'use strict';

// Rasterize adjacent mesh triangles together, with GPU edge ownership rules.
// Canvas clip()+drawImage() anti-aliases each triangle separately and exposes
// the background along shared edges, which looks like a wireframe overlay.
function createRasterizer(context) {
	const document = context.canvas && context.canvas.ownerDocument;
	if (!document || typeof context.getTransform !== 'function') return null;
	const canvas = document.createElement('canvas');
	const gl = canvas.getContext('webgl', {
		alpha: true, premultipliedAlpha: true, antialias: true,
		preserveDrawingBuffer: true, depth: false, stencil: false
	});
	if (!gl) return null;
	function shader(type, source) {
		const shader = gl.createShader(type);
		gl.shaderSource(shader, source); gl.compileShader(shader);
		if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) throw new Error(gl.getShaderInfoLog(shader));
		return shader;
	}
	const vertex = shader(gl.VERTEX_SHADER, `
		attribute vec2 position;
		attribute vec2 uv;
		uniform vec2 resolution;
		varying vec2 textureUV;
		void main() {
			vec2 p = position / resolution * 2.0 - 1.0;
			gl_Position = vec4(p.x, -p.y, 0.0, 1.0);
			textureUV = uv;
		}`);
	const fragment = shader(gl.FRAGMENT_SHADER, `
		precision mediump float;
		uniform sampler2D image;
		uniform float opacity;
		varying vec2 textureUV;
		void main() {
			vec4 color = texture2D(image, textureUV);
			float alpha = color.a * opacity;
			gl_FragColor = vec4(color.rgb * opacity, alpha);
		}`);
	const program = gl.createProgram(); gl.attachShader(program, vertex); gl.attachShader(program, fragment); gl.linkProgram(program);
	if (!gl.getProgramParameter(program, gl.LINK_STATUS)) throw new Error(gl.getProgramInfoLog(program));
	gl.deleteShader(vertex); gl.deleteShader(fragment);
	const buffer = gl.createBuffer();
	const position = gl.getAttribLocation(program, 'position'), uv = gl.getAttribLocation(program, 'uv');
	const resolution = gl.getUniformLocation(program, 'resolution'), opacity = gl.getUniformLocation(program, 'opacity');
	const textures = new Map();
	let coordinates = [], currentImage = null, currentAlpha = 1, currentMode = 'source-over', transform;
	let disposed = false, lost = false;
	canvas.addEventListener('webglcontextlost', event => { event.preventDefault(); lost = true; });
	function texture(image) {
		if (textures.has(image) && image !== context.canvas) return textures.get(image);
		const t = textures.get(image) || gl.createTexture(); gl.bindTexture(gl.TEXTURE_2D, t);
		// Premultiply before interpolation so translucent edges do not darken.
		gl.pixelStorei(gl.UNPACK_PREMULTIPLY_ALPHA_WEBGL, true);
		gl.pixelStorei(gl.UNPACK_FLIP_Y_WEBGL, false);
		gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.LINEAR);
		gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.LINEAR);
		gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_S, gl.CLAMP_TO_EDGE);
		gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_WRAP_T, gl.CLAMP_TO_EDGE);
		gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA, gl.RGBA, gl.UNSIGNED_BYTE, image);
		textures.set(image, t); return t;
	}
	function flush() {
		if (!coordinates.length) return;
		gl.bindTexture(gl.TEXTURE_2D, texture(currentImage));
		gl.uniform1f(opacity, currentAlpha);
		// Premultiplied RGB factors; keep source-over alpha independently.
		const factors = currentMode === 'lighter' ? [gl.ONE, gl.ONE]
			: currentMode === 'multiply' ? [gl.DST_COLOR, gl.ONE_MINUS_SRC_ALPHA]
			: currentMode === 'screen' ? [gl.ONE, gl.ONE_MINUS_SRC_COLOR]
			: [gl.ONE, gl.ONE_MINUS_SRC_ALPHA];
		gl.blendFuncSeparate(factors[0], factors[1], gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
		gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(coordinates), gl.STREAM_DRAW);
		gl.drawArrays(gl.TRIANGLES, 0, coordinates.length / 4);
		coordinates.length = 0;
	}
	return {
		begin() {
			if (disposed || lost || gl.isContextLost()) return false;
			const target = context.canvas;
			if (canvas.width !== target.width) canvas.width = target.width;
			if (canvas.height !== target.height) canvas.height = target.height;
			transform = context.getTransform(); coordinates.length = 0; currentImage = null;
			gl.viewport(0, 0, canvas.width, canvas.height);
			gl.clearColor(0, 0, 0, 0); gl.clear(gl.COLOR_BUFFER_BIT);
			gl.useProgram(program); gl.bindBuffer(gl.ARRAY_BUFFER, buffer);
			gl.enableVertexAttribArray(position); gl.vertexAttribPointer(position, 2, gl.FLOAT, false, 16, 0);
			gl.enableVertexAttribArray(uv); gl.vertexAttribPointer(uv, 2, gl.FLOAT, false, 16, 8);
			gl.uniform2f(resolution, canvas.width, canvas.height);
			gl.activeTexture(gl.TEXTURE0); gl.uniform1i(gl.getUniformLocation(program, 'image'), 0);
			gl.enable(gl.BLEND); gl.blendFunc(gl.ONE, gl.ONE_MINUS_SRC_ALPHA);
			// Seed the same framebuffer with the actual Canvas pixels (solid or
			// checkerboard background). CSS backgrounds cannot participate in blending.
			currentImage = target; currentAlpha = 1; currentMode = 'source-over';
			const w = canvas.width, h = canvas.height;
			coordinates.push(0,0,0,0, w,0,1,0, w,h,1,1, 0,0,0,0, w,h,1,1, 0,h,0,1);
			flush(); currentImage = null;
			return true;
		},
		triangle(image, x0, y0, u0, v0, x1, y1, u1, v1, x2, y2, u2, v2) {
			const alpha = context.globalAlpha;
			const mode = context.globalCompositeOperation || 'source-over';
			if (image !== currentImage || alpha !== currentAlpha || mode !== currentMode) { flush(); currentImage = image; currentAlpha = alpha; currentMode = mode; }
			const t = transform;
			coordinates.push(
				t.a * x0 + t.c * y0 + t.e, t.b * x0 + t.d * y0 + t.f, u0, v0,
				t.a * x1 + t.c * y1 + t.e, t.b * x1 + t.d * y1 + t.f, u1, v1,
				t.a * x2 + t.c * y2 + t.e, t.b * x2 + t.d * y2 + t.f, u2, v2);
		},
		end() {
			flush();
			context.save();
			try { context.setTransform(1, 0, 0, 1, 0, 0); context.globalAlpha = 1; context.globalCompositeOperation = 'copy'; context.drawImage(canvas, 0, 0); }
			finally { context.restore(); }
		},
		dispose() {
			if (disposed) return; disposed = true;
			for (const value of textures.values()) gl.deleteTexture(value);
			textures.clear(); gl.deleteBuffer(buffer); gl.deleteProgram(program);
			const extension = gl.getExtension('WEBGL_lose_context'); if (extension) extension.loseContext();
		}
	};
}

function attach(renderer, context) {
	let rasterizer;
	try { rasterizer = createRasterizer(context); } catch (e) { return renderer; }
	if (!rasterizer) return renderer;
	const draw = renderer.draw.bind(renderer), triangle = renderer.drawTriangle.bind(renderer);
	renderer.draw = skeleton => {
		if (!rasterizer.begin()) return draw(skeleton);
		renderer.drawTriangle = rasterizer.triangle;
		try { draw(skeleton); rasterizer.end(); }
		finally { renderer.drawTriangle = triangle; }
	};
	renderer.dispose = rasterizer.dispose;
	renderer.rasterization = 'webgl';
	return renderer;
}

module.exports = { createRasterizer, attach };