Restore old SOAP foam behaviour and make it work with nonstandard gravity.

src/simulation/Simulation.cpp

@@ -1365,14 +1365,6 @@ int Simulation::try_move(int i, int x, int y, int nx, int ny)
 			return 0;
 		}
 		break;
-	// SOAP slowly floats up inside OIL
-	case PT_SOAP:
-		if (parts[i].type == PT_OIL)
-		{
-			if (rng.chance(19, 20) || std::abs(parts[i].x - nx) > 3 || std::abs(parts[i].y - ny) > 3)
-				return 0;
-		}
-		break;
 	}
 
 	switch (parts[i].type)

src/simulation/SimulationData.cpp

@@ -233,8 +233,6 @@ void SimulationData::init_can_move()
 	can_move[PT_THDR][PT_THDR] = 2;
 	can_move[PT_EMBR][PT_EMBR] = 2;
 	can_move[PT_TRON][PT_SWCH] = 3;
-	can_move[PT_SOAP][PT_OIL] = 0;
-	can_move[PT_OIL][PT_SOAP] = 1;
 }
 
 const CustomGOLData *SimulationData::GetCustomGOLByRule(int rule) const

src/simulation/elements/SOAP.cpp

@@ -243,6 +243,27 @@ static int update(UPDATE_FUNC_ARGS)
 			parts[i].ctype = 1;
 			parts[i].life = 10;
 		}
+
+		//SOAP+OIL foam effect
+		for (auto rx=-2; rx<3; rx++)
+			for (auto ry=-2; ry<3; ry++)
+				if (rx || ry)
+				{
+					auto r = pmap[y+ry][x+rx];
+					if (!r)
+						continue;
+					if (TYP(r) == PT_OIL)
+					{
+						float ax, ay, gx, gy;
+
+						sim->GetGravityField(x, y, elements[PT_SOAP].Gravity, 1.0f, gx, gy);
+
+						ax = ((parts[i].vx-gx)*0.5f + parts[ID(r)].vx)/2;
+						ay = ((parts[i].vy-gy)*0.5f + parts[ID(r)].vy)/2;
+						parts[i].vx = parts[ID(r)].vx = ax;
+						parts[i].vy = parts[ID(r)].vy = ay;
+					}
+				}
 	}
 	for (auto rx = -2; rx <= 2; rx++)
 	{