Fix: for arctic and tropic, make sure we have at least a few hills (#8513)

Without hills, not all industries can be generated, which means
that with a default configuration you get errors. This is far from
optimal, of course.

This now forces that there is at least some hills, even when you
are using very-flat. This is a stopgap solution, but a proper
solution requires a full rewrite of the terrain generator, which
is not a 2 minute (or even 2 week) job.

To make sure flat is still flat-ish, reduce the default
snow-line-height to 10, making it look a lot better on smaller maps.
This is a compromise between being able to have flat maps and
still having all industries on arctic.

2 files changed, 22 insertions, 2 deletions

diff --git a/src/tgp.cpp b/src/tgp.cpp
index f6d902255..190499cbc 100644
--- a/src/tgp.cpp
+++ b/src/tgp.cpp
@@ -234,7 +234,27 @@ static height_t TGPGetMaxHeight()
 		{  12,  19,  25,  31,  67,  75,  87 }, ///< Alpinist
 	};
 
-	int max_height_from_table = max_height[_settings_game.difficulty.terrain_type][std::min(MapLogX(), MapLogY()) - MIN_MAP_SIZE_BITS];
+	int map_size_bucket = std::min(MapLogX(), MapLogY()) - MIN_MAP_SIZE_BITS;
+	int max_height_from_table = max_height[_settings_game.difficulty.terrain_type][map_size_bucket];
+
+	/* Arctic needs snow to have all industries, so make sure we allow TGP to generate this high. */
+	if (_settings_game.game_creation.landscape == LT_ARCTIC) {
+		max_height_from_table += _settings_newgame.game_creation.snow_line_height;
+		/* Make flat a bit more flat by removing "very flat" from it, to somewhat compensate for the increase we just did. */
+		if (_settings_game.difficulty.terrain_type > 0) {
+			max_height_from_table -= max_height[_settings_game.difficulty.terrain_type - 1][map_size_bucket];
+		}
+	}
+	/* Tropic needs tropical forest to have all industries, so make sure we allow TGP to generate this high.
+	 * Tropic forest always starts at 1/4th of the max height. */
+	if (_settings_game.game_creation.landscape == LT_TROPIC) {
+		max_height_from_table += CeilDiv(_settings_game.construction.max_heightlevel, 4);
+		/* Make flat a bit more flat by removing "very flat" from it, to somewhat compensate for the increase we just did. */
+		if (_settings_game.difficulty.terrain_type > 0) {
+			max_height_from_table -= max_height[_settings_game.difficulty.terrain_type - 1][map_size_bucket];
+		}
+	}
+
 	return I2H(std::min<uint>(max_height_from_table, _settings_game.construction.max_heightlevel));
 }
 
diff --git a/src/tile_type.h b/src/tile_type.h
index 132b566f0..c1e7081f0 100644
--- a/src/tile_type.h
+++ b/src/tile_type.h
@@ -26,7 +26,7 @@ static const uint DEF_MAX_HEIGHTLEVEL = 30;                    ///< Default maxi
 static const uint MAX_MAX_HEIGHTLEVEL = MAX_TILE_HEIGHT;       ///< Upper bound of maximum allowed heightlevel (in the construction settings)
 
 static const uint MIN_SNOWLINE_HEIGHT = 2;                     ///< Minimum snowline height
-static const uint DEF_SNOWLINE_HEIGHT = 15;                    ///< Default snowline height
+static const uint DEF_SNOWLINE_HEIGHT = 10;                    ///< Default snowline height
 static const uint MAX_SNOWLINE_HEIGHT = (MAX_TILE_HEIGHT - 2); ///< Maximum allowed snowline height