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.

1 files changed, 32 insertions, 34 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;
 		}