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Use native Math functions in the custom log2 function

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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.
This commit is contained in:
Tim van der Meij 2017-12-09 17:24:31 +01:00
parent 371ca51402
commit c35bbd11b0
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3 changed files with 29 additions and 6 deletions
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11
src/shared/util.js
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@ -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) {