33 lines
1.0 KiB
Plaintext
33 lines
1.0 KiB
Plaintext
module nqueens
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import std/num/int32
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import std/os/env
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alias solution = list<int32>
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alias solutions = list<list<int32>>
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fun safe( queen : int32, diag : int32, ^xs : solution ) : bool
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match xs
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Cons(q,qs) -> (queen != q && queen != (q+diag) && queen != (q - diag) && safe(queen,diag.inc,qs))
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_ -> True
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fun append-safe( queen : int32, xs : solution, xss : solutions ) : div solutions
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if queen <= 0.int32 then xss
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elif safe(queen,1.int32,xs) then append-safe( queen.dec, xs, Cons(Cons(queen,xs),xss) )
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else append-safe( queen.dec, xs, xss )
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fun extend( queen : int32, acc : solutions, xss : solutions ) : div solutions
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match xss
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Cons(xs,rest) -> extend(queen, append-safe(queen,xs,acc), rest)
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Nil -> acc
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fun find-solutions( n : int32, queen : int32 ) : div solutions
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if queen.is-zero
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then [[]]
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else extend(n, [], find-solutions(n,queen.dec))
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pub fun queens( n : int32 ) : div int
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find-solutions(n,n).length
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pub fun main()
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queens(get-args().head("").parse-int.default(13).int32).println
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