// Copyright (c) 2014 The mathutil Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package mathutil

import (
	"sort"
)

// PermutationFirst generates the first permutation of data.
func PermutationFirst(data sort.Interface) {
	sort.Sort(data)
}

// PermutationNext generates the next permutation of data if possible and
// return true.  Return false if there is no more permutation left.  Based on
// the algorithm described here:
// http://en.wikipedia.org/wiki/Permutation#Generation_in_lexicographic_order
func PermutationNext(data sort.Interface) bool {
	var k, l int
	for k = data.Len() - 2; ; k-- { // 1.
		if k < 0 {
			return false
		}

		if data.Less(k, k+1) {
			break
		}
	}
	for l = data.Len() - 1; !data.Less(k, l); l-- { // 2.
	}
	data.Swap(k, l)                             // 3.
	for i, j := k+1, data.Len()-1; i < j; i++ { // 4.
		data.Swap(i, j)
		j--
	}
	return true
}
