Readd walltransfer and toss it behind a define

This is going to be turned into a mapheader option later on to keep compat with RR maps

src/p_mobj.c

@@ -1625,7 +1625,6 @@ void P_XYMovement(mobj_t *mo)
 			S_StartSound(mo, mo->info->activesound);
 
 			//{ SRB2kart - Orbinaut, Ballhog
-			// Ballhog dies on contact with walls
 			if (mo->type == MT_ORBINAUT || mo->type == MT_BALLHOG)
 			{
 				mobj_t *fx;
@@ -1735,12 +1734,44 @@ void P_XYMovement(mobj_t *mo)
 				return;
 			}
 		}
-		if (mo->flags & MF_SLIDEME|MF_PUSHABLE)
-		{
+		if (player || mo->flags & MF_SLIDEME|MF_PUSHABLE)
+		{ // try to slide along it
+#ifdef WALLTRANSFER
+			// Wall transfer part 1.
+			pslope_t *transferslope = NULL;
+			fixed_t transfermomz = 0;
+			if (oldslope && (P_MobjFlip(mo)*(predictedz - mo->z) > 0)) // Only for moving up (relative to gravity), otherwise there's a failed launch when going down slopes and hitting walls
+			{
+				transferslope = ((mo->standingslope) ? mo->standingslope : oldslope);
+				if (((transferslope->zangle < ANGLE_180) ? transferslope->zangle : InvAngle(transferslope->zangle)) >= ANGLE_45) // Prevent some weird stuff going on on shallow slopes.
+					transfermomz = P_GetWallTransferMomZ(mo, transferslope);
+			}
+#endif
+
+
 			P_SlideMove(mo);
 			if (P_MobjWasRemoved(mo))
 				return;
 			xmove = ymove = 0;
+
+#ifdef WALLTRANSFER
+			// Wall transfer part 2.
+			if (transfermomz && transferslope) // Are we "transferring onto the wall" (really just a disguised vertical launch)?
+			{
+				angle_t relation; // Scale transfer momentum based on how head-on it is to the slope.
+				if (mo->momx || mo->momy) // "Guess" the angle of the wall you hit using new momentum
+					relation = transferslope->xydirection - R_PointToAngle2(0, 0, mo->momx, mo->momy);
+				else // Give it for free, I guess.
+					relation = ANGLE_90;
+				transfermomz = FixedMul(transfermomz,
+					abs(FINESINE((relation >> ANGLETOFINESHIFT) & FINEMASK)));
+				if (P_MobjFlip(mo)*(transfermomz - mo->momz) > 2*FRACUNIT) // Do the actual launch!
+				{
+					mo->momz = transfermomz;
+					mo->standingslope = NULL;
+				}
+			}
+#endif
 		}
 		//
 	}

src/p_slopes.c

@@ -1015,6 +1015,41 @@ void P_SlopeLaunch(mobj_t *mo)
 	mo->terrain = NULL;
 }
 
+#ifdef WALLTRANSFER
+//
+// P_GetWallTransferMomZ
+//
+// It would be nice to have a single function that does everything necessary for slope-to-wall transfer.
+// However, it needs to be seperated out in P_XYMovement to take into account momentum before and after hitting the wall.
+// This just performs the necessary calculations for getting the base vertical momentum; the horizontal is already reasonably calculated by P_SlideMove.
+fixed_t P_GetWallTransferMomZ(mobj_t *mo, pslope_t *slope)
+{
+	vector3_t slopemom, axis;
+	angle_t ang;
+
+	if (mo->standingslope->flags & SL_NOPHYSICS)
+		return 0;
+
+	// If there's physics, time for launching.
+	// Doesn't kill the vertical momentum as much as P_SlopeLaunch does.
+	ang = slope->zangle + ANG15*((slope->zangle > 0) ? 1 : -1);
+	if (ang > ANGLE_90 && ang < ANGLE_180)
+		ang = ((slope->zangle > 0) ? ANGLE_90 : InvAngle(ANGLE_90)); // hard cap of directly upwards
+
+	slopemom.x = mo->momx;
+	slopemom.y = mo->momy;
+	slopemom.z = 3*(mo->momz/2);
+
+	axis.x = -slope->d.y;
+	axis.y = slope->d.x;
+	axis.z = 0;
+
+	FV3_Rotate(&slopemom, &axis, ang >> ANGLETOFINESHIFT);
+
+	return 2*(slopemom.z/3);
+}
+#endif
+
 // Function to help handle landing on slopes
 void P_HandleSlopeLanding(mobj_t *thing, pslope_t *slope)
 {

src/p_slopes.h

@@ -101,6 +101,9 @@ boolean P_CanApplySlopePhysics(mobj_t *mo, pslope_t *slope);
 void P_QuantizeMomentumToSlope(vector3_t *momentum, pslope_t *slope);
 void P_ReverseQuantizeMomentumToSlope(vector3_t *momentum, pslope_t *slope);
 void P_SlopeLaunch(mobj_t *mo);
+#ifdef WALLTRANSFER
+fixed_t P_GetWallTransferMomZ(mobj_t *mo, pslope_t *slope);
+#endif
 void P_HandleSlopeLanding(mobj_t *thing, pslope_t *slope);
 void P_ButteredSlope(mobj_t *mo);