String permutations in Go (blakeembrey#179)

solutions/go/string-permutations.go

@@ -0,0 +1,95 @@
+package main
+
+import (
+	"fmt"
+	"sort"
+)
+
+/****************************************************************************
+Generates permutations of sortable sequence changing it in place.
+****************************************************************************/
+
+// PermutationsIter returns a closure, each call to the returned function
+// generates the next permutations of the sequence (changing it in place).
+// Uses Non-recursive lexicographic order (Knuth's L-Algorithm), thus
+// the sequence passed as argument must be sortable.
+func PermutationsIter(a sort.Interface) func() bool {
+	current := int64(0)
+	n := a.Len()
+	return func() bool {
+		if current == 0 {
+			sort.Sort(a)
+			current++
+			return true
+		}
+		// Find largest index k such that a[k] < a[k + 1]
+		k := -1
+		for j := n - 2; j >= 0; j-- {
+			if a.Less(j, j+1) {
+				k = j
+				break
+			}
+		}
+		if k == -1 { // if k not found, all done
+			return false
+		}
+		// Find largest index l greater than k such that a[k] < a[l]
+		l := -1
+		for j := n - 1; j >= k+1; j-- {
+			if a.Less(k, j) {
+				l = j
+				break
+			}
+		}
+		// swap a[k] <-> a[l]
+		a.Swap(k, l)
+		// Reverse a[k+1:n]
+		for i, j := k+1, n-1; i < j; i, j = i+1, j-1 {
+			a.Swap(i, j)
+		}
+		current++
+		return true
+	}
+}
+
+/*************** Create a sortable type ***************/
+
+// MyPermSeq satisfy interface sort.Interface
+type MyPermSeq []byte
+
+// Len of the sequence
+func (ps MyPermSeq) Len() int {
+	return len(ps)
+}
+
+// Less implements <
+func (ps MyPermSeq) Less(i, j int) bool {
+	return ps[i] < ps[j]
+}
+
+// Swap in place
+func (ps MyPermSeq) Swap(i, j int) {
+	ps[i], ps[j] = ps[j], ps[i]
+}
+
+func main() {
+
+	// Let's permutate ABCD in-place: output will be ordered in lexicographical order
+	mySeq := MyPermSeq([]byte("ABCD"))
+	next := PermutationsIter(mySeq)
+	fmt.Println("Generating permutations for", string(mySeq), ": ")
+	for next() {
+		fmt.Print(string(mySeq), " ")
+	}
+	fmt.Println("")
+
+	// Let's permutate ABBB in-place: shows algorithm handles repeated elements well
+	mySeq = MyPermSeq([]byte("ABBB"))
+	next = PermutationsIter(mySeq)
+	fmt.Println("Generating permutations for", string(mySeq), ": ")
+	for next() {
+		fmt.Print(string(mySeq), " ")
+	}
+	fmt.Println("")
+
+}