const vertex_shader_source = `
    attribute vec2 a_pos;
    attribute vec3 a_color;

    uniform vec2 u_scale;
    uniform vec2 u_res;
    uniform vec2 u_translation;
    uniform int u_layer;

    varying vec3 v_color;

    void main() {
        vec2 screen01 = (a_pos * u_scale + u_translation) / u_res;
        vec2 screen02 = screen01 * 2.0;
        screen02.y = 2.0 - screen02.y;
        vec2 screen11 = screen02 - 1.0;
        v_color = a_color;
        gl_Position = vec4(screen11, u_layer, 1);
    }
`;

const fragment_shader_source = `
    precision mediump float;

    varying vec3 v_color;

    void main() {
        gl_FragColor = vec4(v_color, 1.0);
    }
`;

function init_webgl(state, context) {
    context.canvas = document.querySelector('#c');
    context.gl = context.canvas.getContext('webgl', {
        'preserveDrawingBuffer': true,
        'desynchronized': true,
        'antialias': true,
    });

    context.gl.enable(context.gl.BLEND);
    context.gl.blendFunc(context.gl.ONE, context.gl.ONE_MINUS_SRC_ALPHA);

    const vertex_shader = create_shader(context.gl, context.gl.VERTEX_SHADER, vertex_shader_source);
    const fragment_shader = create_shader(context.gl, context.gl.FRAGMENT_SHADER, fragment_shader_source);
    const program = create_program(context.gl, vertex_shader, fragment_shader)

    context.program = program;

    context.locations['a_pos']         = context.gl.getAttribLocation(program, 'a_pos');
    context.locations['a_color']       = context.gl.getAttribLocation(program, 'a_color');

    context.locations['u_res']         = context.gl.getUniformLocation(program, 'u_res');
    context.locations['u_scale']       = context.gl.getUniformLocation(program, 'u_scale');
    context.locations['u_translation'] = context.gl.getUniformLocation(program, 'u_translation');
    context.locations['u_layer']       = context.gl.getUniformLocation(program, 'u_layer');

    context.buffers['b_pos']   = context.gl.createBuffer();
    context.buffers['b_color'] = context.gl.createBuffer();

    const resize_canvas = (entries) => {
        // https://www.khronos.org/webgl/wiki/HandlingHighDPI
        const entry = entries[0];

        let width;
        let height;

        if (entry.devicePixelContentBoxSize) {
            width = entry.devicePixelContentBoxSize[0].inlineSize;
            height = entry.devicePixelContentBoxSize[0].blockSize;
        } else if (entry.contentBoxSize) {
            // fallback for Safari that will not always be correct
            width = Math.round(entry.contentBoxSize[0].inlineSize * devicePixelRatio);
            height = Math.round(entry.contentBoxSize[0].blockSize * devicePixelRatio);
        }

        context.canvas.width = width;
        context.canvas.height = height;

        window.requestAnimationFrame(() => draw(state, context));
    }

    const resize_observer = new ResizeObserver(resize_canvas);
    resize_observer.observe(context.canvas);
}

function create_shader(gl, type, source) {
    const shader = gl.createShader(type);
    
    gl.shaderSource(shader, source);
    gl.compileShader(shader);
    
    if (gl.getShaderParameter(shader, gl.COMPILE_STATUS)) {
        return shader;
    }
     
    console.error(type, ':', gl.getShaderInfoLog(shader));

    gl.deleteShader(shader);
}

function create_program(gl, vs, fs) {
    const program = gl.createProgram();

    gl.attachShader(program, vs);
    gl.attachShader(program, fs);
    gl.linkProgram(program);

    if (gl.getProgramParameter(program, gl.LINK_STATUS)) {
        return program;
    }
    
    console.error('link:', gl.getProgramInfoLog(program));

    gl.deleteProgram(program);
}