June 2018 Update

Task/Permutations-by-swapping/Scala/permutations-by-swapping.scala

@@ -0,0 +1,36 @@
+object JohnsonTrotter extends App {
+
+  private def perm(n: Int): Unit = {
+    val p = new Array[Int](n) // permutation
+    val pi = new Array[Int](n) // inverse permutation
+    val dir = new Array[Int](n) // direction = +1 or -1
+
+    def perm(n: Int, p: Array[Int], pi: Array[Int], dir: Array[Int]): Unit = {
+      if (n >= p.length) for (aP <- p) print(aP)
+      else {
+        perm(n + 1, p, pi, dir)
+        for (i <- 0 until n) { // swap
+          printf("   (%d %d)\n", pi(n), pi(n) + dir(n))
+          val z = p(pi(n) + dir(n))
+          p(pi(n)) = z
+          p(pi(n) + dir(n)) = n
+          pi(z) = pi(n)
+          pi(n) = pi(n) + dir(n)
+          perm(n + 1, p, pi, dir)
+        }
+        dir(n) = -dir(n)
+      }
+    }
+
+    for (i <- 0 until n) {
+      dir(i) = -1
+      p(i) = i
+      pi(i) = i
+    }
+    perm(0, p, pi, dir)
+    print("   (0 1)\n")
+  }
+
+  perm(4)
+
+}