A permutation library for Golang.

Use NewPerm() to generate a Pemutator, the argument k must be a non-nil slice,and the argument less must be a Less function that implements compare functionality of k's element type. If k's element is ordered,argument less can be nil.For ordered in Golang, visit http://golang.org/ref/spec#Comparison_operators

   func NewPerm(k interface{}, less Less) (*Permutator, error) 

After generating a Permutator, the argument k can be modified and deleted,Permutator store a copy of k internel.A Permutator can be used concurrently

Invoke Permutator.Next() to return the next permutation in lexcial order.If all permutations generated,return an error

	func (p *Permutator) Next()(interface{}, error)

Invoke Permutator.NextN() to return the next n permuation in lexical order.

  func (p *Permutator) NextN() interface{}

The returned interface{} can be modified,it does nothing to do with the Permutator

Invoke Permutator.Left() to return the number of ungenerated permutation

	func (p Permutator) Left() int

Invoke Permutator.Index() to return the index of last permutation, which start from 1 to n! (n is the length of slice)

	func (p Permutator) Index() int

Invoke Permutator.Reset() to reset the permutator

  func (p *Permutator) Reset()

An example:

	func main() {
		i := []int{4,3,2,1}
		p,err:=NewPerm(i,nil) //generate a Permutator
		if err != nil {
			fmt.Println(err)
			return
		}
		for i,err:=p.Next();err==nil;i,err=p.Next(){
			fmt.Printf("%3d permutation: %v left %d\n",p.Index()-1,i.([]int),p.Left())
		}
	}

outputs:

  1 permutation: [1 2 3] left 5
  2 permutation: [1 3 2] left 4
  3 permutation: [2 1 3] left 3
  4 permutation: [2 3 1] left 2
  5 permutation: [3 1 2] left 1
  6 permutation: [3 2 1] left 0

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