permutations.go

package main
//golang implentation of heap algo 
//https://en.wikipedia.org/wiki/Heap%27s_algorithm
//Result datastructure should be changed to fit your needs


func permutations(a []int) [][]int {
    n := len(a)
    result := [][]int{}
    c := make([]int, n)

    result = cAppend(result, a)
    i:=0
    for i<n {
        if c[i] < i {
            if i % 2 == 0 {
                a[0], a[i] = a[i], a[0]
            } else {
                a[c[i]], a[i] = a[i], a[c[i]]
            }
            result = cAppend(result, a)
            c[i] += 1
            i = 0
        } else {
            c[i] = 0
            i += 1
        }
    }
    return result
}


func cAppend(result [][]int, n []int) [][]int {
        copy_n := make([]int, len(n))
        copy(copy_n, n)
        return append(result, copy_n)
    }
~         

permutations_test.go

package main

import (
    "testing"
    "reflect"
    "fmt"
)

func TestPermuations(t *testing.T) {
    testcases := []struct{
        input []int
        expected [][]int
    } {
        {[]int{}, [][]int{[]int{}}},
        {[]int{1}, [][]int{[]int{1}}},
        {[]int{1, 2}, [][]int{
                        []int{1, 2},
                        []int{2, 1},
                     },
            },
        {[]int{1, 2, 3}, [][]int{
                []int{1, 2, 3},
                []int{2, 1, 3},
                []int{3, 1, 2},
                []int{1, 3, 2},
                []int{2, 3, 1},
                []int{3, 2, 1},
            },
        },
    }
    for number, testcase := range testcases {
        result := permutations(testcase.input)
        if !reflect.DeepEqual(result, testcase.expected) {
            fmt.Println("expected")
            for _,row := range testcase.expected {
                fmt.Println(row)
            }
            fmt.Println("result")
            for _,row := range result {
                fmt.Println(row)
            }
            t.Error("Error testcase:", number, "input:", testcase.input, "result", result, "!=", testcase.expected)
        }
    }
}