smalltalk: eight-queens classic program (sizes 1/4/5 verified)

lib/smalltalk/tests/programs/eight-queens.st

@@ -0,0 +1,47 @@
+"Eight-queens — classic backtracking search. Counts the number of
+ distinct placements of 8 queens on an 8x8 board with no two attacking.
+ Expected count: 92."
+
+Object subclass: #EightQueens
+  instanceVariableNames: 'columns count size'!
+
+!EightQueens methodsFor: 'init'!
+init
+  size := 8.
+  columns := Array new: size.
+  count := 0.
+  ^ self!
+
+size: n
+  size := n.
+  columns := Array new: n.
+  count := 0.
+  ^ self! !
+
+!EightQueens methodsFor: 'access'!
+count ^ count!
+
+size ^ size! !
+
+!EightQueens methodsFor: 'solve'!
+solve
+  self placeRow: 1.
+  ^ count!
+
+placeRow: row
+  row > size ifTrue: [count := count + 1. ^ self].
+  1 to: size do: [:col |
+    (self isSafe: col atRow: row) ifTrue: [
+      columns at: row put: col.
+      self placeRow: row + 1]]!
+
+isSafe: col atRow: row
+  | r prevCol delta |
+  r := 1.
+  [r < row] whileTrue: [
+    prevCol := columns at: r.
+    prevCol = col ifTrue: [^ false].
+    delta := col - prevCol.
+    delta abs = (row - r) ifTrue: [^ false].
+    r := r + 1].
+  ^ true! !