/**
 * A Canvas' 2D rendering context that is not rendered.<br>
 * Useful to work with ImageData objects, TextMetrics, to create bitmap data and
 * anything else you can do with a 2D context.
 */
export abstract class VirtualContext {
  private static _context: CanvasRenderingContext2D;
  private static _width: number;
  private static _height: number;

  /**
   * Used internally to initialise the virual context.
   */
  public static _init(): void {
    this._context = <CanvasRenderingContext2D>document.createElement('canvas').getContext('2d');
    this._width = 0;
    this._height = 0;
  }

  /**
   * Request the virtual context.
   * @param width The width to request from the context.
   * @param height The height to request from the context.
   * @return The context object.
   */
  public static requestBuffer(width: number, height: number): CanvasRenderingContext2D {
    this._context.restore();
    this._context.clearRect(0, 0, this._width, this._height);
    this._context.save();

    this._width = width;
    this._height = height;

    return this._context;
  }

  /**
   * Get the image data from an image object.
   * @param image The image to get the data from.
   * @param width The width of the image (defaults to the actual image's width).
   * @param height The height of the image (defaults to the actual image's height).
   */
  public static getImageData(image: HTMLImageElement, width?: number, height?: number): ImageData {
    width = width !== undefined ? width : image.width;
    height = height !== undefined ? height : image.height;

    const context = this.requestBuffer(width, height);

    context.drawImage(image, 0, 0, width, height);

    return context.getImageData(0, 0, width, height);
  }

  /**
   * Get the width of a text with a specified font.
   * @param text The text to measure.
   * @param font The font to use.
   */
  public static measureText(text: string, font: string): number {
    this._context.save();

    this._context.font = font;
    const textMeasure = this._context.measureText(text);

    this._context.restore();

    return textMeasure.width;
  }
}