清除最高有效位

Question

I have a file containing two bytes, in Big Endian order, hexdump gives me:

81 50

which is 1000 0001 0101 0000 in binary. However, I want the most significant bit to be a flag, so in golang I have to load the file content, clear the most significant bit, and only then read the value.

So:

valueBuf := make([]byte, 2)
_, err := f.Read(valueBuf) // now printing valueBuf gives me [129 80] in decimal
value := int16(binary.BigEndian.Uint16(valueBuf[0:2])) // now value is -32432

Ok, I have tried to use something like:

func clearBit(n int16, pos uint) int16 {
    mask := ^(1 << pos)
    n &= mask
    return n
}

But it apparently doesn't work as expected. The output value should be 336 in decimal, as normal int, and I cannot get it. How should I do this?

Answer 1

for n &= mask to work, n and mask have to be matching types. So you should write

mask := int16(^(1 << pos))

then, value = clearBit(value, 15) works fine.

Or, since constants are untyped, you can eliminate mask, and also eliminate the assignment to n since it's just returned on the following line, and shorten clearBit to

func clearBit(n int16, pos uint) int16 {
    return n & ^(1 << pos)
}