main.go

// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements.  See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership.  The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License.  You may obtain a copy of the License at

//   http://www.apache.org/licenses/LICENSE-2.0

// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied.  See the License for the
// specific language governing permissions and limitations
// under the License.
package main

import "fmt"

func main() {
	fmt.Println(permutationAtIndex([]string{"0", "1", "2", "3", "4", "5", "6", "7", "8", "9"}, 999999))
}

func permutationAtIndex(digits []string, index int) []string {
	var result []string

	length := len(digits)
	for i := 0; i < length; i++ {
		permutationCountForTheOtherDigits := fact(length - i - 1)

		choice := index / permutationCountForTheOtherDigits
		index = index % permutationCountForTheOtherDigits

		result = append(result, digits[choice])
		digits = removeDigitAt(digits, choice)
	}

	return result
}

func fact(n int) int {
	f := 1
	for i := 2; i <= n; i++ {
		f *= i
	}
	return f
}

func removeDigitAt(digits []string, index int) []string {
	return append(digits[0:index], digits[index+1:]...)
}