EN
JavaScript - get and set bits in byte array
12 points
In this article, we would like to show how in JavaScript get and set bits in a byte array.
To understand the below code it is necessary to know how bytes and bits are indexed.
xxxxxxxxxx1
NOTATION (the way we write and address bytes and bits):2
3
// Byte index: 0 1 24
// Bit index: |7 |0 |15 |8 |23 |165
// v v v v v v6
// DEC values: 170, 175, 10 BIN values: 10101010 10101111 00001010
Practical example:
xxxxxxxxxx1
function getBits(number, length) {2
var text = '';3
for (var i = length - 1; i > -1; --i) {4
text += 0x01 & (number >> i);5
}6
return text;7
}8
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function BitArray(byteArray) {10
this.getSize = function() {11
return 8 * byteArray.length;12
};13
this.getBit = function(index) {14
var byteIndex = Math.floor(index / 8);15
var bitIndex = index - 8 * byteIndex;16
var byteValue = byteArray[byteIndex];17
if (byteValue < 0 || byteValue > 255) {18
throw new Error('Array item must be byte (in range: 0-255).');19
}20
var bitValue = (byteValue >>> bitIndex) & 0x01;21
return bitValue;22
};23
this.setBit = function(index, value) {24
var byteIndex = Math.floor(index / 8);25
var bitIndex = index - 8 * byteIndex;26
var byteValue = byteArray[byteIndex];27
if (byteValue < 0 || byteValue > 255) {28
throw new Error('Array item must be byte (in range: 0-255).');29
}30
var maskValue = 0x01 << bitIndex;31
byteArray[byteIndex] = value ? (byteValue | maskValue) : (byteValue & ~maskValue);32
};33
this.toString = function(delimiter) {34
if (delimiter == null) {35
delimiter = ' ';36
}37
var result = '';38
for (var i = 0; i < byteArray.length; ++i) {39
if (i > 0) {40
result += delimiter;41
}42
var byte = byteArray[i];43
if (byte < 0 || byte > 255) {44
throw new Error('Array item must be byte (in range: 0-255).');45
}46
result += getBits(byte, 8);47
}48
return result;49
};50
}51
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// Usage example:54
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// Byte index: 0 1 256
// Bit index: |7 |0 |15 |8 |23 |1657
// | | | | | |58
// DEC: BIN: v v v v v v59
var byteArray = [170, 175, 10]; // [10101010, 10101111, 00001010]60
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var bitArray = new BitArray(byteArray);62
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// bits getting:64
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console.log(0 + '\t' + bitArray.getBit(0)); // 0 066
console.log(1 + '\t' + bitArray.getBit(1)); // 1 167
console.log(2 + '\t' + bitArray.getBit(2)); // 2 068
console.log(3 + '\t' + bitArray.getBit(3)); // 3 169
70
console.log(bitArray.toString()); // 10101010 10101111 0000101071
72
// bits setting (before: 10101010, after: 10100101):73
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bitArray.setBit(0, 1);75
bitArray.setBit(1, 0);76
bitArray.setBit(2, 1);77
bitArray.setBit(3, 0);78
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console.log(byteArray[0]); // 165 <--- DEC80
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// |7 |082
// | |83
// v v84
console.log(getBits(byteArray[0], 8)); // 10100101 <-- BIN
