From c35bbd11b01f1965f3c5976fe3e0e2fff79d20f8 Mon Sep 17 00:00:00 2001
From: Tim van der Meij <timvandermeij@gmail.com>
Date: Sat, 9 Dec 2017 17:24:31 +0100
Subject: [PATCH] Use native `Math` functions in the custom `log2` function
It is quite confusing that the custom function is called `log2` while it
actually returns the ceiling value and handles zero and negative values
differently than the native function.
To resolve this, we add a comment that explains these differences and
make the function use the native `Math` functions internally instead of
using our own custom logic. To verify that the function does what we
expect, we add unit tests.
All browsers except for IE support `Math.log2` for quite a long time
already (see https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Math/log2).
For IE, we use the core-js polyfill.
According to the microbenchmark at https://jsperf.com/log2-pdfjs/1,
using the native functions should also be faster, in my testing almost
six times as fast.
---
src/shared/compatibility.js | 8 ++++++++
src/shared/util.js | 11 ++++++-----
test/unit/util_spec.js | 16 +++++++++++++++-
3 files changed, 29 insertions(+), 6 deletions(-)
diff --git a/src/shared/compatibility.js b/src/shared/compatibility.js
index 2bfc74c0f..1a9fcdc30 100644
--- a/src/shared/compatibility.js
+++ b/src/shared/compatibility.js
@@ -348,6 +348,14 @@ PDFJS.compatibilityChecked = true;
Array.prototype.includes = require('core-js/fn/array/includes');
})();
+// Provides support for Math.log2 in legacy browsers.
+// Support: IE.
+(function checkMathLog2() {
+ if (Math.log2) {
+ return;
+ }
+ Math.log2 = require('core-js/fn/math/log2');
+})();
// Provides support for Number.isNaN in legacy browsers.
// Support: IE.
diff --git a/src/shared/util.js b/src/shared/util.js
index 0dd597e2c..93d6f1583 100644
--- a/src/shared/util.js
+++ b/src/shared/util.js
@@ -600,13 +600,14 @@ function string32(value) {
(value >> 8) & 0xff, value & 0xff);
}
+// Calculate the base 2 logarithm of the number `x`. This differs from the
+// native function in the sense that it returns the ceiling value and that it
+// returns 0 instead of `Infinity`/`NaN` for `x` values smaller than/equal to 0.
function log2(x) {
- var n = 1, i = 0;
- while (x > n) {
- n <<= 1;
- i++;
+ if (x <= 0) {
+ return 0;
}
- return i;
+ return Math.ceil(Math.log2(x));
}
function readInt8(data, start) {
diff --git a/test/unit/util_spec.js b/test/unit/util_spec.js
index 798404bd0..c5e18d564 100644
--- a/test/unit/util_spec.js
+++ b/test/unit/util_spec.js
@@ -15,7 +15,7 @@
import {
bytesToString, isArrayBuffer, isBool, isEmptyObj, isNum, isSpace, isString,
- ReadableStream, removeNullCharacters, stringToBytes, stringToPDFString
+ log2, ReadableStream, removeNullCharacters, stringToBytes, stringToPDFString
} from '../../src/shared/util';
describe('util', function() {
@@ -136,6 +136,20 @@ describe('util', function() {
});
});
+ describe('log2', function() {
+ it('handles values smaller than/equal to zero', function() {
+ expect(log2(0)).toEqual(0);
+ expect(log2(-1)).toEqual(0);
+ });
+
+ it('handles values larger than zero', function() {
+ expect(log2(1)).toEqual(0);
+ expect(log2(2)).toEqual(1);
+ expect(log2(3)).toEqual(2);
+ expect(log2(3.14)).toEqual(2);
+ });
+ });
+
describe('stringToBytes', function() {
it('handles non-string arguments', function() {
expect(function() {