I need to convert one byte into java script signed integer. I have checked this link which converts 2 bytes , but how could I downgrade and convert only one byte?
for example 0x0A would be 10, how about 0xD4?
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parseInt("0xD4", 16) // from 16 bit , parseInt("473", 8) // from 8 bit , parseInt("110011", 2) // from 2 bit ,Arshpreet Wadehra– Arshpreet Wadehra2019-06-13 10:01:49 +00:00Commented Jun 13, 2019 at 10:01
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1Please read the question carefully, your solution is working only for unsigned values, for some reason I know that 0xD4 should be a negative valueEhsan Zargar Ershadi– Ehsan Zargar Ershadi2019-06-13 10:04:07 +00:00Commented Jun 13, 2019 at 10:04
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3 Answers
Javascript handles bitwise operators as signed 32 bit integers.
Knowing this, if you want to sign extend a byte, you need to propagate the top bit of the byte (which indicates the sign) to all of the higher bits in the thirty two bit integer. This can be achieved with:
value << 24 >> 24
What this does is shift the byte to the top of the 32 bit integer, then shift it back down preserving the sign bit.
Similarly, you can sign extend a 16 bit value with:
value << 16 >> 16
1 Comment
tekHedd
Or, if you're working with MIDI (like me) you can sign extend a 7-bit 2's complement with value << 25 >> 25. Never thought I'd be doing that! :D
This solution converts using 2's complement for negative numbers, which is the standard way negative numbers are encoded.
function uncomplement(val, bitwidth) {
var isnegative = val & (1 << (bitwidth - 1));
var boundary = (1 << bitwidth);
var minval = -boundary;
var mask = boundary - 1;
return isnegative ? minval + (val & mask) : val;
}
uncomplement(0xFF, 8) returns -1
uncomplement(0x80, 8) returns -128
uncomplement(0x7F, 8) returns 127
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I came up with this solution:
function convertToSignedInt(signedByte) {
var sign = signedByte & (1 << 7);
return (signedByte & 0x7f) * (sign !== 0 ? -1 : 1);
}
Hope it could save others some time. convertToSignedInt(0x81) // returns -1
answered Jun 13, 2019 at 10:42
Ehsan Zargar Ershadi
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