/*---------------------------------------------------------------------------------------------
 *  Copyright (c) Microsoft Corporation. All rights reserved.
 *  Licensed under the MIT License. See License.txt in the project root for license information.
 *--------------------------------------------------------------------------------------------*/

import { INavigator, ArrayNavigator } from '../../../vs/base/common/navigator';

export class HistoryNavigator<T> implements INavigator<T> {
  private _history!: Set<T>;
  private _limit: number;
  private _navigator!: ArrayNavigator<T>;

  constructor(history: readonly T[] = [], limit: number = 10) {
    this._initialize(history);
    this._limit = limit;
    this._onChange();
  }

  public add(t: T) {
    this._history.delete(t);
    this._history.add(t);
    this._onChange();
  }

  public next(): T | null {
    if (this._currentPosition() !== this._elements.length - 1) {
      return this._navigator.next();
    }
    return null;
  }

  public previous(): T | null {
    if (this._currentPosition() !== 0) {
      return this._navigator.previous();
    }
    return null;
  }

  public current(): T | null {
    return this._navigator.current();
  }

  public first(): T | null {
    return this._navigator.first();
  }

  public last(): T | null {
    return this._navigator.last();
  }

  public has(t: T): boolean {
    return this._history.has(t);
  }

  private _onChange() {
    this._reduceToLimit();
    const elements = this._elements;
    this._navigator = new ArrayNavigator(
      elements,
      0,
      elements.length,
      elements.length
    );
  }

  private _reduceToLimit() {
    const data = this._elements;
    if (data.length > this._limit) {
      this._initialize(data.slice(data.length - this._limit));
    }
  }

  private _currentPosition(): number {
    const currentElement = this._navigator.current();
    if (!currentElement) {
      return -1;
    }

    return this._elements.indexOf(currentElement);
  }

  private _initialize(history: readonly T[]): void {
    this._history = new Set();
    for (const entry of history) {
      this._history.add(entry);
    }
  }

  private get _elements(): T[] {
    const elements: T[] = [];
    this._history.forEach((e) => elements.push(e));
    return elements;
  }
}