Function CalculateCoverageLine

src/landscape.cpp:1521–1585

* Calculate what height would be needed to cover N% of the landmass. * * The function allows both snow and desert/tropic line to be calculated. It * tries to find the closest height which covers N% of the landmass; it can * be below or above it. * * Tropic has a mechanism where water and tropic tiles in mountains grow * inside the desert. To better approximate the requested coverage, this i

 * @return The estimated best height to use to cover N% of the landmass.
 */
static uint CalculateCoverageLine(uint coverage, uint edge_multiplier)
{
	/* Histogram of how many tiles per height level exist. */
	std::array<int, MAX_TILE_HEIGHT + 1> histogram = {};
	/* Histogram of how many neighbour tiles are lower than the tiles of the height level. */
	std::array<int, MAX_TILE_HEIGHT + 1> edge_histogram = {};

	/* Build a histogram of the map height. */
	for (const auto tile : Map::Iterate()) {
		uint h = TileHeight(tile);
		histogram[h]++;

		if (edge_multiplier != 0) {
			/* Check if any of our neighbours is below us. */
			for (DiagDirection dir = DIAGDIR_BEGIN; dir != DIAGDIR_END; dir++) {
				TileIndex neighbour_tile = AddTileIndexDiffCWrap(tile, TileIndexDiffCByDiagDir(dir));
				if (IsValidTile(neighbour_tile) && TileHeight(neighbour_tile) < h) {
					edge_histogram[h]++;
				}
			}
		}
	}

	/* The amount of land we have is the map size minus the first (sea) layer. */
	uint land_tiles = Map::Size() - histogram[0];
	int best_score = land_tiles;

	/* Our goal is the coverage amount of the land-mass. */
	int goal_tiles = land_tiles * coverage / 100;

	/* We scan from top to bottom. */
	uint h = MAX_TILE_HEIGHT;
	uint best_h = h;

	int current_tiles = 0;
	for (; h > 0; h--) {
		current_tiles += histogram[h];
		int current_score = goal_tiles - current_tiles;

		/* Tropic grows from water and mountains into the desert. This is a
		 * great visual, but it also means we* need to take into account how
		 * much less desert tiles are being created if we are on this
		 * height-level. We estimate this based on how many neighbouring
		 * tiles are below us for a given length, assuming that is where
		 * tropic is growing from.
		 */
		if (edge_multiplier != 0 && h > 1) {
			/* From water tropic tiles grow for a few tiles land inward. */
			current_score -= edge_histogram[1] * edge_multiplier;
			/* Tropic tiles grow into the desert for a few tiles. */
			current_score -= edge_histogram[h] * edge_multiplier;
		}

		if (std::abs(current_score) < std::abs(best_score)) {
			best_score = current_score;
			best_h = h;
		}