Question about function Decode in package hex

Sexydynamite · November 2, 2019, 1:51pm

Hi Golang!

I have two questions for following Decode function.

golang/go/blob/master/src/encoding/hex/hex.go#L58


// Specifically, it returns x / 2.
func DecodedLen(x int) int { return x / 2 }


// Decode decodes src into DecodedLen(len(src)) bytes,
// returning the actual number of bytes written to dst.
//
// Decode expects that src contains only hexadecimal
// characters and that src has even length.
// If the input is malformed, Decode returns the number
// of bytes decoded before the error.
func Decode(dst, src []byte) (int, error) {
	i, j := 0, 1
	for ; j < len(src); j += 2 {
		a, ok := fromHexChar(src[j-1])
		if !ok {
			return i, InvalidByteError(src[j-1])
		}
		b, ok := fromHexChar(src[j])
		if !ok {
			return i, InvalidByteError(src[j])
		}

One question

Why do a and b need but one by one?

Other question

About L69
dst[i] = (a << 4) | b
Why does this code slide 4?
Recently, I start to read source code and it’s difficult to understand.
Please help me.
Thank you.

calmh · November 2, 2019, 2:27pm

A hex digit represents the range 0-15 which is four bits of data. Two digits can represent eight bits, or a byte. Hence a and b, and shifting a four bits to the left.

system · January 31, 2020, 2:40pm

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