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66 lines
1.7 KiB
Text
66 lines
1.7 KiB
Text
app [main!] { pf: platform "platform/main.roc" }
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import pf.Host
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main! : {} => {}
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main! = \{} ->
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{ value, is_error } = Host.get_int!({})
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input_result =
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if is_error then
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Err(GetIntError)
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else
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Ok(value)
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when input_result is
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Ok(n) ->
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queens(n) # original koka 13
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|> Num.toStr
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|> Host.put_line!
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Err(GetIntError) ->
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Host.put_line!("Error: Failed to get Integer from stdin.")
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ConsList a : [Nil, Cons a (ConsList a)]
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queens = \n -> length(find_solutions(n, n))
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find_solutions = \n, k ->
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if k <= 0 then
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# should we use U64 as input type here instead?
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Cons(Nil, Nil)
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else
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extend(n, Nil, find_solutions(n, (k - 1)))
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extend = \n, acc, solutions ->
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when solutions is
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Nil -> acc
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Cons(soln, rest) -> extend(n, append_safe(n, soln, acc), rest)
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append_safe : I64, ConsList I64, ConsList (ConsList I64) -> ConsList (ConsList I64)
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append_safe = \k, soln, solns ->
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if k <= 0 then
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solns
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else if safe(k, 1, soln) then
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append_safe((k - 1), soln, Cons(Cons(k, soln), solns))
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else
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append_safe((k - 1), soln, solns)
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safe : I64, I64, ConsList I64 -> Bool
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safe = \queen, diagonal, xs ->
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when xs is
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Nil -> Bool.true
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Cons(q, t) ->
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if queen != q && queen != q + diagonal && queen != q - diagonal then
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safe(queen, (diagonal + 1), t)
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else
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Bool.false
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length : ConsList a -> I64
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length = \xs ->
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length_help(xs, 0)
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length_help : ConsList a, I64 -> I64
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length_help = \foobar, acc ->
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when foobar is
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Cons(_, lrest) -> length_help(lrest, (1 + acc))
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Nil -> acc
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