// Package uniuri generates random strings good for use in URIs to identify
// unique objects.
//
// Example usage:
//
//	s := uniuri.New() // s is now "apHCJBl7L1OmC57n"
//
// A standard string created by New() is 16 bytes in length and consists of
// Latin upper and lowercase letters, and numbers (from the set of 62 allowed
// characters), which means that it has ~95 bits of entropy. To get more
// entropy, you can use NewLen(UUIDLen), which returns 20-byte string, giving
// ~119 bits of entropy, or any other desired length.
//
// Functions read from crypto/rand random source, and panic if they fail to
// read from it.
package uniuri

import "crypto/rand"

const (
	// Standard length of uniuri string to achive ~95 bits of entropy.
	StdLen = 16
	// Length of uniurl string to achive ~119 bits of entropy, closest
	// to what can be losslessly converted to UUIDv4 (122 bits).
	UUIDLen = 20
)

// Standard characters allowed in uniuri string.
var StdChars = []byte("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890")

// New returns a new random string of the standard length, consisting of
// standard characters.
func New() string {
	return NewLenChars(StdLen, StdChars)
}

// NewLen returns a new random string of the provided length, consisting of
// standard characters.
func NewLen(length int) string {
	return NewLenChars(length, StdChars)
}

// NewLenChars returns a new random string of the provided length, consisting
// of the provided byte slice of allowed characters (maximum 256).
func NewLenChars(length int, chars []byte) string {
	b := make([]byte, length)
	if _, err := rand.Reader.Read(b); err != nil {
		panic("error reading from random source: " + err.String())
	}
	alen := byte(len(chars))
	for i, c := range b {
		b[i] = chars[c%alen]
	}
	return string(b)
}