September 2017 Update

Task/Permutations-by-swapping/Kotlin/permutations-by-swapping.kotlin

@@ -0,0 +1,46 @@
+// version 1.1.2
+
+fun johnsonTrotter(n: Int): Pair<List<IntArray>, List<Int>> {
+    val p = IntArray(n) { it }  // permutation
+    val q = IntArray(n) { it }  // inverse permutation
+    val d = IntArray(n) { -1 }  // direction = 1 or -1
+    var sign = 1
+    val perms = mutableListOf<IntArray>()
+    val signs = mutableListOf<Int>()
+
+    fun permute(k: Int) {
+        if (k >= n) {
+            perms.add(p.copyOf())
+            signs.add(sign)
+            sign *= -1
+            return
+        }
+        permute(k + 1)
+        for (i in 0 until k) {
+            val z = p[q[k] + d[k]]
+            p[q[k]] = z
+            p[q[k] + d[k]] = k
+            q[z] = q[k]
+            q[k] += d[k]
+            permute(k + 1)
+        }
+        d[k] *= -1
+    }
+
+    permute(0)
+    return perms to signs
+}
+
+fun printPermsAndSigns(perms: List<IntArray>, signs: List<Int>) {
+    for ((i, perm) in perms.withIndex()) {
+        println("${perm.contentToString()} -> sign = ${signs[i]}")
+    }
+}
+
+fun main(args: Array<String>) {
+    val (perms, signs) = johnsonTrotter(3)
+    printPermsAndSigns(perms, signs)
+    println()
+    val (perms2, signs2) = johnsonTrotter(4)
+    printPermsAndSigns(perms2, signs2)
+}