desk2/client/speed.js

async function init_wasm(state) {
    const results = await WebAssembly.instantiateStreaming(fetch('wasm/lod.wasm'));
    state.wasm.exports = results.instance.exports;
    state.wasm.exports.memory.grow(4096);
}

function rdp_find_max(state, zoom, coords_from, start, end) {
    // Finds a point from the range [start, end) with the maximum distance from the line (start--end) that is also further than EPS
    const EPS = 1.0 / zoom;

    let result = -1;
    let max_dist = 0;

    const ax = state.coordinates.data[coords_from + start * 2 + 0];
    const ay = state.coordinates.data[coords_from + start * 2 + 1];
    const bx = state.coordinates.data[coords_from + end * 2 + 0];
    const by = state.coordinates.data[coords_from + end * 2 + 1];

    const dx = bx - ax;
    const dy = by - ay;

    const dist_ab = Math.sqrt(dx * dx + dy * dy);
    const dir_nx = dy / dist_ab;
    const dir_ny = -dx / dist_ab;

    for (let i = start + 1; i < end; ++i) {
        const px = state.coordinates.data[coords_from + i * 2 + 0];
        const py = state.coordinates.data[coords_from + i * 2 + 1];
        
        const apx = px - ax;
        const apy = py - ay;

        const dist = Math.abs(apx * dir_nx + apy * dir_ny);

        if (dist > EPS && dist > max_dist) {
            result = i;
            max_dist = dist;
        }
    }

    state.stats.rdp_max_count++;
    state.stats.rdp_segments += end - start - 1;

    return result;
}

function do_lod_wasm(state, context) {
    state.wasm.exports.total_free();

    const clipped_indices = state.wasm.exports.alloc(context.clipped_indices.size * 4); 
    const stroke_coords_from = state.wasm.exports.alloc(state.coords_from.size * 4); 
    const stroke_coords_to = state.wasm.exports.alloc(state.coords_to.size * 4); 
    const line_threshold = state.wasm.exports.alloc(state.line_threshold.size * 4);
    const segments_from = state.wasm.exports.alloc((context.clipped_indices.size + 1) * 4); 
    const segments = state.wasm.exports.alloc(state.segments.capacity * 4); 
    const coordinates = state.wasm.exports.alloc(state.coordinates.size * 4);
    const mem = new Uint8Array(state.wasm.exports.memory.buffer);

    mem.set(tv_bytes(context.clipped_indices), clipped_indices);
    mem.set(tv_bytes(state.coords_from), stroke_coords_from);
    mem.set(tv_bytes(state.coords_to), stroke_coords_to);
    mem.set(tv_bytes(state.line_threshold), line_threshold);
    mem.set(tv_bytes(state.coordinates), coordinates);

    const segment_count = state.wasm.exports.do_lod(
        clipped_indices, context.clipped_indices.size, state.canvas.zoom,
        stroke_coords_from, stroke_coords_to,
        line_threshold, coordinates,
        segments_from, segments
    );

    // copy result back
    const wasm_points = new Float32Array(state.wasm.exports.memory.buffer, coordinates + state.coordinates.size * 4, segment_count * 2);
    const wasm_ids = new Uint32Array(state.wasm.exports.memory.buffer, coordinates + (state.coordinates.size + segment_count * 2) * 4, segment_count);
    
    context.instance_data_points.data = wasm_points;
    context.instance_data_points.size = segment_count * 2;

    context.instance_data_ids.data = wasm_ids;
    context.instance_data_ids.size = segment_count;

    return segment_count;
}

function do_lod(state, context) {
    if (state.debug.use_wasm) {
        return do_lod_wasm(state, context);
    }

    const zoom = state.canvas.zoom;
    const segments_data = state.segments.data;
    
    let segments_head = 0;

    for (let i = 0; i < context.clipped_indices.size; ++i) {
        const stroke_index = context.clipped_indices.data[i];
        const stroke = state.events[stroke_index];
        const point_count = (stroke.coords_to - stroke.coords_from) / 2;
        const coords_from = stroke.coords_from;

        if (point_count > state.rdp_traverse_stack.length) {
            //console.size('allocate')
            state.rdp_traverse_stack = new Uint32Array(round_to_pow2(point_count, 4096));
        }

        const stack = state.rdp_traverse_stack;

        // Basic CSR crap
        state.segments_from.data[i] = segments_head;

        if (state.canvas.zoom <= state.line_threshold.data[stroke_index]) {
            segments_data[segments_head++] = 0;
            segments_data[segments_head++] = point_count - 1;
        } else {
            let segment_count = 2;

            segments_data[segments_head++] = 0;
            
            let head = 0;
            // Using stack.push() allocates even if the stack is pre-allocated!
            
            stack[head++] = 0;
            stack[head++] = 0;
            stack[head++] = point_count - 1;

            while (head > 0) {
                const end = stack[--head];
                const value = start = stack[--head];
                const type = stack[--head];

                if (type === 1) {
                    segments_data[segments_head++] = value;
                } else {
                    const max = rdp_find_max(state, zoom, coords_from, start, end);
                    if (max !== -1) {
                        segment_count += 1;
                        
                        stack[head++] = 0;
                        stack[head++] = max;
                        stack[head++] = end;

                        stack[head++] = 1;
                        stack[head++] = max;
                        stack[head++] = -1;
                        
                        stack[head++] = 0;
                        stack[head++] = start;
                        stack[head++] = max;
                    }
                }
            }

            segments_data[segments_head++] = point_count - 1;

            if (segment_count === 2 && state.canvas.zoom > state.line_threshold.data[stroke_index]) {
                state.line_threshold.data[stroke_index] = state.canvas.zoom;
            }
        }
    }

    state.segments_from.data[context.clipped_indices.size] = segments_head;
    state.segments_from.size = context.clipped_indices.size + 1;
    state.segments.size = segments_head;

    write_coordinates(state, context); 

    return segments_head;
}

function write_coordinates(state, context) {
    tv_ensure(context.instance_data_points, state.segments.size * 2);
    tv_ensure(context.instance_data_ids, state.segments.size);

    tv_clear(context.instance_data_points);
    tv_clear(context.instance_data_ids);

    const clipped = context.clipped_indices.data;
    const segments_from = state.segments_from.data;
    const segments = state.segments.data;
    const coords = state.coordinates.data;
    const events = state.events;
    
    // TODO: move this loop to WASM
    for (let i = 0; i < state.segments_from.size - 1; ++i) {
        const stroke_index = clipped[i];
        const coords_from = state.events[stroke_index].coords_from;
        const from = segments_from[i];
        const to = segments_from[i + 1];

        for (let j = from; j < to; ++j) {
            const base_this = segments[j];

            const ax = coords[coords_from + base_this * 2 + 0];
            const ay = coords[coords_from + base_this * 2 + 1];
           
            tv_add(context.instance_data_points, ax);
            tv_add(context.instance_data_points, ay);

            // Pack 1 into highest bit of stroke_index if we should not draw a segemtn from this
            // point to the next one
            if (j != to - 1) {
                tv_add(context.instance_data_ids, stroke_index);
            } else {
                tv_add(context.instance_data_ids, stroke_index | (1 << 31));
            }
        }
    }
}