1set/gut

package yrand

import (
    "crypto/rand"
    "errors"
    "math/big"
)

var (
    errShuffleNegative = errors.New("n should be non-negative")
)

// Shuffle randomizes the order of elements.
//
// n is the number of elements. swap swaps the elements with indexes i and j.
//
// The algorithm is the Fisher-Yates Shuffle and its complexity is O(n).
func Shuffle(n int, swap ShuffleSwapFunc) (err error) {
    if n < 0 {
        return errShuffleNegative
    } else if n <= 1 {
        return
    }

    var (
        randBig   = new(big.Int)
        randBytes = make([]byte, 8)
    )
    for i := uint64(n - 1); i > 0; {
        if _, err = rand.Read(randBytes); err != nil {
            return
        }

        randBig.SetBytes(randBytes)
        for num := randBig.Uint64(); num > i && i > 0; i-- {
            max := i + 1
            j := int(num % max)
            num /= max
            swap(int(i), j)
        }
    }

    return
}

// ChoiceInt returns a random element from the non-empty slice of int.
func ChoiceInt(list []int) (n int, err error) {
    var idx int
    if idx, err = randomIndex(len(list)); err == nil {
        n = list[idx]
    }
    return
}

// ChoiceString returns a random element from the non-empty slice of string.
func ChoiceString(list []string) (s string, err error) {
    var idx int
    if idx, err = randomIndex(len(list)); err == nil {
        s = list[idx]
    }
    return
}