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feat(std/io): add LimitedReader (#6026)

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hastri 2020-06-02 00:37:59 +02:00 committed by GitHub
parent c9aded05a6
commit b075f55d58
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2 changed files with 59 additions and 1 deletions

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@ -1,4 +1,10 @@
// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
// Based on https://github.com/golang/go/blob/0452f9460f50f0f0aba18df43dc2b31906fb66cc/src/io/io.go
// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
type Reader = Deno.Reader;
import { encode } from "../encoding/utf8.ts";
@ -40,3 +46,30 @@ export class MultiReader implements Reader {
return result;
}
}
/**
* A `LimitedReader` reads from `reader` but limits the amount of data returned to just `limit` bytes.
* Each call to `read` updates `limit` to reflect the new amount remaining.
* `read` returns `null` when `limit` <= `0` or
* when the underlying `reader` returns `null`.
*/
export class LimitedReader implements Deno.Reader {
constructor(public reader: Deno.Reader, public limit: number) {}
async read(p: Uint8Array): Promise<number | null> {
if (this.limit <= 0) {
return null;
}
if (p.length > this.limit) {
p = p.subarray(0, this.limit);
}
const n = await this.reader.read(p);
if (n == null) {
return null;
}
this.limit -= n;
return n;
}
}

View file

@ -1,6 +1,6 @@
const { copy, test } = Deno;
import { assertEquals } from "../testing/asserts.ts";
import { MultiReader, StringReader } from "./readers.ts";
import { LimitedReader, MultiReader, StringReader } from "./readers.ts";
import { StringWriter } from "./writers.ts";
import { copyN } from "./ioutil.ts";
import { decode } from "../encoding/utf8.ts";
@ -36,3 +36,28 @@ test("ioMultiReader", async function (): Promise<void> {
await copy(r, w);
assertEquals(w.toString(), "abcdef");
});
test("ioLimitedReader", async function (): Promise<void> {
let sr = new StringReader("abc");
let r = new LimitedReader(sr, 2);
let buffer = await Deno.readAll(r);
assertEquals(decode(buffer), "ab");
assertEquals(decode(await Deno.readAll(sr)), "c");
sr = new StringReader("abc");
r = new LimitedReader(sr, 3);
buffer = await Deno.readAll(r);
assertEquals(decode(buffer), "abc");
assertEquals((await Deno.readAll(r)).length, 0);
sr = new StringReader("abc");
r = new LimitedReader(sr, 4);
buffer = await Deno.readAll(r);
assertEquals(decode(buffer), "abc");
assertEquals((await Deno.readAll(r)).length, 0);
});
test("ioLimitedReader", async function (): Promise<void> {
const rb = new StringReader("abc");
const wb = new StringWriter();
await Deno.copy(new LimitedReader(rb, -1), wb);
assertEquals(wb.toString(), "");
});