// Compile a WebGL program from a vertex shader and a fragment shader
compile = (gl, vshader, fshader) => {
    var vs = gl.createShader(gl.VERTEX_SHADER);
    gl.shaderSource(vs, vshader);
    gl.compileShader(vs);
    var fs = gl.createShader(gl.FRAGMENT_SHADER);
    gl.shaderSource(fs, fshader);
    gl.compileShader(fs);
    var program = gl.createProgram();
    gl.attachShader(program, vs);
    gl.attachShader(program, fs);
    gl.linkProgram(program);
    gl.useProgram(program);
    console.log('vertex shader:', gl.getShaderInfoLog(vs) || 'OK');
    console.log('fragment shader:', gl.getShaderInfoLog(fs) || 'OK');
    console.log('program:', gl.getProgramInfoLog(program) || 'OK');
    return program;
}

// Bind a data buffer to an attribute, fill it with data and enable it
buffer = (gl, data, program, attribute, size, type) => {
    gl.bindBuffer(gl.ARRAY_BUFFER, gl.createBuffer());
    gl.bufferData(gl.ARRAY_BUFFER, data, gl.STATIC_DRAW);
    var a = gl.getAttribLocation(program, attribute);
    gl.vertexAttribPointer(a, size, type, false, 0, 0);
    gl.enableVertexAttribArray(a);
}

// Draw a box
drawBox = (gl, n, viewProjMatrix, u_MvpMatrix, u_NormalMatrix) => {

    // Compute mvp matrix
    g_mvpMatrix.set(viewProjMatrix);
    g_mvpMatrix.multiply(g_modelMatrix);
    gl.uniformMatrix4fv(u_MvpMatrix, false, g_mvpMatrix.elements);

    // Compute inverse transform
    g_normalMatrix.setInverseOf(g_modelMatrix);
    g_normalMatrix.transpose();
    gl.uniformMatrix4fv(u_NormalMatrix, false, g_normalMatrix.elements);

    // Draw
    gl.drawElements(gl.TRIANGLES, n, gl.UNSIGNED_BYTE, 0);
}