Method CalcListClipping

tutorials/common/imgui/imgui.cpp:2570–2611

Legacy helper to calculate coarse clipping of large list of evenly sized items. This legacy API is not ideal because it assumes we will return a single contiguous rectangle. Prefer using ImGuiListClipper which can returns non-contiguous ranges.

// This legacy API is not ideal because it assumes we will return a single contiguous rectangle.
// Prefer using ImGuiListClipper which can returns non-contiguous ranges.
void ImGui::CalcListClipping(int items_count, float items_height, int* out_items_display_start, int* out_items_display_end)
{
    ImGuiContext& g = *GImGui;
    ImGuiWindow* window = g.CurrentWindow;
    if (g.LogEnabled)
    {
        // If logging is active, do not perform any clipping
        *out_items_display_start = 0;
        *out_items_display_end = items_count;
        return;
    }
    if (GetSkipItemForListClipping())
    {
        *out_items_display_start = *out_items_display_end = 0;
        return;
    }

    // We create the union of the ClipRect and the scoring rect which at worst should be 1 page away from ClipRect
    // We don't include g.NavId's rectangle in there (unless g.NavJustMovedToId is set) because the rectangle enlargement can get costly.
    ImRect rect = window->ClipRect;
    if (g.NavMoveScoringItems)
        rect.Add(g.NavScoringNoClipRect);
    if (g.NavJustMovedToId && window->NavLastIds[0] == g.NavJustMovedToId)
        rect.Add(WindowRectRelToAbs(window, window->NavRectRel[0])); // Could store and use NavJustMovedToRectRel

    const ImVec2 pos = window->DC.CursorPos;
    int start = (int)((rect.Min.y - pos.y) / items_height);
    int end = (int)((rect.Max.y - pos.y) / items_height);

    // When performing a navigation request, ensure we have one item extra in the direction we are moving to
    // FIXME: Verify this works with tabbing
    const bool is_nav_request = (g.NavMoveScoringItems && g.NavWindow && g.NavWindow->RootWindowForNav == window->RootWindowForNav);
    if (is_nav_request && g.NavMoveClipDir == ImGuiDir_Up)
        start--;
    if (is_nav_request && g.NavMoveClipDir == ImGuiDir_Down)
        end++;

    start = ImClamp(start, 0, items_count);
    end = ImClamp(end + 1, start, items_count);
    *out_items_display_start = start;
    *out_items_display_end = end;
}
#endif

static void ImGuiListClipper_SortAndFuseRanges(ImVector<ImGuiListClipperRange>& ranges, int offset = 0)