A permutation library based on Paul Hankin's from https://stackoverflow.com/a/30230552/14660

permutation.go

package permutation

// Based on https://stackoverflow.com/a/30230552/14660 by Paul Hankin

// perm := NewPermutation( slice )
// for next := perm.Next(); next != nil; next = perm.Next() {
//    ...
// }
//
// or
//
// permCh := day09.NewPermutationChan( test.arg )
// for next := range ch {
//     ...
// }

type Permutation struct {
    orig []int
    perm []int
}

func NewPermutation( orig []int ) *Permutation {
    self := Permutation{ orig: orig, perm: make([]int, len(orig)) }
    return &self
}

func NewPermutationChan( orig []int ) chan []int {
    perm := NewPermutation( orig )
    ch := make( chan []int )
    
    go func() {
        defer close(ch)
        for next := perm.Next(); next != nil; next = perm.Next() {
            ch <- next
        }
    }()
    
    return ch
}

func (self *Permutation) nextPerm() {
    for i := len(self.perm) - 1; i >= 0; i-- {
        if i == 0 || self.perm[i] < len(self.perm)-i-1 {
            self.perm[i]++
            return
        }
        self.perm[i] = 0
    }
}

func (self *Permutation) Next() []int {
    defer self.nextPerm()

	if len(self.perm) == 0 || self.perm[0] >= len(self.perm) {
		return nil
	}
	
    result := append([]int{}, self.orig...)
    for i, v := range self.perm {
        result[i], result[i+v] = result[i+v], result[i]
    }
    return result
}