static size_t calc_n_verts(ImDrawData *draw_data) {
    size_t total = 0;

    for (int i = 0; i < draw_data->CmdListsCount; i++) {
        auto cmd_list = draw_data->CmdLists[i];

        for (int j = 0; j < cmd_list->CmdBuffer.Size; j++) {
            auto pcmd = &cmd_list->CmdBuffer[j];

            if (!pcmd->UserCallback) {
                total += pcmd->ElemCount;
            }
        }
    }

    return total;
}

static glm::vec4 rgba_to_vec(uint32_t rgba) {
    float r, g, b, a;
    r = static_cast<float>(rgba >> 24) / 255.f;
    g = static_cast<float>((rgba >> 16) & 255) / 255.f;
    b = static_cast<float>((rgba >> 8) & 255) / 255.f;
    a = static_cast<float>(rgba & 255) / 255.f;

    return {r, g, b, a};
}

size_t imgui_drawer::generate_mesh(ImDrawData *draw_data, vertex *verts) {
    size_t total = 0;

    for (int i = 0; i < draw_data->CmdListsCount; i++) {
        auto cmd_list = draw_data->CmdLists[i];

        for (int j = 0; j < cmd_list->CmdBuffer.Size; j++) {
            auto pcmd = &cmd_list->CmdBuffer[j];

            if (pcmd->UserCallback)
                continue;

            assert(reinterpret_cast<uintptr_t>(pcmd->TextureId) == _tex.get_id());
            for (size_t n = 0; n < pcmd->ElemCount; n++) {
                vertex &v = verts[n];
                ImDrawVert &iv = cmd_list->VtxBuffer.Data[cmd_list->IdxBuffer.Data[pcmd->IdxOffset + n]];
                v.pos = {iv.pos.x, iv.pos.y, 0.f};
                v.uv = {iv.uv.x, iv.uv.y};
                v.color = rgba_to_vec(iv.col);
            }

            verts += pcmd->ElemCount;
            total += pcmd->ElemCount;
        }
    }

    return total;
}

void imgui_drawer::update_mesh() {
    size_t n_verts = calc_n_verts(ImGui::GetDrawData());

    _mesh.resize(n_verts);
    _mesh.update_sync(&imgui_drawer::generate_mesh, this, ImGui::GetDrawData());
}