import { initBuffers } from "./init-buffers.js";
import { drawScene } from "./draw-scene.js";

main();

/* TODO: Avoid using alerts! Check return values instead,
 * TODO: or create/use a Result like object.
 */

// 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 program = gl.createProgram();
  gl.attachShader(program, vertexShader);
  gl.attachShader(program, fragmentShader);
  gl.linkProgram(program);

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

  return program;
}

// Creates a shader of the given type, uploads the source and compiles
function loadShader(gl, type, source) {
  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;
}

function main() {
  const canvas = document.querySelector("#gl-canvas");
  // Initialize the GL context
  const gl = canvas.getContext("webgl");

  // Only continue if WebGL is available and working
  if (gl === null) {
    alert(
      "Unable to initialize WebGL. Your browser or machine may not support it.",
    );
    return;
  }

  // Vertex shader program
  const vsSource = `
      attribute vec4 aVertexPosition;
      uniform mat4 uModelViewMatrix;
      uniform mat4 uProjectionMatrix;
      void main() {
        gl_Position = uProjectionMatrix * uModelViewMatrix * aVertexPosition;
      }
    `;

  const fsSource = `
      void main() {
        gl_FragColor = vec4(1.0, 1.0, 1.0, 1.0);
      }
    `;
  
  const shaderProgram = initShaderProgram(gl, vsSource, fsSource);

  // Collect all the info needed to use the shader program.
  // Look up which attribute our shader program is using
  // for aVertexPosition and 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"),
    },
  };

  // Here's where we call the routine that builds all the
  // objects we'll be drawing.
  const buffers = initBuffers(gl);

  // Draw the scene
  drawScene(gl, programInfo, buffers);
}