Fix spectator hand jittering in 1.12.2 and below

versions/1.12.2/src/main/java/com/replaymod/replay/mixin/Mixin_FixHandOffsetTickDelta.java

@@ -0,0 +1,46 @@
+package com.replaymod.replay.mixin;
+
+import com.replaymod.replay.camera.CameraEntity;
+import net.minecraft.client.entity.EntityPlayerSP;
+import net.minecraft.client.renderer.ItemRenderer;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Redirect;
+
+/**
+ * In 1.12.2 and below, Vanilla does not respect the tickDelta value when getting the yaw/pitch of the player for
+ * computing the hand offset.
+ * This causes the hand movement to be jittery when spectating another player in a replay.
+ */
+@Mixin(ItemRenderer.class)
+public abstract class Mixin_FixHandOffsetTickDelta {
+    @Redirect(method = "rotateArm", at = @At(value = "FIELD", target = "Lnet/minecraft/client/entity/EntityPlayerSP;rotationYaw:F"))
+    private float getYaw(
+            EntityPlayerSP instance,
+            //#if MC<10900
+            //$$ EntityPlayerSP arg,
+            //#endif
+            float tickDelta
+    ) {
+        if (instance instanceof CameraEntity) {
+            return instance.prevRotationYaw + (instance.rotationYaw - instance.prevRotationYaw) * tickDelta;
+        } else {
+            return instance.rotationYaw;
+        }
+    }
+
+    @Redirect(method = "rotateArm", at = @At(value = "FIELD", target = "Lnet/minecraft/client/entity/EntityPlayerSP;rotationPitch:F"))
+    private float getPitch(
+            EntityPlayerSP instance,
+            //#if MC<10900
+            //$$ EntityPlayerSP arg,
+            //#endif
+            float tickDelta
+    ) {
+        if (instance instanceof CameraEntity) {
+            return instance.prevRotationPitch + (instance.rotationPitch - instance.prevRotationPitch) * tickDelta;
+        } else {
+            return instance.rotationPitch;
+        }
+    }
+}