var shader_time = 0.0;

window.addEventListener("load", main);
function main() {
  const canvas = document.querySelector('#webgl-canvas');
  const gl = canvas.getContext('webgl');
  // If we don't have a GL context, give up now
  if (!gl) {
    alert('Unable to initialize WebGL. Your browser or machine may not support it.');
    return;
  }

  const vsSource = document.querySelector("#webgl-canvas #vertex-shader").innerHTML; // Vertex shader program
  const fsSource = document.querySelector("#webgl-canvas #fragment-shader").innerHTML; // fragment shader program
  const shaderProgram = initShaderProgram(gl, vsSource, fsSource);
  // Collect all the info needed to use the shader program.
  // Look up which attributes our shader program is using
  // for aVertexPosition, aVevrtexColor and also
  // look up uniform locations.
  const programInfo = {
    program: shaderProgram,
    attribLocations: {
      vertexPosition: gl.getAttribLocation(shaderProgram, 'aVertexPosition'),
    },
    uniformLocations: {
      projectionMatrix: gl.getUniformLocation(shaderProgram, 'uProjectionMatrix'),
      modelViewMatrix: gl.getUniformLocation(shaderProgram, 'uModelViewMatrix'),
      uResolution: gl.getUniformLocation(shaderProgram, 'uResolution'),
      uTime: gl.getUniformLocation(shaderProgram, 'uTime'),
    },
  };

  // Here's where we call the routine that builds all the
  // objects we'll be drawing.
  const buffers = initBuffers(gl);
  var then = 0;
  // Draw the scene repeatedly
  function render(now) {
    now *= 0.001;  // convert to seconds
    const deltaTime = now - then;
    then = now;
    drawScene(gl, programInfo, buffers, deltaTime);
    requestAnimationFrame(render);
  }
  requestAnimationFrame(render);
}

//
// initBuffers
//
// Initialize the buffers we'll need. For this demo, we just
// have one object -- a simple two-dimensional square.
//
function initBuffers(gl) {
  // 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);
  return {
    position: positionBuffer,
  };
}

//
// Draw the scene.
//
function drawScene(gl, programInfo, buffers, deltaTime) {
  gl.clearColor(0.0, 0.0, 0.0, 1.0);  // Clear to black, fully opaque
  gl.clearDepth(1.0);                 // Clear everything
  gl.enable(gl.DEPTH_TEST);           // Enable depth testing
  gl.depthFunc(gl.LEQUAL);            // Near things obscure far things

  // Clear the canvas before we start drawing on it.
  gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

  // Create a orthographic matrix
  const aspect = gl.canvas.clientWidth / gl.canvas.clientHeight;
  const zNear = 0.0;
  const zFar = 100.0;
  const projectionMatrix = mat4.create();

  // note: glmatrix.js always has the first argument as the destination to receive the result.
  mat4.ortho(projectionMatrix, -1.0, 1.0, -1.0, 1.0, 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,     // destination matrix
  //                modelViewMatrix,     // matrix to translate
  //                [-0.0, 0.0, -0.0]);  // amount to translate
  // mat4.rotate(modelViewMatrix,  // destination matrix
  //             modelViewMatrix,  // matrix to rotate
  //             squareRotation,   // amount to rotate in radians
  //             [0, 0, 1]);       // axis to rotate around

  // 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 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);
  var size = [1280.0, 720.0]    //TODO: make this size of canvas and change on window resize
  gl.uniform2f(programInfo.uniformLocations.uResolution, size[0], size[1]);
  gl.uniform1f(programInfo.uniformLocations.uTime, shader_time);
  {
    const offset = 0;
    const vertexCount = 4;
    gl.drawArrays(gl.TRIANGLE_STRIP, offset, vertexCount);
  }
  // Update time:
  shader_time += deltaTime;
}

//
// Initialize a shader program, so WebGL knows how to draw our data
//
function initShaderProgram(gl, vsSource, fsSource) {
  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;
}

//
// creates a shader of the given type, uploads the source and compiles it.
//
function loadShader(gl, type, source) {
  const shader = gl.createShader(type);
  gl.shaderSource(shader, source); // Send the source to the shader object
  gl.compileShader(shader); // Compile the shader program
  // 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;
}