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48 lines
1009 B
Smalltalk
48 lines
1009 B
Smalltalk
"Eight-queens — classic backtracking search. Counts the number of
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distinct placements of 8 queens on an 8x8 board with no two attacking.
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Expected count: 92."
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Object subclass: #EightQueens
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instanceVariableNames: 'columns count size'!
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!EightQueens methodsFor: 'init'!
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init
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size := 8.
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columns := Array new: size.
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count := 0.
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^ self!
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size: n
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size := n.
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columns := Array new: n.
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count := 0.
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^ self! !
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!EightQueens methodsFor: 'access'!
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count ^ count!
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size ^ size! !
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!EightQueens methodsFor: 'solve'!
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solve
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self placeRow: 1.
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^ count!
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placeRow: row
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row > size ifTrue: [count := count + 1. ^ self].
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1 to: size do: [:col |
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(self isSafe: col atRow: row) ifTrue: [
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columns at: row put: col.
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self placeRow: row + 1]]!
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isSafe: col atRow: row
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| r prevCol delta |
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r := 1.
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[r < row] whileTrue: [
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prevCol := columns at: r.
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prevCol = col ifTrue: [^ false].
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delta := col - prevCol.
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delta abs = (row - r) ifTrue: [^ false].
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r := r + 1].
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^ true! !
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