102 lines
2.6 KiB
Forth
102 lines
2.6 KiB
Forth
# http://rosettacode.org/wiki/N-queens_problem
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# port of the Arc solution at http://arclanguage.org/item?id=19743
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# run with tracing turned on:
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# ./mu --trace nqueens.mu
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container square [
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rank:num
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file:num
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]
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def nqueens n:num, queens:&:list:square -> result:num, queens:&:list:square [
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local-scope
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load-inputs
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# if 'queens' is already long enough, print it and return
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added-so-far:num <- length queens
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{
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done?:bool <- greater-or-equal added-so-far, n
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break-unless done?
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stash queens
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return 1
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}
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# still work to do
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next-rank:num <- copy 0
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{
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break-unless queens
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first:square <- first queens
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existing-rank:num <- get first, rank:offset
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next-rank <- add existing-rank, 1
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}
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result <- copy 0
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next-file:num <- copy 0
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{
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done?:bool <- greater-or-equal next-file, n
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break-if done?
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curr:square <- merge next-rank, next-file
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{
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curr-conflicts?:bool <- conflict? curr, queens
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break-if curr-conflicts?
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new-queens:&:list:square <- push curr, queens
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sub-result:num <- nqueens n, new-queens
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result <- add result, sub-result
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}
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next-file <- add next-file, 1
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loop
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}
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]
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# check if putting a queen on 'curr' conflicts with any of the existing
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# queens
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# assumes that 'curr' is on a non-conflicting rank, and checks for conflict
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# only in files and diagonals
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def conflict? curr:square, queens:&:list:square -> result:bool [
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local-scope
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load-inputs
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result <- conflicting-file? curr, queens
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return-if result
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result <- conflicting-diagonal? curr, queens
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]
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def conflicting-file? curr:square, queens:&:list:square -> result:bool [
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local-scope
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load-inputs
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curr-file:num <- get curr, file:offset
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{
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break-unless queens
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q:square <- first queens
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qfile:num <- get q, file:offset
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file-match?:bool <- equal curr-file, qfile
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return-if file-match?, true/conflict-found
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queens <- rest queens
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loop
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}
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return false/no-conflict-found
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]
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def conflicting-diagonal? curr:square, queens:&:list:square -> result:bool [
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local-scope
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load-inputs
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curr-rank:num <- get curr, rank:offset
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curr-file:num <- get curr, file:offset
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{
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break-unless queens
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q:square <- first queens
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qrank:num <- get q, rank:offset
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qfile:num <- get q, file:offset
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rank-delta:num <- subtract qrank, curr-rank
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file-delta:num <- subtract qfile, curr-file
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rank-delta <- abs rank-delta
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file-delta <- abs file-delta
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diagonal-match?:bool <- equal rank-delta, file-delta
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return-if diagonal-match?, true/conflict-found
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queens <- rest queens
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loop
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}
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return false/no-conflict-found
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]
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def main [
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nqueens 4
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$dump-trace [app]
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]
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