Allow multiple cave liquids in a biome definition (#8481)

This allows games to specify biome cave liquids and avoid the old
hardcoded behaviour, but preserves the ability to have multiple
cave liquids in one biome, such as lava and water.

When multiple cave liquids are defined by the biome definition,
make each entire cave use a randomly chosen liquid, instead of
every small cave segment using a randomly chosen liquid.

Plus an optimisation:
Don't place nodes if cave liquid is defined as 'air'

1 files changed, 29 insertions, 24 deletions

diff --git a/src/mapgen/cavegen.cpp b/src/mapgen/cavegen.cpp
index e54d76e08..4fa3e009d 100644
--- a/src/mapgen/cavegen.cpp
+++ b/src/mapgen/cavegen.cpp
@@ -321,9 +321,26 @@ void CavesRandomWalk::makeCave(MMVManip *vm, v3s16 nmin, v3s16 nmax,
 
 	this->ystride = nmax.X - nmin.X + 1;
 
+	flooded = ps->range(1, 2) == 2;
+
+	// If flooded:
+	// Get biome at mapchunk midpoint. If cave liquid defined for biome, use it.
+	// If defined liquid is "air", disable 'flooded' to avoid placing "air".
+	use_biome_liquid = false;
+	if (flooded && bmgn) {
+		v3s16 midp = node_min + (node_max - node_min) / v3s16(2, 2, 2);
+		Biome *biome = (Biome *)bmgn->getBiomeAtPoint(midp);
+		if (biome->c_cave_liquid[0] != CONTENT_IGNORE) {
+			use_biome_liquid = true;
+			c_biome_liquid =
+				biome->c_cave_liquid[ps->range(0, biome->c_cave_liquid.size() - 1)];
+			if (c_biome_liquid == CONTENT_AIR)
+				flooded = false;
+		}
+	}
+
 	// Set initial parameters from randomness
 	int dswitchint = ps->range(1, 14);
-	flooded = ps->range(1, 2) == 2;
 
 	if (large_cave) {
 		part_max_length_rs = ps->range(2, 4);
@@ -502,31 +519,19 @@ void CavesRandomWalk::carveRoute(v3f vec, float f, bool randomize_xz)
 	fp.Z += 0.1f * ps->range(-10, 10);
 	v3s16 cp(fp.X, fp.Y, fp.Z);
 
-	// Get biome at 'cp + of', the absolute centre point of this route
-	v3s16 cpabs = cp + of;
+	// Choose cave liquid
 	MapNode liquidnode = CONTENT_IGNORE;
 
-	if (bmgn) {
-		Biome *biome = nullptr;
-		if (cpabs.X < node_min.X || cpabs.X > node_max.X ||
-				cpabs.Z < node_min.Z || cpabs.Z > node_max.Z)
-			// Point is outside heat and humidity noise maps so use point noise
-			// calculations.
-			biome = (Biome *)bmgn->calcBiomeAtPoint(cpabs);
-		else
-			// Point is inside heat and humidity noise maps so use them
-			biome = (Biome *)bmgn->getBiomeAtPoint(cpabs);
-
-		if (biome->c_cave_liquid != CONTENT_IGNORE)
-			liquidnode = biome->c_cave_liquid;
-	}
-
-	if (liquidnode == CONTENT_IGNORE) {
-		// Fallback to classic behaviour using point 'startp'
-		float nval = NoisePerlin3D(np_caveliquids, startp.X,
-			startp.Y, startp.Z, seed);
-		liquidnode = (nval < 0.40f && node_max.Y < lava_depth) ?
-			lavanode : waternode;
+	if (flooded) {
+		if (use_biome_liquid) {
+			liquidnode = c_biome_liquid;
+		} else {
+			// If cave liquid not defined by biome, fallback to old hardcoded behaviour
+			float nval = NoisePerlin3D(np_caveliquids, startp.X,
+				startp.Y, startp.Z, seed);
+			liquidnode = (nval < 0.40f && node_max.Y < lava_depth) ?
+				lavanode : waternode;
+		}
 	}
 
 	s16 d0 = -rs / 2;