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September 2017 Update

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Ingy döt Net 2017-09-23 10:01:46 +02:00
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59
Task/N-queens-problem/REXX/n-queens-problem.rexx
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/*REXX program places N queens on an NxN chessboard (the eight queens problem). */
parse arg N . /*obtain optional argument from the CL.*/
if N=='' | N=="," then N=8 /*Not specified: Then use the default.*/
if N<1 then call noSol /*display a message, the board is bad. */
if N<1 then signal nOK /*display a message, the board is bad. */
rank=1; file=1; #=0 /*starting rank&file; #≡number queens.*/
@.=0; pad=left('', 9* (N<18)) /*define empty board; set indentation.*/
do while #<N; @.file.rank=1 /*keep placing queens until we're done.*/
if ok(file,rank) then do; #=#+1 /*Queen not being attacked? Then eureka*/
file=1 /*use another attempt at another file. */
rank=rank+1 /*and also bump the rank counter. */
iterate /*go and try another queen placement. */
end /* [↑] found a good queen placement. */
/* [↓] rank&file ≡ chessboard row&cols*/
do while #<N; @.file.rank=1 /*keep placing queens until we're done.*/
if ok(file, rank) then do; #=#+1; rank=rank+1 /*Queen not being attacked? Then eureka*/
file=1 /*use another attempt at another file. */
iterate /*go and try another queen placement. */
end /* [↑] found a good queen placement. */
@.file.rank=0 /*It isn't safe. So remove this queen.*/
file=file+1 /*So, try the next (higher) file. */
do while file>N; rank=rank-1; if rank==0 then call noSol
do j=1 for N; if \@.j.rank then iterate /*occupied?*/
file=j; @.file.rank=0; #=#-1; file=j+1; leave /*j*/
file=file+1 /*So, try the next (higher) chess file.*/
do while file>N; rank=rank-1; if rank==0 then call nOK
do j=1 for N; if \@.j.rank then iterate /*¿ocupado?*/
@.j.rank=0; #=#-1; file=j+1; leave
end /*j*/
end /*while file>N*/
end /*while #<N*/
say 'A solution for' N "queens:"; a = substr( copies("┼───", N) ,2); say
say pad translate('┌'a"┐", '┬', "┼") /*display the top rank (of the board).*/
line = '├'a"┤"; dither='░' /*define a line (bar) for cell boundry.*/
bar = '│' ; queen='Q' /*kinds: horizontal, vertical, salad. */
Bqueen = dither || queen || dither /*glyph befitting a black-square queen.*/
Wqueen = ' 'queen" " /* " " " white-square " */
say 'A solution for ' N " queens:"; g=substr( copies("╬═══", N) ,2); say
say pad translate('╔'g"╗", '╦', "╬") /*display the top rank (of the board).*/
line = '╠'g"╣"; dither='░' /*define a line (bar) for cell boundry.*/
bar = '║' ; queen='Q' /*kinds: horizontal, vertical, salad. */
Bqueen = dither || queen || dither /*glyph befitting a black─square queen.*/
Wqueen = ' 'queen" " /* " " " white─square " */
do rank=1 for N; if rank\==1 then say pad line; _= /*display sep for rank.*/
do file=1 for N; B=(file+rank)//2 /*is the square black? */
Qgylph=Wqueen; if B then Qgylph=Bqueen /*use a dithered queen.*/
if @.file.rank then _=_ || bar || Qgylph /*3-char queen symbol. */
do rank=1 for N; if rank\==1 then say pad line; _= /*display sep for rank.*/
do file=1 for N; B = (file+rank) // 2 /*is the square black? */
Qgylph=Wqueen; if B then Qgylph=Bqueen /*use a dithered queen.*/
if @.file.rank then _=_ || bar || Qgylph /*3─char queen symbol. */
else if B then _=_ || bar || copies(dither,3) /*use dithering for sq.*/
else _=_ || bar || copies(' ' ,3) /* " 3 blanks " " */
end /*file*/ /* [↑] preserve square-ish chessboard.*/
end /*file*/ /* [↑] preserve square─ish chessboard.*/
say pad _ || bar /*show a single rank of the chessboard.*/
end /*rank*/ /*80 cols can view a 19x19 chessboard.*/
say pad translate('└'a"┘", '┴', "┼") /*display the last rank (of the board).*/
say pad translate('╚'g"╝", '╩', "╬") /*display the last rank (of the board).*/
exit 1 /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
noSol: say; say "No solution for" N 'queens.'; say; exit 0
nOK: say; say "No solution for" N 'queens.'; say; exit 0
/*──────────────────────────────────────────────────────────────────────────────────────*/
ok: parse arg f,r; rm=r-1; fm=f-1; fp=f+1
do k=1 for rm; if @.f.k then return 0; end
f=fm; do k=rm by -1 for rm while f\==0; if @.f.k then return 0; f=f-1; end
f=fp; do k=rm by -1 for rm while f <=N; if @.f.k then return 0; f=f+1; end
return 1 /* ↑↑↑↑↑↑↑↑ is queen under attack?*/
ok: parse arg f,r; fp=f+1; rm=r-1 /*if return≡0, then queen isn't safe. */
do k=1 for rm; if @.f.k then return 0; end
f=f-1; do k=rm by -1 for rm while f\==0; if @.f.k then return 0; f=f-1; end
f=fp; do k=rm by -1 for rm while f <=N; if @.f.k then return 0; f=f+1; end
return 1 /*1≡queen is safe. */ /* ↑↑↑↑↑↑↑↑ is queen under attack? */