Function FlowRiver

src/landscape.cpp:1373–1450

* Try to flow the river down from a given begin. * @param spring The springing point of the river. * @param begin The begin point we are looking from; somewhere down hill from the spring. * @param min_river_length The minimum length for the river. * @return First element: True iff a river could/has been built, otherwise false; second element: River ends at sea. */

 * @return First element: True iff a river could/has been built, otherwise false; second element: River ends at sea.
 */
static std::tuple<bool, bool> FlowRiver(TileIndex spring, TileIndex begin, uint min_river_length)
{
	if (IsWaterTile(begin)) {
		return { DistanceManhattan(spring, begin) > min_river_length, GetTileZ(begin) == 0 };
	}

	int height_begin = TileHeight(begin);

	std::unordered_set<TileIndex> marks;
	marks.insert(begin);

	std::vector<TileIndex> queue;
	queue.push_back(begin);

	/* Breadth first search for the closest tile we can flow down to.
	 * We keep the queue to find the Nth tile for lake guessing. The tiles
	 * in the queue are neatly ordered by insertion.
	 */
	bool found = false;
	TileIndex end;
	for (size_t i = 0; i != queue.size(); i++) {
		end = queue[i];

		int height_end;
		if (IsTileFlat(end, &height_end) && (height_end < height_begin || (height_end == height_begin && IsWaterTile(end)))) {
			if (IsWaterTile(end) && GetWaterClass(end) == WaterClass::Sea) {
				/* If we've found the sea, make sure it's large enough. Scale by the map size but set a cap to avoid performance issues on large maps. */
				const uint MAX_SEA_SIZE_THRESHOLD = 1024;
				const uint SEA_SIZE_THRESHOLD = std::min(static_cast<uint>(2 * std::sqrt(Map::SizeX() * Map::SizeY())), MAX_SEA_SIZE_THRESHOLD);
				std::unordered_set<TileIndex> sea;
				/* Count the connected tiles, if the sea is large we can end the river here. */
				bool found_edge = CountConnectedSeaTiles(end, sea, SEA_SIZE_THRESHOLD);
				if (found_edge || sea.size() > SEA_SIZE_THRESHOLD) {
					found = true;
					break;
				} else {
					/* Sea is too small, flatten it so the river keeps looking or forms a lake / wetland. */
					for (TileIndex sea_tile : sea) {
						Command<CMD_TERRAFORM_LAND>::Do(DoCommandFlag::Execute, sea_tile, SLOPE_ELEVATED, false);
						Slope slope = ComplementSlope(GetTileSlope(sea_tile));
						Command<CMD_TERRAFORM_LAND>::Do(DoCommandFlag::Execute, sea_tile, slope, true);
					}
				}
			} else {
				/* We've found a river. */
				found = true;
				break;
			}
		}

		for (DiagDirection d = DIAGDIR_BEGIN; d < DIAGDIR_END; d++) {
			TileIndex t = end + TileOffsByDiagDir(d);
			if (IsValidTile(t) && !marks.contains(t) && RiverFlowsDown(end, t)) {
				marks.insert(t);
				queue.push_back(t);
			}
		}
	}