .x#___#: .x#___#: .x#####: ..x..... :..xxxxx
The : represent nodes it did not even look at, the . those added but never gone back to, obviously x represent the path, and together _ and x all nodes actually analysed.
Extra credit
Well, why not. Note this does not reuse/share any code with the above, although I presume the task author assumed it would, instead the main loop uses a priority queue to obtain the next lowest cost and a simple dictionary to avoid re-examination/inifinte recursion. <lang Phix>--set_rand(3) -- (for consistent output) constant optimal = false,
mtm = true, -- mutli-tile metrics target = {1,2,3,4,5,6,7,8,0}, -- <-tile found 0..8-> mcost = {{0,0,1,2,1,2,3,2,3}, -- position 1 {0,1,0,1,2,1,2,3,2}, {0,2,1,0,3,2,1,4,3}, {0,1,2,3,0,1,2,1,2}, {0,2,1,2,1,0,1,2,1}, -- ... {0,3,2,1,2,1,0,3,2}, {0,2,3,4,1,2,3,0,1}, {0,3,2,3,2,1,2,1,0}, {0,4,3,2,3,2,1,2,1}}, -- position 9 udlr = "udlr", dirs = {+3,-3,+1,-1}, -- udlr lims = {{9,9,9,9,9,9,9,9,9}, -- up {1,1,1,1,1,1,1,1,1}, -- down {3,3,3,6,6,6,9,9,9}, -- left {1,1,1,4,4,4,7,7,7}} -- right
function get_moves(sequence grid, bool mtm)
sequence valid = {} integer p0 = find(0,grid) for dx=1 to length(dirs) do integer step = dirs[dx], lim = lims[dx][p0], count = 1 for i=p0+step to lim by step do valid = append(valid,{step,i,udlr[dx],count}) if not mtm then exit end if count += 1 end for end for return valid
end function
function make_move(sequence grid, move)
integer p0 = find(0,grid), {step,lim} = move for i=p0+step to lim by step do grid[p0] = grid[i] grid[i] = 0 p0 = i end for return grid
end function
function manhattan(sequence grid)
integer res = 0 for i=1 to 9 do res += mcost[i][grid[i]+1] end for return res
end function
sequence problem, grid, new_grid,
moves, next_moves, move
procedure show_grid()
printf(1,"%s\n",join_by(sq_add(grid,'0'),1,3,""))
end procedure
grid = target for i=1 to 1000 do
-- (initially shuffle as if mtm==true, otherwise -- output compares answers to different puzzles) moves = get_moves(grid,true) move = moves[rand(length(moves))] grid = make_move(grid,move)
end for problem = grid printf(1,"problem (manhattan cost is %d):\n",manhattan(grid)) show_grid()
integer todo = pq_new(),
seen = new_dict()
pq_add({{grid,{}},iff(optimal?0:manhattan(grid))},todo) setd(grid,true,seen) atom t1 = time()+1 bool found = false integer count = 0, mc while not found do
if pq_size(todo)=0 then ?"impossible" exit end if {{grid,moves},mc} = pq_pop(todo) if time()>t1 then string m = iff(optimal?"moves":"manhattan") printf(1,"searching (count=%d, %s=%d)\r",{count,m,mc}) t1 = time()+1 end if next_moves = get_moves(grid,mtm) count += length(next_moves) integer l = length(moves) for i=1 to length(next_moves) do move = next_moves[i] new_grid = make_move(grid,move) mc = manhattan(new_grid) if mc=0 then if new_grid!=target then ?9/0 end if moves = append(moves,move) found = true exit end if if getd_index(new_grid,seen)=NULL then if optimal then mc = l+1 end if pq_add({{new_grid,append(moves,move)},mc},todo) setd(new_grid,true,seen) end if end for
end while if found then
string s = iff(length(moves)=1?"":"s") if optimal then s &= sprintf(" (max shd be %d)",iff(mtm?24:31)) end if grid = problem string soln = "" for i=1 to length(moves) do move = moves[i] grid = make_move(grid,move) integer {{},{},ch,c} = move soln &= ch if c>1 then soln&='0'+c end if
-- show_grid() -- (set the initial shuffle to eg 5 first!)
end for
-- show_grid() -- (not very educational!)
if grid!=target then ?9/0 end if printf(1,"solved in %d move%s:%s\n",{length(moves),s,soln})
end if printf(1,"count:%d, seen:%d, queue:%d\n",{count,dict_size(seen),pq_size(todo)})</lang>
- Output:
Note: The solutions are non-optimal (far from it, in fact), since it searches lowest manhattan() first.
In fact that set_rand(3), used for all the results below, is somewhat worse than 0, 1, and 2, and the
first to breach optimal limits, ie 31/24, but obviously only when the optimal flag is set to false, as
well as being the first to hint at the potential thousand-fold-or-more performance gains on offer.
An optimal solution can instead be found by searching fewest moves first, albeit significantly slower!
Note this approach is not really suitable for solving 15-puzzles (or larger).
with optimal false and mtm false:
problem (manhattan cost is 20): 546 807 321 solved in 88 moves:ulddruurdluldrdluurrddlurulldrrdlulurrddlurulldrdlururdllurrdlulddrurdlurdlulurrddlurull count:592, seen:371, queue:155
with optimal false and mtm true:
solved in 45 moves:uld2r2u2l2d2r2u2ld2rul2dru2rdl2urdrdlu2rd2luruld2ru2l2dr2uldlu count:328, seen:164, queue:82
with optimal true and mtm false:
solved in 26 moves (max shd be 31):rulldrdruulddruullddrruull count:399996, seen:163976, queue:13728
with optimal true and mtm true:
solved in 17 moves (max shd be 24):rul2drdru2ld2ru2l2d2r2u2l2 count:298400, seen:106034, queue:31434
PowerShell
<lang powershell>function CreateGrid($h, $w, $fill) {
$grid = 0..($h - 1) | ForEach-Object { , (, $fill * $w) } return $grid
}
function EstimateCost($a, $b) {
$xd = [Math]::Abs($a.Item1 - $b.Item1) $yd = [Math]::Abs($a.Item2 - $b.Item2) return [Math]::Max($xd, $yd)
}
function AStar($costs, $start, $goal) {
# ValueTuples can be used to index a Hashtable: $start = [ValueTuple]::Create($start[0], $start[1]) $goal = [ValueTuple]::Create($goal[0], $goal[1])
$rows = $costs.Length $cols = $costs[0].Length
$cameFrom = CreateGrid $rows $cols $null $openSet = @{$start = (EstimateCost $start $goal), 0} $closedSet = @{}
while ($openSet.Count -gt 0) { # find the value in openSet with the lowest fScore $curFScore = [int]::MaxValue
foreach ($p in $openSet.Keys) { $fScore, $gScore = $openSet[$p] if ($fScore -lt $curFScore) { $curFScore = $fScore $curGScore = $gScore $cur = $p } }
if ($cur -eq $goal) { $totalCost = $curGScore break }
$openSet.Remove($cur) $closedSet.Add($cur, 0) $r, $c = $cur.Item1, $cur.Item2
# iterate over each cell in the 3x3 neighborhood foreach ($i in [Math]::Max($r - 1, 0)..[Math]::Min($r + 1, $rows - 1)) { foreach ($j in [Math]::Max($c - 1, 0)..[Math]::Min($c + 1, $cols - 1)) { $neighbor = [ValueTuple]::Create($i, $j) if ($closedSet.ContainsKey($neighbor)) { continue }
$newGScore = $curGScore + $costs[$i][$j] $newFScore = $newGScore + (EstimateCost $neighbor $goal)
if (-not $openSet.ContainsKey($neighbor)) { $openSet[$neighbor] = $newFScore, $newGScore } else { $fs, $gs = $openSet[$neighbor] if ($newGScore -ge $gs) { continue } }
$cameFrom[$i][$j] = $cur } } }
# Walk back from the goal $route = @(, ($goal.Item1, $goal.Item2)) $cur = $goal
while ($cur -ne $start) { $cur = $cameFrom[$cur.Item1][$cur.Item2] $route += , ($cur.Item1, $cur.Item2) }
[array]::Reverse($route) return $route, $totalCost
}
$grid = CreateGrid 8 8 1 $grid[2][4] = 100 $grid[2][5] = 100 $grid[2][6] = 100 $grid[3][6] = 100 $grid[4][6] = 100 $grid[5][6] = 100 $grid[5][5] = 100 $grid[5][4] = 100 $grid[5][3] = 100 $grid[5][2] = 100 $grid[4][2] = 100 $grid[3][2] = 100
$route, $cost = AStar $grid (0, 0) (7, 7) $displayGrid = CreateGrid 8 8 '.'
foreach ($i in 0..7) {
foreach ($j in 0..7) { if ($grid[$i][$j] -gt 1) { $displayGrid[$i][$j] = '#' } }
}
foreach ($step in $route) {
$displayGrid[$step[0]][$step[1]] = 'x'
}
Write-Output ($displayGrid | ForEach-Object { $_ -join }) Write-Output "Cost: $cost" $routeString = ($route | ForEach-Object { "($($_[0]), $($_[1]))" }) -join ', ' Write-Output "Route: $routeString"</lang>
- Output:
x....... .x...... ..x.###. .x#...#. .x#...#. .x#####. ..x.x.x. ...x.x.x Cost: 11 Route: (0, 0), (1, 1), (2, 2), (3, 1), (4, 1), (5, 1), (6, 2), (7, 3), (6, 4), (7, 5), (6, 6), (7, 7)
Python
<lang python>from __future__ import print_function import matplotlib.pyplot as plt
class AStarGraph(object): #Define a class board like grid with two barriers
def __init__(self): self.barriers = [] self.barriers.append([(2,4),(2,5),(2,6),(3,6),(4,6),(5,6),(5,5),(5,4),(5,3),(5,2),(4,2),(3,2)])
def heuristic(self, start, goal): #Use Chebyshev distance heuristic if we can move one square either #adjacent or diagonal D = 1 D2 = 1 dx = abs(start[0] - goal[0]) dy = abs(start[1] - goal[1]) return D * (dx + dy) + (D2 - 2 * D) * min(dx, dy)
def get_vertex_neighbours(self, pos): n = [] #Moves allow link a chess king for dx, dy in [(1,0),(-1,0),(0,1),(0,-1),(1,1),(-1,1),(1,-1),(-1,-1)]: x2 = pos[0] + dx y2 = pos[1] + dy if x2 < 0 or x2 > 7 or y2 < 0 or y2 > 7: continue n.append((x2, y2)) return n
def move_cost(self, a, b): for barrier in self.barriers: if b in barrier: return 100 #Extremely high cost to enter barrier squares return 1 #Normal movement cost
def AStarSearch(start, end, graph):
G = {} #Actual movement cost to each position from the start position F = {} #Estimated movement cost of start to end going via this position
#Initialize starting values G[start] = 0 F[start] = graph.heuristic(start, end)
closedVertices = set() openVertices = set([start]) cameFrom = {}
while len(openVertices) > 0: #Get the vertex in the open list with the lowest F score current = None currentFscore = None for pos in openVertices: if current is None or F[pos] < currentFscore: currentFscore = F[pos] current = pos
#Check if we have reached the goal if current == end: #Retrace our route backward path = [current] while current in cameFrom: current = cameFrom[current] path.append(current) path.reverse() return path, F[end] #Done!
#Mark the current vertex as closed openVertices.remove(current) closedVertices.add(current)
#Update scores for vertices near the current position for neighbour in graph.get_vertex_neighbours(current): if neighbour in closedVertices: continue #We have already processed this node exhaustively candidateG = G[current] + graph.move_cost(current, neighbour)
if neighbour not in openVertices: openVertices.add(neighbour) #Discovered a new vertex elif candidateG >= G[neighbour]: continue #This G score is worse than previously found
#Adopt this G score cameFrom[neighbour] = current G[neighbour] = candidateG H = graph.heuristic(neighbour, end) F[neighbour] = G[neighbour] + H
raise RuntimeError("A* failed to find a solution")
if __name__=="__main__": graph = AStarGraph() result, cost = AStarSearch((0,0), (7,7), graph) print ("route", result) print ("cost", cost) plt.plot([v[0] for v in result], [v[1] for v in result]) for barrier in graph.barriers: plt.plot([v[0] for v in barrier], [v[1] for v in barrier]) plt.xlim(-1,8) plt.ylim(-1,8) plt.show()</lang>
- Output:
route [(0, 0), (1, 1), (2, 2), (3, 1), (4, 1), (5, 1), (6, 2), (7, 3), (6, 4), (7, 5), (6, 6), (7, 7)] cost 11
Racket
This code is lifted from: this blog post. Read it, it's very good.
<lang racket>#lang scribble/lp @(chunk
<graph-sig> (define-signature graph^ (node? edge? node-edges edge-src edge-cost edge-dest)))
@(chunk
<map-generation> (define (make-map N) ;; Jay's random algorithm ;; (build-matrix N N (λ (x y) (random 3))) ;; RC version (matrix [[0 0 0 0 0 0 0 0] [0 0 0 0 0 0 0 0] [0 0 0 0 1 1 1 0] [0 0 1 0 0 0 1 0] [0 0 1 0 0 0 1 0] [0 0 1 1 1 1 1 0] [0 0 0 0 0 0 0 0] [0 0 0 0 0 0 0 0]])))
@(chunk
<map-graph-rep> (struct map-node (M x y) #:transparent) (struct map-edge (src dx dy dest)))
@(chunk
<map-graph-cost> (define (edge-cost e) (match-define (map-edge _ _ _ (map-node M x y)) e) (match (matrix-ref M x y) [0 1] [1 100] [2 1000])))
@(chunk
<map-graph-edges> (define (node-edges n) (match-define (map-node M x y) n) (append* (for*/list ([dx (in-list '(1 0 -1))] [dy (in-list '(1 0 -1))] #:when (and (not (and (zero? dx) (zero? dy))) ;; RC -- allowed to move diagonally, so not this clause ;;(or (zero? dx) (zero? dy)) )) (cond [(and (<= 0 (+ dx x) (sub1 (matrix-num-cols M))) (<= 0 (+ dy y) (sub1 (matrix-num-rows M)))) (define dest (map-node M (+ dx x) (+ dy y))) (list (map-edge n dx dy dest))] [else empty])))))
@(chunk
<a-star> (define (A* graph@ initial node-cost) (define-values/invoke-unit graph@ (import) (export graph^)) (define count 0) <a-star-setup> (begin0 (let/ec esc <a-star-loop> #f) (printf "visited ~a nodes\n" count))))
@(chunk
<a-star-setup> <a-star-setup-closed> <a-star-setup-open>)
@(chunk
<a-star-setup-closed> (define node->best-path (make-hash)) (define node->best-path-cost (make-hash)) (hash-set! node->best-path initial empty) (hash-set! node->best-path-cost initial 0))
@(chunk
<a-star-setup-open> (define (node-total-estimate-cost n) (+ (node-cost n) (hash-ref node->best-path-cost n))) (define (node-cmp x y) (<= (node-total-estimate-cost x) (node-total-estimate-cost y))) (define open-set (make-heap node-cmp)) (heap-add! open-set initial))
@(chunk
<a-star-loop> (for ([x (in-heap/consume! open-set)]) (set! count (add1 count)) <a-star-loop-body>))
@(chunk
<a-star-loop-stop?> (define h-x (node-cost x)) (define path-x (hash-ref node->best-path x)) (when (zero? h-x) (esc (reverse path-x))))
@(chunk
<a-star-loop-body> <a-star-loop-stop?> (define g-x (hash-ref node->best-path-cost x)) (for ([x->y (in-list (node-edges x))]) (define y (edge-dest x->y)) <a-star-loop-per-neighbor>))
@(chunk
<a-star-loop-per-neighbor> (define new-g-y (+ g-x (edge-cost x->y))) (define old-g-y (hash-ref node->best-path-cost y +inf.0)) (when (< new-g-y old-g-y) (hash-set! node->best-path-cost y new-g-y) (hash-set! node->best-path y (cons x->y path-x)) (heap-add! open-set y)))
@(chunk
<map-display> (define map-scale 15) (define (type-color ty) (match ty [0 "yellow"] [1 "green"] [2 "red"])) (define (cell-square ty) (square map-scale "solid" (type-color ty))) (define (row-image M row) (apply beside (for/list ([col (in-range (matrix-num-cols M))]) (cell-square (matrix-ref M row col))))) (define (map-image M) (apply above (for/list ([row (in-range (matrix-num-rows M))]) (row-image M row)))))
@(chunk
<path-display-line> (define (edge-image-on e i) (match-define (map-edge (map-node _ sx sy) _ _ (map-node _ dx dy)) e) (add-line i (* (+ sy 0.5) map-scale) (* (+ sx 0.5) map-scale) (* (+ dy 0.5) map-scale) (* (+ dx 0.5) map-scale) "black")))
@(chunk
<path-display> (define (path-image M path) (foldr edge-image-on (map-image M) path)))
@(chunk
<map-graph> (define-unit map@ (import) (export graph^) (define node? map-node?) (define edge? map-edge?) (define edge-src map-edge-src) (define edge-dest map-edge-dest) <map-graph-cost> <map-graph-edges>))
@(chunk
<map-node-cost> (define ((make-node-cost GX GY) n) (match-define (map-node M x y) n) ;; Jay's #;(+ (abs (- x GX)) (abs (- y GY))) ;; RC -- diagonal movement (max (abs (- x GX)) (abs (- y GY)))))
@(chunk
<map-example> (define N 8) (define random-M (make-map N)) (define random-path (time (A* map@ (map-node random-M 0 0) (make-node-cost (sub1 N) (sub1 N))))))
@(chunk
<*> (require rackunit math/matrix racket/unit racket/match racket/list data/heap 2htdp/image racket/runtime-path) <graph-sig> <map-generation> <map-graph-rep> <map-graph> <a-star> <map-node-cost> <map-example> (printf "path is ~a long\n" (length random-path)) (printf "path is: ~a\n" (map (match-lambda [(map-edge src dx dy dest) (cons dx dy)]) random-path)) <map-display> <path-display-line> <path-display>
(path-image random-M random-path))</lang>
- Output:
visited 35 nodes cpu time: 94 real time: 97 gc time: 15 path is 11 long path is: ((1 . 1) (1 . 1) (1 . -1) (1 . 0) (1 . 0) (1 . 1) (1 . 1) (0 . 1) (-1 . 1) (1 . 1) (0 . 1)) .
A diagram is also output, but you'll need to run this in DrRacket to see it.
Raku
<lang perl6># 20200427 Raku programming solution
class AStarGraph {
has @.barriers = <2 4>,<2 5>,<2 6>,<3 6>,<4 6>,<5 6>,<5 5>,<5 4>,<5 3>,<5 2>,<4 2>,<3 2>;
method heuristic(\start, \goal) { my (\D1,\D2) = 1, 1; my (\dx,\dy) = ( start.list »-« goal.list )».abs; return (D1 * (dx + dy)) + (D2 - 2*D1) * min dx, dy }
method get_vertex_neighbours(\pos) { gather { for <1 0>,<-1 0>,<0 1>,<0 -1>,<1 1>,<-1 1>,<1 -1>,<-1 -1> -> \d { my (\x2,\y2) = pos.list »+« d.list; (x2 < 0 || x2 > 7 || y2 < 0 || y2 > 7) && next; take x2, y2; } } }
method move_cost(\a,\b) { (b ~~ any self.barriers) ?? 100 !! 1 }
}
sub AStarSearch(\start, \end, \graph) {
my (%G,%F);
%G{start.Str} = 0; %F{start.Str} = graph.heuristic(start, end);
my @closedVertices = []; my @openVertices = [].push(start); my %cameFrom;
while (@openVertices.Bool) { my $current = Nil; my $currentFscore = Inf;
for @openVertices -> \pos { if (%F{pos.Str} < $currentFscore) { $currentFscore = %F{pos.Str}; $current = pos } }
if $current ~~ end { my @path = [].push($current); while %cameFrom{$current.Str}:exists { $current = %cameFrom{$current.Str}; @path.push($current) } return @path.reverse, %F{end.Str} }
@openVertices .= grep: * !eqv $current; @closedVertices.push($current);
for (graph.get_vertex_neighbours($current)) -> \neighbour { next if neighbour ~~ any @closedVertices; my \candidateG = %G{$current.Str}+graph.move_cost($current,neighbour);
if !(neighbour ~~ any @openVertices) { @openVertices.push(neighbour) } elsif (candidateG ≥ %G{neighbour.Str}) { next }
%cameFrom{neighbour.Str} = $current; %G{neighbour.Str} = candidateG; my \H = graph.heuristic(neighbour, end); %F{neighbour.Str} = %G{neighbour.Str} + H; } } die "A* failed to find a solution"
}
my \graph = AStarGraph.new; my (\route, \cost) = AStarSearch(<0 0>, <7 7>, graph);
my \w = my \h = 10;
my @grid = [ ['.' xx w ] xx h ]; for ^h -> \y { @grid[y;0] = "█"; @grid[y;*-1] = "█" } for ^w -> \x { @grid[0;x] = "█"; @grid[*-1;x] = "█" }
for (graph.barriers) -> \d { @grid[d[0]+1][d[1]+1] = "█" } for @(route) -> \d { @grid[d[0]+1][d[1]+1] = "x" }
.join.say for @grid ;
say "Path cost : ", cost, " and route : ", route;</lang>
- Output:
███████████x.......█ █.x......█ █..x.███.█ █.x█...█.█ █.x█...█.█ █.x█████.█ █..xxxxx.█ █.......x█ ██████████
Path cost : 11 and route : ((0 0) (1 1) (2 2) (3 1) (4 1) (5 1) (6 2) (6 3) (6 4) (6 5) (6 6) (7 7))
REXX
<lang rexx>/*REXX program solves the A* search problem for a (general) NxN grid. */ parse arg N sCol sRow . /*obtain optional arguments from the CL*/ if N== | N=="," then N=8 /*No grid size specified? Use default.*/ if sCol== | sCol=="," then sCol=1 /*No starting column given? " " */ if sRow== | sRow=="," then sRow=1 /* " " row " " " */ beg= '─0─' /*mark the start of the journey in grid*/ o.=.; p.=0 /*list of optimum start journey starts.*/ times=0 /*cntr/pos for number of optimizations.*/
Pc = ' 1 1 0 0 1 -1 -1 -1 ' /*the possible column moves for a path.*/ Pr = ' 1 0 1 -1 -1 0 1 -1 ' /* " " row " " " " */
Pcm=words(Pc) /* [↑] optimized for moving right&down*/ $.=1e6; OK=0; min$=$. /*# possible directions; cost; solution*/ @Aa= " A* search algorithm on" /*a handy─dandy literal for the SAYs. */ flasher= '@. $. min$ N o. p. Pc. Pcm Pr. sCol sRow times' /*a literal list for EXPOSE.*/ call path 0 /*find a possible solution for the grid*/ @NxN= 'a ' N"x"N ' grid' /*a literal used for a SAY statement.*/ if OK then say 'A solution for the' @Aa @NxN "with a score of " @.N.N':'
else say 'No' @Aa "solution for" @NxN'.'
call show 1 /*invoke subroutine to display the grid*/ exit /*stick a fork in it, we're all done. */ /*──────────────────────────────────────────────────────────────────────────────────────*/ @: parse arg x,y,aChar; if arg()==3 then @.x.y=aChar; return @.x.y @p: parse arg x,y; if datatype(@.x.y, 'W') then return @.x.y<m-1; return 0 /*──────────────────────────────────────────────────────────────────────────────────────*/ barr: $=2.4 2.5 2.6 3.6 4.6 5.6 5.5 5.4 5.3 5.2 4.2 3.2 /*locations of barriers on grid*/
do b=1 for words($); _=word($, b); parse var _ c '.' r; call @ c+1,r+1,"█" end /*b*/; return
/*──────────────────────────────────────────────────────────────────────────────────────*/ move: procedure expose (flasher); parse arg m,col,row /*obtain move,col,row.*/
do t=1 for Pcm; nc=col + Pc.t; nr=row + Pr.t /*a new path position. */ if @.nc.nr==. then do; if opti() then iterate /*Costlier path? Next.*/ @.nc.nr=m; p.1.m=nc nr /*Empty? A legal path.*/ p.pcm.m=nr nc-1 /*used for a fast path.*/ if nc==N then if nr==N then return 1 /*last move? */ if move(m + 1, nc, nr) then return 1 /* " " */ @.nc.nr=. /*undo the above move. */ end /*try a different move.*/ end /*t*/ /* [↑] all moves tried*/ return 0 /*path isn't possible. */
/*──────────────────────────────────────────────────────────────────────────────────────*/ opti: ncm=nc-1; nrm=nr-1; if @p(ncm, nrm) then return 1
if @p(ncm, nr ) then return 1 if @p(nc, nrm) then return 1 ncp=nc+1; nrp=nr+1; if @p(ncp, nr ) then return 1 if @p(ncp, nrm) then return 1 if @p(nc, nrp) then return 1 if @p(ncm, nrp) then return 1 if @p(ncp, nrp) then return 1; return 0
/*──────────────────────────────────────────────────────────────────────────────────────*/ path: parse arg z; t=times /*initial move can only be one of eight*/
do #=1 for Pcm; @.= /*optimize for each degree of movement.*/ if z\==0 then if #\==z then iterate /*This a particular low─cost request ? */ do c=1 for N; do r=1 for N; @.c.r=.; end /*r*/ end /*c*/ iCol=sCol; iRow=sRow; @.sCol.sRow= beg /*all path's initial starting position*/ call barr /*place the barriers on the grid. */ Pco=subword(Pc Pc, #, Pcm); Pro=subword(Pr Pr, #, Pcm) parse var Pco Pc.1 Pc.2 Pc.3 Pc.4 Pc.5 Pc.6 Pc.7 Pc.8 /*possible directions.*/ parse var Pro Pr.1 Pr.2 Pr.3 Pr.4 Pr.5 Pr.6 Pr.7 Pr.8 /* " " */ do o=1 for times; parse var o.o c r; @.c.r=o; iRow=r; iCol=c end /*o*/ fp=move(1+times, iCol, iRow); sol=@N.N\==. & fp if sol then do; $.#=@.N.N /*Found a solution? Remember the cost.*/ OK=1; min$=min(min$, $.#) end end /*#*/ wp=1e7; wg=0; do g=1 for Pcm; if $.g<wp & $.g>0 & t\=2 then do; wg=g; wp=$.g; end end /*g*/ /* [↑] find minimum non-zero path cost*/ if wg==0 then wg=8 /*Not found? Then use last cost found.*/ times=times + 1 /*bump # times a marker has been placed*/ o.times= p.wg.times /*remember this move location for PATH.*/ if times<4 then call path 0 /*only do memoization for first 3 moves*/ return
/*──────────────────────────────────────────────────────────────────────────────────────*/ show: ind=left(, 9 * (n<18) ); say /*the indentation of the displayed grid*/
_=substr(copies("┼───", N),2); say ind translate('┌'_"┐", '┬', "┼") /*grid top.*/ /* [↓] build a display for the grid. */ do c=1 for N; if c\==1 & arg(1) then say ind '├'_"┤"; L=@. do r=1 for N; ?=@.c.r; if c ==N & r==N & ?\==. then ?='end'; L=L"│"center(?, 3) end /*r*/ /*done with rank of the grid. */ say ind translate(L'│', , .) /*display a " " " " */ end /*c*/ /*a 19x19 grid can be shown 80 columns.*/ say ind translate('└'_"┘",'┴',"┼"); return /*display the very bottom of the grid. */</lang>
- output when using the default input:
A solution for the A* search algorithm on a 8x8 grid with a score of 11: ┌───┬───┬───┬───┬───┬───┬───┬───┐ │─0─│ │ │ │ │ │ │ │ ├───┼───┼───┼───┼───┼───┼───┼───┤ │ │ 1 │ │ │ │ │ │ │ ├───┼───┼───┼───┼───┼───┼───┼───┤ │ │ │ 2 │ │ █ │ █ │ █ │ │ ├───┼───┼───┼───┼───┼───┼───┼───┤ │ │ 3 │ █ │ │ │ │ █ │ │ ├───┼───┼───┼───┼───┼───┼───┼───┤ │ │ 4 │ █ │ │ │ │ █ │ │ ├───┼───┼───┼───┼───┼───┼───┼───┤ │ │ 5 │ █ │ █ │ █ │ █ │ █ │ │ ├───┼───┼───┼───┼───┼───┼───┼───┤ │ │ │ 6 │ │ │ │ │ │ ├───┼───┼───┼───┼───┼───┼───┼───┤ │ │ │ │ 7 │ 8 │ 9 │10 │end│ └───┴───┴───┴───┴───┴───┴───┴───┘
SequenceL
<lang sequencel> import <Utilities/Set.sl>; import <Utilities/Math.sl>; import <Utilities/Sequence.sl>;
Point ::= (x : int, y : int);
State ::= (open : Point(1), closed : Point(1), cameFrom : Point(2), estimate : int(2), actual : int(2));
allNeighbors := [(x : -1, y : -1), (x : 1, y : -1), (x : -1, y : 1), (x : 1, y : 1), (x : 0, y : -1), (x : -1, y : 0), (x : 0, y : 1), (x : 1, y : 0)];
defaultBarriers := [(x : 3, y : 5),(x : 3, y : 6),(x : 3, y : 7),(x : 4, y : 7), (x : 5, y : 7),(x : 6, y : 7),(x : 6, y : 6),(x : 6, y : 5),(x : 6, y : 4), (x : 6, y : 3),(x : 5, y : 3),(x : 4, y : 3)];
defaultWidth := 8; defaultHeight := 8;
main(args(2)) := aStar(defaultWidth, defaultHeight, defaultBarriers, (x : 1, y : 1), (x : defaultWidth, y : defaultHeight));
aStar(width, height, barriers(1), start, end) := let newEstimate[i,j] := heuristic(start, end) when i = start.x and j = start.y else 0 foreach i within 1...width, j within 1 ... height; newActual[i,j] := 0 foreach i within 1...width, j within 1...height; newCameFrom[i,j] := (x : 0, y : 0) foreach i within 1...width, j within 1...height;
searchResults := search((open : [start], closed : [], estimate : newEstimate, actual : newActual, cameFrom : newCameFrom), barriers, end); shortestPath := path(searchResults.cameFrom, start, end) ++ [end]; in "No Path Found" when size(searchResults.open) = 0 else "Path: " ++ toString(shortestPath) ++ "\nCost:" ++ toString(searchResults.actual[end.x, end.y]) ++ "\nMap:\n" ++ join(appendNT(drawMap(barriers,shortestPath,width, height),"\n"));
path(cameFrom(2), start, current) := let next := cameFrom[current.x, current.y]; in [] when current = start else path(cameFrom, start, next) ++ [next];
drawMap(barriers(1), path(1), width, height)[i,j] := '#' when elementOf((x:i, y:j), barriers) else 'X' when elementOf((x:i, y:j), path) else '.' foreach i within 1 ... width, j within 1 ... height;
search(state, barriers(1), end) := let nLocation := smallestEstimate(state.open, state.estimate, 2, 1, state.estimate[state.open[1].x, state.open[1].y]); n := state.open[nLocation]; neighbors := createNeighbors(n, allNeighbors, size(state.actual), size(state.actual[1])); startState := (open : state.open[1...nLocation-1] ++ state.open[nLocation+1 ... size(state.open)], closed : state.closed ++ [n], cameFrom : state.cameFrom, estimate : state.estimate, actual : state.actual); newState := findOpenNeighbors(n, startState, barriers, end, neighbors); in state when size(state.open) = 0 else state when n = end else search(newState, barriers, end);
smallestEstimate(open(1), estimate(2), index, minIndex, minEstimate) := let newEstimate := estimate[open[index].x, open[index].y]; in minIndex when index > size(open) else smallestEstimate(open, estimate, index + 1, minIndex, minEstimate) when newEstimate > minEstimate else smallestEstimate(open, estimate, index + 1, index, newEstimate);
findOpenNeighbors(n, state, barriers(1), end, neighbors(1)) := let neighbor := head(neighbors); cost := 1 + n.cost; candidate := state.actual[n.x, n.y] + calculateCost(barriers, n, neighbor); in state when size(neighbors) = 0 else findOpenNeighbors(n, state, barriers, end, tail(neighbors)) when elementOf(neighbor, state.closed) else findOpenNeighbors(n, state, barriers, end, tail(neighbors)) when elementOf(neighbor, state.open) and candidate >= state.actual[neighbor.x, neighbor.y] else findOpenNeighbors(n, (open : state.open ++ [neighbor], closed : state.closed, cameFrom : setMap(state.cameFrom, neighbor, n), estimate : setMap(state.estimate, neighbor, candidate + heuristic(neighbor, end)), actual : setMap(state.actual, neighbor, candidate)), barriers, end, tail(neighbors));
createNeighbors(n, p, w, h) := let x := n.x + p.x; y := n.y + p.y; in (x : x, y : y) when x >= 1 and x <= w and y >= 1 and y <= h;
calculateCost(barriers(1), start, end) := 100 when elementOf(end, barriers) else 1;
heuristic(start, end) := let dx := abs(start.x - end.x); dy := abs(start.y - end.y); in (dx + dy) - min(dx, dy);
setMap(map(2), point, value)[i,j] := value when point.x = i and point.y = j else map[i,j] foreach i within 1 ... size(map), j within 1 ... size(map[1]); </lang>
- Output
Path: [(x:1,y:1),(x:2,y:2),(x:3,y:3),(x:4,y:2),(x:5,y:2),(x:6,y:2),(x:7,y:3),(x:7,y:4),(x:7,y:5),(x:7,y:6),(x:7,y:7),(x:8,y:8)] Cost:11 Map: X....... .X...... ..X.###. .X#...#. .X#...#. .X#####. ..XXXXX. .......X
Sidef
<lang ruby>class AStarGraph {
has barriers = [ [2,4],[2,5],[2,6],[3,6],[4,6],[5,6],[5,5],[5,4],[5,3],[5,2],[4,2],[3,2] ]
method heuristic(start, goal) { var (D1 = 1, D2 = 1) var dx = abs(start[0] - goal[0]) var dy = abs(start[1] - goal[1]) (D1 * (dx + dy)) + ((D2 - 2*D1) * Math.min(dx, dy)) }
method get_vertex_neighbours(pos) { gather { for dx, dy in [[1,0],[-1,0],[0,1],[0,-1],[1,1],[-1,1],[1,-1],[-1,-1]] { var x2 = (pos[0] + dx) var y2 = (pos[1] + dy) (x2<0 || x2>7 || y2<0 || y2>7) && next take([x2, y2]) } } }
method move_cost(_a, b) { barriers.contains(b) ? 100 : 1 }
}
func AStarSearch(start, end, graph) {
var G = Hash() var F = Hash()
G{start} = 0 F{start} = graph.heuristic(start, end)
var closedVertices = [] var openVertices = [start] var cameFrom = Hash()
while (openVertices) {
var current = nil var currentFscore = Inf
for pos in openVertices { if (F{pos} < currentFscore) { currentFscore = F{pos} current = pos } }
if (current == end) { var path = [current] while (cameFrom.contains(current)) { current = cameFrom{current} path << current } path.flip! return (path, F{end}) }
openVertices.remove(current) closedVertices.append(current)
for neighbour in (graph.get_vertex_neighbours(current)) { if (closedVertices.contains(neighbour)) { next } var candidateG = (G{current} + graph.move_cost(current, neighbour))
if (!openVertices.contains(neighbour)) { openVertices.append(neighbour) } elsif (candidateG >= G{neighbour}) { next }
cameFrom{neighbour} = current G{neighbour} = candidateG var H = graph.heuristic(neighbour, end) F{neighbour} = (G{neighbour} + H) } }
die "A* failed to find a solution"
}
var graph = AStarGraph() var (route, cost) = AStarSearch([0,0], [7,7], graph)
var w = 10 var h = 10
var grid = h.of { w.of { "." } } for y in (^h) { grid[y][0] = "█"; grid[y][-1] = "█" } for x in (^w) { grid[0][x] = "█"; grid[-1][x] = "█" }
for x,y in (graph.barriers) { grid[x+1][y+1] = "█" } for x,y in (route) { grid[x+1][y+1] = "x" }
grid.each { .join.say }
say "Path cost #{cost}: #{route}"</lang>
- Output:
██████████ █x.......█ █.x......█ █..x.███.█ █.x█...█.█ █.x█...█.█ █.x█████.█ █..xxxxx.█ █.......x█ ██████████ Path cost 11: [[0, 0], [1, 1], [2, 2], [3, 1], [4, 1], [5, 1], [6, 2], [6, 3], [6, 4], [6, 5], [6, 6], [7, 7]]
UNIX Shell
<lang bash>
- !/bin/bash
- This option will make the script exit when there is an error
set -o errexit
- This option will make the script exit when it tries to use an unset variable
set -o nounset
declare -A grid declare -A cell_type=(
["empty"]=0 ["barrier"]=1 ["start"]=2 ["end"]=3 ["path"]=4 ["right"]=5 ["left"]=6 ["up"]=7 ["down"]=8 ["left_up"]=9 ["left_down"]=10 ["right_up"]=11 ["right_down"]=12 )
grid_size=(10 10)
generate_rosetta_grid(){
grid_size=(8 8) start=(0 0) end=(7 7) for (( i = 0; i < grid_size[0]; i++ )); do for (( j = 0; j < grid_size[1]; j++ )); do grid[$i,$j]=${cell_type[empty]} done done barriers=( "2,4" "2,5" "2,6" "3,6" "4,6" "5,6" "5,5" "5,4" "5,3" "5,2" "4,2" "3,2") for barrier in ${barriers[*]};do grid["$barrier"]=${cell_type[barrier]} done grid[${start[0]},${start[1]}]=${cell_type[start]} grid[${end[0]},${end[1]}]=${cell_type[end]}
}
abs(){
# Number asbolute value. # Params: # ------ # $1 -> number # Return: # number abs if $1 -gt 0 ; then echo "$1" else echo "$((-$1))" fi
}
print_table(){
# Print table using unicode symbols. # Symbols: # " " -> empty cell # ◼ -> barrier # ◉ -> start position # ✪ -> goal # arrows -> path from start to goal printf ' ' # Print letters at top. for ((i=0;i< grid_size[1];i++)) do printf "%s" $i done echo for ((i=0;i < grid_size[0];i++)) do # Print numbers. printf "%s" $i for ((j=0;j < grid_size[1];j++)) do cell=${grid[$i,$j]} if [[ $cell -eq ${cell_type[empty]} ]]; then # If cell is empty prints space printf " " elif [[ $cell -eq ${cell_type[barrier]} ]]; then # If cell is a barrier printf "■" elif [[ $cell -eq ${cell_type[start]} ]]; then # Print start and end position printf "◉" elif [[ $cell -eq ${cell_type[end]} ]]; then # Print end position printf "✪" elif [[ $cell -eq ${cell_type[path]} ]]; then # Print path printf "*" elif [[ $cell -eq ${cell_type[up]} ]]; then # Print path printf "↑" elif [[ $cell -eq ${cell_type[down]} ]]; then # Print path printf "↓" elif [[ $cell -eq ${cell_type[right]} ]]; then # Print path printf "→" elif [[ $cell -eq ${cell_type[left]} ]]; then # Print path printf "←" elif [[ $cell -eq ${cell_type[right_up]} ]]; then # Print path printf "↗" elif [[ $cell -eq ${cell_type[right_down]} ]]; then # Print path printf "↙" elif [[ $cell -eq ${cell_type[left_up]} ]]; then # Print path printf "↖" elif [[ $cell -eq ${cell_type[left_down]} ]]; then # Print path printf "↘" fi done echo done
}
get_neighbours(){
# Calculates all point's neighbours # Params: # ------ # $1 -> "x,y" formatted point position # Return: # ------ # array of available positions # Skips nonexistent indices.
neighbours=() for i in {-1..1},{-1..1}; do if [[ ( ${i%,*} -eq 0 ) && ( ${i#*,} -eq 0 ) ]]; then continue fi dx=${i%,*} dy=${i#*,} x=$((${1%,*}+dx)) y=$((${1#*,}+dy)) if $x -lt 0 || [[ $x -ge ${grid_size[0]} ]]; then continue fi if | $y -ge ${grid_size[1]} ; then continue fi neighbours+=("$x,$y") done echo "${neighbours[*]}"
}
move_cost(){
# Calculates how much will it cost # to travel to point b. # return 100 if b is barrier # # Params: # ------ # $1 -> a # $2 -> b # Return: # ------ # movement cost.
barrier=${cell_type[barrier]} if [[ ${grid[${2%,*},${2#*,}]} -eq barrier ]]; then echo 100 else echo 1 fi
}
print_raw(){
# Print raw grid values.
for ((i=0;i < grid_size[0];i++)) do for ((j=0;j < grid_size[1];j++)) do printf "%s" "${grid[$i,$j]}" done echo done
}
minimum(){
# Minimum between two numbers # Params: # ------ # $1 -> a # $2 -> b # Return: # ------ # less value
if $1 -lt $2 ; then echo "$1" else echo "$2" fi
}
heuristic_cost(){
# Chebyshev distance heuristic score # if we can move one square either # adjacent or diagonal
d=1 d2=1 dx=$(abs $((${1#*,} - ${2#*,}))) dy=$(abs $((${1%,*} - ${2%,*}))) echo "$(((d*(dx + dy))+(d2 - 2 * d)*$(minimum dx dy)))"
}
contains(){
for el in "${2[@]}"; do echo "$el" done
}
contains_value() {
# Check if element exists in array # Params: # ------ # $1 -> array # $2 -> element to find. # Returns: # 1 if element exists in array # 0 otherwise.
local array="$1[@]" arr=("${!array}") local seeking=$2 local in=0 for element in ${arr[*]}; do if [ "$element" = "$seeking" ]; then in=1 break fi done echo "$in"
}
reverse_array(){
# Reverse given array. # Params: # ------ # $1 -> array # Return: # ------ # reversed array. local array="$1[@]" arr=("${!array}") result=() for (( idx=${#arr[@]}-1 ; idx>=0 ; idx-- )) ; do result+=("${arr[$idx]}") done echo "${result[@]}"
}
find_path(){
declare -A fScore declare -A gScore declare -A cameFrom declare -a openVertices declare -a closedVertices for (( i = 0; i < grid_size[0]; i++ )); do for (( j = 0; j < grid_size[1]; j++ )); do gScore[$i,$j]=$((1<<62)) fScore[$i,$j]=$((1<<62)) done done gScore["${start[0]},${start[1]}"]=0 fScore["${start[0]},${start[1]}"]=$(heuristic_cost "${start[0]},${start[1]}" "${end[0]},${end[1]}") openVertices+=("${start[0]},${start[1]}")
while [[ -n "${openVertices[*]}" ]]; do
current=-1 currentFscore=0 for pos in ${openVertices[*]}; do if $current -eq -1 || [[ ${fScore["$pos"]} -lt $currentFscore ]]; then currentFscore=${fScore["$pos"]} current=$pos fi done if [[ "$current" = "${end[0]},${end[1]}" ]]; then path=( "$current" ) while [ ${cameFrom["$current"]+_} ]; do current=${cameFrom["$current"]} path+=("$current") done reverse_array path return 0 fi openVertices=( "$( echo "${openVertices[@]/$current}" | xargs )" ) closedVertices+=( "$current" ) neighbours=( "$(get_neighbours "$current")" )
for neighbour in ${neighbours[*]}; do if $(contains_value closedVertices "$neighbour") -eq 1 ; then continue fi mCost="$(move_cost "$current" "$neighbour")" candidateG=$(( ${gScore["$current"]}+mCost )) if $candidateG -gt 100 ; then continue fi if $(contains_value openVertices "$neighbour") -eq 0 ; then openVertices+=("$neighbour") elif [[ $candidateG -gt ${gScore[$neighbour]} ]]; then continue fi cameFrom["$neighbour"]="$current" gScore["$neighbour"]=$candidateG heuristic_score=$(heuristic_cost "$neighbour" "${end[0]},${end[1]}") fScore["$neighbour"]=$(( candidateG+heuristic_score )) done done
}
map_to_arrows(){
local array="$1[@]" arr=("${!array}") last="${start[0]},${start[1]}" for el in ${arr[*]}; do if [[ $((${el#*,}-${last#*,})) -eq -1 ]] && [[ $((${el%,*}-${last%,*})) -eq -1 ]]; then grid["$last"]=${cell_type[left_up]} elif [[ $((${el#*,}-${last#*,})) -eq -1 ]] && [[ $((${el%,*}-${last%,*})) -eq 1 ]]; then grid["$last"]=${cell_type[right_down]} elif [[ $((${el#*,}-${last#*,})) -eq 1 ]] && [[ $((${el%,*}-${last%,*})) -eq -1 ]]; then grid["$last"]=${cell_type[right_up]} elif [[ $((${el#*,}-${last#*,})) -eq 1 ]] && [[ $((${el%,*}-${last%,*})) -eq 1 ]]; then grid["$last"]=${cell_type[left_down]} elif [[ $((${el#*,}-${last#*,})) -eq -1 ]];then grid["$last"]=${cell_type[left]} elif [[ $((${el%,*}-${last%,*})) -eq -1 ]];then grid["$last"]=${cell_type[up]} elif [[ $((${el#*,}-${last#*,})) -eq 1 ]];then grid["$last"]=${cell_type[right]} elif [[ $((${el%,*}-${last%,*})) -eq 1 ]];then grid["$last"]=${cell_type[down]} else grid["$last"]=${cell_type[path]} fi last=$el done grid[${start[0]},${start[1]}]=${cell_type[start]} grid[${end[0]},${end[1]}]=${cell_type[end]}
}
main(){
generate_rosetta_grid path=( "$(find_path)" ) pstr="$(echo "${path[*]}" | xargs | sed "s/space:/ → /g")" echo path: "$pstr" if -z $pstr ; then echo "No path found." else map_to_arrows path print_table fi
}
main "$@"
</lang>
- Output:
path: 0,0 → 1,0 → 2,0 → 3,0 → 4,0 → 5,1 → 6,2 → 7,3 → 7,4 → 7,5 → 7,6 → 7,7 01234567 0◉ 1↓ 2↓ ■■■ 3↓ ■ ■ 4↘ ■ ■ 5 ↘■■■■■ 6 ↘ 7 →→→→✪
zkl
<lang zkl> // we use strings as hash keys: (x,y)-->"x,y", keys are a single pair fcn toKey(xy){ xy.concat(",") }
fcn AStarSearch(start,end,graph){
G:=Dictionary(); # Actual movement cost to each position from the start position F:=Dictionary(); # Estimated movement cost of start to end going via this position #Initialize starting values kstart:=toKey(start); G[kstart]=0; F[kstart]=graph.heuristic(start,end); closedVertices,openVertices,cameFrom := List(),List(start),Dictionary(); while(openVertices){ # Get the vertex in the open list with the lowest F score current,currentFscore := Void, Void; foreach pos in (openVertices){ kpos:=toKey(pos); if(current==Void or F[kpos]<currentFscore)
currentFscore,current = F[kpos],pos;
# Check if we have reached the goal if(current==end){ # Yes! Retrace our route backward path,kcurrent := List(current),toKey(current); while(current = cameFrom.find(kcurrent)){ path.append(current); kcurrent=toKey(current); } return(path.reverse(),F[toKey(end)]) # Done! }
# Mark the current vertex as closed openVertices.remove(current); if(not closedVertices.holds(current)) closedVertices.append(current);
# Update scores for vertices near the current position foreach neighbor in (graph.get_vertex_neighbors(current)){ if(closedVertices.holds(neighbor)) continue; # We have already processed this node exhaustively kneighbor:=toKey(neighbor); candidateG:=G[toKey(current)] + graph.move_cost(current, neighbor);
if(not openVertices.holds(neighbor)) openVertices.append(neighbor); # Discovered a new vertex else if(candidateG>=G[kneighbor]) continue; # This G score is worse than previously found
# Adopt this G score cameFrom[kneighbor]=current; G[kneighbor]=candidateG; F[kneighbor]=G[kneighbor] + graph.heuristic(neighbor,end); }
} } // while throw(Exception.AssertionError("A* failed to find a solution"));
}</lang> <lang zkl>class [static] AStarGraph{ # Define a class board like grid with barriers
var [const] barriers = T( T(3,2),T(4,2),T(5,2), // T is RO List
T(5,3), T(2,4), T(5,4), T(2,5), T(5,5), T(2,6),T(3,6),T(4,6),T(5,6) );
fcn heuristic(start,goal){ // (x,y),(x,y) # Use Chebyshev distance heuristic if we can move one square either # adjacent or diagonal D,D2,dx,dy := 1,1, (start[0] - goal[0]).abs(), (start[1] - goal[1]).abs(); D*(dx + dy) + (D2 - 2*D)*dx.min(dy); } fcn get_vertex_neighbors([(x,y)]){ # Move like a chess king var moves=Walker.cproduct([-1..1],[-1..1]).walk(); // 8 moves + (0,0) moves.pump(List,'wrap([(dx,dy)]){
x2,y2 := x + dx, y + dy; if((dx==dy==0) or x2 < 0 or x2 > 7 or y2 < 0 or y2 > 7) Void.Skip; else T(x2,y2);
}) } fcn move_cost(a,b){ // ( (x,y),(x,y) ) if(barriers.holds(b))
return(100); # Extremely high cost to enter barrier squares
1 # Normal movement cost }
}</lang> <lang zkl>graph:=AStarGraph; route,cost := AStarSearch(T(0,0), T(7,7), graph); println("Route: ", route.apply(fcn(xy){ String("(",toKey(xy),")") }).concat(",")); println("Cost: ", cost);
// graph the solution:
grid:=(10).pump(List,List.createLong(10," ").copy); foreach x,y in (graph.barriers){ grid[x][y]="#" } foreach x,y in (route){ grid[x][y]="+" } grid[0][0] = "S"; grid[7][7] = "E"; foreach line in (grid){ println(line.concat()) }</lang>
- Output:
Route: (0,0),(1,1),(2,2),(3,1),(4,0),(5,1),(6,2),(7,3),(7,4),(7,5),(7,6),(7,7) Cost: 11 S + + ### +# # + # # +##### + ++++E