const simple_vs_src = `#version 300 es
    in vec2 a_pos;

    uniform vec2 u_scale;
    uniform vec2 u_res;
    uniform vec2 u_translation;
    
    out vec2 v_uv;
    flat out int v_quad_id;

    void main() {
        vec2 screen01 = (a_pos * u_scale + u_translation) / u_res;
        vec2 screen02 = screen01 * 2.0;
        screen02.y = 2.0 - screen02.y;

        int vertex_index = gl_VertexID % 6;

        if (vertex_index == 0) {
            v_uv = vec2(0.0, 0.0);
        } else if (vertex_index == 1 || vertex_index == 5) {
            v_uv = vec2(1.0, 0.0);
        } else if (vertex_index == 2 || vertex_index == 4) {
            v_uv = vec2(0.0, 1.0);
        } else {
            v_uv = vec2(1.0, 1.0);
        }

        v_quad_id = gl_VertexID / 6;

        gl_Position = vec4(screen02 - 1.0, 0.0, 1.0); 
    }
`;

const simple_fs_src = `#version 300 es
    precision highp float;
    in vec2 v_uv;
    flat in int v_quad_id;
    layout(location = 0) out vec4 FragColor;
    void main() {
        vec2 pixel = fwidth(v_uv);
        vec2 border = 2.0 * pixel;

        if (border.x <= v_uv.x && v_uv.x <= 1.0 - border.x && border.y <= v_uv.y && v_uv.y <= 1.0 - border.y) {
            discard;
        } else {
            vec3 color = vec3(float(v_quad_id * 869363 % 255) / 255.0, float(v_quad_id * 278975 % 255) / 255.0, float(v_quad_id * 587286 % 255) / 255.0);
            float alpha = 0.5;
            FragColor = vec4(color * alpha, alpha);
        }
    }
`;

const opaque_vs_src = `#version 300 es
    in vec3 a_pos; // .z is radius
    in vec4 a_line;
    
    in int a_stroke_id;

    uniform vec2 u_scale;
    uniform vec2 u_res;
    uniform vec2 u_translation;

    uniform int u_stroke_count;

    flat out int v_stroke_id;

    void main() {
        // Do not inflate quad (as opposed to the full sdf shader), thus only leaving the opaque part

        // Shrink to not include the caps
        vec2 line_dir = normalize(a_line.zw - a_line.xy);

        int vertex_index = gl_VertexID % 4;
        vec2 pos = a_pos.xy;

        if (vertex_index == 0 || vertex_index == 2) {
            // vertices on the "beginning" side of the line
            pos.xy += line_dir * a_pos.z / 2.0;
        } else {
            // on the "ending" side of the line
            pos.xy -= line_dir * a_pos.z / 2.0;
        }

        vec2 screen01 = (pos * u_scale + u_translation) / u_res;
        vec2 screen02 = screen01 * 2.0;
        screen02.y = 2.0 - screen02.y;

        v_stroke_id = a_stroke_id;

        gl_Position = vec4(screen02 - 1.0, (float(a_stroke_id) / float(u_stroke_count)) * 2.0 - 1.0, 1.0);
    }
`;

const nop_fs_src = `#version 300 es
    precision highp float;
    flat in int v_stroke_id;
    layout(location = 0) out vec4 FragColor;
    void main() {
        vec3 color = vec3(float(v_stroke_id * 3245 % 255) / 255.0, float(v_stroke_id * 7343 % 255) / 255.0, float(v_stroke_id * 5528 % 255) / 255.0);
        FragColor = vec4(color, 1.0);
    }
`;

const sdf_vs_src = `#version 300 es
    in vec2 a_a; // point from
    in vec2 a_b; // point to
    in int a_stroke_id;
    in float a_pressure;

    uniform vec2 u_scale;
    uniform vec2 u_res;
    uniform vec2 u_translation;
    uniform int u_stroke_count;
    uniform int u_stroke_texture_size;

    uniform highp usampler2D u_stroke_data;

    out vec4 v_line;
    out vec2 v_texcoord;
    out vec3 v_color;

    flat out float v_thickness;

    void main() {
        vec2 screen02; 
        float apron = 1.0; // google "futanari inflation rule 34"
        
        int stroke_data_y = a_stroke_id / u_stroke_texture_size;
        int stroke_data_x = a_stroke_id % u_stroke_texture_size;

        uvec4 stroke_data = texelFetch(u_stroke_data, ivec2(stroke_data_x, stroke_data_y), 0);
        float radius = float(stroke_data.w);

        vec2 line_dir = normalize(a_b - a_a);
        vec2 up_dir = vec2(line_dir.y, -line_dir.x);
        vec2 pixel = vec2(2.0) / u_res * apron;
        float rscale = apron / u_scale.x;

        int vertex_index = gl_VertexID % 6;

        vec2 outwards;
        vec2 origin;

        if (vertex_index == 0) {
            // "top left" aka "p1"
            origin = a_a;
            outwards = up_dir - line_dir; 
        } else if (vertex_index == 1 || vertex_index == 5) {
            // "top right" aka "p2"
            origin = a_b;
            outwards = up_dir + line_dir;
        } else if (vertex_index == 2 || vertex_index == 4) {
            // "bottom left" aka "p3"
            origin = a_a;
            outwards = -up_dir - line_dir;
        } else {
            // "bottom right" aka "p4"
            origin = a_b;
            outwards = -up_dir + line_dir;
        }
      
        vec2 pos = origin + normalize(outwards) * radius * 2.0; // doubling is to account for max possible pressure
        screen02 = (pos.xy * u_scale + u_translation) / u_res * 2.0 + outwards * pixel;
        v_texcoord = pos.xy + outwards * rscale;

        screen02.y = 2.0 - screen02.y;

        v_line = vec4(a_a, a_b);        
        v_thickness = radius * a_pressure * 2.0; // pressure 0.5 is the "neutral" pressure
        v_color = vec3(stroke_data.xyz) / 255.0;

        if (a_stroke_id >> 31 != 0) {
            screen02 += vec2(100.0); // shift offscreen
        }

        gl_Position = vec4(screen02 - 1.0, (float(a_stroke_id) / float(u_stroke_count)) * 2.0 - 1.0, 1.0);
    }
`;

const sdf_fs_src = `#version 300 es
    precision highp float;

    uniform int u_debug_mode;

    in vec4 v_line;
    in vec2 v_texcoord;
    in vec3 v_color;

    flat in float v_thickness;

    layout(location = 0) out vec4 FragColor;

    void main() {
        if (u_debug_mode == 0) {
            vec2 a = v_line.xy;
            vec2 b = v_line.zw;

            vec2 pa = v_texcoord - a.xy, ba = b.xy - a.xy;
            float h = clamp(dot(pa, ba) / dot(ba, ba), 0.0, 1.0);
            float dist = length(pa - ba * h) - v_thickness / 2.0;

            float fade = 0.5 * length(fwidth(v_texcoord));
            float alpha = 1.0 - smoothstep(-fade, fade, dist);

            FragColor = vec4(v_color * alpha, alpha);
        } else {
            FragColor = vec4(0.2, 0.0, 0.0, 0.2);
        }
    }
`;

const tquad_vs_src = `#version 300 es
    in vec2 a_pos;
    in vec2 a_texcoord;
     
    uniform vec2 u_scale;
    uniform vec2 u_res;
    uniform vec2 u_translation;
     
    out 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, 0, 1);
    }
`;

const tquad_fs_src = `#version 300 es
    precision highp float;
     
    in vec2 v_texcoord;
     
    uniform sampler2D u_texture;
    uniform bool u_outline;

    layout(location = 0) out vec4 FragColor;

    void main() {
        if (!u_outline) {
            FragColor = texture(u_texture, v_texcoord);
        } else {
            FragColor = mix(texture(u_texture, v_texcoord), vec4(0.7, 0.7, 0.95, 1), 0.5);
        }
    }
`;

function init_webgl(state, context) {
    context.canvas = document.querySelector('#c');
    context.gl = context.canvas.getContext('webgl2', {
        'preserveDrawingBuffer': true,
        'desynchronized': true,
        'antialias': false,
    });

    const gl = context.gl;

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

    gl.enable(gl.DEPTH_TEST);
    gl.depthFunc(gl.GEQUAL);

    context.gpu_timer_ext = gl.getExtension('EXT_disjoint_timer_query_webgl2');
    if (context.gpu_timer_ext === null) {
        context.gpu_timer_ext = gl.getExtension('EXT_disjoint_timer_query');
    }
    
    const quad_vs = create_shader(gl, gl.VERTEX_SHADER, tquad_vs_src);
    const quad_fs = create_shader(gl, gl.FRAGMENT_SHADER, tquad_fs_src);

    const sdf_vs = create_shader(gl, gl.VERTEX_SHADER, sdf_vs_src);
    const sdf_fs = create_shader(gl, gl.FRAGMENT_SHADER, sdf_fs_src);

    const opaque_vs = create_shader(gl, gl.VERTEX_SHADER, opaque_vs_src);
    const nop_fs = create_shader(gl, gl.FRAGMENT_SHADER, nop_fs_src);

    const simple_vs = create_shader(gl, gl.VERTEX_SHADER, simple_vs_src);
    const simple_fs = create_shader(gl, gl.FRAGMENT_SHADER, simple_fs_src);

    context.programs['image'] = create_program(gl, quad_vs, quad_fs);
    context.programs['debug'] = create_program(gl, simple_vs, simple_fs);
    context.programs['sdf'] = {
        'opaque': create_program(gl, opaque_vs, nop_fs),
        'main': create_program(gl, sdf_vs, sdf_fs),
    };

    context.locations['debug'] = {
        'a_pos':             gl.getAttribLocation(context.programs['debug'], 'a_pos'),

        'u_res':             gl.getUniformLocation(context.programs['debug'], 'u_res'),
        'u_scale':           gl.getUniformLocation(context.programs['debug'], 'u_scale'),
        'u_translation':     gl.getUniformLocation(context.programs['debug'], 'u_translation'),
    };

    context.locations['sdf'] = {
        'opaque': {
            'a_pos':             gl.getAttribLocation(context.programs['sdf'].opaque, 'a_pos'),
            'a_line':            gl.getAttribLocation(context.programs['sdf'].opaque, 'a_line'),
            'a_stroke_id':       gl.getAttribLocation(context.programs['sdf'].opaque, 'a_stroke_id'),

            'u_res':             gl.getUniformLocation(context.programs['sdf'].opaque, 'u_res'),
            'u_scale':           gl.getUniformLocation(context.programs['sdf'].opaque, 'u_scale'),
            'u_translation':     gl.getUniformLocation(context.programs['sdf'].opaque, 'u_translation'),
            'u_stroke_count':    gl.getUniformLocation(context.programs['sdf'].opaque, 'u_stroke_count'),
        },

        'main': {
            'a_a':               gl.getAttribLocation(context.programs['sdf'].main, 'a_a'),
            'a_b':               gl.getAttribLocation(context.programs['sdf'].main, 'a_b'),
            'a_stroke_id':       gl.getAttribLocation(context.programs['sdf'].main, 'a_stroke_id'),
            'a_pressure':        gl.getAttribLocation(context.programs['sdf'].main, 'a_pressure'),

            'u_res':             gl.getUniformLocation(context.programs['sdf'].main, 'u_res'),
            'u_scale':           gl.getUniformLocation(context.programs['sdf'].main, 'u_scale'),
            'u_translation':     gl.getUniformLocation(context.programs['sdf'].main, 'u_translation'),
            'u_debug_mode':      gl.getUniformLocation(context.programs['sdf'].main, 'u_debug_mode'),
            'u_stroke_count':    gl.getUniformLocation(context.programs['sdf'].main, 'u_stroke_count'),
            'u_stroke_data':     gl.getUniformLocation(context.programs['sdf'].main, 'u_stroke_data'),
            'u_stroke_texture_size': gl.getUniformLocation(context.programs['sdf'].main, 'u_stroke_texture_size'),
        }
    };

    context.buffers['debug'] = {
        'b_packed': gl.createBuffer(),
    };

    context.buffers['sdf'] = {
        'b_instance': gl.createBuffer(),
        'b_dynamic_instance': gl.createBuffer(),
    };

    context.textures = {
        'stroke_data': gl.createTexture(),
        'dynamic_stroke_data': gl.createTexture(),
    };

    gl.bindTexture(gl.TEXTURE_2D, context.textures['stroke_data']);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA16UI, config.stroke_texture_size, config.stroke_texture_size, 0, gl.RGBA_INTEGER, gl.UNSIGNED_SHORT, null);

    gl.bindTexture(gl.TEXTURE_2D, context.textures['dynamic_stroke_data']);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MIN_FILTER, gl.NEAREST);
    gl.texParameteri(gl.TEXTURE_2D, gl.TEXTURE_MAG_FILTER, gl.NEAREST);
    gl.texImage2D(gl.TEXTURE_2D, 0, gl.RGBA16UI, config.dynamic_stroke_texture_size, config.dynamic_stroke_texture_size, 0, gl.RGBA_INTEGER, gl.UNSIGNED_SHORT, null);

    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);
}