// @ts-ignore
import mat4 from 'gl-mat4';

main();

/**
 * The program purpose is encapsulated in a main function
 */
function main() {
	const canvas: any = document.querySelector('#glCanvas')!;
	const gl = canvas.getContext('webgl');

	if (gl == null) {
		canvas.parentNode.removeChild(canvas);
		document.getElementById('root')!.insertAdjacentHTML('beforeend',
			`<p>Unable to initialize WebGL. Your browser or machine may not
			support it.</p>`);
	}

	const vsSource = `
		attribute vec4 aVertexPosition;
		attribute vec4 aVertexColor;

		uniform mat4 uModelViewMatrix;
		uniform mat4 uProjectionMatrix;

		varying lowp vec4 vColor;

		void main() {
			gl_Position = uProjectionMatrix *
				uModelViewMatrix *
				aVertexPosition;
			vColor = aVertexColor;
		}
	`;
	const fsSource = `
		varying lowp vec4 vColor;

		void main() {
			gl_FragColor = vColor;
		}
	`;


	const shaderProgram = initShaderProgram(gl, vsSource, fsSource);

	const programInfo: any = {
		program: shaderProgram,
		attribLocations: {
			vertexPosition: gl.getAttribLocation(shaderProgram,
				'aVertexPosition'),
			vertexColor: gl.getAttribLocation(shaderProgram, 'aVertexColor'),
		},
		uniformLocations: {
			projectionMatrix: gl.getUniformLocation(
				shaderProgram, 'uProjectionMatrix'),
			modelViewMatrix: gl.getUniformLocation(
				shaderProgram, 'uModelViewMatrix'),
		},
	};

	const buffers = initBuffers(gl);
	drawScene(gl, programInfo, buffers);
}

/**
 * Initialize a shader program, so WebGL knows how to draw our data
 * @param {any} gl the WebGL context
 * @param {string} vsSource the vertex shader source
 * @param {string} fsSource the fragment shader source
 * @return {any} the shader program
 */
function initShaderProgram(gl: any, vsSource: string, fsSource: string) {
	const vertexShader = loadShader(gl, gl.VERTEX_SHADER, vsSource);
	const fragmentShader = loadShader(gl, gl.FRAGMENT_SHADER, fsSource);

	// Create the shader program
	const shaderProgram = gl.createProgram();
	gl.attachShader(shaderProgram, vertexShader);
	gl.attachShader(shaderProgram, fragmentShader);
	gl.linkProgram(shaderProgram);

	// If creating the shader program failed, alert
	if (!gl.getProgramParameter(shaderProgram, gl.LINK_STATUS)) {
		alert('Unable to initialize the shader program: ' +
			gl.getProgramInfoLog(shaderProgram));
		return null;
	}
	return shaderProgram;
}

/**
 * load a GL shader
 * @param {any} gl the WebGL context
 * @param {any} type type of shader to load
 * @param {string} source source code of shader
 * @return {any} the loaded shader
 */
function loadShader(gl: any, type: any, source: string) {
	const shader = gl.createShader(type);
	// Send the source to the shader object
	gl.shaderSource(shader, source);
	// Compile the shader program
	gl.compileShader(shader);
	// See if it compiled successfully
	if (!gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
		alert('An error occurred compiling the shaders: ' +
			gl.getShaderInfoLog(shader));
		gl.deleteShader(shader);
		return null;
	}
	return shader;
}

/**
 * init buffers to create a square
 * @param {any} gl the web gl context
 * @return {any} the buffer
 */
function initBuffers(gl: any) {
	// Create a buffer for the square's positions.
	const positionBuffer = gl.createBuffer();
	// Select the positionBuffer as the one to apply buffer
	// operations to from here out.
	gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
	// Now create an array of positions for the square.
	const positions = [
		-1.0, 1.0,
		1.0, 1.0,
		-1.0, -1.0,
		1.0, -1.0,
	];
	// Now pass the list of positions into WebGL to build the
	// shape. We do this by creating a Float32Array from the
	// JavaScript array, then use it to fill the current buffer.
	gl.bufferData(
		gl.ARRAY_BUFFER,
		new Float32Array(positions),
		gl.STATIC_DRAW);

	const colors = [
		1.0, 1.0, 1.0, 1.0,
		1.0, 0.0, 0.0, 1.0,
		0.0, 1.0, 0.0, 1.0,
		0.0, 0.0, 1.0, 1.0,
	];

	const colorBuffer = gl.createBuffer();
	gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
	gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(colors), gl.STATIC_DRAW);

	return {
		position: positionBuffer,
		color: colorBuffer,
	};
}

/**
 * Draw a webgl scene
 * @param {any} gl the WebGL context
 * @param {any} programInfo WebGL program information
 * @param {any} buffers the buffers to draw
 */
function drawScene(gl: any, programInfo: any, buffers: any) {
	gl.clearColor(0.0, 0.0, 0.0, 1.0);
	gl.clearDepth(1.0);
	gl.enable(gl.DEPTH_TEST);
	gl.depthFunc(gl.LEQUAL);
	// Clear the canvas before we start drawing on it.
	gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

	// Create a perspective matrix, a special matrix that is
	// used to simulate the distortion of perspective in a camera.
	// Our field of view is 45 degrees, with a width/height
	// ratio that matches the display size of the canvas
	// and we only want to see objects between 0.1 units
	// and 100 units away from the camera.
	const fieldOfView = 45 * Math.PI / 180;
	const aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
	const zNear = 0.1;
	const zFar = 100.0;
	const projectionMatrix = mat4.create();

	// note: glmatrix.js always has the first argument
	// as the destination to receive the result.
	mat4.perspective(
		projectionMatrix,
		fieldOfView,
		aspect,
		zNear,
		zFar);

	// Set the drawing position to the "identity" point, which is
	// the center of the scene.
	const modelViewMatrix = mat4.create();

	// Now move the drawing position a bit to where we want to
	// start drawing the square.
	mat4.translate(
		modelViewMatrix,
		modelViewMatrix,
		[-0.0, 0.0, -6.0]);

	// Tell WebGL how to pull out the positions from the position
	// buffer into the vertexPosition attribute.
	{
		const numComponents = 2;
		const type = gl.FLOAT;
		const normalize = false;
		const stride = 0;
		const offset = 0;
		gl.bindBuffer(gl.ARRAY_BUFFER, buffers.position);
		gl.vertexAttribPointer(
			programInfo.attribLocations.vertexPosition,
			numComponents,
			type,
			normalize,
			stride,
			offset);
		gl.enableVertexAttribArray(
			programInfo.attribLocations.vertexPosition);
	}

	// Tell WebGL how to pull out the positions from the position
	// buffer into the vertexPosition attribute.
	{
		const numComponents = 4;
		const type = gl.FLOAT;
		const normalize = false;
		const stride = 0;
		const offset = 0;
		gl.bindBuffer(gl.ARRAY_BUFFER, buffers.color);
		gl.vertexAttribPointer(
			programInfo.attribLocations.vertexColor,
			numComponents,
			type,
			normalize,
			stride,
			offset);
		gl.enableVertexAttribArray(
			programInfo.attribLocations.vertexColor);
	}

	// Tell WebGL to use our program when drawing
	gl.useProgram(programInfo.program);
	// Set the shader uniforms
	gl.uniformMatrix4fv(
		programInfo.uniformLocations.projectionMatrix,
		false,
		projectionMatrix);
	gl.uniformMatrix4fv(
		programInfo.uniformLocations.modelViewMatrix,
		false,
		modelViewMatrix);

	{
		const offset = 0;
		const vertexCount = 4;
		gl.drawArrays(gl.TRIANGLE_STRIP, offset, vertexCount);
	}
}