Day 12: Garden Groups
Megathread guidelines
- Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
- You can send code in code blocks by using three backticks, the code, and then three backticks or use something such as https://topaz.github.io/paste/ if you prefer sending it through a URL
FAQ
- What is this?: Here is a post with a large amount of details: https://programming.dev/post/6637268
- Where do I participate?: https://adventofcode.com/
- Is there a leaderboard for the community?: We have a programming.dev leaderboard with the info on how to join in this post: https://programming.dev/post/6631465
You are viewing a single thread.
View all comments 
2 points
Ended up oversleeping somewhat, so I did the first part on the way to work using flood fills over a global visited set, and now that work’s over I’ve sat down to expand that solution to do corner counting for part two as well.
C#
char[] map = new char[0];
(int X, int Y) size = (0, 0);
public void Input(IEnumerable<string> lines)
{
map = string.Concat(lines).ToCharArray();
size = (lines.First().Length, lines.Count());
}
Dictionary<HashSet<(int,int)>,int> areas = new Dictionary<HashSet<(int,int)>,int>();
public void PreCalc()
{
HashSet<(int, int)> visited = new HashSet<(int, int)>();
for (int y = 0; y < size.Y; ++y)
for (int x = 0; x < size.X; ++x)
{
var at = (x, y);
if (visited.Contains(at))
continue;
var area = Flood((x, y), visited);
areas[area.Area] = area.Perim;
}
}
public void Part1()
{
int sum = areas.Select(kv => kv.Key.Count * kv.Value).Sum();
Console.WriteLine($"Fencing total: {sum}");
}
public void Part2()
{
int sum = areas.Select(kv => kv.Key.Count * countCorners(kv.Key)).Sum();
Console.WriteLine($"Fencing total: {sum}");
}
readonly (int dX, int dY)[] links = new[] { (1, 0), (0, 1), (-1, 0), (0, -1) };
(HashSet<(int,int)> Area, int Perim) Flood((int X, int Y) from, HashSet<(int X, int Y)> visited)
{
char at = map[from.Y * size.X + from.X];
(HashSet<(int,int)> Area, int Perim) ret = (new HashSet<(int,int)>(), 0);
visited.Add(from);
ret.Area.Add(from);
foreach (var link in links)
{
(int X, int Y) newAt = (from.X + link.dX, from.Y + link.dY);
char offset ;
if (newAt.X < 0 || newAt.X >= size.X || newAt.Y < 0 || newAt.Y >= size.Y)
offset = '\0';
else
offset = map[newAt.Y * size.X + newAt.X];
if (offset == at)
{
if (visited.Contains(newAt))
continue;
var nextArea = Flood(newAt, visited);
ret.Area.UnionWith(nextArea.Area);
ret.Perim += nextArea.Perim;
}
else
{
ret.Perim += 1;
}
}
return ret;
}
readonly (int dX, int dY)[] cornerPoints = new[] { (0, 0), (1, 0), (1, 1), (0, 1) };
readonly int[] diagonalValues = new[] { (2 << 0) + (2 << 2), (2 << 1) + (2 << 3) };
int countCorners(HashSet<(int X, int Y)> points)
{
int corners = 0;
var bounds = findBounds(points);
for (int y = bounds.minY - 1; y < bounds.maxY + 1; ++y)
for (int x = bounds.minX - 1; x < bounds.maxX + 1; ++x)
{
var atPoint = cornerPoints.Select(c => points.Contains((x + c.dX, y + c.dY)));
var before = corners;
if (atPoint.Where(c => c).Count() % 2 == 1)
corners++;
else if (diagonalValues.Contains(atPoint.Select((c, i) => c ? (2 << i) : 0).Sum()))
corners += 2;
}
return corners;
}
(int minX, int minY, int maxX, int maxY) findBounds(HashSet<(int X, int Y)> points)
{
(int minX, int minY, int maxX, int maxY) ret = (int.MaxValue, int.MaxValue, int.MinValue, int.MinValue);
foreach (var point in points)
{
ret.minX = Math.Min(ret.minX, point.X);
ret.minY = Math.Min(ret.minY, point.Y);
ret.maxX = Math.Max(ret.maxX, point.X);
ret.maxY = Math.Max(ret.maxY, point.Y);
}
return ret;
}
