// Copyright 2014 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

export interface HistoryEntry {
  valid(): boolean;

  reveal(): void;
}

export class SimpleHistoryManager {
  #entries: HistoryEntry[];
  #activeEntryIndex: number;
  #coalescingReadonly: number;
  readonly #historyDepth: number;
  constructor(historyDepth: number) {
    this.#entries = [];
    this.#activeEntryIndex = -1;

    // Lock is used to make sure that reveal() does not
    // make any changes to the history while we are
    // rolling back or rolling over.
    this.#coalescingReadonly = 0;
    this.#historyDepth = historyDepth;
  }

  private readOnlyLock(): void {
    ++this.#coalescingReadonly;
  }

  private releaseReadOnlyLock(): void {
    --this.#coalescingReadonly;
  }

  private getPreviousValidIndex(): number {
    if (this.empty()) {
      return -1;
    }

    let revealIndex = this.#activeEntryIndex - 1;
    while (revealIndex >= 0 && !this.#entries[revealIndex].valid()) {
      --revealIndex;
    }
    if (revealIndex < 0) {
      return -1;
    }

    return revealIndex;
  }

  private getNextValidIndex(): number {
    let revealIndex = this.#activeEntryIndex + 1;

    while (revealIndex < this.#entries.length && !this.#entries[revealIndex].valid()) {
      ++revealIndex;
    }
    if (revealIndex >= this.#entries.length) {
      return -1;
    }

    return revealIndex;
  }

  private readOnly(): boolean {
    return Boolean(this.#coalescingReadonly);
  }

  empty(): boolean {
    return !this.#entries.length;
  }

  active(): HistoryEntry|null {
    return this.empty() ? null : this.#entries[this.#activeEntryIndex];
  }

  push(entry: HistoryEntry): void {
    if (this.readOnly()) {
      return;
    }
    if (!this.empty()) {
      this.#entries.splice(this.#activeEntryIndex + 1);
    }
    this.#entries.push(entry);
    if (this.#entries.length > this.#historyDepth) {
      this.#entries.shift();
    }
    this.#activeEntryIndex = this.#entries.length - 1;
  }

  canRollback(): boolean {
    return this.getPreviousValidIndex() >= 0;
  }

  canRollover(): boolean {
    return this.getNextValidIndex() >= 0;
  }

  rollback(): boolean {
    const revealIndex = this.getPreviousValidIndex();
    if (revealIndex === -1) {
      return false;
    }
    this.readOnlyLock();
    this.#activeEntryIndex = revealIndex;
    this.#entries[revealIndex].reveal();
    this.releaseReadOnlyLock();

    return true;
  }

  rollover(): boolean {
    const revealIndex = this.getNextValidIndex();
    if (revealIndex === -1) {
      return false;
    }

    this.readOnlyLock();
    this.#activeEntryIndex = revealIndex;
    this.#entries[revealIndex].reveal();
    this.releaseReadOnlyLock();

    return true;
  }
}