import Control.Arrow
import Data.List
import Data.Map (Map)
import Data.Map qualified as Map
import Data.Set (Set)
import Data.Set qualified as Set

readInput :: String -> Map (Int, Int) Char
readInput s = Map.fromList [((i, j), c) | (i, l) <- zip [0 ..] (lines s), (j, c) <- zip [0 ..] l]

(i1, j1) .+. (i2, j2) = (i1 + i2, j1 + j2)

(i1, j1) .-. (i2, j2) = (i1 - i2, j1 - j2)

directions = [(0, 1), (1, 0), (0, -1), (-1, 0)] :: [(Int, Int)]

edges = zip ps (drop 1 ps) :: [((Int, Int), (Int, Int))]
  where
    ps = [(0, 1), (1, 1), (1, 0), (0, 0), (0, 1)]

regions :: Map (Int, Int) Char -> [Set (Int, Int)]
regions = unfoldr (fmap (uncurry removeRegion) . Map.minViewWithKey)
  where
    removeRegion (p, t) = go Set.empty (Set.singleton p)
      where
        go r ps plots
          | Set.null ps = (r, plots)
          | otherwise =
              let ps' =
                    Set.filter (\p -> plots Map.!? p == Just t) $
                      Set.fromList (concatMap adjacent ps) Set.\\ ps
               in go (Set.union r ps) ps' (Map.withoutKeys plots ps')
        adjacent = (`map` directions) . (.+.)

boundary :: Set (Int, Int) -> Set ((Int, Int), (Int, Int))
boundary region =
  Set.fromList $
    [ (p .+. e1, p .+. e2)
      | p <- Set.elems region,
        (d, (e1, e2)) <- zip directions edges,
        p .+. d `Set.notMember` region
    ]

perimeter :: Set (Int, Int) -> [[(Int, Int)]]
perimeter = unfoldr (fmap (uncurry removeChain) . Set.minView) . boundary
  where
    removeChain e@(e1, e2) es = first (e1 :) $ go [] e es
    go c e@(e1, e2) es =
      case find ((== e2) . fst) es of
        Nothing -> (e1 : c, es)
        Just e' -> go (e1 : c) e' (Set.delete e' es)

countSides :: [(Int, Int)] -> Int
countSides ps = length $ group $ zipWith (.-.) (drop 1 ps) ps

main = do
  input <- readInput <$> readFile "input12"
  let rs = map (Set.size &&& perimeter) $ regions input
  print . sum $ map (\(a, p) -> a * sum (map (subtract 1 . length) p)) rs
  print . sum $ map (\(a, p) -> a * sum (map countSides p)) rs

import 'dart:math';
import 'package:collection/collection.dart';
import 'package:more/more.dart';

const List<Point> n4 = [Point(0, 1), Point(0, -1), Point(1, 0), Point(-1, 0)];
List<Point> n8 = n4 + [Point(1, 1), Point(1, -1), Point(-1, 1), Point(-1, -1)];
const List<Point> c4 = [Point(0, 0), Point(0, 1), Point(1, 0), Point(1, 1)];

(Map<Point, String>, Map<Point, List<Point>>) parse(ls) {
  var nodes = {
    for (var y in 0.to(ls.length))
      for (var x in 0.to(ls.first.length)) Point<num>(x, y): ls[y][x] as String
  };
  var nexts = Map.fromEntries(nodes.keys.map((n) => MapEntry(
      n,
      n4
          .map((d) => n + d)
          .where((d) => (nodes[d] ?? '') == nodes[n]!)
          .toList())));
  return (nodes, nexts);
}

(int, Set<Point>) survey(
    Point here, String target, Map<Point<num>, List<Point>> nexts,
    [Set sofar = const {}]) {
  seen.add(here);
  var fences = 4 - nexts[here]!.length;
  var area = {here};
  for (var f in nexts[here]!.where((e) => !seen.contains(e))) {
    var (fs, a) = survey(f, target, nexts, sofar.toSet()..add(f));
    fences += fs;
    area.addAll(a);
  }
  return (fences, area);
}

late Set<Point> seen;
List<(int, Set<Point<num>>)> costs(List<String> lines) {
  seen = {};
  var ret = <(int, Set<Point<num>>)>[];
  var (nodes, nexts) = parse(lines);
  var toVisit = nodes.keys.toSet();
  while (toVisit.isNotEmpty) {
    var here = toVisit.first;
    toVisit.remove(here);
    ret.add(survey(here, nodes[here]!, nexts));
    toVisit.removeAll(seen);
  }
  return ret;
}

Function eq = const ListEquality().equals;
int corners(Set<Point> points) {
  var border = points.map((e) => n8.map((n) => n + e)).flattenedToSet
    ..addAll(points);
  // A corner is where a 2x2 grid contains one/three in-shape points, or
  // two diagonally-opposite cells
  var corners = 0;
  for (var cell in border) {
    var count = c4.map((e) => points.contains(e + cell)).toList();
    if (count.count((e) => e) % 2 == 1) {
      corners += 1;
    } else {
      if (eq(count, [true, false, false, true]) ||
          eq(count, [false, true, true, false])) {
        corners += 2;
      }
    }
  }
  return corners;
}

part1(lines) => costs(lines).map((e) => e.first * e.last.length).sum;
part2(lines) => costs(lines).map((e) => corners(e.last) * e.last.length).sum;

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;
}