Heap's Algorithm

Author: kevinburchfield

DIRECTORY.md

@@ -123,6 +123,9 @@
   * Sieve
     * [Sieve](https://github.com/TheAlgorithms/Go/blob/master/math/sieve/Sieve.go)
     * [Sieve Test](https://github.com/TheAlgorithms/Go/blob/master/math/sieve/sieve_test.go)
+  * Permutation
+    * [Heap's Algorithm](https://github.com/TheAlgorithms/Go/blob/master/math/permutation/heaps.go)
+    * [Heap's Algorithm Test](https://github.com/TheAlgorithms/Go/blob/master/math/permutation/heaps_test.go)
 
 ## Other
   * Maxsubarraysum

math/permutation/heaps.go

@@ -0,0 +1,43 @@
+package permutation
+
+import (
+	"strings"
+)
+
+// Heap's Algorithm for generating all permutations of n objects
+func Heaps(out chan []string, n int) {
+	elementSetCh := make(chan []string)
+	go GenerateElementSet(elementSetCh, n)
+	elementSet := <-elementSetCh
+
+	var recursiveGenerate func([]string, int, []string)
+	var permutations []string
+	recursiveGenerate = func(previousIteration []string, n int, elements []string) {
+		if n == 1 {
+			permutations = append(permutations, strings.Join(elements, ""))
+		} else {
+			for i := 0; i < n; i++ {
+				recursiveGenerate(previousIteration, n - 1, elements)
+				if n % 2 == 1 {
+					tmp := elements[i]
+					elements[i] = elements[n - 1]
+					elements[n - 1] = tmp
+				} else {
+					tmp := elements[0]
+					elements[0] = elements[n - 1]
+					elements[n - 1] = tmp
+				}
+			}
+		}
+	}
+	recursiveGenerate(permutations, n, elementSet)
+	out <- permutations
+}
+
+func GenerateElementSet(out chan []string, n int) {
+	elementSet := make([]string, n)
+	for i, _ := range elementSet {
+		elementSet[i] = string(i + 49) // Adjust this if you want to change your charset
+	}
+	out <- elementSet
+}

math/permutation/heaps_test.go

@@ -0,0 +1,31 @@
+package permutation
+
+import (
+	"reflect"
+	"sort"
+	"testing"
+)
+
+func TestHeaps(t *testing.T) {
+	t.Run("Should generate permutations for various size sets", func (t *testing.T) {
+		expectedValues := [][]string {
+			{"1"},
+			{"12", "21"},
+			{"123", "213", "321", "231", "312", "132"},
+			{"1234", "1243", "1324", "1342", "1423", "1432", "2134", "2143", "2314", "2341", "2413", "2431", "3124", "3142", "3214", "3241", "3412", "3421", "4123", "4132", "4213", "4231", "4312", "4321"},
+			{"12345", "12354", "12435", "12453", "12534", "12543", "13245", "13254", "13425", "13452", "13524", "13542", "14235", "14253", "14325", "14352", "14523", "14532", "15234", "15243", "15324", "15342", "15423", "15432", "21345", "21354", "21435", "21453", "21534", "21543", "23145", "23154", "23415", "23451", "23514", "23541", "24135", "24153", "24315", "24351", "24513", "24531", "25134", "25143", "25314", "25341", "25413", "25431", "31245", "31254", "31425", "31452", "31524", "31542", "32145", "32154", "32415", "32451", "32514", "32541", "34125", "34152", "34215", "34251", "34512", "34521", "35124", "35142", "35214", "35241", "35412", "35421", "41235", "41253", "41325", "41352", "41523", "41532", "42135", "42153", "42315", "42351", "42513", "42531", "43125", "43152", "43215", "43251", "43512", "43521", "45123", "45132", "45213", "45231", "45312", "45321", "51234", "51243", "51324", "51342", "51423", "51432", "52134", "52143", "52314", "52341", "52413", "52431", "53124", "53142", "53214", "53241", "53412", "53421", "54123", "54132", "54213", "54231", "54312", "54321"},
+		}
+		permutationsCh := make(chan []string)
+		var value []string
+
+		for i := 1; i <= 5; i++ {
+			go Heaps(permutationsCh, i)
+			value = <- permutationsCh
+			sort.Strings(value)
+			sort.Strings(expectedValues[i - 1])
+			if !reflect.DeepEqual(value, expectedValues[i - 1]) {
+				t.Errorf("Permutation set is incorrect for element size of %v. Expected (%v) and received (%v)", i, expectedValues[i - 1], value)
+			}
+		}
+	})
+}