var lengthOfLongestSubstring = function(s) {
    var res = 0;
    var str = "";
    var len = s.length;
    for(var i = 0; i < len; i++) {
      var char = s.charAt(i);
      var index = str.indexOf(char);
      if(index === -1) {
        str += char;
        res = res < str.length ? str.length : res;
      } else {
        str = str.substr(index + 1) + char;
      }
    }
    return res; 
};

/**
 * Created by hushhw on 17/12/14.
 */
//RotatingTriangle.js
var VSHADER_SOURCE =
    'attribute vec4 a_Position;\n' +
    'uniform mat4 u_ModelMatrix;\n' +
    'void main() {\n' +
    'gl_Position = u_ModelMatrix * a_Position;\n' +
    '}\n';

var FSHADER_SOURCE=
    'void main(){'+
    'gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);'+
    '}';

var ANGLE_STEP = 45.0;

function main() {

    var canvas = document.getElementById("webgl");
    if (!canvas) {
        console.log("Failed to retrieve the <canvas> element");
        return;
    }

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

    if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
        console.log("Failed to initialize shaders.");
        return;
    }

    //设置顶点位置
    var n = initVertexBuffers(gl);
    if (n < 0) {
        console.log('Failed to set the positions of the vertices');
        return;
    }

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

    var u_ModelMatrix = gl.getUniformLocation(gl.program, 'u_ModelMatrix');
    if (u_ModelMatrix < 0) {
        console.log("Failed to get the storage location of u_xformMatrix");
        return;
    }
    var modelMatrix = new Matrix4();

    var currentAngle = 0.0;

    var tick = function () {
        currentAngle = animate(currentAngle);
        draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix);
        requestAnimationFrame(tick);
    };
    tick();
}

function initVertexBuffers(gl) {
    var vertices = new Float32Array(
        [0.0, 0.5, -0.5, -0.5, 0.5, -0.5]
    );
    var n=3; //点的个数

    //创建缓冲区对象
    var vertexBuffer = gl.createBuffer();
    if(!vertexBuffer){
        console.log("Failed to create thie buffer object");
        return -1;
    }

    //将缓冲区对象保存到目标上
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);

    //向缓存对象写入数据
    gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

    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;
    }

    //将缓冲区对象分配给a_Postion变量
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);

    //连接a_Postion变量与分配给它的缓冲区对象
    gl.enableVertexAttribArray(a_Position);

    return n;
}

function draw(gl, n, currentAngle, modelMatrix, u_ModelMatrix)
{
    modelMatrix.setRotate(currentAngle, 0, 0, 1);
    gl.uniformMatrix4fv( u_ModelMatrix, false, modelMatrix.elements);
    gl.clear(gl.COLOR_BUFFER_BIT);
    gl.drawArrays(gl.TRIANGLES, 0, n);
}

var g_last = Date.now();
function animate(angle)
{
    var now = Date.now();
    var elapsed = now - g_last;
    g_last = now;
    var newAngle = angle + (ANGLE_STEP * elapsed) / 1000.0;
    return newAngle %= 360;
}