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Improve performance of peeking bytes by copying subarrays instead of copying byte-by-byte

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codedread 2018-01-26 08:59:51 -08:00
parent 23e3921072
commit 950dc65b56
2 changed files with 82 additions and 13 deletions
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54
io/bitstream.js
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@ -33,13 +33,43 @@ bitjs.io.BitStream = class {
const offset = opt_offset || 0;
const length = opt_length || ab.byteLength;
/**
* The current page of bytes in the stream.
* @type {Uint8Array}
* @private
*/
this.bytes = new Uint8Array(ab, offset, length);
/**
* The next pages of bytes in the stream.
* @type {Array<Uint8Array>}
* @private
*/
this.pages_ = [];
this.bytePtr = 0; // tracks which byte we are on
/**
* The byte in the current page that we are currently on.
* @type {Number}
* @private
*/
this.bytePtr = 0;
/**
* The bit in the current byte that we will read next (can have values 0 through 7).
* @type {Number}
* @private
*/
this.bitPtr = 0; // tracks which bit we are on (can have values 0 through 7)
this.peekBits = rtl ? this.peekBits_rtl : this.peekBits_ltr;
// An ever-increasing number.
/**
* An ever-increasing number.
* @type {Number}
* @private
*/
this.bitsRead_ = 0;
this.peekBits = rtl ? this.peekBits_rtl : this.peekBits_ltr;
}
/**
@ -257,14 +287,22 @@ bitjs.io.BitStream = class {
const movePointers = opt_movePointers || false;
const result = new Uint8Array(num);
let curPage = this.bytes;
let ptr = this.bytePtr;
let bytesLeftToCopy = num;
let pageIndex = 0;
let bytePtr = this.bytePtr;
for (let i = 0; i < num; ++i) {
result[i] = curPage[bytePtr++];
if (bytePtr >= curPage.length) {
while (bytesLeftToCopy > 0) {
const bytesLeftInPage = curPage.length - ptr;
const sourceLength = Math.min(bytesLeftToCopy, bytesLeftInPage);
result.set(curPage.subarray(ptr, ptr + sourceLength), num - bytesLeftToCopy);
ptr += sourceLength;
if (ptr >= curPage.length) {
curPage = this.pages_[pageIndex++];
bytePtr = 0;
ptr = 0;
}
bytesLeftToCopy -= sourceLength;
}
if (movePointers) {