From ca7ae45518f8e51a70da0d3e5302e6337704894f Mon Sep 17 00:00:00 2001
From: Erik <erik.nihlen@gmail.com>
Date: Mon, 13 Apr 2026 12:31:32 +0200
Subject: [PATCH] fix(anim): handle reversed frame range in BuildBlendedFrame

Backward-playback nodes (TurnLeft, SideStepLeft) have LowFrame >
HighFrame after multiply_framerate swaps them. Math.Clamp(x, 19, 0)
throws because min > max.

Fix: compute rangeLo/rangeHi from Min/Max of Low/High, use those
for clamping. Also step nextIdx in the playback direction (forward
+1, backward -1) instead of always +1.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
---
 .../Physics/AnimationSequencer.cs             | 23 +++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/src/AcDream.Core/Physics/AnimationSequencer.cs b/src/AcDream.Core/Physics/AnimationSequencer.cs
index 4c2226f..12f1abf 100644
--- a/src/AcDream.Core/Physics/AnimationSequencer.cs
+++ b/src/AcDream.Core/Physics/AnimationSequencer.cs
@@ -487,11 +487,26 @@ public sealed class AnimationSequencer
         int numPartFrames = curr.Anim.PartFrames.Count;
 
         int frameIdx = (int)Math.Floor(_frameNum);
-        frameIdx = Math.Clamp(frameIdx, curr.LowFrame, Math.Min(curr.HighFrame, numPartFrames - 1));
+        // For backward playback, LowFrame > HighFrame. Use actual min/max
+        // of the two to get a valid range for clamping.
+        int rangeLo = Math.Min(curr.LowFrame, curr.HighFrame);
+        int rangeHi = Math.Min(Math.Max(curr.LowFrame, curr.HighFrame), numPartFrames - 1);
+        frameIdx = Math.Clamp(frameIdx, rangeLo, rangeHi);
 
-        int nextIdx = frameIdx + 1;
-        if (nextIdx > curr.HighFrame || nextIdx >= numPartFrames)
-            nextIdx = curr.LowFrame;
+        // Next frame for interpolation: step in the playback direction.
+        int nextIdx;
+        if (curr.Framerate >= 0f)
+        {
+            nextIdx = frameIdx + 1;
+            if (nextIdx > rangeHi || nextIdx >= numPartFrames)
+                nextIdx = rangeLo;  // wrap forward
+        }
+        else
+        {
+            nextIdx = frameIdx - 1;
+            if (nextIdx < rangeLo)
+                nextIdx = rangeHi;  // wrap backward
+        }
 
         float t = _frameNum - (float)Math.Floor(_frameNum);
         if (t < 0f) t = 0f;