From 4d4a80891feb7e249fdc28ba126a4c13edbb95f3 Mon Sep 17 00:00:00 2001
From: Pheenoh <pheenoh@gmail.com>
Date: Fri, 8 May 2026 20:21:37 -0600
Subject: [PATCH] frame interp: fix obj_fchain

---
 include/d/actor/d_a_obj_fchain.h | 12 ++++++++++
 src/d/actor/d_a_obj_fchain.cpp   | 39 ++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/include/d/actor/d_a_obj_fchain.h b/include/d/actor/d_a_obj_fchain.h
index 1bd7b9a810..7c4ae74faf 100644
--- a/include/d/actor/d_a_obj_fchain.h
+++ b/include/d/actor/d_a_obj_fchain.h
@@ -31,6 +31,10 @@ public:
     csXyz* getAngle() { return field_0x8a4; }
     J3DModelData* getModelData() { return mModelData; }
 
+#if TARGET_PC
+    void onInterpCallback();
+#endif
+
 private:
     /* 0x568 */ request_of_phase_process_class mPhase;
     /* 0x570 */ J3DModelData* mModelData;
@@ -42,6 +46,14 @@ private:
     /* 0x694 */ cXyz field_0x694[22];
     /* 0x79C */ cXyz field_0x79c[22];
     /* 0x8A4 */ csXyz field_0x8a4[22];
+
+#if TARGET_PC
+    static const int CHAIN_COUNT = 22;
+    cXyz mChainInterpPrev[CHAIN_COUNT];
+    cXyz mChainInterpCurr[CHAIN_COUNT];
+    bool mChainInterpPrevValid;
+    bool mChainInterpCurrValid;
+#endif
 };
 
 STATIC_ASSERT(sizeof(daObjFchain_c) == 0x928);
diff --git a/src/d/actor/d_a_obj_fchain.cpp b/src/d/actor/d_a_obj_fchain.cpp
index e681fec4e7..024b4ecf8b 100644
--- a/src/d/actor/d_a_obj_fchain.cpp
+++ b/src/d/actor/d_a_obj_fchain.cpp
@@ -10,6 +10,8 @@
 #include "JSystem/J3DGraphBase/J3DDrawBuffer.h"
 #include "SSystem/SComponent/c_math.h"
 #include "d/d_com_inf_game.h"
+#include "dusk/frame_interpolation.h"
+#include "dusk/settings.h"
 #include <cstring>
 
 static char const l_arcName[] = "Fchain";
@@ -65,6 +67,10 @@ int daObjFchain_c::create() {
             local_48++;
         }
         rv = cPhs_COMPLEATE_e;
+#if TARGET_PC
+        mChainInterpPrevValid = false;
+        mChainInterpCurrValid = false;
+#endif
         break;
     }
     return rv;
@@ -289,6 +295,26 @@ void daObjFchain_shape_c::draw() {
     }
 }
 
+#if TARGET_PC
+static void fchain_interp_callback(bool isSimFrame, void* pUserWork) {
+    static_cast<daObjFchain_c*>(pUserWork)->onInterpCallback();
+}
+
+void daObjFchain_c::onInterpCallback() {
+    if (!mChainInterpPrevValid || !mChainInterpCurrValid) {
+        return;
+    }
+
+    const f32 alpha = dusk::frame_interp::get_interpolation_step();
+
+    for (int i = 0; i < CHAIN_COUNT; i++) {
+        const cXyz& p0 = mChainInterpPrev[i];
+        const cXyz& p1 = mChainInterpCurr[i];
+        field_0x694[i] = p0 + (p1 - p0) * alpha;
+    }
+}
+#endif
+
 int daObjFchain_c::draw() {
     if (field_0x584 != 0) {
         g_env_light.settingTevStruct(0, &current.pos, &tevStr);
@@ -297,6 +323,19 @@ int daObjFchain_c::draw() {
             return 1;
         }
         dComIfGd_getOpaListDark()->entryImm(&mShape, 0);
+
+#if TARGET_PC
+        if (dusk::getSettings().game.enableFrameInterpolation) {
+            if (mChainInterpCurrValid) {
+                memcpy(mChainInterpPrev, mChainInterpCurr, sizeof(mChainInterpCurr));
+                mChainInterpPrevValid = true;
+            }
+
+            memcpy(mChainInterpCurr, field_0x694, sizeof(mChainInterpCurr));
+            mChainInterpCurrValid = true;
+            dusk::frame_interp::add_interpolation_callback(&fchain_interp_callback, this);
+        }
+#endif
     }
     return 1;
 }