const VSHADER_SOURCE = `
attribute vec4 a_Position;

void main() {
  gl_Position = a_Position;
  gl_PointSize = 10.0;
}`;

const FSHADER_SOURCE = `
precision mediump float;
uniform vec4 u_FragColor;

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

function initVertexBuffers(gl, vertices) {
  const vertexBuffer = gl.createBuffer();
  if (!vertexBuffer) {
    console.error("Gagal dalam membuat buffer object!");
    return -1;
  }

  gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

  const a_Position = gl.getAttribLocation(gl.program, "a_Position");
  gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);
  gl.enableVertexAttribArray(a_Position);

  const n = Math.floor(vertices.length / 2);
  return n;
}

function setColor(gl, color) {
  const u_FragColor = gl.getUniformLocation(gl.program, "u_FragColor");
  gl.uniform4f(u_FragColor, ...color);
}

function drawShape(gl, mode, vertices) {
  const n = initVertexBuffers(gl, vertices);
  if (n < 0) {
    alert("Gagal dalam menginisiasi buffer object!");
    return;
  }

  gl.drawArrays(mode, 0, n);
}

function main() {
  const canvas = document.getElementById("canvas");
  const gl = canvas.getContext("webgl");

  if (!initShaderProgram(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
    alert("Gagal dalam menginisialisasi shader program!");
    return;
  }
  gl.useProgram(gl.program);

  gl.clearColor(0.0, 0.0, 0.0, 1.0);
  gl.clear(gl.COLOR_BUFFER_BIT);

  const vertices = new Float32Array([
    0.0, 0.0,
    0.5, 0.5,
    0.0, 0.7,
    -0.5, 0.5,
  ]);

  setColor(gl, [1.0, 0.0, 0.0, 1.0]);
  drawShape(gl, gl.TRIANGLE_FAN, vertices);
}