Addition chains

From Rosetta Code
Addition chains is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

An addition chain of length r for n is a sequence 1 = a(0) < a(1) < a(2) ... < a(r) = n , such as a(k) = a(i) + a(j) ( i < k and j < k , i may be = j) . Each member is the sum of two earlier members, not necessarily distincts.

A Brauer chain for n is an addition chain where a(k) = a(k-1) + a(j) with j < k. Each member uses the previous member as a summand.

We are interested in chains of minimal length L(n).

Task

For each n in {7,14,21,29,32,42,64} display the following : L(n), the count of Brauer chains of length L(n), an example of such a Brauer chain, the count of non-brauer chains of length L(n), an example of such a chain. (NB: counts may be 0 ).

Extra-credit: Same task for n in {47, 79, 191, 382 , 379, 12509}

References

  • OEIS sequences A079301, A079302. [1]
  • Richard K. Guy - Unsolved problems in Number Theory - C6 - Addition chains.

Example

  • minimal chain length l(19) = 6
  • brauer-chains(19) : count = 31 Ex: ( 1 2 3 4 8 11 19)
  • non-brauer-chains(19) : count = 2 Ex: ( 1 2 3 6 7 12 19)

EchoLisp[edit]

 
;; 2^n
(define exp2 (build-vector 32 (lambda(i)(expt 2 i))))
 
;; counters and results
(define-values (*minlg* *counts* *chains* *calls*) '(0 null null 0))
 
(define (register-hit chain lg )
(define idx (if (brauer? chain lg) 0 1))
(when (< lg *minlg*)
(set! *counts* (make-vector 2 0))
(set! *chains* (make-vector 2 ""))
(set! *minlg* lg))
(vector+= *counts* idx 1)
(vector-set! *chains* idx (vector->list chain)))
 
;; is chain a brauer chain ?
(define (brauer? chain lg)
(for [(i (in-range 1 lg))]
#:break (not (vector-search* (- [chain i] [chain (1- i)]) chain)) => #f
#t))
 
;; all min chains to target n (brute force)
(define (chains n chain lg (a) (top) (tops null))
(++ *calls*)
(set! top [chain lg])
(cond
[(> lg *minlg*) #f] ;; too long
[(= n top) (register-hit chain lg)] ;; hit
[(< n top) #f] ;; too big
[(and (< *minlg* 32) (< (* top [exp2 (- *minlg* lg)]) n)) #f] ;; too small
[else
(for* ([i (in-range lg -1 -1)] [j (in-range lg (1- i) -1)])
(set! a (+ [chain i] [chain j]))
#:continue (<= a top) ;; increasing sequence
#:continue (memq a tops) ;; prevent duplicates
(set! tops (cons a tops))
(vector-push chain a)
(chains n chain (1+ lg))
(vector-pop chain))]))
 
 
(define (task n)
(set!-values (*minlg* *calls*) '(Infinity 0 ))
(chains n (make-vector 1 1) 0)
(printf "L(%d) = %d - brauer-chains: %d non-brauer: %d chains: %a %a "
n *minlg* [*counts* 0] [*counts* 1] [*chains* 0] [*chains* 1]))
 
Output:
(for-each task {7 14 21 29 32 42 64})

L(7) = 4 - brauer-chains: 5 non-brauer: 0 chains: (1 2 3 4 7)  
L(14) = 5 - brauer-chains: 14 non-brauer: 0 chains: (1 2 3 4 7 14)  
L(21) = 6 - brauer-chains: 26 non-brauer: 3 chains: (1 2 3 4 7 14 21) (1 2 4 5 8 13 21) 
L(29) = 7 - brauer-chains: 114 non-brauer: 18 chains: (1 2 3 4 7 11 18 29) (1 2 3 6 9 11 18 29) 
L(32) = 5 - brauer-chains: 1 non-brauer: 0 chains: (1 2 4 8 16 32)  
L(42) = 7 - brauer-chains: 78 non-brauer: 6 chains: (1 2 3 4 7 14 21 42) (1 2 4 5 8 13 21 42) 
L(64) = 6 - brauer-chains: 1 non-brauer: 0 chains: (1 2 4 8 16 32 64) 

;; a few extras
(task 47)
L(47) = 8 - brauer-chains: 183 non-brauer: 37 chains: (1 2 3 4 7 10 20 27 47) (1 2 3 5 7 14 19 28 47) 
(task 79)
L(79) = 9 - brauer-chains: 492 non-brauer: 129 chains: (1 2 3 4 7 9 18 36 43 79) (1 2 3 5 7 12 24 31 48 79) 

zkl[edit]

Translation of: EchoLisp
var exp2=(32).pump(List,(2).pow),   // 2^n, n=0..31
_minlg, _counts, _chains; // counters and results
 
fcn register_hit(chain,lg){ // save [upto 2] chains
idx:=(if(isBrauer(chain,lg)) 0 else 1);
if(lg<_minlg) _counts,_chains,_minlg=List(0,0), List("",""), lg;
_counts[idx]+=1;
_chains[idx]=chain.copy();
}
// is chain a brauer chain ?
fcn isBrauer(chain,lg){
foreach i in (lg){
if(not chain.holds(chain[i+1] - chain[i])) return(False);
}
True
}
// all min chains to target n (brute force)
fcn chains(n,chain,lg){
top,tops:=chain[lg], List();
if(lg>_minlg) {} // too long
else if(n==top) register_hit(chain,lg); // hit
else if(n<top) {} // too big
else if((_minlg<32) and (top*exp2[_minlg - lg]<n)){} // too small
else{
foreach i,j in ([lg..0,-1],[lg..i,-1]){
a:=chain[i] + chain[j];
if(a<=top) continue; // increasing sequence
if(tops.holds(a)) continue; // prevent duplicates
tops.append(a);
chain.append(a);
self.fcn(n,chain,lg+1); // recurse
chain.pop();
}
}
}
fcn task(n){
_minlg=(0).MAX;
chains(n,List(1),0);
println("L(%2d) = %d; Brauer-chains: %3d; non-brauer: %3d; chains: %s"
.fmt(n,_minlg,_counts.xplode(),_chains.filter()));
}
T(7,14,21,29,32,42,64,47,79).apply2(task);
Output:
L( 7) = 4; Brauer-chains:   5; non-brauer:   0; chains: L(L(1,2,3,4,7))
L(14) = 5; Brauer-chains:  14; non-brauer:   0; chains: L(L(1,2,3,4,7,14))
L(21) = 6; Brauer-chains:  26; non-brauer:   3; chains: L(L(1,2,3,4,7,14,21),L(1,2,4,5,8,13,21))
L(29) = 7; Brauer-chains: 114; non-brauer:  18; chains: L(L(1,2,3,4,7,11,18,29),L(1,2,3,6,9,11,18,29))
L(32) = 5; Brauer-chains:   1; non-brauer:   0; chains: L(L(1,2,4,8,16,32))
L(42) = 7; Brauer-chains:  78; non-brauer:   6; chains: L(L(1,2,3,4,7,14,21,42),L(1,2,4,5,8,13,21,42))
L(64) = 6; Brauer-chains:   1; non-brauer:   0; chains: L(L(1,2,4,8,16,32,64))
L(47) = 8; Brauer-chains: 183; non-brauer:  37; chains: L(L(1,2,3,4,7,10,20,27,47),L(1,2,3,5,7,14,19,28,47))
L(79) = 9; Brauer-chains: 492; non-brauer: 129; chains: L(L(1,2,3,4,7,9,18,36,43,79),L(1,2,3,5,7,12,24,31,48,79))