1set/gut

package yrand

import (
    "errors"
    "strings"
    "unicode/utf8"
)

var (
    errStringAlphabet = errors.New("length of alphabet should be greater than one")
    errStringLength   = errors.New("length of string should be positive")
    alphabetLetters   = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    alphabetBase36    = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
    alphabetBase62    = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
)

// String returns a random string of given length with given ASCII chars only.
func String(alphabet string, length int) (s string, err error) {
    base := uint64(len(alphabet))
    if base <= 1 {
        err = errStringAlphabet
    } else if length <= 0 {
        err = errStringLength
    }
    if err != nil {
        return
    }

    sb := strings.Builder{}
    sb.Grow(length)
    writeByteBack := func(num uint64) error {
        return sb.WriteByte(alphabet[int(num)])
    }

    if err = iterateRandomNumbers(length, base, writeByteBack); err == nil {
        s = sb.String()
    }
    return
}

// Runes returns a random string of given length with given Unicode chars only.
func Runes(alphabet string, length int) (s string, err error) {
    base := uint64(utf8.RuneCountInString(alphabet))
    if base <= 1 {
        err = errStringAlphabet
    } else if length <= 0 {
        err = errStringLength
    }
    if err != nil {
        return
    }

    sb := strings.Builder{}
    sb.Grow(length * 4)
    abRunes := []rune(alphabet)
    writeRuneBack := func(num uint64) error {
        _, e := sb.WriteRune(abRunes[int(num)])
        return e
    }

    if err = iterateRandomNumbers(length, base, writeRuneBack); err == nil {
        s = sb.String()
    }
    return
}

// StringLetters returns a random string of given length with A-Z chars only.
func StringLetters(length int) (s string, err error) {
    return String(alphabetLetters, length)
}

// StringBase36 returns a random string of given length with A-Z0-9 chars only.
func StringBase36(length int) (s string, err error) {
    return String(alphabetBase36, length)
}

// StringBase62 returns a random string of given length with a-zA-Z0-9 chars only.
func StringBase62(length int) (s string, err error) {
    return String(alphabetBase62, length)
}