Mgvalleys / cavegen: Place riverbed nodes under river water

When a CavesNoiseIntersection tunnel intersects a river place biome
'riverbed' nodes in tunnel entrance instead of biome 'top' nodes.

src/cavegen.cpp

@@ -79,6 +79,7 @@ void CavesNoiseIntersection::generateCaves(MMVManip *vm,
 	for (s16 z = nmin.Z; z <= nmax.Z; z++)
 	for (s16 x = nmin.X; x <= nmax.X; x++, index2d++) {
 		bool column_is_open = false;  // Is column open to overground
+		bool is_under_river = false;  // Is column under river water
 		bool is_tunnel = false;  // Is tunnel or tunnel floor
 		u32 vi = vm->m_area.index(x, nmax.Y, z);
 		u32 index3d = (z - nmin.Z) * m_zstride_1d + m_csize.Y * m_ystride +
@@ -99,6 +100,10 @@ void CavesNoiseIntersection::generateCaves(MMVManip *vm,
 					c == biome->c_water) {
 				column_is_open = true;
 				continue;
+			} else if (c == biome->c_river_water) {
+				column_is_open = true;
+				is_under_river = true;
+				continue;
 			}
 			// Ground
 			float d1 = contour(noise_cave1->result[index3d]);
@@ -111,9 +116,13 @@ void CavesNoiseIntersection::generateCaves(MMVManip *vm,
 			} else {
 				// Not in tunnel or not ground content
 				if (is_tunnel && column_is_open &&
-						(c == biome->c_filler || c == biome->c_stone))
+						(c == biome->c_filler || c == biome->c_stone)) {
 					// Tunnel entrance floor
-					vm->m_data[vi] = MapNode(biome->c_top);
+					if (is_under_river)
+						vm->m_data[vi] = MapNode(biome->c_riverbed);
+					else
+						vm->m_data[vi] = MapNode(biome->c_top);
+				}
 
 				column_is_open = false;
 				is_tunnel = false;

src/mapgen_valleys.cpp

@@ -665,6 +665,7 @@ void MapgenValleys::generateCaves(s16 max_stone_y, s16 large_cave_depth)
 	for (s16 x = node_min.X; x <= node_max.X; x++, index_2d++) {
 		Biome *biome = (Biome *)m_bmgr->getRaw(biomemap[index_2d]);
 		bool tunnel_air_above = false;
+		bool is_under_river = false;
 		bool underground = false;
 		u32 index_data = vm->m_area.index(x, node_max.Y, z);
 		u32 index_3d = (z - node_min.Z) * zstride_1d + csize.Y * ystride + (x - node_min.X);
@@ -696,14 +697,13 @@ void MapgenValleys::generateCaves(s16 max_stone_y, s16 large_cave_depth)
 			}
 
 			content_t c = vm->m_data[index_data].getContent();
+			// Detect river water to place riverbed nodes in tunnels
+			if (c == biome->c_river_water)
+				is_under_river = true;
+
 			float d1 = contour(noise_cave1->result[index_3d]);
 			float d2 = contour(noise_cave2->result[index_3d]);
 
-			// River water is not set as ground content
-			// in the default game. This can produce strange results
-			// when a tunnel undercuts a river. However, that's not for
-			// the mapgen to correct. Fix it in lua.
-
 			if (d1 * d2 > cave_width && ndef->get(c).is_ground_content) {
 				// in a tunnel
 				vm->m_data[index_data] = n_air;
@@ -716,8 +716,10 @@ void MapgenValleys::generateCaves(s16 max_stone_y, s16 large_cave_depth)
 					vm->m_area.add_y(em, j, 1);
 
 					if (sr > terrain - y) {
-						// Put dirt in tunnels near the surface.
-						if (underground)
+						// Put biome nodes in tunnels near the surface
+						if (is_under_river)
+							vm->m_data[index_data] = MapNode(biome->c_riverbed);
+						else if (underground)
 							vm->m_data[index_data] = MapNode(biome->c_filler);
 						else
 							vm->m_data[index_data] = MapNode(biome->c_top);