const stroke_vs_src = `
    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 stroke_fs_src = `
    precision mediump float;

    varying vec3 v_color;

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

const tquad_vs_src = `
    attribute vec2 a_pos;
    attribute vec2 a_texcoord;
     
    uniform vec2 u_scale;
    uniform vec2 u_res;
    uniform vec2 u_translation;
    uniform int u_layer;
     
    varying vec2 v_texcoord;
     
    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_texcoord = a_texcoord;
        gl_Position = vec4(screen11, u_layer, 1);
    }
`;

const tquad_fs_src = `
    precision mediump float;
     
    varying vec2 v_texcoord;
     
    uniform sampler2D u_texture;
     
    void main() {
       gl_FragColor = texture2D(u_texture, v_texcoord);
    }
`;

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

    const gl = context.gl;

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

    const stroke_vs = create_shader(gl, gl.VERTEX_SHADER, stroke_vs_src);
    const stroke_fs = create_shader(gl, gl.FRAGMENT_SHADER, stroke_fs_src);

    const quad_vs = create_shader(gl, gl.VERTEX_SHADER, tquad_vs_src);
    const quad_fs = create_shader(gl, gl.FRAGMENT_SHADER, tquad_fs_src);

    context.programs['stroke'] = create_program(gl, stroke_vs, stroke_fs);
    context.programs['quad'] = create_program(gl, quad_vs, quad_fs);

    context.locations['stroke'] = {
        'a_pos':         gl.getAttribLocation(context.programs['stroke'], 'a_pos'),
        'a_color':       gl.getAttribLocation(context.programs['stroke'], 'a_color'),
        'u_res':         gl.getUniformLocation(context.programs['stroke'], 'u_res'),
        'u_scale':       gl.getUniformLocation(context.programs['stroke'], 'u_scale'),
        'u_translation': gl.getUniformLocation(context.programs['stroke'], 'u_translation'),
        'u_layer':       gl.getUniformLocation(context.programs['stroke'], 'u_layer'),
    };

    context.locations['quad'] = {
        'a_pos':         gl.getAttribLocation(context.programs['quad'], 'a_pos'),
        'a_texcoord':    gl.getAttribLocation(context.programs['quad'], 'a_texcoord'),
        'u_res':         gl.getUniformLocation(context.programs['quad'], 'u_res'),
        'u_scale':       gl.getUniformLocation(context.programs['quad'], 'u_scale'),
        'u_translation': gl.getUniformLocation(context.programs['quad'], 'u_translation'),
        'u_layer':       gl.getUniformLocation(context.programs['quad'], 'u_layer'),
    };

    context.buffers['stroke'] = {
        'b_pos': context.gl.createBuffer(),
        'b_color': context.gl.createBuffer(),
    };

    context.buffers['quad'] = {
        'b_pos': context.gl.createBuffer(),
        'b_texcoord': 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;

        schedule_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);
}