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# http://rosettacode.org/wiki/N-queens_problem
# port of the Arc solution at http://arclanguage.org/item?id=19743
container square [
rank:number
file:number
]
def nqueens n:number, queens:address:list:square -> result:number [
local-scope
load-ingredients
# if 'queens' is already long enough, print it and return
added-so-far:number <- length queens
{
done?:boolean <- greater-or-equal added-so-far, n
break-unless done?
stash queens
return 1
}
# still work to do
next-rank:number <- copy 0
{
break-unless queens
first:square <- first queens
existing-rank:number <- get first, rank:offset
next-rank <- add existing-rank, 1
}
result <- copy 0
next-file:number <- copy 0
{
done?:boolean <- greater-or-equal next-file, n
break-if done?
curr:square <- merge next-rank, next-file
{
curr-conflicts?:boolean <- conflict? curr, queens
break-if curr-conflicts?
new-queens:address:list:square <- push curr, queens
sub-result:number <- nqueens n, new-queens
result <- add result, sub-result
}
next-file <- add next-file, 1
loop
}
]
def conflict? curr:square, queens:address:list:square -> result:boolean [
local-scope
load-ingredients
result1:boolean <- conflicting-file? curr, queens
reply-if result1, result1
result2:boolean <- conflicting-diagonal? curr, queens
reply result2
]
def conflicting-file? curr:square, queens:address:list:square -> result:boolean [
local-scope
load-ingredients
curr-file:number <- get curr, file:offset
{
break-unless queens
q:square <- first queens
qfile:number <- get q, file:offset
file-match?:boolean <- equal curr-file, qfile
reply-if file-match?, 1/conflict-found
queens <- rest queens
loop
}
reply 0/no-conflict-found
]
def conflicting-diagonal? curr:square, queens:address:list:square -> result:boolean [
local-scope
load-ingredients
curr-rank:number <- get curr, rank:offset
curr-file:number <- get curr, file:offset
{
break-unless queens
q:square <- first queens
qrank:number <- get q, rank:offset
qfile:number <- get q, file:offset
rank-delta:number <- subtract qrank, curr-rank
file-delta:number <- subtract qfile, curr-file
rank-delta <- abs rank-delta
file-delta <- abs file-delta
diagonal-match?:boolean <- equal rank-delta, file-delta
reply-if diagonal-match?, 1/conflict-found
queens <- rest queens
loop
}
reply 0/no-conflict-found
]
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