Mgvalleys: Make river depth variation and humidity drop optional (#7532)

Add 2 new mapgen flags to make river depth variation and humidity drop
with altitude independently optional, instead of both being enabled by
the 'humid rivers' flag.

Simplify and clarify related code by removing a low priority
optimisation regarding 't_heat'.
Remove unnecessary optimisation bools and use spflags directly instead.
Improve and fix documentation in settingtypes.txt.
A few minor code cleanups.

2 files changed, 47 insertions, 45 deletions

diff --git a/src/mapgen/mapgen_valleys.cpp b/src/mapgen/mapgen_valleys.cpp
index 341ce3137..0a0f93a75 100644
--- a/src/mapgen/mapgen_valleys.cpp
+++ b/src/mapgen/mapgen_valleys.cpp
@@ -44,10 +44,12 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include <cmath>
 
 
-static FlagDesc flagdesc_mapgen_valleys[] = {
-	{"altitude_chill", MGVALLEYS_ALT_CHILL},
-	{"humid_rivers",   MGVALLEYS_HUMID_RIVERS},
-	{NULL,             0}
+FlagDesc flagdesc_mapgen_valleys[] = {
+	{"altitude_chill",   MGVALLEYS_ALT_CHILL},
+	{"humid_rivers",     MGVALLEYS_HUMID_RIVERS},
+	{"vary_river_depth", MGVALLEYS_VARY_RIVER_DEPTH},
+	{"altitude_dry",     MGVALLEYS_ALT_DRY},
+	{NULL,               0}
 };
 
 
@@ -94,8 +96,6 @@ MapgenValleys::MapgenValleys(int mapgenid, MapgenValleysParams *params,
 	MapgenBasic::np_cave2   = params->np_cave2;
 	MapgenBasic::np_cavern  = params->np_cavern;
 
-	humid_rivers       = (spflags & MGVALLEYS_HUMID_RIVERS);
-	use_altitude_chill = (spflags & MGVALLEYS_ALT_CHILL);
 	humidity_adjust    = bp->np_humidity.offset - 50.0f;
 }
 
@@ -298,10 +298,10 @@ void MapgenValleys::calculateNoise()
 	float heat_offset = 0.0f;
 	float humidity_scale = 1.0f;
 	// Altitude chill tends to reduce the average heat.
-	if (use_altitude_chill)
+	if (spflags & MGVALLEYS_ALT_CHILL)
 		heat_offset = 5.0f;
 	// River humidity tends to increase the humidity range.
-	if (humid_rivers)
+	if (spflags & MGVALLEYS_HUMID_RIVERS)
 		humidity_scale = 0.8f;
 
 	for (s32 index = 0; index < csize.X * csize.Z; index++) {
@@ -477,17 +477,16 @@ int MapgenValleys::generateTerrain()
 		if (surface_y > surface_max_y)
 			surface_max_y = std::ceil(surface_y);
 
-		if (humid_rivers) {
-			// Derive heat from (base) altitude. This will be most correct
-			// at rivers, since other surface heights may vary below.
-			if (use_altitude_chill && (surface_y > 0.0f || river_y > 0.0f))
-				t_heat -= alt_to_heat *
-					std::fmax(surface_y, river_y) / altitude_chill;
-
-			// If humidity is low or heat is high, lower the water table
+		// Optionally vary river depth according to heat and humidity
+		if (spflags & MGVALLEYS_VARY_RIVER_DEPTH) {
+			float heat = ((spflags & MGVALLEYS_ALT_CHILL) &&
+				(surface_y > 0.0f || river_y > 0.0f)) ?
+				t_heat - alt_to_heat *
+					std::fmax(surface_y, river_y) / altitude_chill :
+				t_heat;
 			float delta = m_bgen->humidmap[index_2d] - 50.0f;
 			if (delta < 0.0f) {
-				float t_evap = (t_heat - 32.0f) / evaporation;
+				float t_evap = (heat - 32.0f) / evaporation;
 				river_y += delta * std::fmax(t_evap, 0.08f);
 			}
 		}
@@ -534,33 +533,32 @@ int MapgenValleys::generateTerrain()
 			}
 		}
 
-		if (humid_rivers) {
-			// Use base ground (water table) in a riverbed, to avoid an
-			// unnatural rise in humidity.
+		// Optionally increase humidity around rivers
+		if (spflags & MGVALLEYS_HUMID_RIVERS) {
+			// Ground height ignoring riverbeds
 			float t_alt = std::fmax(noise_rivers->result[index_2d],
 				(float)heightmap[index_2d]);
-			float humid = m_bgen->humidmap[index_2d];
 			float water_depth = (t_alt - river_y) / humidity_dropoff;
-			humid *= 1.0f + std::pow(0.5f, std::fmax(water_depth, 1.0f));
+			m_bgen->humidmap[index_2d] *=
+				1.0f + std::pow(0.5f, std::fmax(water_depth, 1.0f));
+		}
 
-			// Reduce humidity with altitude (ignoring riverbeds)
+		// Optionally decrease humidity with altitude
+		if (spflags & MGVALLEYS_ALT_DRY) {
+			// Ground height ignoring riverbeds
+			float t_alt = std::fmax(noise_rivers->result[index_2d],
+				(float)heightmap[index_2d]);
 			if (t_alt > 0.0f)
-				humid -= alt_to_humid * t_alt / altitude_chill;
-
-			m_bgen->humidmap[index_2d] = humid;
+				m_bgen->humidmap[index_2d] -=
+					alt_to_humid * t_alt / altitude_chill;
 		}
 
-		// Assign the heat adjusted by any changed altitudes.
-		// The altitude will change about half the time.
-		if (use_altitude_chill) {
+		// Optionally decrease heat with altitude
+		if (spflags & MGVALLEYS_ALT_CHILL) {
 			// Ground height ignoring riverbeds
 			float t_alt = std::fmax(noise_rivers->result[index_2d],
 				(float)heightmap[index_2d]);
-
-			if (humid_rivers && heightmap[index_2d] == (s16)myround(surface_y))
-				// The altitude hasn't changed. Use the first result
-				m_bgen->heatmap[index_2d] = t_heat;
-			else if (t_alt > 0.0f)
+			if (t_alt > 0.0f)
 				m_bgen->heatmap[index_2d] -=
 					alt_to_heat * t_alt / altitude_chill;
 		}
diff --git a/src/mapgen/mapgen_valleys.h b/src/mapgen/mapgen_valleys.h
index f6a486d86..8bde7a622 100644
--- a/src/mapgen/mapgen_valleys.h
+++ b/src/mapgen/mapgen_valleys.h
@@ -28,9 +28,11 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #include "mapgen.h"
 
-////////////// Mapgen Valleys flags
-#define MGVALLEYS_ALT_CHILL    0x01
-#define MGVALLEYS_HUMID_RIVERS 0x02
+/////////////////// Mapgen Valleys flags
+#define MGVALLEYS_ALT_CHILL        0x01
+#define MGVALLEYS_HUMID_RIVERS     0x02
+#define MGVALLEYS_VARY_RIVER_DEPTH 0x04
+#define MGVALLEYS_ALT_DRY          0x08
 
 // Feed only one variable into these
 #define MYSQUARE(x) (x) * (x)
@@ -39,12 +41,15 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 class BiomeManager;
 class BiomeGenOriginal;
 
+extern FlagDesc flagdesc_mapgen_valleys[];
+
 
 struct MapgenValleysParams : public MapgenParams {
-	u32 spflags = MGVALLEYS_HUMID_RIVERS | MGVALLEYS_ALT_CHILL;
-	u16 altitude_chill = 90; // The altitude at which temperature drops by 20C
-	u16 river_depth = 4; // How deep to carve river channels
-	u16 river_size = 5; // How wide to make rivers
+	u32 spflags = MGVALLEYS_ALT_CHILL | MGVALLEYS_HUMID_RIVERS |
+		MGVALLEYS_VARY_RIVER_DEPTH | MGVALLEYS_ALT_DRY;
+	u16 altitude_chill = 90;
+	u16 river_depth = 4;
+	u16 river_size = 5;
 
 	float cave_width = 0.09f;
 	s16 large_cave_depth = -33;
@@ -53,7 +58,7 @@ struct MapgenValleysParams : public MapgenParams {
 	s16 cavern_taper = 192;
 	float cavern_threshold = 0.6f;
 	s16 dungeon_ymin = -31000;
-	s16 dungeon_ymax = 63; // No higher than surface mapchunks
+	s16 dungeon_ymax = 63;
 
 	NoiseParams np_filler_depth;
 	NoiseParams np_inter_valley_fill;
@@ -88,7 +93,8 @@ struct TerrainNoise {
 class MapgenValleys : public MapgenBasic {
 public:
 
-	MapgenValleys(int mapgenid, MapgenValleysParams *params, EmergeManager *emerge);
+	MapgenValleys(int mapgenid, MapgenValleysParams *params,
+		EmergeManager *emerge);
 	~MapgenValleys();
 
 	virtual MapgenType getType() const { return MAPGEN_VALLEYS; }
@@ -100,8 +106,6 @@ private:
 	BiomeGenOriginal *m_bgen;
 
 	float altitude_chill;
-	bool humid_rivers;
-	bool use_altitude_chill;
 	float humidity_adjust;
 	float river_depth_bed;
 	float river_size_factor;