83 lines
2.9 KiB
Markdown
83 lines
2.9 KiB
Markdown
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# flatstr
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Flattens the underlying C structures of a concatenated JavaScript string
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## About
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If you're doing lots of string concatenation and then writing that
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string somewhere, you may find that passing your string through
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`flatstr` vastly improves performance.
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## Usage
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```js
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var flatstr = require('flatstr')
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flatstr(someHeavilyConcatenatedString)
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```
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## Benchmarks
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Benchmarks test flat vs non-flat strings being written to
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an `fs.WriteStream`.
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```
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unflattenedManySmallConcats*10000: 147.540ms
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flattenedManySmallConcats*10000: 105.994ms
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unflattenedSeveralLargeConcats*10000: 287.901ms
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flattenedSeveralLargeConcats*10000: 226.121ms
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unflattenedExponentialSmallConcats*10000: 410.533ms
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flattenedExponentialSmallConcats*10000: 219.973ms
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unflattenedExponentialLargeConcats*10000: 2774.230ms
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flattenedExponentialLargeConcats*10000: 1862.815ms
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```
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In each case, flattened strings win,
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here's the performance gains from using `flatstr`
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```
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ManySmallConcats: 28%
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SeveralLargeConcats: 21%
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ExponentialSmallConcats: 46%
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ExponentialLargeConcats: 33%
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```
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## How does it work
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In the v8 C++ layer, JavaScript strings can be represented in two ways.
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1. As an array
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2. As a tree
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When JavaScript strings are concatenated, tree structures are used
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to represent them. For the concat operation, this is cheaper than
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reallocating a larger array. However, performing other operations
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on the tree structures can become costly (particularly where lots of
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concatenation has occurred).
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V8 has a a method called `String::Flatten`which converts the tree into a C array. This method is typically called before operations that walk through the bytes of the string (for instance, when testing against a regular expression). It may also be called if a string is accessed many times over,
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as an optimization on the string. However, strings aren't always flattened. One example is when we pass a string into a `WriteStream`, at some point the string will be converted to a buffer, and this may be expensive if the underlying representation is a tree.
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`String::Flatten` is not exposed as a JavaScript function, but it can be triggered as a side effect.
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There are several ways to indirectly call `String::Flatten` (see `alt-benchmark.js`),
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but coercion to a number appears to be (one of) the cheapest.
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However since Node 10 the V8 version has stopped using Flatten in all
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places identified. Thus the code has been updated to seamlessly
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use the native runtime function `%FlattenString` without having to use
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the `--allow-natives-syntax` flag directly.
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One final note: calling flatstr too much can in fact negatively effect performance. For instance, don't call it every time you concat (if that
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was performant, v8 wouldn't be using trees in the first place). The best
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place to use flatstr is just prior to passing it to an API that eventually
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runs non-v8 code (such as `fs.WriteStream`, or perhaps `xhr` or DOM apis in the browser).
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## Acknowledgements
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* Sponsored by nearForm
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## License
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MIT
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