Implement check_offset for decorations

doc/lua_api.md

@@ -9564,8 +9564,13 @@ See [Decoration types]. Used by `minetest.register_decoration`.
 
     spawn_by = "default:water",
     -- Node (or list of nodes) that the decoration only spawns next to.
-    -- Checks the 8 neighboring nodes on the same Y, and also the ones
-    -- at Y+1, excluding both center nodes.
+    -- Checks the 8 neighboring nodes on the same height,
+    -- and also the ones at the height plus the check_offset, excluding both center nodes.
+
+    check_offset = -1,
+    -- Specifies the offset that spawn_by should also check
+    -- The default value of -1 is useful to e.g check for water next to the base node.
+    -- 0 disables additional checks, valid values: {-1, 0, 1}
 
     num_spawn_by = 1,
     -- Number of spawn_by nodes that must be surrounding the decoration

src/mapgen/mg_decoration.cpp

@@ -87,6 +87,9 @@ void Decoration::resolveNodeNames()
 
 bool Decoration::canPlaceDecoration(MMVManip *vm, v3s16 p)
 {
+	// Note that `p` refers to the node the decoration will be placed ontop of,
+	// not to the decoration itself.
+
 	// Check if the decoration can be placed on this node
 	u32 vi = vm->m_area.index(p);
 	if (!CONTAINS(c_place_on, vm->m_data[vi].getContent()))
@@ -97,16 +100,7 @@ bool Decoration::canPlaceDecoration(MMVManip *vm, v3s16 p)
 		return true;
 
 	int nneighs = 0;
-	static const v3s16 dirs[16] = {
-		v3s16( 0, 0,  1),
-		v3s16( 0, 0, -1),
-		v3s16( 1, 0,  0),
-		v3s16(-1, 0,  0),
-		v3s16( 1, 0,  1),
-		v3s16(-1, 0,  1),
-		v3s16(-1, 0, -1),
-		v3s16( 1, 0, -1),
-
+	static const v3s16 dirs[8] = {
 		v3s16( 0, 1,  1),
 		v3s16( 0, 1, -1),
 		v3s16( 1, 1,  0),
@@ -117,6 +111,7 @@ bool Decoration::canPlaceDecoration(MMVManip *vm, v3s16 p)
 		v3s16( 1, 1, -1)
 	};
 
+
 	// Check these 16 neighboring nodes for enough spawnby nodes
 	for (size_t i = 0; i != ARRLEN(dirs); i++) {
 		u32 index = vm->m_area.index(p + dirs[i]);
@@ -127,6 +122,19 @@ bool Decoration::canPlaceDecoration(MMVManip *vm, v3s16 p)
 			nneighs++;
 	}
 
+	if (check_offset != 0) {
+		const v3s16 dir_offset(0, check_offset,  0);
+
+		for (size_t i = 0; i != ARRLEN(dirs); i++) {
+			u32 index = vm->m_area.index(p + dirs[i] + dir_offset);
+			if (!vm->m_area.contains(index))
+				continue;
+
+			if (CONTAINS(c_spawnby, vm->m_data[index].getContent()))
+				nneighs++;
+		}
+
+	}
 	if (nneighs < nspawnby)
 		return false;
 
@@ -280,6 +288,7 @@ void Decoration::cloneTo(Decoration *def) const
 	def->flags = flags;
 	def->mapseed = mapseed;
 	def->c_place_on = c_place_on;
+	def->check_offset = check_offset;
 	def->sidelen = sidelen;
 	def->y_min = y_min;
 	def->y_max = y_max;

src/mapgen/mg_decoration.h

@@ -73,6 +73,7 @@ class Decoration : public ObjDef, public NodeResolver {
 	std::vector<content_t> c_spawnby;
 	s16 nspawnby;
 	s16 place_offset_y = 0;
+	s16 check_offset = -1;
 
 	std::unordered_set<biome_t> biomes;
 

src/script/lua_api/l_mapgen.cpp

@@ -1097,6 +1097,7 @@ int ModApiMapgen::l_register_decoration(lua_State *L)
 	deco->y_max          = getintfield_default(L, index, "y_max", 31000);
 	deco->nspawnby       = getintfield_default(L, index, "num_spawn_by", -1);
 	deco->place_offset_y = getintfield_default(L, index, "place_offset_y", 0);
+	deco->check_offset   = getintfield_default(L, index, "check_offset", -1);
 	deco->sidelen        = getintfield_default(L, index, "sidelen", 8);
 	if (deco->sidelen <= 0) {
 		errorstream << "register_decoration: sidelen must be "
@@ -1131,6 +1132,10 @@ int ModApiMapgen::l_register_decoration(lua_State *L)
 		errorstream << "register_decoration: no spawn_by nodes defined,"
 			" but num_spawn_by specified" << std::endl;
 	}
+	if (deco->check_offset < -1 || deco->check_offset > 1) {
+		delete deco;
+		luaL_error(L, "register_decoration: check_offset out of range!  Allowed values: [-1, 0, 1]");
+	}
 
 	//// Handle decoration type-specific parameters
 	bool success = false;