bench-read-byte-int-vs-loop-bytes.js

/**
 * SPDX-FileCopyrightText: 2022 Petr Pucil <[email protected]>
 *
 * SPDX-License-Identifier: Apache-2.0
 */

"use strict";

// Base `readBitsIntLe` implemenation: https://github.com/kaitai-io/kaitai_struct_javascript_runtime/blob/abf5542/KaitaiStream.js#L486-L525
// Diff between `readBitsIntLe` and `readBitsIntLeV2` is as follows:
/*
--- 1/readBitsIntLe.js
+++ 2/readBitsIntLeV2.js
@@ -1,4 +1,4 @@
-KaitaiStream.prototype.readBitsIntLe = function(n) {
+KaitaiStream.prototype.readBitsIntLeV2 = function(n) {
   // JS only supports bit operations on 32 bits
   if (n > 32) {
     throw new RangeError("readBitsIntLe: the maximum supported bit length is 32 (tried to read " + n + " bits)");
@@ -11,9 +11,21 @@ KaitaiStream.prototype.readBitsIntLe = function(n) {
     // 8 bits => 1 byte
     // 9 bits => 2 bytes
     var bytesNeeded = ((bitsNeeded - 1) >> 3) + 1; // `ceil(bitsNeeded / 8)` (NB: `x >> 3` is `floor(x / 8)`)
-    var buf = this.readBytes(bytesNeeded);
-    for (var i = 0; i < bytesNeeded; i++) {
-      res |= buf[i] << (i * 8);
+    var shift = 0;
+    switch (bytesNeeded) {
+      case 4:
+        res = this.readU4le();
+        break;
+      case 3:
+        res = this.readU1();
+        shift = 8;
+        // fallthrough
+      case 2:
+        res |= this.readU2le() << shift;
+        break;
+      case 1:
+        res = this.readU1();
+        break;
     }

     // NB: in JavaScript, bit shift operators always shift by modulo 32 of the right-hand operand (see
*/

const KaitaiStream = require('./KaitaiStream');
const b = require('benny');

const randomData = Uint8Array.from(new Array(24), v => (Math.random() * 0x1_00) >>> 0);

const _io = new KaitaiStream(randomData);
const funcRef = KaitaiStream.prototype.readBitsIntLe.bind(_io);
const funcV2 = KaitaiStream.prototype.readBitsIntLeV2.bind(_io);

function benchIter(io, func) {
    io.seek(0);
    io.alignToByte();

    const rand = (Math.random() * 0x1_0000_0000) >>> 0;

    const a = func(1 + (rand & 0b11111));
    const b = func(1 + ((rand & 0b11111_00000) >>> 5));
    const c = func(1 + ((rand & 0b11111_00000_00000) >>> 10));
    const d = func(1 + ((rand & 0b11111_00000_00000_00000) >>> 15));
    const e = func(1 + ((rand & 0b11111_00000_00000_00000_00000) >>> 20));
    const f = func(1 + ((rand & 0b11111_00000_00000_00000_00000_00000) >>> 25));
}

function run() {
    return b.suite(
        'readBitsIntLe suite',
        b.add('KaitaiStream#readBitsIntLe', benchIter.bind(null, _io, funcRef)),
        b.add('KaitaiStream#readBitsIntLeV2', benchIter.bind(null, _io, funcV2)),
        b.cycle(),
        b.complete(),
    );
}
run();