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Added the ability to specify leftMostBitFirst

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h345983745 2019-08-24 19:07:44 +01:00
parent 54286784a7
commit 3ebec7117b
1 changed files with 42 additions and 19 deletions
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61
src/core/lib/Stream.mjs
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@ -93,40 +93,63 @@ export default class Stream {
/** /**
* Reads a number of bits from the buffer. * Reads a number of bits from the buffer.
* *
* @TODO Add endianness
*
* @param {number} numBits * @param {number} numBits
* @param {bool} [leftMostBitFirst=false]
* @returns {number} * @returns {number}
*/ */
readBits(numBits) { readBits(numBits, leftMostBitFirst=false) {
if (this.position > this.length) return undefined; if (this.position > this.length) return undefined;
let bitBuf = 0, let bitBuf = 0,
bitBufLen = 0; bitBufLen = 0;
// Add remaining bits from current byte if (leftMostBitFirst) {
bitBuf = (this.bytes[this.position++] & bitMask(this.bitPos)) >>> this.bitPos; // Add remaining bits from current byte
bitBufLen = 8 - this.bitPos; const mask = ((1 << (8 - this.bitPos)) -1);
this.bitPos = 0; bitBuf = this.bytes[this.position++] & mask;
bitBufLen = 8 - this.bitPos;
this.bitPos = 0;
// Not enough bits yet // Not enough bits yet
while (bitBufLen < numBits) { while (bitBufLen < numBits) {
bitBuf |= this.bytes[this.position++] << bitBufLen; bitBuf = (bitBuf << 8) | this.bytes[this.position++];
bitBufLen += 8; bitBufLen += 8;
} }
// Reverse back to numBits // Reverse back to numBits
if (bitBufLen > numBits) {
const excess = bitBufLen - numBits; const excess = bitBufLen - numBits;
bitBuf &= (1 << numBits) - 1; bitBuf = bitBuf >> excess;
bitBufLen -= excess; bitBufLen -= excess;
this.position--; this.position--;
this.bitPos = 8 - excess; this.bitPos = 8 - excess;
return bitBuf;
} else {
// Add remaining bits from current byte
bitBuf = (this.bytes[this.position++] & bitMask(this.bitPos)) >>> this.bitPos;
bitBufLen = 8 - this.bitPos;
this.bitPos = 0;
// Not enough bits yet
while (bitBufLen < numBits) {
bitBuf |= this.bytes[this.position++] << bitBufLen;
bitBufLen += 8;
}
// Reverse back to numBits
if (bitBufLen > numBits) {
const excess = bitBufLen - numBits;
bitBuf &= (1 << numBits) - 1;
bitBufLen -= excess;
this.position--;
this.bitPos = 8 - excess;
}
return bitBuf;
} }
/**
return bitBuf;
/**
* Calculates the bit mask based on the current bit position. * Calculates the bit mask based on the current bit position.
* *
* @param {number} bitPos * @param {number} bitPos