Data commit

Task/N-queens-problem/Julia/n-queens-problem.julia

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+"""
+# EightQueensPuzzle
+
+Ported to **Julia** from examples in several languages from
+here: https://hbfs.wordpress.com/2009/11/10/is-python-slow
+"""
+module EightQueensPuzzle
+
+export Board, solve!
+
+mutable struct Board
+    cols::Int
+    nodes::Int
+    diag45::Int
+    diag135::Int
+    solutions::Int
+
+    Board() = new(0, 0, 0, 0, 0)
+end
+
+"Marks occupancy."
+function mark!(b::Board, k::Int, j::Int)
+    b.cols    ⊻= (1 << j)
+    b.diag135 ⊻= (1 << (j+k))
+    b.diag45  ⊻= (1 << (32+j-k))
+end
+
+"Tests if a square is menaced."
+function test(b::Board, k::Int, j::Int)
+    b.cols    & (1 << j)        +
+    b.diag135 & (1 << (j+k))    +
+    b.diag45  & (1 << (32+j-k)) == 0
+end
+
+"Backtracking solver."
+function solve!(b::Board, niv::Int, dx::Int)
+    if niv > 0
+        for i in 0:dx-1
+            if test(b, niv, i) == true
+                mark!(b, niv, i)
+                solve!(b, niv-1, dx)
+                mark!(b, niv, i)
+            end
+        end
+    else
+        for i in 0:dx-1
+            if test(b, 0, i) == true
+                b.solutions += 1
+            end
+        end
+    end
+    b.nodes += 1
+    b.solutions
+end
+
+end # module
+
+using  .EightQueensPuzzle
+
+for n = 1:17
+    b = Board()
+    @show n
+    print("elapsed:")
+    solutions = @time solve!(b, n-1, n)
+    @show solutions
+    println()
+end