// 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";

/** Reader utility for strings */
export class StringReader implements Reader {
  private offs = 0;
  private buf = new Uint8Array(encode(this.s));

  constructor(private readonly s: string) {}

  read(p: Uint8Array): Promise<number | null> {
    const n = Math.min(p.byteLength, this.buf.byteLength - this.offs);
    p.set(this.buf.slice(this.offs, this.offs + n));
    this.offs += n;
    if (n === 0) {
      return Promise.resolve(null);
    }
    return Promise.resolve(n);
  }
}

/** Reader utility for combining multiple readers */
export class MultiReader implements Reader {
  private readonly readers: Reader[];
  private currentIndex = 0;

  constructor(...readers: Reader[]) {
    this.readers = readers;
  }

  async read(p: Uint8Array): Promise<number | null> {
    const r = this.readers[this.currentIndex];
    if (!r) return null;
    const result = await r.read(p);
    if (result === null) {
      this.currentIndex++;
      return 0;
    }
    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;
  }
}