I'm attempting to write a Go program to parse ans.1 BER two's complement integer encoding. However, the integer can have either 1, 2, 3 or 4 byte length encoding (depending on its size).
According to the specification (http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf) the leftmost bit is always the complement.
What's a clean way to do this?
func ParseInt(b []byte) (int64, error) {
switch len(b) {
case 1:
// this works
return int64(b[0]&0x7f) - int64(b[0]&0x80), nil
case 2:
// left most byte of b[0] is -32768
case 3:
// left most byte of b[0] is -8388608
case 4:
// left most byte of b[0] is -2147483648 (and so on for 5, 6, 7, 8)
case 5:
case 6:
case 7:
case 8:
default:
return 0, errors.New("value does not fit in a int64")
}
}
ParseInt([]byte{0xfe}) // should return (-2, nil)
ParseInt([]byte{0xfe, 0xff}) // should return (-257, nil)
ParseInt([]byte{0x01, 0x00}) // should return (256, nil)