Method TableMergeDrawChannels

external/imgui/imgui_tables.cpp:2627–2795

This function reorder draw channels based on matching clip rectangle, to facilitate merging them. Called by EndTable(). For simplicity we call it TableMergeDrawChannels() but in fact it only reorder channels + overwrite ClipRect, actual merging is done by table->DrawSplitter.Merge() which is called right after TableMergeDrawChannels(). Columns where the contents didn't stray off their local clip

//
// This function is particularly tricky to understand.. take a breath.
void ImGui::TableMergeDrawChannels(ImGuiTable* table)
{
    ImGuiContext& g = *GImGui;
    ImDrawListSplitter* splitter = table->DrawSplitter;
    const bool has_freeze_v = (table->FreezeRowsCount > 0);
    const bool has_freeze_h = (table->FreezeColumnsCount > 0);
    IM_ASSERT(splitter->_Current == 0);

    // Track which groups we are going to attempt to merge, and which channels goes into each group.
    struct MergeGroup
    {
        ImRect          ClipRect;
        int             ChannelsCount = 0;
        ImBitArrayPtr   ChannelsMask = NULL;
    };
    int merge_group_mask = 0x00;
    MergeGroup merge_groups[4];

    // Use a reusable temp buffer for the merge masks as they are dynamically sized.
    const int max_draw_channels = (4 + table->ColumnsCount * 2);
    const int size_for_masks_bitarrays_one = (int)ImBitArrayGetStorageSizeInBytes(max_draw_channels);
    g.TempBuffer.reserve(size_for_masks_bitarrays_one * 5);
    memset(g.TempBuffer.Data, 0, size_for_masks_bitarrays_one * 5);
    for (int n = 0; n < IM_COUNTOF(merge_groups); n++)
        merge_groups[n].ChannelsMask = (ImBitArrayPtr)(void*)(g.TempBuffer.Data + (size_for_masks_bitarrays_one * n));
    ImBitArrayPtr remaining_mask = (ImBitArrayPtr)(void*)(g.TempBuffer.Data + (size_for_masks_bitarrays_one * 4));

    // 1. Scan channels and take note of those which can be merged
    for (int column_n = 0; column_n < table->ColumnsCount; column_n++)
    {
        if (!IM_BITARRAY_TESTBIT(table->VisibleMaskByIndex, column_n))
            continue;
        ImGuiTableColumn* column = &table->Columns[column_n];

        const int merge_group_sub_count = has_freeze_v ? 2 : 1;
        for (int merge_group_sub_n = 0; merge_group_sub_n < merge_group_sub_count; merge_group_sub_n++)
        {
            const int channel_no = (merge_group_sub_n == 0) ? column->DrawChannelFrozen : column->DrawChannelUnfrozen;

            // Don't attempt to merge if there are multiple draw calls within the column
            ImDrawChannel* src_channel = &splitter->_Channels[channel_no];
            if (src_channel->_CmdBuffer.Size > 0 && src_channel->_CmdBuffer.back().ElemCount == 0 && src_channel->_CmdBuffer.back().UserCallback == NULL) // Equivalent of PopUnusedDrawCmd()
                src_channel->_CmdBuffer.pop_back();
            if (src_channel->_CmdBuffer.Size != 1)
                continue;

            // Find out the width of this merge group and check if it will fit in our column
            // (note that we assume that rendering didn't stray on the left direction. we should need a CursorMinPos to detect it)
            if (!(column->Flags & ImGuiTableColumnFlags_NoClip))
            {
                float content_max_x;
                if (!has_freeze_v)
                    content_max_x = ImMax(column->ContentMaxXUnfrozen, column->ContentMaxXHeadersUsed); // No row freeze
                else if (merge_group_sub_n == 0)
                    content_max_x = ImMax(column->ContentMaxXFrozen, column->ContentMaxXHeadersUsed);   // Row freeze: use width before freeze
                else
                    content_max_x = column->ContentMaxXUnfrozen;                                        // Row freeze: use width after freeze
                if (content_max_x > column->ClipRect.Max.x)