Data commit

Task/Permutations-by-swapping/Swift/permutations-by-swapping.swift

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+// Implementation of Heap's algorithm.
+// See https://en.wikipedia.org/wiki/Heap%27s_algorithm#Details_of_the_algorithm
+func generate<T>(array: inout [T], output: (_: [T], _: Int) -> Void) {
+    let n = array.count
+    var c = Array(repeating: 0, count: n)
+    var i = 1
+    var sign = 1
+    output(array, sign)
+    while i < n {
+        if c[i] < i {
+            if (i & 1) == 0 {
+                array.swapAt(0, i)
+            } else {
+                array.swapAt(c[i], i)
+            }
+            sign = -sign
+            output(array, sign)
+            c[i] += 1
+            i = 1
+        } else {
+            c[i] = 0
+            i += 1
+        }
+    }
+}
+
+func printPermutation<T>(array: [T], sign: Int) {
+    print("\(array) \(sign)")
+}
+
+print("Permutations and signs for three items:")
+var a = [0, 1, 2]
+generate(array: &a, output: printPermutation)
+
+print("\nPermutations and signs for four items:")
+var b = [0, 1, 2, 3]
+generate(array: &b, output: printPermutation)