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Improve perf of fromCharCode(), fixes #71 #72

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Aug 15, 2015
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Improve perf of fromCharCode(), fixes #71 #72

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28 changes: 16 additions & 12 deletions index.js
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Original file line number Diff line number Diff line change
Expand Up @@ -622,14 +622,14 @@ function base64Slice (buf, start, end) {
}
}

function decodeCodePointsArray (array) {
// Based on http://stackoverflow.com/a/22747272/680742, the browser with
// the lowest argument limit is Chrome, with 0x10000 args.
if (array.length < 0x10000) return String.fromCharCode.apply(String, array)
function decodeCodePointsArray (buf) {
var len = buf.length

if (len <= 0x10000) {
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I might pull this constant out and put the explanation comment on it, that way its not magically here.

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thanks, that's a good idea!

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The issue isn't manual string concatenation; it's doing an unnecessary Array.slice() when the buffer.length is < 0x10000. I think it's pretty clear that doing slice() is slower than not doing slice(), although a JSPerf would tell us exactly how slow.

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I can indeed verify that slice is slower than no-slice: http://jsperf.com/string-fromcharcode-apply-vs-string-fromcharcode-using-/4

return String.fromCharCode.apply(String, buf) // avoid extra slice()
}

// If above that limit, decode using string concatenation
// to avoid "call stack size exceeded".
return binarySlice(array, 0, array.length)
return binarySlice(buf, 0, len)
}

function utf8Slice (buf, start, end) {
Expand Down Expand Up @@ -723,13 +723,17 @@ function asciiSlice (buf, start, end) {
}

function binarySlice (buf, start, end) {
var ret = ''
end = Math.min(buf.length, end)
var chunk = 0x10000
var res = ''
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Will re-add this, shouldn't have been removed.
Probably need to add a test for it.

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ah, my bad. sorry!


for (var i = start; i < end; i++) {
ret += String.fromCharCode(buf[i])
// Decode in chunks to avoid "call stack size exceeded".
// Based on http://stackoverflow.com/a/22747272/680742, the browser with
// the lowest limit is Chrome, with 0x10000 args.
for (var i = start; i < end; i += chunk) {
var chunkEnd = Math.min(end, i + chunk)
res += String.fromCharCode.apply(String, buf.slice(i, chunkEnd))
}
return ret
return res
}

function hexSlice (buf, start, end) {
Expand Down