B2: add Dalamud-free CardClipPlanner variable-height clip-plan helper

HellionChat/Ui/Components/CardClipPlanner.cs

@@ -0,0 +1,74 @@
+using System.Collections.Generic;
+
+namespace HellionChat.Ui.Components;
+
+// B2 (PERF-B2): variable-height clip plan for card mode. ImGuiListClipper needs
+// a constant row height, so card mode (rows wrap to arbitrary heights) computes
+// its own plan from the cached per-row heights: a lead dummy for the rows above
+// the viewport, the [first..last] index range that overlaps the viewport, and
+// an end dummy for the rows below. This is the prefix-sum analogue of
+// OtterGui's GetNecessarySkips, but for non-uniform heights — kept Dalamud-free
+// so the Build Suite pins every edge case.
+// TEST-MIRROR: ../../../../Hellion Build test/Ui/CardClipPlanTests.cs
+internal readonly record struct CardClipPlan(
+    int FirstVisible,
+    int LastVisible,
+    float LeadDummyHeight,
+    float EndDummyHeight
+);
+
+internal static class CardClipPlanner
+{
+    // heights[i] is the cached, spacing-inclusive height of row i in draw order.
+    // A row overlaps the viewport iff rowTop < windowBottom && rowBottom >
+    // windowTop. FirstVisible/LastVisible are -1 when nothing overlaps (empty
+    // list); callers then submit a single end dummy of the full content height
+    // so the scrollbar stays correct.
+    internal static CardClipPlan Plan(
+        IReadOnlyList<float> heights,
+        float scrollY,
+        float viewportHeight
+    )
+    {
+        var count = heights.Count;
+        if (count == 0)
+            return new CardClipPlan(-1, -1, 0f, 0f);
+
+        var windowTop = scrollY;
+        var windowBottom = scrollY + viewportHeight;
+
+        var first = -1;
+        var last = -1;
+        var leadDummy = 0f;
+        var endDummy = 0f;
+
+        var cursor = 0f; // running prefix sum = top edge of the current row
+        for (var i = 0; i < count; i++)
+        {
+            var rowTop = cursor;
+            var rowBottom = cursor + heights[i];
+            var overlaps = rowTop < windowBottom && rowBottom > windowTop;
+
+            if (overlaps)
+            {
+                if (first < 0)
+                    first = i;
+                last = i;
+            }
+            else if (first < 0)
+            {
+                // Still above the visible window -> grows the lead dummy.
+                leadDummy += heights[i];
+            }
+            else
+            {
+                // Already past the visible window -> grows the end dummy.
+                endDummy += heights[i];
+            }
+
+            cursor = rowBottom;
+        }
+
+        return new CardClipPlan(first, last, leadDummy, endDummy);
+    }
+}