perf(streams): add single-character fast path for DelimiterStream()
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iuioiua
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Oct 29, 2023
streams/delimiter_stream.ts
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| let transform: ( | ||
| chunk: Uint8Array, | ||
| controller: TransformStreamDefaultController<Uint8Array>, | ||
| ) => void; | ||
| const flush = ( | ||
| controller: TransformStreamDefaultController<Uint8Array>, | ||
| ) => { | ||
| const bufs = this.#bufs; | ||
| const length = bufs.length; | ||
| if (length === 0) { | ||
| controller.enqueue(new Uint8Array()); | ||
| } else if (length === 1) { | ||
| controller.enqueue(bufs[0]); | ||
| } else { | ||
| controller.enqueue(concat(...bufs)); | ||
| } | ||
| }; | ||
|
|
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| let lps: Uint8Array | null = null; | ||
|
|
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| if (delimiter.length === 1) { | ||
| // Delimiter is a single char | ||
| transform = (chunk, controller) => this.#handleChar(chunk, controller); | ||
| } else { | ||
| transform = (chunk, controller) => this.#handle(chunk, controller); | ||
| lps = createLPS(delimiter); | ||
| } | ||
| super({ | ||
| transform: (chunk, controller) => { | ||
| this.#handle(chunk, controller); | ||
| }, | ||
| flush: (controller) => { | ||
| controller.enqueue(concat(...this.#bufs)); | ||
| }, | ||
| transform, | ||
| flush, | ||
| }); | ||
|
|
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| this.#delimiter = delimiter; | ||
| this.#delimLPS = createLPS(delimiter); | ||
| this.#delimLPS = lps; | ||
| this.#disp = options?.disposition ?? "discard"; | ||
| } |
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We can simplify the constructor by doing something like:
constructor(
delimiter: Uint8Array,
options?: DelimiterStreamOptions,
) {
super({
transform: (chunk, controller) =>
delimiter.length === 1
? this.#handleChar(chunk, controller)
: this.#handle(chunk, controller),
flush: (controller) => this.#flush(controller),
});
this.#delimiter = delimiter;
this.#delimLPS = delimiter.length > 1 ? createLPS(delimiter) : null;
this.#disp = options?.disposition ?? "discard";
}I think we should also factor out the flush() logic into its own separate #flush() method to match the style of the rest of the class. WDYT?
iuioiua
changed the title
DelimiterStream()
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DelimiterStream is often used to delimit formats that use a single char as the separator such as most commonly used flavours of CSV, new-line delimited streamable JSON and others. Multi-character delimiters require extra handling that is entirely unnecessary for char delimiters. As such, it makes sense to special-case char delimiter streams for extra performance.
The performance benefit is at most a few percentage points.