combinations.go

package main

import "fmt"

func main() {
	result := make(chan string, 1000)
	entrophy := "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789"
	go combinations(entrophy, 7, result)
	i := 1
	for val := range result {
		i++
		fmt.Print(val, " - ", i, "\n")
	}
}

func combinations(input string, size int, result chan string) {
	defer close(result)
	if size == 0 {
		return
	}
	l := make([]int, size)
	pool := []rune(input)
	result <- makeResult(pool, l)
	for {
		l[0] += 1
		if overFlow(l, len(pool)) {
			break
		}
		result <- makeResult(pool, l)
	}
}

func overFlow(l []int, size int) bool {
	for i := 0; i < len(l); i++ {
		if l[i] >= size {
			if i+1 >= len(l) {
				return true
			}
			l[i], l[i+1] = 0, l[i+1]+1
		}
	}
	return false
}

func makeResult(pool []rune, l []int) string {
	result := []rune{}
	for i := 0; i < len(l); i++ {
		result = append(result, pool[l[i]])
	}
	return string(result)
}

combinations_test.go

package main

import (
	"reflect"
	"sort"
	"testing"
)

func TestPermuations(t *testing.T) {
	testcases := []struct {
		iString  string
		iSize    int
		expected []string
	}{
		{"1", 1, []string{"1"}},
		{"12", 2, []string{"11", "21", "12", "22"}},
		{"ABCD", 2, []string{"AA", "BA", "CA", "DA", "AB", "BB", "CB", "DB", "AC", "BC", "CC", "DC", "AD", "BD", "CD", "DD"}},
		{"123", 3, []string{"111", "211", "311", "121", "221", "321", "131", "231", "331", "112", "212", "312", "122", "222", "322", "132", "232", "332", "113", "213", "313", "123", "223", "323", "133", "233", "333"}},
	}
	for number, testcase := range testcases {
		resultChan := make(chan string)
		go combinations(testcase.iString, testcase.iSize, resultChan)
		result := []string{}
		for item := range resultChan {
			result = append(result, item)
		}
		// order of the permutation function should not matter
		sort.Strings(result)
		sort.Strings(testcase.expected)
		if !reflect.DeepEqual(result, testcase.expected) {
			t.Error("Error testcase:", number, "input:", testcase.iString, "result", result, "!=", testcase.expected)
		}
	}
}