Two lists of tiles, I want to set rainforest on tiles that are 49 squared dist away from desert tiles

landscape.cpp

static void CreateDesertOrRainForest(uint desert_tropic_line)
{
	TileIndex update_freq = Map::Size() / 2;
	std::vector<std::pair<TileIndex, TileIndex>> queue;

	for (TileIndex tile = 0; tile != Map::Size(); ++tile) {
		if ((tile % update_freq) == 0) IncreaseGeneratingWorldProgress(GWP_LANDSCAPE);

		if (!IsValidTile(tile)) continue;

		if (TileHeight(tile) >= desert_tropic_line || IsTileType(tile, MP_WATER)) {
			SetTropicZone(tile, TROPICZONE_RAINFOREST);
			queue.push_back(std::make_pair(tile, tile));
		} else {
			SetTropicZone(tile, TROPICZONE_DESERT);
		}
	}

	IncreaseGeneratingWorldProgress(GWP_LANDSCAPE);

	std::vector<TileIndex> normal_tiles;
	static const TileIndexDiffC neighbours[] = { {-1, 0}, {1, 0}, {0, -1}, {0, 1} };
	auto i = 0;
	while (i != queue.size()) {
		auto p = queue[i];
		for (auto &d : neighbours) {
			auto t = AddTileIndexDiffCWrap(p.second, d);
			if (t == INVALID_TILE) continue;
			if (GetTropicZone(t) != TROPICZONE_DESERT) continue;

			auto dist = DistanceSquare(t, p.first);
			if (dist < 49) {
				queue.push_back(std::make_pair(p.first, t));
				if (dist == 0) {
					SetTropicZone(t, TROPICZONE_RAINFOREST);
				} else {
					if (IsValidTile(t)) normal_tiles.emplace_back(t);
					SetTropicZone(t, TROPICZONE_NORMAL);
				}
			}
		}
		i++;
	}

	IncreaseGeneratingWorldProgress(GWP_LANDSCAPE);

	TileIndex middle = TileXY(Map::SizeX() / 2, Map::SizeY() / 2);
	TileArea ta(middle, 1, 1);
	ta.Expand(6);

	std::vector<TileIndexDiffC> diffs;
	for (auto &tile : ta) {
		if (DistanceSquare(tile, middle) <= 50) {
			TileIndexDiffC diff = TileIndexToTileIndexDiffC(tile, middle);
			diffs.emplace_back(diff);
		}
	}

	update_freq = normal_tiles.size() / 4;
	auto count = 0;
	for (auto &tile : normal_tiles) {
		count++;
		if (count == update_freq) {
			IncreaseGeneratingWorldProgress(GWP_LANDSCAPE);
			count = 0;
		}
		bool make_rainforest = true;
		for (auto &diff : diffs) {
			auto t = AddTileIndexDiffCWrap(tile, diff);

			if (IsValidTile(t) && GetTropicZone(t) == TROPICZONE_DESERT) {
				make_rainforest = false;
				break;
			}
		}
		if (make_rainforest) {
			SetTropicZone(tile, TROPICZONE_RAINFOREST);
		}
	}

	for (uint i = 0; i != 256; i++) {
		if ((i % 64) == 0) IncreaseGeneratingWorldProgress(GWP_LANDSCAPE);

		RunTileLoop();
	}
}