# Particle fountain

Particle fountain
You are encouraged to solve this task according to the task description, using any language you may know.

Implement a particle fountain.

Emulate a fountain of water droplets in a gravitational field being sprayed up and then falling back down.

The particle fountain should be generally ordered but individually chaotic; the particles should be going mostly in the same direction, but should have slightly different vectors.

Your fountain should have at least several hundred particles in motion at any one time, and ideally several thousand.

It is optional to have the individual particle interact with each other.

If at all possible, link to a short video clip of your fountain in action.

## C++

Library: SDL
Translation of: Raku
```#include <SDL2/SDL.h>

#include <algorithm>
#include <chrono>
#include <cmath>
#include <iostream>
#include <memory>
#include <random>
#include <tuple>
#include <vector>

auto now() {
using namespace std::chrono;
auto time = system_clock::now();
return duration_cast<milliseconds>(time.time_since_epoch()).count();
}

auto hsv_to_rgb(int h, double s, double v) {
double hp = h / 60.0;
double c = s * v;
double x = c * (1 - std::abs(std::fmod(hp, 2) - 1));
double m = v - c;
double r = 0, g = 0, b = 0;
if (hp <= 1) {
r = c;
g = x;
} else if (hp <= 2) {
r = x;
g = c;
} else if (hp <= 3) {
g = c;
b = x;
} else if (hp <= 4) {
g = x;
b = c;
} else if (hp <= 5) {
r = x;
b = c;
} else {
r = c;
b = x;
}
r += m;
g += m;
b += m;
return std::make_tuple(Uint8(r * 255), Uint8(g * 255), Uint8(b * 255));
}

class ParticleFountain {
public:
ParticleFountain(int particles, int width, int height);
void run();

private:
struct WindowDeleter {
void operator()(SDL_Window* window) const { SDL_DestroyWindow(window); }
};
struct RendererDeleter {
void operator()(SDL_Renderer* renderer) const {
SDL_DestroyRenderer(renderer);
}
};
struct PointInfo {
double x = 0;
double y = 0;
double vx = 0;
double vy = 0;
double lifetime = 0;
};

void update(double df);
bool handle_event();
void render();
double rand() { return dist_(rng_); }
double reciprocate() const {
return reciprocate_ ? range_ * std::sin(now() / 1000.0) : 0.0;
}

std::unique_ptr<SDL_Window, WindowDeleter> window_;
std::unique_ptr<SDL_Renderer, RendererDeleter> renderer_;
int width_;
int height_;
std::vector<PointInfo> point_info_;
std::vector<SDL_Point> points_;
int num_points_ = 0;
double saturation_ = 0.4;
double spread_ = 1.5;
double range_ = 1.5;
bool reciprocate_ = false;
std::mt19937 rng_;
std::uniform_real_distribution<> dist_;
};

ParticleFountain::ParticleFountain(int n, int width, int height)
: width_(width), height_(height), point_info_(n), points_(n, {0, 0}),
rng_(std::random_device{}()), dist_(0.0, 1.0) {
window_.reset(SDL_CreateWindow(
"C++ Particle System!", SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED,
width, height, SDL_WINDOW_RESIZABLE));
if (window_ == nullptr)
throw std::runtime_error(SDL_GetError());

renderer_.reset(
SDL_CreateRenderer(window_.get(), -1, SDL_RENDERER_ACCELERATED));
if (renderer_ == nullptr)
throw std::runtime_error(SDL_GetError());
}

void ParticleFountain::run() {
for (double df = 0.0001;;) {
auto start = now();
if (!handle_event())
break;
update(df);
render();
df = (now() - start) / 1000.0;
}
}

void ParticleFountain::update(double df) {
int pointidx = 0;
for (PointInfo& point : point_info_) {
bool willdraw = false;
if (point.lifetime <= 0.0) {
if (rand() < df) {
point.x = width_ / 20.0;
point.y = height_ / 10.0;
point.vx =
(spread_ * rand() - spread_ / 2 + reciprocate()) * 10.0;
point.vy = (rand() - 2.9) * height_ / 20.5;
willdraw = true;
}
} else {
if (point.y > height_ / 10.0 && point.vy > 0)
point.vy *= -0.3;
point.vy += (height_ / 10.0) * df;
point.x += point.vx * df;
point.y += point.vy * df;
willdraw = true;
}
if (willdraw) {
points_[pointidx].x = std::floor(point.x * 10.0);
points_[pointidx].y = std::floor(point.y * 10.0);
++pointidx;
}
}
num_points_ = pointidx;
}

bool ParticleFountain::handle_event() {
bool result = true;
SDL_Event event;
while (result && SDL_PollEvent(&event)) {
switch (event.type) {
case SDL_QUIT:
result = false;
break;
case SDL_WINDOWEVENT:
if (event.window.event == SDL_WINDOWEVENT_RESIZED) {
width_ = event.window.data1;
height_ = event.window.data2;
}
break;
case SDL_KEYDOWN:
switch (event.key.keysym.scancode) {
case SDL_SCANCODE_UP:
saturation_ = std::min(saturation_ + 0.1, 1.0);
break;
case SDL_SCANCODE_DOWN:
saturation_ = std::max(saturation_ - 0.1, 0.0);
break;
case SDL_SCANCODE_PAGEUP:
break;
case SDL_SCANCODE_PAGEDOWN:
break;
case SDL_SCANCODE_RIGHT:
range_ = std::min(range_ + 0.1, 2.0);
break;
case SDL_SCANCODE_LEFT:
range_ = std::max(range_ - 0.1, 0.1);
break;
case SDL_SCANCODE_SPACE:
reciprocate_ = !reciprocate_;
break;
case SDL_SCANCODE_Q:
result = false;
break;
default:
break;
}
break;
}
}
return result;
}

void ParticleFountain::render() {
SDL_Renderer* renderer = renderer_.get();
SDL_SetRenderDrawColor(renderer, 0x0, 0x0, 0x0, 0xff);
SDL_RenderClear(renderer);
auto [red, green, blue] = hsv_to_rgb((now() % 5) * 72, saturation_, 1);
SDL_SetRenderDrawColor(renderer, red, green, blue, 0x7f);
SDL_RenderDrawPoints(renderer, points_.data(), num_points_);
SDL_RenderPresent(renderer);
}

int main() {
std::cout << "Use UP and DOWN arrow keys to modify the saturation of the "
"particle colors.\n"
"Use PAGE UP and PAGE DOWN keys to modify the \"spread\" of "
"the particles.\n"
"Toggle reciprocation off / on with the SPACE bar.\n"
"Use LEFT and RIGHT arrow keys to modify angle range for "
"reciprocation.\n"
"Press the \"q\" key to quit.\n";

if (SDL_Init(SDL_INIT_VIDEO) != 0) {
std::cerr << "ERROR: " << SDL_GetError() << '\n';
return EXIT_FAILURE;
}

try {
ParticleFountain pf(3000, 800, 800);
pf.run();
} catch (const std::exception& ex) {
std::cerr << "ERROR: " << ex.what() << '\n';
SDL_Quit();
return EXIT_FAILURE;
}

SDL_Quit();
return EXIT_SUCCESS;
}
```

## EasyLang

```rad = 0.125
n = 6000
#
len x[] n ; len y[] n
len vx[] n ; len vy[] n
background 479
color 999
on animate
for i = 1 to 32
ind = (ind + 1) mod1 n
x[ind] = 50 + randomf
y[ind] = i / 4
vx[ind] = (randomf - 0.5) * 0.4
vy[ind] = 2 + randomf * 0.1
.
clear
for i = 1 to n
move x[i] y[i]
x[i] += vx[i] ; y[i] += vy[i]
vy[i] -= 0.025
.
.```

## Julia

Translation of: Raku
```using Dates, Colors, SimpleDirectMediaLayer.LibSDL2

mutable struct ParticleFountain
particlenum::Int
positions::Vector{Float64}
velocities::Vector{Float64}
points::Vector{SDL_Point}
numpoints::Int
saturation::Float64
range::Float64
reciprocate::Bool
ParticleFountain(N) = new(N, zeros(2N), zeros(2N), zeros(N), fill(SDL_Point(0, 0), N),
0, 0.4, 1.5, 1.5, false)
end

function update(pf, w, h, df)
xidx, yidx, pointidx = 1, 2, 0
recip() = pf.reciprocate ? pf.range * sin(Dates.value(now()) / 1000) : 0.0
for idx in 1:pf.particlenum
willdraw = false
if pf.lifetimes[idx] <= 0.0
if rand() < df
pf.lifetimes[idx]   = 2.5;                       # time to live
pf.positions[xidx]  = (w / 20)                   # starting position x
pf.positions[yidx]  = (h / 10)                   # and y
pf.velocities[xidx] = 10 * (pf.spread * rand() - pf.spread / 2 + recip()) # starting velocity x
pf.velocities[yidx] = (rand() - 2.9) * h / 20.5; # and y (randomized slightly so points reach different heights)
willdraw = true
end
else
if pf.positions[yidx] > h / 10 && pf.velocities[yidx] > 0
pf.velocities[yidx] *= -0.3                  # "bounce"
end
pf.velocities[yidx] += df * h / 10                  # adjust velocity
pf.positions[xidx]  += pf.velocities[xidx] * df     # adjust position x
pf.positions[yidx]  += pf.velocities[yidx] * df     # and y
willdraw = true
end

if willdraw # gather all of the points that are going to be rendered
pointidx += 1
pf.points[pointidx] = SDL_Point(Cint(floor(pf.positions[xidx] * 10)),
Cint(floor(pf.positions[yidx] * 10)))
end
xidx += 2
yidx = xidx + 1
pf.numpoints = pointidx
end
return pf
end

function fountain(particlenum = 3000, w = 800, h = 800)
SDL_Init(SDL_INIT_VIDEO)
window = SDL_CreateWindow("Julia Particle System!", SDL_WINDOWPOS_CENTERED_MASK,
SDL_WINDOWPOS_CENTERED_MASK, w, h, SDL_WINDOW_RESIZABLE)
renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED)
SDL_ClearError()
df = 0.0001
pf = ParticleFountain(3000)
overallstart, close, frames = now(), false, 0
while !close
dfstart = now()
event_ref = Ref{SDL_Event}()
while Bool(SDL_PollEvent(event_ref))
event_type = event_ref[].type
evt = event_ref[]
if event_type == SDL_QUIT
close = true
break
end
if event_type == SDL_WINDOWEVENT
if evt.window.event == 5
w = evt.window.data1
h = evt.window.data2
end
end
if event_type == SDL_KEYDOWN
comm = evt.key.keysym.scancode
if comm == SDL_SCANCODE_UP
saturation = min(pf.saturation + 0.1, 1.0)
elseif comm == SDL_SCANCODE_DOWN
saturation = max(pf.saturation - 0.1, 0.0)
elseif comm == SDL_SCANCODE_PAGEUP
elseif comm == SDL_SCANCODE_PAGEDOWN
elseif comm == SDL_SCANCODE_LEFT
range = min(pf.range + 0.1, 12.0)
elseif comm == SDL_SCANCODE_RIGHT
range = max(pf.range - 0.1, 0.1)
elseif comm == SDL_SCANCODE_SPACE
pf.reciprocate = !pf.reciprocate
elseif comm == SDL_SCANCODE_Q
close = true
break
end
end
end
pf = update(pf, w, h, df)
SDL_SetRenderDrawColor(renderer, 0x0, 0x0, 0x0, 0xff)
SDL_RenderClear(renderer)
rgb = parse(UInt32, hex(HSL((Dates.value(now()) % 5) * 72, pf.saturation, 0.5)), base=16)
red, green, blue = rgb & 0xff, (rgb >> 8) & 0xff, (rgb >>16) & 0xff
SDL_SetRenderDrawColor(renderer, red, green, blue, 0x7f)
SDL_RenderDrawPoints(renderer, pf.points, pf.numpoints)
SDL_RenderPresent(renderer)
frames += 1
df = Float64(Dates.value(now()) - Dates.value(dfstart)) / 1000
elapsed = Float64(Dates.value(now()) - Dates.value(overallstart)) / 1000
elapsed > 0.5 && print("\r", ' '^20, "\rFPS: ", round(frames / elapsed, digits=1))
end
SDL_Quit()
end

println("""
Use UP and DOWN arrow keys to modify the saturation of the particle colors.
Use PAGE UP and PAGE DOWN keys to modify the "spread" of the particles.
Toggle reciprocation off / on with the SPACE bar.
Use LEFT and RIGHT arrow keys to modify angle range for reciprocation.
Press the "q" key to quit.
""")

fountain()
```

## Lua

Library: LÖVE
```-- Returns canvas of given width and height containing a circle
function initCanvas (width, height)
local c = love.graphics.newCanvas(width, height)
love.graphics.setCanvas(c) -- Switch to drawing on canvas 'c'
love.graphics.circle("fill", width / 2, height / 2, 2, 100)
love.graphics.setCanvas() -- Switch back to drawing on main screen
return c
end

-- Returns particle system with given canvas
function initPartSys (image, maxParticles)
local ps = love.graphics.newParticleSystem(image, maxParticles)
ps:setParticleLifetime(3, 5) -- (min, max)
ps:setDirection(math.pi * 1.5)
ps:setSpeed(700)
ps:setLinearAcceleration(-100, 500, 100, 700) -- (minX, minY, maxX, maxY)
ps:setEmissionRate(1000)
ps:setPosition(400, 550)
ps:setColors(1, 1, 1, 1, 0, 0, 1, 0) -- Start solid white, fade to transluscent blue
return ps
end

-- LÖVE callback that runs on program start
love.window.setTitle("Lua particle fountain")
local canvas = initCanvas(10, 10)
psystem = initPartSys(canvas, 10000)
end

-- LÖVE callback to update values before each frame
function love.update (dt)
psystem:update(dt)
end

-- LÖVE callback to draw each frame to the screen
function love.draw ()
love.graphics.draw(psystem)
end
```

## Nim

Translation of: Julia
Library: SDL2

Note that for key events, the scan code doesn’t take in account the keyboard layout. So we check the "sym" value instead.

```import std/[lenientops, math, monotimes, random, times]
import sdl2

type ParticleFountain[N: static Positive] = object
positions: array[1..2 * N, float]
velocities: array[1..2 * N, float]
points: array[1..N, Point]
numPoints: int
saturation: float
range: float
reciprocate: bool

proc initParticleFountain[N: static Positive](): ParticleFountain[N] =
ParticleFountain[N](saturation: 0.4, spread: 1.5, range: 1.5)

proc update(pf: var ParticleFountain; w, h: cint; df: float) =
var
xidx = 1
yidx = 2
pointidx = 0

template recip(pf: ParticleFountain): float =
if pf.reciprocate: pf.range * sin(epochTime() / 1000) else: 0.0

for idx in 1..pf.N:
var willDraw = false
if pf.lifetimes[idx] <= 0:
if rand(1.0) < df:
pf.lifetimes[idx] = 2.5   # Time to live.
# Starting position.
pf.positions[xidx] = w / 20
pf.positions[yidx] = h / 10
# Starting velocity.
pf.velocities[xidx] = 10 * (pf.spread * rand(1.0) - pf.spread / 2 + pf.recip())
pf.velocities[yidx] = (rand(1.0) - 2.9) * h / 20.5
willDraw = true
else:
if pf.positions[yidx] > h / 10 and pf.velocities[yidx] > 0:
pf.velocities[yidx] *= -0.3   # "Bounce".
pf.velocities[yidx] += df * h / 10                  # Adjust velocity.
pf.positions[xidx] += pf.velocities[xidx] * df      # Adjust position x.
pf.positions[yidx] += pf.velocities[yidx] * df      # Adjust position y.
willDraw = true

if willDraw:
# Gather all of the points that are going to be rendered.
inc pointIdx
pf.points[pointidx] = (cint(pf.positions[xidx] * 10), cint(pf.positions[yidx] * 10))
inc xidx, 2
yidx = xidx + 1
pf.numPoints = pointidx

func hsvToRgb(h, s, v: float): (byte, byte, byte) =
let hp = h / 60.0
let c = s * v
let x = c * (1 - abs(hp mod 2 - 1))
let m = v - c
var (r, g, b) = if hp <= 1: (c, x, 0.0)
elif hp <= 2: (x, c, 0.0)
elif hp <= 3: (0.0, c, x)
elif hp <= 4: (0.0, x, c)
elif hp <= 5: (x, 0.0, c)
else: (c, 0.0, x)
r += m
g += m
b += m
result = (byte(r * 255), byte(g * 255), byte(b * 255))

proc fountain(particleNum = 3000; w = 800; h = 800) =
var w = w.cint
var h = h.cint
discard sdl2.init(INIT_VIDEO or INIT_EVENTS)
let window = createWindow("Nim Particle System!", SDL_WINDOWPOS_CENTERED_MASK,
SDL_WINDOWPOS_CENTERED_MASK, w, h, SDL_WINDOW_RESIZABLE)
let renderer = createRenderer(window, -1, 0)
clearError()
var df = 0.0001
var pf = initParticleFountain[3000]()
var close = false
var frames = 0
block Simulation:
while not close:
let dfStart = getMonoTime()
var event: Event
while bool(pollEvent(event)):
case event.kind
of QuitEvent:
break Simulation
of WindowEvent:
if event.window.event == WindowEvent_Resized:
w = event.window.data1
h = event.window.data2
of KeyDown:
let comm = event.key.keysym.sym
case comm
of K_UP:
pf.saturation = min(pf.saturation + 0.1, 1.0)
of K_DOWN:
pf.saturation = max(pf.saturation - 0.1, 0.0)
of K_PAGEUP:
of K_PAGEDOWN:
of K_LEFT:
pf.range = min(pf.range + 0.1, 12.0)
of K_RIGHT:
pf.range = max(pf.range - 0.1, 0.1)
of K_SPACE:
pf.reciprocate = not pf.reciprocate
of K_Q:
break Simulation
else:
else:

pf.update(w, h, df)
renderer.setDrawColor(0x0, 0x0, 0x0, 0xff)
renderer.clear()
let (red, green, blue) = hsvToRgb(epochTime() mod 5 * 72, pf.saturation, 1.0)
renderer.setDrawColor(red, green, blue, 0x7f)
renderer.present()
inc frames
df = (getMonoTime() - dfStart).inMilliseconds.float / 1000

sdl2.quit()

randomize()
echo """
Use UP and DOWN arrow keys to modify the saturation of the particle colors.
Use PAGE UP and PAGE DOWN keys to modify the "spread" of the particles.
Toggle reciprocation off / on with the SPACE bar.
Use LEFT and RIGHT arrow keys to modify angle range for reciprocation.
Press the "q" key to quit.
"""
fountain()
```

## Perl

```#!/usr/bin/perl

use strict; # https://rosettacode.org/wiki/Particle_fountain
use warnings;
use Tk;

my \$size = 900;
my @particles;
my \$maxparticles = 500;
my @colors = qw( red green blue yellow cyan magenta orange white );

my \$mw = MainWindow->new;
my \$c = \$mw->Canvas( -width => \$size, -height => \$size, -bg => 'black',
)->pack;
\$mw->Button(-text => 'Exit', -command => sub {\$mw->destroy},
)->pack(-fill => 'x');

step();
MainLoop;
-M \$0 < 0 and exec \$0;

sub step
{
\$c->delete('all');
\$c->createLine(\$size / 2 - 10, \$size, \$size / 2, \$size - 10,
\$size / 2 + 10, \$size, -fill => 'white' );
for ( @particles )
{
my (\$ox, \$oy, \$vx, \$vy, \$color) = @\$_;
my \$x = \$ox + \$vx;
my \$y = \$oy + \$vy;
\$c->createRectangle(\$ox, \$oy, \$x, \$y, -fill => \$color, -outline => \$color);
if( \$y < \$size )
{
\$_->[0] = \$x;
\$_->[1] = \$y;
\$_->[3] += 0.006; # gravity :)
}
else { \$_ = undef }
}
@particles = grep defined, @particles;
if( @particles < \$maxparticles and --\$| )
{
push @particles, [ \$size >> 1, \$size - 10,
(1 - rand 2) / 2.5 , -3 - rand 0.05, \$colors[rand @colors] ];
}
\$mw->after(1 => \&step);
}
```

## Phix

Translation of: Raku
Library: Phix/pGUI
Library: Phix/online

You can run this online here.

```--
-- demo\rosetta\Particle_fountain.exw
-- ==================================
--
with javascript_semantics
include pGUI.e

Ihandle dlg, canvas
cdCanvas cddbuffer, cdcanvas

constant title = "Particle fountain"
constant help_text = """
Uparrow increases the saturation of the particle colors,
downarrow decreases saturation until they all become white.
PageUp sprays the particles out at a wider angle/spread,
PageDown makes the jet narrower.
Space toggles reciprocation (wobble) on and off (straight up).
Left arrow decreases the angle range for reciprocation,
right arrow increases the angle range for reciprocation.
Press the "q" key to quit.
"""

constant particlenum = 3000
-- each particle is {x,y,color,life,dx,dy}
sequence particles = repeat({0,0,0,0,0,0},particlenum)
atom t1 = time()+1
integer fps = 0
bool reciprocate = true
atom range = 1.5,
saturation = 0.4,
start = time(),
df = 0.0001

function redraw_cb(Ihandle /*ih*/, integer /*posx*/, /*posy*/)
integer {w, h} = IupGetIntInt(canvas, "DRAWSIZE")
cdCanvasActivate(cddbuffer)
cdCanvasClear(cddbuffer)
for i=1 to length(particles) do
atom {x,y,color,life} = particles[i]
if life>0 then
cdCanvasPixel(cddbuffer, x, h/10-y, color)
end if
end for
cdCanvasFlush(cddbuffer)
return IUP_DEFAULT
end function

function map_cb(Ihandle ih)
cdcanvas = cdCreateCanvas(CD_IUP, ih)
cddbuffer = cdCreateCanvas(CD_DBUFFER, cdcanvas)
cdCanvasSetBackground(cddbuffer, CD_BLACK)
return IUP_DEFAULT
end function

function timer_cb(Ihandle /*ih*/)
integer {w, h} = IupGetIntInt(canvas, "DRAWSIZE")
fps += 1
df = time()-start
start = time()
for i=1 to particlenum do
atom {x,y,color,life,dx,dy} = particles[i]
if life<=0 then
if rnd()<df then
life = 2.5          -- time to live
x = w/2             -- starting position x
y = h/10            --               and y
-- randomize velocity so points reach different heights:
atom r = iff(reciprocate?range*sin(time()):0)
dx = (spread*rnd()-spread/2+r)*50   -- starting velocity x
dy = (rnd()-2.9) * h/20.5           --               and y
color = hsv_to_rgb(round(remainder(time(),5)/5,100), saturation, 1)
end if
else
if y>h/10 and dy>0 then
dy *= -0.3  -- "bounce"
end if
dy += (h/10)*df -- adjust velocity
x += dx*df      -- adjust position x
y += dy*df*8    --             and y
life -= df
end if
particles[i] = {x,y,color,life,dx,dy}
end for
IupRedraw(canvas)
if time()>t1 then
IupSetStrAttribute(dlg,"TITLE","%s (%d, %d fps/s [%dx%d])",{title,particlenum,fps,w,h})
t1 = time()+1
fps = 0
end if
return IUP_DEFAULT
end function

function key_cb(Ihandle /*dlg*/, atom c)
if c=K_ESC or lower(c)='q' then return IUP_CLOSE
elsif c=K_F1 then   IupMessage(title,help_text)
elsif c=K_UP then   saturation = min(saturation+0.1,1)
elsif c=K_DOWN then saturation = max(saturation-0.1,0)
elsif c=K_RIGHT then range = min(range+0.1,2)
elsif c=K_LEFT then range = max(range-0.1,0.1)
elsif c=K_SP then reciprocate = not reciprocate
end if
return IUP_CONTINUE
end function

procedure main()
IupOpen()
canvas = IupGLCanvas("RASTERSIZE=400x300")
IupSetCallbacks({canvas}, {"ACTION", Icallback("redraw_cb"),
"MAP_CB", Icallback("map_cb")})
dlg = IupDialog(canvas,`TITLE="%s"`,{title})
IupSetCallback(dlg, "KEY_CB", Icallback("key_cb"))
Ihandle timer = IupTimer(Icallback("timer_cb"), 1000/25)
IupShowXY(dlg,IUP_CENTER,IUP_CENTER)
IupSetAttribute(canvas, "RASTERSIZE", NULL)
if platform()!=JS then
IupMainLoop()
IupClose()
end if
end procedure

main()
```

## Python

Library: SDL
Translation of: C++

Use arrow keys, PageUp, PageDown and Space to vary fountain parameters.

```# Using SDL2 library: # pip install PySDL2

import sys
import random
import time
import math

import sdl2
import sdl2.ext

FPS = 60
NEW_PARTICLES_PER_FRAME = 10
MAX_PARTICLES = 5_000
GRAVITY = 100
WIDTH = 640
HEIGHT = 480

def clamp(value, min_, max_):
"""Return value clamped between min and max"""
return max(min_, min(value, max_))

class Particle:
"""Particle obeying gravity law."""

def __init__(self):
self.x = 0
self.y = 0
self.v_x = 0
self.v_y = 0

def update(self, dtime: float) -> None:
"""Move particle and update speed with gravity"""
self.x = self.x + self.v_x * dtime
self.y = self.y + self.v_y * dtime
self.v_y = self.v_y + GRAVITY * dtime

def set(self, x, y, v_x, v_y):
"""Set particle values"""
self.x = x
self.y = y
self.v_x = v_x
self.v_y = v_y

class Fountain:
"""The fountain"""

def __init__(self, max_particles: int, particles_per_frame: int):
self.particles_per_frame = particles_per_frame
self.max_particles = max_particles
self.range = math.sqrt(2 * GRAVITY * (HEIGHT - 20 - self.spread))
self.saturation = 155
self.reciprocate = False
self.reciprocating_time = 0.0
self.particles = [
self.init_particle(Particle()) for _ in range(self.particles_per_frame)
]

def update(self, dtime) -> None:
"""Update particles"""
if self.reciprocate:
self.reciprocating_time += dtime

for particle in self.particles:
particle.update(dtime)
if particle.y > HEIGHT - 10:
self.init_particle(particle)

if len(self.particles) < self.max_particles:
for _ in range(self.particles_per_frame):
self.particles.append(self.init_particle(Particle()))
# print(len(particles))

def render(self, renderer: sdl2.ext.renderer.Renderer) -> None:
"""Render particles"""
points = [(particle.x, particle.y) for particle in self.particles]

renderer.clear()
renderer.draw_point(
points, sdl2.ext.Color(self.saturation, self.saturation, 255)
)
renderer.present()

def step_parameter(self, param, step):
"""Change parameters"""
if param == "spread":
self.spread = clamp(self.spread + step, 0, 50)
elif param == "range":
self.range = clamp(self.range + step, 0, 300)
elif param == "color":
self.saturation = clamp(self.saturation + step, 0, 255)
elif param == "reciprocate":
self.reciprocate = not self.reciprocate
self.reciprocating_time = 0.0

def init_particle(self, particle: Particle) -> Particle:
"""Move particle at initial position with a random-y speed"""
direction = random.random() * math.pi * 2
v_x = radius * math.cos(direction) + math.sin(self.reciprocating_time) * 20.0
v_y = -self.range + radius * math.sin(direction)
particle.set(WIDTH // 2, HEIGHT - 10, v_x, v_y)
return particle

def make_renderer() -> sdl2.ext.renderer.Renderer:
"""Initialise SDL and make renderer"""
sdl2.ext.init()

window = sdl2.ext.Window("Particle Fountain", size=(WIDTH, HEIGHT))
window.show()

renderer = sdl2.ext.renderer.Renderer(window)

return renderer

def limit_frame_rate(fps: float, cur_time: int) -> bool:
"""Limit frame rate"""
dtime = time.monotonic_ns() - cur_time
frame_duration = 1e9 / fps
if dtime < frame_duration:
time.sleep((frame_duration - dtime) / 1e9)
return True
return False

def handle_events(fountain: Fountain):
"""Act on events"""
key_actions = {
sdl2.SDL_SCANCODE_PAGEUP: lambda: fountain.step_parameter("color", 5),
sdl2.SDL_SCANCODE_PAGEDOWN: lambda: fountain.step_parameter("color", -5),
sdl2.SDL_SCANCODE_UP: lambda: fountain.step_parameter("range", 1),
sdl2.SDL_SCANCODE_DOWN: lambda: fountain.step_parameter("range", -1),
sdl2.SDL_SCANCODE_LEFT: lambda: fountain.step_parameter("spread", -1),
sdl2.SDL_SCANCODE_RIGHT: lambda: fountain.step_parameter("spread", 1),
sdl2.SDL_SCANCODE_SPACE: lambda: fountain.step_parameter("reciprocate", 1),
}

events = sdl2.ext.get_events()
for event in events:
if event.type == sdl2.SDL_QUIT:
return False
if event.type == sdl2.SDL_KEYDOWN:
if event.key.keysym.scancode in key_actions:
key_actions[event.key.keysym.scancode]()
elif event.key.keysym.scancode == sdl2.SDL_SCANCODE_Q:
return False
return True

def main_loop(renderer: sdl2.ext.renderer.Renderer, fountain: Fountain) -> None:
"""Main animation loop"""
running = True

cur_time = time.monotonic_ns()
while running:
running = handle_events(fountain)

fountain.render(renderer)

if not limit_frame_rate(FPS, cur_time):
print(f"Didn't make it in time with {len(fountain.particles)} particles.")

dtime = (time.monotonic_ns() - cur_time) / 1e9  # in seconds
fountain.update(dtime)
cur_time = time.monotonic_ns()

sdl2.ext.quit()

def run():
"""Start!"""

renderer = make_renderer()
fountain = Fountain(MAX_PARTICLES, NEW_PARTICLES_PER_FRAME)

main_loop(renderer, fountain)

return 0

if __name__ == "__main__":
sys.exit(run())
```

## Raku

Has options to vary the direction at which the fountain sprays, the "spread" angle and the color of the emitted particles.

```use NativeCall;
use SDL2::Raw;

my int (\$w, \$h) = 800, 800;
my SDL_Window \$window;
my SDL_Renderer \$renderer;

my int \$particlenum = 3000;

SDL_Init(VIDEO);
\$window = SDL_CreateWindow(
"Raku Particle System!",
\$w, \$h,
RESIZABLE
);
\$renderer = SDL_CreateRenderer( \$window, -1, ACCELERATED );

SDL_ClearError();

my num @positions  = 0e0 xx (\$particlenum * 2);
my num @velocities = 0e0 xx (\$particlenum * 2);
my num @lifetimes  = 0e0 xx  \$particlenum;

my CArray[int32] \$points .= new;
my int \$numpoints;
my Num \$saturation = 4e-1;
my Num \$spread = 15e-1;
my &reciprocate = sub { 0 }
my \$range = 1.5;

sub update (num \df) {
my int \$xidx = 0;
my int \$yidx = 1;
my int \$pointidx = 0;
loop (my int \$idx = 0; \$idx < \$particlenum; \$idx = \$idx + 1) {
my int \$willdraw = 0;
if (@lifetimes[\$idx] <= 0e0) {
if (rand < df) {
@lifetimes[\$idx]   = 25e-1;                       # time to live
@positions[\$xidx]  = (\$w / 20e0).Num;             # starting position x
@positions[\$yidx]  = (\$h / 10).Num;               # and y
@velocities[\$xidx] = (\$spread * rand - \$spread/2 + reciprocate()) * 10; # starting velocity x
@velocities[\$yidx] = (rand - 2.9e0) * \$h / 20.5;    # and y (randomized slightly so points reach different heights)
\$willdraw = 1;
}
} else {
if @positions[\$yidx] > \$h / 10 && @velocities[\$yidx] > 0 {
@velocities[\$yidx] = @velocities[\$yidx] * -0.3e0; # "bounce"
}

@velocities[\$yidx] = @velocities[\$yidx] + \$h/10.Num * df;         # adjust velocity
@positions[\$xidx]  = @positions[\$xidx] + @velocities[\$xidx] * df; # adjust position x
@positions[\$yidx]  = @positions[\$yidx] + @velocities[\$yidx] * df; # and y

\$willdraw = 1;
}

if (\$willdraw) {
\$points[\$pointidx++] = (@positions[\$xidx] * 10).floor; # gather all of the points that
\$points[\$pointidx++] = (@positions[\$yidx] * 10).floor; # are still going to be rendered
}

\$xidx = \$xidx + 2;
\$yidx = \$xidx + 1;
}
\$numpoints = (\$pointidx - 1) div 2;
}

sub render {
SDL_SetRenderDrawColor(\$renderer, 0x0, 0x0, 0x0, 0xff);
SDL_RenderClear(\$renderer);

SDL_SetRenderDrawColor(\$renderer, |hsv2rgb(((now % 5) / 5).round(.01), \$saturation, 1), 0x7f);
SDL_RenderDrawPoints(\$renderer, \$points, \$numpoints);

SDL_RenderPresent(\$renderer);
}

enum KEY_CODES (
K_UP     => 82,
K_DOWN   => 81,
K_LEFT   => 80,
K_RIGHT  => 79,
K_SPACE  => 44,
K_PGUP   => 75,
K_PGDN   => 78,
K_Q      => 20,
);

say q:to/DOCS/;
Use UP and DOWN arrow keys to modify the saturation of the particle colors.
Use PAGE UP and PAGE DOWN keys to modify the "spread" of the particles.
Toggle reciprocation off / on with the SPACE bar.
Use LEFT and RIGHT arrow keys to modify angle range for reciprocation.
Press the "q" key to quit.
DOCS

my \$event = SDL_Event.new;

my num \$df = 0.0001e0;

main: loop {
my \$start = now;

while SDL_PollEvent(\$event) {
my \$casted_event = SDL_CastEvent(\$event);

given \$casted_event {
when *.type == QUIT {
last main;
}
when *.type == WINDOWEVENT {
if .event == RESIZED {
\$w = .data1;
\$h = .data2;
}
}
when *.type == KEYDOWN {
if KEY_CODES(.scancode) -> \$comm {
given \$comm {
when 'K_UP'    { \$saturation = ((\$saturation + .1) min 1e0) }
when 'K_DOWN'  { \$saturation = ((\$saturation - .1) max 0e0) }
when 'K_PGUP'  { \$spread = ((\$spread + .1) min 5e0) }
when 'K_PGDN'  { \$spread = ((\$spread - .1) max 2e-1) }
when 'K_RIGHT' { \$range = ((\$range + .1) min 2e0) }
when 'K_LEFT'  { \$range = ((\$range - .1) max 1e-1) }
when 'K_SPACE' { &reciprocate = reciprocate() == 0 ?? sub { \$range * sin(now) } !! sub { 0 } }
when 'K_Q'     { last main }
}
}
}
}
}

update(\$df);

render();

\$df = (now - \$start).Num;

print fps();
}

say '';

sub fps {
state \$fps-frames = 0;
state \$fps-now    = now;
state \$fps        = '';
\$fps-frames++;
if now - \$fps-now >= 1 {
\$fps = [~] "\r", ' ' x 20, "\r",
sprintf "FPS: %5.1f  ", (\$fps-frames / (now - \$fps-now));
\$fps-frames = 0;
\$fps-now = now;
}
\$fps
}

sub hsv2rgb ( \$h, \$s, \$v ){
state %cache;
%cache{"\$h|\$s|\$v"} //= do {
my \$c = \$v * \$s;
my \$x = \$c * (1 - abs( ((\$h*6) % 2) - 1 ) );
my \$m = \$v - \$c;
[(do given \$h {
when   0..^1/6 { \$c, \$x, 0 }
when 1/6..^1/3 { \$x, \$c, 0 }
when 1/3..^1/2 { 0, \$c, \$x }
when 1/2..^2/3 { 0, \$x, \$c }
when 2/3..^5/6 { \$x, 0, \$c }
when 5/6..1    { \$c, 0, \$x }
} ).map: ((*+\$m) * 255).Int]
}
}
```

## Wren

Translation of: Julia
Library: DOME
Library: Wren-dynamic
```import "dome" for Window, Platform, Process
import "graphics" for Canvas, Color
import "math" for Math, Point
import "random" for Random
import "input" for Keyboard
import "./dynamic" for Struct

var Start  = Platform.time
var Rand   = Random.new()

var fields = [
"particleNum",
"positions",
"velocities",
"points",
"numPoints",
"saturation",
"range",
"reciprocate"
]
var ParticleFountain = Struct.create("ParticleFountain", fields)

class ParticleDisplay {
construct new(particleNum, width, height) {
Window.resize(width, height)
Canvas.resize(width, height)
Window.title = "Wren Particle System!"
_pn = particleNum
_w = width
_h = height
_df = 1 / 200 // say
_pf = ParticleFountain.new(
_pn,                      // particleNum
List.filled(_pn * 2, 0),  // positions
List.filled(_pn * 2, 0),  // velocities
List.filled(_pn, 0),      // lifetimes
List.filled(_pn, null),   // points
0,                        // numPoints
0.4,                      // saturation
1.5,                      // range
false                     // reciprocate
)
for (i in 0..._pn) _pf.points[i] = Point.new(0, 0)
}

init() {
Canvas.cls()
_frames = 0
}

updatePF() {
var xidx = 0
var yidx = 1
var pointIdx = 0
var recip = Fn.new { _pf.reciprocate ? _pf.range * Math.sin(Platform.time/1000) : 0 }
for (idx in 0..._pf.particleNum) {
var willDraw = false
if (_pf.lifetimes[idx] <= 0) {
if (Rand.float() < _df) {
_pf.lifetimes[idx]  = 2.5       // time to live
_pf.positions[xidx] = _w / 20   // starting position x
_pf.positions[yidx] = _h / 10   // and y

// starting velocities x and y
// randomized slightly so points reach different heights
_pf.velocities[xidx] = 10 * (_pf.spread * Rand.float() - _pf.spread / 2 + recip.call())
_pf.velocities[yidx] = (Rand.float() - 2.9) * _h / 20.5
_willDraw = true
}
} else {
if (_pf.positions[yidx] > _h/10 && _pf.velocities[yidx] > 0) {
_pf.velocities[yidx] = _pf.velocities[yidx] * (-0.3)   // bounce
}
_pf.velocities[yidx] = _pf.velocities[yidx] + _df * _h / 10             // adjust velocity
_pf.positions[xidx]  = _pf.positions[xidx] + _pf.velocities[xidx] * _df // adjust position x
_pf.positions[yidx]  = _pf.positions[yidx] + _pf.velocities[yidx] * _df // and y
willDraw = true
}
if (willDraw) {  // gather all the points that are going to be rendered
_pf.points[pointIdx] = Point.new((_pf.positions[xidx] * 10).floor,
(_pf.positions[yidx] * 10).floor)
pointIdx = pointIdx + 1
}
xidx = xidx + 2
yidx = xidx + 1
_pf.numPoints = pointIdx
}
}

update() {
if (Keyboard["Up"].justPressed) {
_pf.saturation = Math.min(_pf.saturation + 0.1, 1)
} else if (Keyboard["Down"].justPressed) {
_pf.saturation = Math.max(_pf.saturation - 0.1, 0)
} else if (Keyboard["PageUp"].justPressed) {
} else if (Keyboard["PageDown"].justPressed) {
} else if (Keyboard["Left"].justPressed) {
_pf.range = Math.min(_pf.range + 0.1, 12)
} else if (Keyboard["Right"].justPressed) {
_pf.range = Math.max(_pf.range - 0.1, 0.1)
} else if (Keyboard["Space"].justPressed) {
_pf.reciprocate = !_pf.reciprocate
} else if (Keyboard["Q"].justPressed) {
Process.exit()
}
updatePF()
}

draw(alpha) {
var c = Color.hsv((Platform.time % 5) * 72, _pf.saturation, 0.5, 0x7f)
for (i in 0..._pf.numPoints) {
Canvas.pset(_pf.points[i].x, _pf.points[i].y, c)
}
_frames = _frames + 1
var now = Platform.time
if (now - Start >= 1) {
Start = now
Window.title = "Wren Particle System!    (FPS = %(_frames))"
_frames = 0
}
}
}

System.print("""

Use UP and DOWN arrow keys to modify the saturation of the particle colors.
Use PAGE UP and PAGE DOWN keys to modify the "spread" of the particles.
Toggle reciprocation off / on with the SPACE bar.
Use LEFT and RIGHT arrow keys to modify angle range for reciprocation.
Press the "q" key to quit.
""")

var Game = ParticleDisplay.new(3000, 800, 800)```