class Canvas {
	constructor(rootId) {
		const root = document.getElementById(rootId);

		this.element = document.createElement('canvas');
		this.context = this.element.getContext('2d');
		this.fillWindow();
		window.addEventListener('resize', () => this.fillWindow());

		root.appendChild(this.element);

		this.ondraw = null;

		/* automatically compute drawing-space mouse coordinates */
		this.mousePos = { x: 0, y: 0 };
		this.onmousemove = null;
		this.element.addEventListener('mousemove', e => {
			const matrix = this.context.getTransform();
			matrix.invertSelf();

			const screenX = e.offsetX; const screenY = e.offsetY;
			this.mousePos.x = matrix.a*screenX + matrix.b*screenY;
			this.mousePos.y = matrix.c*screenX + matrix.d*screenY;

			if (this.onmousemove) this.onmousemove(this.mousePos);
		});

	}

	fillWindow() {
		const width = window.innerWidth;
		const height = window.innerHeight;
		const scale = Math.max(width, height);

		this.element.width = width; this.element.height = height;
		this.context.scale(scale, scale);

		this.draw();
	}

	draw() {
		this.context.clearRect(0, 0, 1, 1);
		if (this.ondraw) this.ondraw(this.context);
	}
}

export default Canvas;