Mapgen: Fix biome Y calculation regression

BiomeGen::getNextTransitionY(y) did not guarantee the condition (y < biome_y_min)
of the next loop because the function may return the value (biome_y_min - 1).
Hence, the biome was not updated until one Y coordinate after.

src/mapgen/cavegen.cpp

@@ -84,7 +84,7 @@ void CavesNoiseIntersection::generateCaves(MMVManip *vm,
 		u16 depth_riverbed = biome->depth_riverbed;
 		u16 nplaced = 0;
 
-		s16 biome_y_min = m_bmgn->getNextTransitionY(nmax.Y);
+		s16 biome_y_next = m_bmgn->getNextTransitionY(nmax.Y);
 
 		// Don't excavate the overgenerated stone at nmax.Y + 1,
 		// this creates a 'roof' over the tunnel, preventing light in
@@ -94,13 +94,13 @@ void CavesNoiseIntersection::generateCaves(MMVManip *vm,
 				index3d -= m_ystride,
 				VoxelArea::add_y(em, vi, -1)) {
 			// We need this check to make sure that biomes don't generate too far down
-			if (y < biome_y_min) {
+			if (y <= biome_y_next) {
 				biome = m_bmgn->getBiomeAtIndex(index2d, v3s16(x, y, z));
-				biome_y_min = m_bmgn->getNextTransitionY(y);
+				biome_y_next = m_bmgn->getNextTransitionY(y);
 
 				if (x == nmin.X && z == nmin.Z && false) {
 					dstream << "cavegen: biome at " << y << " is " << biome->name
-						<< ", next at " << biome_y_min << std::endl;
+						<< ", next at " << biome_y_next << std::endl;
 				}
 			}
 

src/mapgen/mapgen.cpp

@@ -644,7 +644,7 @@ void MapgenBasic::generateBiomes()
 		u16 depth_riverbed = 0;
 		u32 vi = vm->m_area.index(x, node_max.Y, z);
 
-		s16 biome_y_min = biomegen->getNextTransitionY(node_max.Y);
+		s16 biome_y_next = biomegen->getNextTransitionY(node_max.Y);
 
 		// Check node at base of mapchunk above, either a node of a previously
 		// generated mapchunk or if not, a node of overgenerated base terrain.
@@ -661,23 +661,29 @@ void MapgenBasic::generateBiomes()
 
 		for (s16 y = node_max.Y; y >= node_min.Y; y--) {
 			content_t c = vm->m_data[vi].getContent();
+			const bool biome_outdated = !biome || y <= biome_y_next;
 			// Biome is (re)calculated:
 			// 1. At the surface of stone below air or water.
 			// 2. At the surface of water below air.
 			// 3. When stone or water is detected but biome has not yet been calculated.
 			// 4. When stone or water is detected just below a biome's lower limit.
 			bool is_stone_surface = (c == c_stone) &&
-				(air_above || water_above || !biome || y < biome_y_min); // 1, 3, 4
+				(air_above || water_above || biome_outdated); // 1, 3, 4
 
 			bool is_water_surface =
 				(c == c_water_source || c == c_river_water_source) &&
-				(air_above || !biome || y < biome_y_min); // 2, 3, 4
+				(air_above || biome_outdated); // 2, 3, 4
 
 			if (is_stone_surface || is_water_surface) {
-				if (!biome || y < biome_y_min) {
+				if (biome_outdated) {
 					// (Re)calculate biome
 					biome = biomegen->getBiomeAtIndex(index, v3s16(x, y, z));
-					biome_y_min = biomegen->getNextTransitionY(y);
+					biome_y_next = biomegen->getNextTransitionY(y);
+
+					if (x == node_min.X && z == node_min.Z && false) {
+						dstream << "biomegen: biome at " << y << " is " << biome->name
+							<< ", next at " << biome_y_next << std::endl;
+					}
 				}
 
 				// Add biome to biomemap at first stone surface detected

src/mapgen/mg_biome.cpp

@@ -129,7 +129,10 @@ BiomeGenOriginal::BiomeGenOriginal(BiomeManager *biomemgr,
 	for (size_t i = 0; i < m_bmgr->getNumObjects(); i++) {
 		Biome *b = (Biome *)m_bmgr->getRaw(i);
 		values.push_back(b->max_pos.Y);
-		values.push_back(b->min_pos.Y);
+		// We scan for biomes from high Y to low Y (top to bottom). Hence,
+		// biomes effectively transition at (min_pos.Y - 1).
+		if (b->min_pos.Y > -MAX_MAP_GENERATION_LIMIT)
+			values.push_back(b->min_pos.Y - 1);
 	}
 
 	std::sort(values.begin(), values.end(), std::greater<>());

src/mapgen/mg_biome.h

@@ -197,7 +197,8 @@ private:
 	Noise *noise_heat_blend;
 	Noise *noise_humidity_blend;
 
-	// ordered descending
+	/// Y values at which biomes may transition.
+	/// This array may only be used for downwards scanning!
 	std::vector<s16> m_transitions_y;
 };
 

src/unittest/test_mapgen.cpp

@@ -93,11 +93,10 @@ void TestMapgen::testBiomeGen(IGameDef *gamedef)
 			const char *name;
 			s16 next_y;
 		} expected_biomes[] = {
-			{ MAX_MAP_GENERATION_LIMIT, "deciduous_forest", 1 },
-			// ^ FIXME: next_y should be 0 (min_pos.Y - 1)
+			{ MAX_MAP_GENERATION_LIMIT, "deciduous_forest", 0 },
 			{ 1, "deciduous_forest", 0 },
-			{ 0, "deciduous_forest_shore", -MAX_MAP_GENERATION_LIMIT },
-			{ -100, "deciduous_forest_shore", -MAX_MAP_GENERATION_LIMIT },
+			{    0, "deciduous_forest_shore", S16_MIN },
+			{ -100, "deciduous_forest_shore", S16_MIN },
 		};
 		for (const auto expected : expected_biomes) {
 			Biome *biome = biomegen->getBiomeAtIndex(