// RotatingTranslatedTriangle.js (c) 2012 matsuda

const vertex_shader = `
  attribute vec4 a_Position;
  uniform mat4 u_ModelMatrix;
  
  void main() 
  {
    gl_Position = u_ModelMatrix * a_Position;
  }	`;


const fragment_shader = `
  void main() 
  {
    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
  }	`;

// Rotation angle (degrees/second)
var ANGLE_STEP = 45.0;

function main() 
{
  // Retrieve <canvas> element
  var canvas = document.getElementById('webgl');

  // Get the rendering context for WebGL
  var gl = getWebGLContext(canvas);
  if (!gl) {
    console.log('Failed to get the rendering context for WebGL');
    return;
  }

  // Initialize shaders
  if (!initShaders(gl, vertex_shader, fragment_shader))
  {
    console.log('Failed to intialize shaders.');
    return;
  }

  // Write the positions of vertices to a vertex shader
  var n = initVertexBuffers(gl);
  if (n < 0) 
  {
    console.log('Failed to set the positions of the vertices');
    return;
  }

  // Specify the color for clearing <canvas>
  gl.clearColor(0, 0, 0, 1);

  // Get storage location of u_ModelMatrix
  var u_ModelMatrix = gl.getUniformLocation(gl.program, 'u_ModelMatrix');
  if (!u_ModelMatrix) 
  { 
    console.log('Failed to get the storage location of u_ModelMatrix');
    return;
  }

  // Current rotation angle
  var currentAngle = 0.0;
  
  // Model matrix
  var modelMatrix = new Matrix4();

  // Start drawing
  var tick = function() 
  {
    currentAngle = animate(currentAngle);  // Update the rotation angle
    draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix);   // Draw the triangle
    requestAnimationFrame(tick, canvas);   // Request that the browser ?calls tick
  };
  tick();
}

function initVertexBuffers(gl) 
{
  var vertices = new Float32Array ([
    0, 0.5,   -0.5, -0.5,   0.5, -0.5
  ]);
  var n = 3;   // The number of vertices

  // Create a buffer object
  var vertexBuffer = gl.createBuffer();
  if (!vertexBuffer) 
  {
    console.log('Failed to create the buffer object');
    return -1;
  }

  // Bind the buffer object to target
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
  
  // Write date into the buffer object
  gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

  // Assign the buffer object to a_Position variable
  var a_Position = gl.getAttribLocation(gl.program, 'a_Position');
  if(a_Position < 0) 
  {
    console.log('Failed to get the storage location of a_Position');
    return -1;
  }
  gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);

  // Enable the assignment to a_Position variable
  gl.enableVertexAttribArray(a_Position);

  return n;
}

function draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix) 
{
  // Set the rotation matrix
  modelMatrix.setRotate(currentAngle, 0, 0, 1);
  modelMatrix.translate(0.35, 0, 0);
 
  // Pass the rotation matrix to the vertex shader
  gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);

  // Clear <canvas>
  gl.clear(gl.COLOR_BUFFER_BIT);

  // Draw the rectangle
  gl.drawArrays(gl.TRIANGLES, 0, n);
}

// Last time that this function was called
var g_last = Date.now();
function animate(angle) 
{
  // Calculate the elapsed time
  var now = Date.now();
  var elapsed = now - g_last;
  g_last = now;
  // Update the current rotation angle (adjusted by the elapsed time)
  var newAngle = angle + (ANGLE_STEP * elapsed) / 1000.0;
  return newAngle %= 360;
}

function up() 
{
  ANGLE_STEP += 10; 
}

function down() 
{
  ANGLE_STEP -= 10; 
}