Day10.java

package com.sbaars.adventofcode.year23.days;

import com.sbaars.adventofcode.common.Direction;
import com.sbaars.adventofcode.common.grid.InfiniteGrid;
import com.sbaars.adventofcode.common.location.Loc;
import com.sbaars.adventofcode.year23.Day2023;

import java.util.stream.Stream;

import static com.sbaars.adventofcode.common.Direction.*;

public class Day10 extends Day2023 {
  public Day10() {
    super(10);
  }

  public static void main(String[] args) {
    new Day10().printParts();
  }

  @Override
  public Object part1() {
    var grid = new InfiniteGrid(dayGrid());
    Loc start = grid.find('S');
    Direction comeFrom = four()
        .filter(d -> grid.contains(d.move(start)))
        .filter(d -> connections(grid.get(d.move(start)).get()).anyMatch(d2 -> d2.equals(d.turnDegrees(180))))
        .findAny()
        .get();
    Loc currentLoc = comeFrom.move(start);
    int numWalked = 1;
    while (!currentLoc.equals(start)) {
      Direction nextDir = comeFrom;
      comeFrom = connections(grid.get(currentLoc).get())
          .filter(d -> !d.equals(nextDir.turnDegrees(180)))
          .findAny()
          .get();
      currentLoc = comeFrom.move(currentLoc);
      numWalked++;
    }
    return numWalked / 2;
  }

  public Stream<Direction> connections(char c) {
    return switch (c) {
      case 'F' -> Stream.of(EAST, SOUTH);
      case '7' -> Stream.of(SOUTH, WEST);
      case 'L' -> Stream.of(NORTH, EAST);
      case 'J' -> Stream.of(NORTH, WEST);
      case '-' -> Stream.of(EAST, WEST);
      case '|' -> Stream.of(NORTH, SOUTH);
      default -> Stream.empty();
    };
  }

  @Override
  public Object part2() {
    var grid = new InfiniteGrid(dayGrid());
    var visited = new InfiniteGrid(' ', grid.width() * 2, grid.height() * 2);
    Loc start = grid.find('S');
    Direction comeFrom = four().filter(d -> grid.contains(d.move(start))).filter(d -> connections(grid.get(d.move(start)).get()).anyMatch(d2 -> d2.equals(d.turnDegrees(180)))).findAny().get();
    Loc currentLoc = moveLoc(start, comeFrom, visited);
    while (!currentLoc.equals(start)) {
      Direction nextDir = comeFrom;
      comeFrom = connections(grid.get(currentLoc).get()).filter(d -> !d.equals(nextDir.turnDegrees(180))).findAny().get();
      currentLoc = moveLoc(currentLoc, comeFrom, visited);
    }

    var pathGrid = new InfiniteGrid('.', grid.width(), grid.height());
    visited.findAll('X').forEach(l -> pathGrid.set(l.translate(l2 -> l2 / 2), 'X'));

    var borderGrid = pathGrid.withBorder(1, '.');
    var borderVisited = visited.withBorder(2, ' ');
    return borderGrid.findAll('.').count() - borderVisited.floodFill(new Loc(0, 0), c -> c == ' ').stream().filter(l -> l.x % 2 == 0 && l.y % 2 == 0).count();
  }

  private static Loc moveLoc(Loc currentLoc, Direction comeFrom, InfiniteGrid visited) {
    currentLoc = comeFrom.move(currentLoc);
    Loc zoomed = currentLoc.translate(l -> l * 2);
    visited.set(zoomed, 'X');
    Loc prevPos = comeFrom.turnDegrees(180).move(zoomed);
    visited.set(prevPos, 'X');
    return currentLoc;
  }
}