const sdf_vs_src = `#version 300 es
    in vec2 a_pos;
    in vec3 a_color;

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

    out vec2 v_texcoord;
    out vec3 v_color;
    flat out int v_vertexid;

    void main() {
        vec2 screen01 = (a_pos * u_scale + u_translation) / u_res;
        vec2 screen02 = screen01 * 2.0;
        screen02.y = 2.0 - screen02.y;
        
        v_texcoord = a_pos + vec2(0.5);
        v_vertexid = gl_VertexID;
        v_color = a_color;

        gl_Position = vec4(screen02 - 1.0, 0, 1);
    }
`;

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

    uniform sampler2D u_texture_points;
    uniform highp usampler2D u_texture_indices;

    in vec2 v_texcoord;
    in vec3 v_color;
    flat in int v_vertexid;

    out vec4 FragColor;

    void main() {
        float mindist = 99999.9;
        float th = 5.0;

        uvec4 indices = texelFetch(u_texture_indices, ivec2(v_vertexid / 6, 0), 0);

        uint v_from = indices.x;
        uint v_to = indices.y;

        for (uint i = v_from; i < v_to - uint(1); ++i) {
            vec4 a = texelFetch(u_texture_points, ivec2(i, 0), 0);
            vec4 b = texelFetch(u_texture_points, ivec2(i + uint(1), 0), 0);

            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) - th;

            mindist = min(mindist, dist);
        }

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

        FragColor = vec4(v_color * alpha, 0.1 + alpha);
        // FragColor = vec4(v_color, 1.0);
    }
`;

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 mediump float;
     
    in vec2 v_texcoord;
     
    uniform sampler2D u_texture;
    uniform bool u_outline;

    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);

    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);

    context.programs['image'] = create_program(gl, quad_vs, quad_fs);
    context.programs['sdf'] = create_program(gl, sdf_vs, sdf_fs);

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

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

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

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

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

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

    context.textures['sdf'] = {
        'points': gl.createTexture(),
        'indices': gl.createTexture()
    };

    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);
}