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Add better documentation for how MTL and LTM modes work for BitStream

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This commit is contained in:
Jeff Schiller 2023-02-16 20:31:51 -08:00
parent 95d630abda
commit 4854b9754f
4 changed files with 80 additions and 5 deletions
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27
build/io/bitstream-def.js
View file

@ -18,12 +18,34 @@
* Note that this stream is optimized, and thus, will *NOT* throw an error if
* the end of the stream is reached. Only use this in scenarios where you
* already have all the bits you need.
*
* Bit reading always proceeds from the first byte in the buffer, to the
* second byte, and so on. The MTL flag controls which bit is considered
* first *inside* the byte.
*
* An Example for how Most-To-Least vs Least-to-Most mode works:
*
* If you have an ArrayBuffer with the following two Uint8s:
* 185 (0b10111001) and 66 (0b01000010)
* and you perform a series of readBits: 2 bits, then 3, then 5, then 6.
*
* A BitStream in "mtl" mode will yield the following:
* - readBits(2) => 2 ('10')
* - readBits(3) => 7 ('111')
* - readBits(5) => 5 ('00101')
* - readBits(6) => 2 ('000010')
*
* A BitStream in "ltm" mode will yield the following:
* - readBits(2) => 1 ('01')
* - readBits(3) => 6 ('110')
* - readBits(5) => 21 ('10101')
* - readBits(6) => 16 ('010000')
*/
class BitStream {
/**
* @param {ArrayBuffer} ab An ArrayBuffer object or a Uint8Array.
* @param {boolean} mtl Whether the stream reads bits from the byte starting with the
* most-significant-bit (bit 7) to least-significant (bit 0). False means the direciton is
* most-significant-bit (bit 7) to least-significant (bit 0). False means the direction is
* from least-significant-bit (bit 0) to most-significant (bit 7).
* @param {Number} opt_offset The offset into the ArrayBuffer
* @param {Number} opt_length The length of this BitStream
@ -69,6 +91,7 @@
/**
* Returns how many bites have been read in the stream since the beginning of time.
* @returns {number}
*/
getNumBitsRead() {
return this.bitsRead_;
@ -76,6 +99,7 @@
/**
* Returns how many bits are currently in the stream left to be read.
* @returns {number}
*/
getNumBitsLeft() {
const bitsLeftInByte = 8 - this.bitPtr;
@ -201,6 +225,7 @@
* Bit at (bytePtr,bitPtr) has the highest position in returning data.
* Taken from getbits.hpp in unrar.
* TODO: Move this out of BitStream and into unrar.
* @returns {number}
*/
getBits() {
return (((((this.bytes[this.bytePtr] & 0xff) << 16) +

27
io/bitstream-worker.js
View file

@ -22,12 +22,34 @@ bitjs.io.BitStream =
* Note that this stream is optimized, and thus, will *NOT* throw an error if
* the end of the stream is reached. Only use this in scenarios where you
* already have all the bits you need.
*
* Bit reading always proceeds from the first byte in the buffer, to the
* second byte, and so on. The MTL flag controls which bit is considered
* first *inside* the byte.
*
* An Example for how Most-To-Least vs Least-to-Most mode works:
*
* If you have an ArrayBuffer with the following two Uint8s:
* 185 (0b10111001) and 66 (0b01000010)
* and you perform a series of readBits: 2 bits, then 3, then 5, then 6.
*
* A BitStream in "mtl" mode will yield the following:
* - readBits(2) => 2 ('10')
* - readBits(3) => 7 ('111')
* - readBits(5) => 5 ('00101')
* - readBits(6) => 2 ('000010')
*
* A BitStream in "ltm" mode will yield the following:
* - readBits(2) => 1 ('01')
* - readBits(3) => 6 ('110')
* - readBits(5) => 21 ('10101')
* - readBits(6) => 16 ('010000')
*/
class BitStream {
/**
* @param {ArrayBuffer} ab An ArrayBuffer object or a Uint8Array.
* @param {boolean} mtl Whether the stream reads bits from the byte starting with the
* most-significant-bit (bit 7) to least-significant (bit 0). False means the direciton is
* most-significant-bit (bit 7) to least-significant (bit 0). False means the direction is
* from least-significant-bit (bit 0) to most-significant (bit 7).
* @param {Number} opt_offset The offset into the ArrayBuffer
* @param {Number} opt_length The length of this BitStream
@ -73,6 +95,7 @@ bitjs.io.BitStream =
/**
* Returns how many bites have been read in the stream since the beginning of time.
* @returns {number}
*/
getNumBitsRead() {
return this.bitsRead_;
@ -80,6 +103,7 @@ bitjs.io.BitStream =
/**
* Returns how many bits are currently in the stream left to be read.
* @returns {number}
*/
getNumBitsLeft() {
const bitsLeftInByte = 8 - this.bitPtr;
@ -205,6 +229,7 @@ bitjs.io.BitStream =
* Bit at (bytePtr,bitPtr) has the highest position in returning data.
* Taken from getbits.hpp in unrar.
* TODO: Move this out of BitStream and into unrar.
* @returns {number}
*/
getBits() {
return (((((this.bytes[this.bytePtr] & 0xff) << 16) +

27
io/bitstream.js
View file

@ -20,12 +20,34 @@ export const BitStream =
* Note that this stream is optimized, and thus, will *NOT* throw an error if
* the end of the stream is reached. Only use this in scenarios where you
* already have all the bits you need.
*
* Bit reading always proceeds from the first byte in the buffer, to the
* second byte, and so on. The MTL flag controls which bit is considered
* first *inside* the byte.
*
* An Example for how Most-To-Least vs Least-to-Most mode works:
*
* If you have an ArrayBuffer with the following two Uint8s:
* 185 (0b10111001) and 66 (0b01000010)
* and you perform a series of readBits: 2 bits, then 3, then 5, then 6.
*
* A BitStream in "mtl" mode will yield the following:
* - readBits(2) => 2 ('10')
* - readBits(3) => 7 ('111')
* - readBits(5) => 5 ('00101')
* - readBits(6) => 2 ('000010')
*
* A BitStream in "ltm" mode will yield the following:
* - readBits(2) => 1 ('01')
* - readBits(3) => 6 ('110')
* - readBits(5) => 21 ('10101')
* - readBits(6) => 16 ('010000')
*/
class BitStream {
/**
* @param {ArrayBuffer} ab An ArrayBuffer object or a Uint8Array.
* @param {boolean} mtl Whether the stream reads bits from the byte starting with the
* most-significant-bit (bit 7) to least-significant (bit 0). False means the direciton is
* most-significant-bit (bit 7) to least-significant (bit 0). False means the direction is
* from least-significant-bit (bit 0) to most-significant (bit 7).
* @param {Number} opt_offset The offset into the ArrayBuffer
* @param {Number} opt_length The length of this BitStream
@ -71,6 +93,7 @@ export const BitStream =
/**
* Returns how many bites have been read in the stream since the beginning of time.
* @returns {number}
*/
getNumBitsRead() {
return this.bitsRead_;
@ -78,6 +101,7 @@ export const BitStream =
/**
* Returns how many bits are currently in the stream left to be read.
* @returns {number}
*/
getNumBitsLeft() {
const bitsLeftInByte = 8 - this.bitPtr;
@ -203,6 +227,7 @@ export const BitStream =
* Bit at (bytePtr,bitPtr) has the highest position in returning data.
* Taken from getbits.hpp in unrar.
* TODO: Move this out of BitStream and into unrar.
* @returns {number}
*/
getBits() {
return (((((this.bytes[this.bytePtr] & 0xff) << 16) +

4
tests/io-bitstream.spec.js
View file

@ -20,7 +20,7 @@ describe('bitjs.io.BitStream', () => {
});
it('BitPeekAndRead_RTL', () => {
const stream = new BitStream(array.buffer, true /* rtl */);
const stream = new BitStream(array.buffer, true /* mtl */);
// 0110 = 2 + 4 = 6
expect(stream.readBits(4)).equals(6);
// 0101 011 = 1 + 2 + 8 + 32 = 43
@ -35,7 +35,7 @@ describe('bitjs.io.BitStream', () => {
});
it('BitPeekAndRead_LTR', () => {
const stream = new BitStream(array.buffer, false /* rtl */);
const stream = new BitStream(array.buffer, false /* mtl */);
// 0101 = 2 + 4 = 6
expect(stream.peekBits(4)).equals(5);