package ints

import "strconv"

// Digit returns the n'th digit of i
func Digit(n int, i uint) int {
	return int(Digit64(int64(n), i))
}

// Digit64 returns the n'th digit of i
func Digit64(n int64, i uint) int64 {
	for i >= 9 {
		n /= 1000000000
		i -= 9
	}
	for i >= 6 {
		n /= 1000000
		i -= 6
	}
	for i >= 3 {
		n /= 1000
		i -= 3
	}
	for i > 0 {
		n /= 10
		i--
	}
	return n % 10
}

// Faculty returns the faculty of i.
func Faculty(i int) int {
	if i < 2 {
		return 1
	}
	return i * Faculty(i-1)
}

// Faculty64 returns the faculty of i.
func Faculty64(i int64) int64 {
	if i < 2 {
		return 1
	}
	return i * Faculty64(i-1)
}

// IsPalindromic returns if the integer n is palindromic (reads the same backwards).
func IsPalindromic(n int) bool {
	if n < 10 {
		return true
	}
	if n%10 == 0 {
		return false
	}
	s := strconv.Itoa(n)
	max := (len(s) + 1) / 2
	for i := 0; i < max; i++ {
		if s[i] != s[len(s)-i-1] {
			return false
		}
	}
	return true
}

// Permutations returns all possible permutations of the the set integers.
func Permutations(set []int) [][]int {
	var n = len(set)
	if n == 0 {
		return nil
	}
	var res = make([][]int, 0, Faculty(n))
	var p = append([]int(nil), set...)

	var c = make([]int, n)
	res = append(res, append([]int(nil), p...))
	i := 0
	for i < n {
		if c[i] < i {
			if i%2 == 0 {
				p[0], p[i] = p[i], p[0]
			} else {
				p[c[i]], p[i] = p[i], p[c[i]]
			}
			res = append(res, append([]int(nil), p...))
			c[i] = c[i] + 1
			i = 0
		} else {
			c[i] = 0
			i++
		}
	}
	return res
}

// SumDigits returns the sum of the digits of n.
func SumDigits(n int) int {
	var sum int
	for n > 0 {
		sum += n % 10
		n /= 10
	}
	return int(sum)
}

// SumDigits64 returns the sum of the digits of n.
func SumDigits64(n int64) int {
	var sum int64
	for n > 0 {
		sum += n % 10
		n /= 10
	}
	return int(sum)
}