// Ported to Deno from:
// 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.

import { Reader, ReadResult } from "deno";
import { assert, charCode, copyBytes } from "./util.ts";

const DEFAULT_BUF_SIZE = 4096;
const MIN_BUF_SIZE = 16;
const MAX_CONSECUTIVE_EMPTY_READS = 100;
const CR = charCode("\r");
const LF = charCode("\n");

export type BufState = null | "EOF" | "BufferFull" | "NoProgress" | Error;

/** BufReader implements buffering for a Reader object. */
export class BufReader implements Reader {
  private buf: Uint8Array;
  private rd: Reader; // Reader provided by caller.
  private r = 0; // buf read position.
  private w = 0; // buf write position.
  private lastByte: number;
  private lastCharSize: number;
  private err: BufState;

  constructor(rd: Reader, size = DEFAULT_BUF_SIZE) {
    if (size < MIN_BUF_SIZE) {
      size = MIN_BUF_SIZE;
    }
    this._reset(new Uint8Array(size), rd);
  }

  /** Returns the size of the underlying buffer in bytes. */
  get byteLength(): number {
    return this.buf.byteLength;
  }

  buffered(): number {
    return this.w - this.r;
  }

  private _readErr(): BufState {
    const err = this.err;
    this.err = null;
    return err;
  }

  // Reads a new chunk into the buffer.
  private async _fill(): Promise<void> {
    // Slide existing data to beginning.
    if (this.r > 0) {
      this.buf.copyWithin(0, this.r, this.w);
      this.w -= this.r;
      this.r = 0;
    }

    if (this.w >= this.buf.byteLength) {
      throw Error("bufio: tried to fill full buffer");
    }

    // Read new data: try a limited number of times.
    for (let i = MAX_CONSECUTIVE_EMPTY_READS; i > 0; i--) {
      let rr: ReadResult;
      try {
        rr = await this.rd.read(this.buf.subarray(this.w));
      } catch (e) {
        this.err = e;
        return;
      }
      assert(rr.nread >= 0, "negative read");
      this.w += rr.nread;
      if (rr.eof) {
        this.err = "EOF";
        return;
      }
      if (rr.nread > 0) {
        return;
      }
    }
    this.err = "NoProgress";
  }

  /** Discards any buffered data, resets all state, and switches
   * the buffered reader to read from r.
   */
  reset(r: Reader): void {
    this._reset(this.buf, r);
  }

  private _reset(buf: Uint8Array, rd: Reader): void {
    this.buf = buf;
    this.rd = rd;
    this.lastByte = -1;
    // this.lastRuneSize = -1;
  }

  /** reads data into p.
   * It returns the number of bytes read into p.
   * The bytes are taken from at most one Read on the underlying Reader,
   * hence n may be less than len(p).
   * At EOF, the count will be zero and err will be io.EOF.
   * To read exactly len(p) bytes, use io.ReadFull(b, p).
   */
  async read(p: ArrayBufferView): Promise<ReadResult> {
    let rr: ReadResult = { nread: p.byteLength, eof: false };
    if (rr.nread === 0) {
      if (this.err) {
        throw this._readErr();
      }
      return rr;
    }

    if (this.r === this.w) {
      if (this.err) {
        throw this._readErr();
      }
      if (p.byteLength >= this.buf.byteLength) {
        // Large read, empty buffer.
        // Read directly into p to avoid copy.
        rr = await this.rd.read(p);
        assert(rr.nread >= 0, "negative read");
        if (rr.nread > 0) {
          this.lastByte = p[rr.nread - 1];
          // this.lastRuneSize = -1;
        }
        if (this.err) {
          throw this._readErr();
        }
        return rr;
      }
      // One read.
      // Do not use this.fill, which will loop.
      this.r = 0;
      this.w = 0;
      try {
        rr = await this.rd.read(this.buf);
      } catch (e) {
        this.err = e;
      }
      assert(rr.nread >= 0, "negative read");
      if (rr.nread === 0) {
        if (this.err) {
          throw this._readErr();
        }
        return rr;
      }
      this.w += rr.nread;
    }

    // copy as much as we can
    rr.nread = copyBytes(p as Uint8Array, this.buf.subarray(this.r, this.w), 0);
    this.r += rr.nread;
    this.lastByte = this.buf[this.r - 1];
    // this.lastRuneSize = -1;
    return rr;
  }

  /** Returns the next byte [0, 255] or -1 if EOF. */
  async readByte(): Promise<number> {
    while (this.r === this.w) {
      await this._fill(); // buffer is empty.
      if (this.err == "EOF") {
        return -1;
      }
      if (this.err != null) {
        throw this._readErr();
      }
    }
    const c = this.buf[this.r];
    this.r++;
    this.lastByte = c;
    return c;
  }

  /** readString() reads until the first occurrence of delim in the input,
   * returning a string containing the data up to and including the delimiter.
   * If ReadString encounters an error before finding a delimiter,
   * it returns the data read before the error and the error itself (often io.EOF).
   * ReadString returns err != nil if and only if the returned data does not end in
   * delim.
   * For simple uses, a Scanner may be more convenient.
   */
  async readString(delim: string): Promise<string> {
    throw new Error("Not implemented");
  }

  /** readLine() is a low-level line-reading primitive. Most callers should use
   * readBytes('\n') or readString('\n') instead or use a Scanner.
   *
   * readLine tries to return a single line, not including the end-of-line bytes.
   * If the line was too long for the buffer then isPrefix is set and the
   * beginning of the line is returned. The rest of the line will be returned
   * from future calls. isPrefix will be false when returning the last fragment
   * of the line. The returned buffer is only valid until the next call to
   * ReadLine. ReadLine either returns a non-nil line or it returns an error,
   * never both.
   *
   * The text returned from ReadLine does not include the line end ("\r\n" or "\n").
   * No indication or error is given if the input ends without a final line end.
   * Calling UnreadByte after ReadLine will always unread the last byte read
   * (possibly a character belonging to the line end) even if that byte is not
   * part of the line returned by ReadLine.
   */
  async readLine(): Promise<[Uint8Array, boolean, BufState]> {
    let [line, err] = await this.readSlice(LF);

    if (err === "BufferFull") {
      // Handle the case where "\r\n" straddles the buffer.
      if (line.byteLength > 0 && line[line.byteLength - 1] === CR) {
        // Put the '\r' back on buf and drop it from line.
        // Let the next call to ReadLine check for "\r\n".
        assert(this.r > 0, "bufio: tried to rewind past start of buffer");
        this.r--;
        line = line.subarray(0, line.byteLength - 1);
      }
      return [line, true, null];
    }

    if (line.byteLength === 0) {
      return [line, false, err];
    }
    err = null;

    if (line[line.byteLength - 1] == LF) {
      let drop = 1;
      if (line.byteLength > 1 && line[line.byteLength - 2] === CR) {
        drop = 2;
      }
      line = line.subarray(0, line.byteLength - drop);
    }
    return [line, false, err];
  }

  /** readSlice() reads until the first occurrence of delim in the input,
   * returning a slice pointing at the bytes in the buffer. The bytes stop
   * being valid at the next read. If readSlice() encounters an error before
   * finding a delimiter, it returns all the data in the buffer and the error
   * itself (often io.EOF).  readSlice() fails with error ErrBufferFull if the
   * buffer fills without a delim. Because the data returned from readSlice()
   * will be overwritten by the next I/O operation, most clients should use
   * readBytes() or readString() instead. readSlice() returns err != nil if and
   * only if line does not end in delim.
   */
  async readSlice(delim: number): Promise<[Uint8Array, BufState]> {
    let s = 0; // search start index
    let line: Uint8Array;
    let err: BufState;
    while (true) {
      // Search buffer.
      let i = this.buf.subarray(this.r + s, this.w).indexOf(delim);
      if (i >= 0) {
        i += s;
        line = this.buf.subarray(this.r, this.r + i + 1);
        this.r += i + 1;
        break;
      }

      // Pending error?
      if (this.err) {
        line = this.buf.subarray(this.r, this.w);
        this.r = this.w;
        err = this._readErr();
        break;
      }

      // Buffer full?
      if (this.buffered() >= this.buf.byteLength) {
        this.r = this.w;
        line = this.buf;
        err = "BufferFull";
        break;
      }

      s = this.w - this.r; // do not rescan area we scanned before

      await this._fill(); // buffer is not full
    }

    // Handle last byte, if any.
    let i = line.byteLength - 1;
    if (i >= 0) {
      this.lastByte = line[i];
      // this.lastRuneSize = -1
    }

    return [line, err];
  }

  /** Peek returns the next n bytes without advancing the reader. The bytes stop
   * being valid at the next read call. If Peek returns fewer than n bytes, it
   * also returns an error explaining why the read is short. The error is
   * ErrBufferFull if n is larger than b's buffer size.
   */
  async peek(n: number): Promise<[Uint8Array, BufState]> {
    if (n < 0) {
      throw Error("negative count");
    }

    while (
      this.w - this.r < n &&
      this.w - this.r < this.buf.byteLength &&
      this.err == null
    ) {
      await this._fill(); // this.w - this.r < len(this.buf) => buffer is not full
    }

    if (n > this.buf.byteLength) {
      return [this.buf.subarray(this.r, this.w), "BufferFull"];
    }

    // 0 <= n <= len(this.buf)
    let err: BufState;
    let avail = this.w - this.r;
    if (avail < n) {
      // not enough data in buffer
      n = avail;
      err = this._readErr();
      if (!err) {
        err = "BufferFull";
      }
    }
    return [this.buf.subarray(this.r, this.r + n), err];
  }
}