Function fastDecodeBase62

base62.go:102–175

This function decodes the base 62 representation of the src KSUID to the binary form into dst. In order to support a couple of optimizations the function assumes that src is 27 bytes long and dst is 20 bytes long. Any unused bytes in dst will be set to zero.

(dst []byte, src []byte)

//
// Any unused bytes in dst will be set to zero.
func fastDecodeBase62(dst []byte, src []byte) error {
	const srcBase = 62
	const dstBase = 4294967296

	// This line helps BCE (Bounds Check Elimination).
	// It may be safely removed.
	_ = src[26]

	parts := [27]byte{
		base62Value(src[0]),
		base62Value(src[1]),
		base62Value(src[2]),
		base62Value(src[3]),
		base62Value(src[4]),
		base62Value(src[5]),
		base62Value(src[6]),
		base62Value(src[7]),
		base62Value(src[8]),
		base62Value(src[9]),

		base62Value(src[10]),
		base62Value(src[11]),
		base62Value(src[12]),
		base62Value(src[13]),
		base62Value(src[14]),
		base62Value(src[15]),
		base62Value(src[16]),
		base62Value(src[17]),
		base62Value(src[18]),
		base62Value(src[19]),

		base62Value(src[20]),
		base62Value(src[21]),
		base62Value(src[22]),
		base62Value(src[23]),
		base62Value(src[24]),
		base62Value(src[25]),
		base62Value(src[26]),
	}

	n := len(dst)
	bp := parts[:]
	bq := [stringEncodedLength]byte{}

	for len(bp) > 0 {
		quotient := bq[:0]
		remainder := uint64(0)

		for _, c := range bp {
			value := uint64(c) + uint64(remainder)*srcBase
			digit := value / dstBase
			remainder = value % dstBase

			if len(quotient) != 0 || digit != 0 {
				quotient = append(quotient, byte(digit))
			}
		}