Advent Of Code - Year 2024 - Day 15 - Java

package aoc2024;

import ak.CharGrid;
import ak.Fn;
import ak.I1m;
import ak.I2;
import ak.I2m;
import ak.Set;

public class D15 {

  public static boolean canPushUnit (CharGrid grid, I2 unit, I2 step, Set<I2> seen, boolean p2) {
    if (seen.contains(unit)) { return true; }
    seen.add(unit);
    if (p2) {
      if (grid.check(unit, '[')) {
        if (!canPushUnit(grid, unit.add(I2.right), step, seen, p2)) { return false; }
      }
      else {
        if (!canPushUnit(grid, unit.add(I2.left), step, seen, p2)) { return false; }
      }
    }
    I2 next = unit.add(step);
    if (grid.check(next, '#')) { return false; }
    if (grid.check(next, '.')) { return true; }
    return canPushUnit(grid, next, step, seen, p2);
  }

  public static void pushUnit (CharGrid grid, I2 unit, I2 step, Set<I2> seen, boolean p2) {
    if (seen.contains(unit)) { return; }
    seen.add(unit);
    if (p2) {
      if (grid.check(unit, '[')) {
        pushUnit(grid, unit.add(I2.right), step, seen, p2);
      }
      else {
        pushUnit(grid, unit.add(I2.left), step, seen, p2);
      }
    }
    char v = grid.get(unit);
    grid.set(unit, '.');
    I2 next = unit.add(step);
    if (grid.check(next, '#')) { return; }
    if (grid.checkAny(next, '[', ']', 'O')) { pushUnit(grid, next, step, seen, p2); }
    grid.set(next, v);
  }

  public static void useRobotCommand (CharGrid grid, I2m robot, char command, boolean p2) {
    I2 step = commandToDir(command);
    I2 previous = robot.close();
    I2 next = previous.add(step);
    if (grid.check(next, '#')) {
      return;
    }
    else if (grid.checkAny(next, '[', ']', 'O')) {
      boolean pushable = canPushUnit(grid, next, step, new Set<>(), p2);
      if (!pushable) { return; }
      pushUnit(grid, next, step, new Set<>(), p2);
    }
    grid.set(previous, '.');
    grid.set(next, '@');
    robot.set(next);
  }

  public static I2 commandToDir (char command) {
    return switch (command) {
      case '>' -> I2.right;
      case '<' -> I2.left;
      case 'v' -> I2.up;
      case '^' -> I2.down;
      default -> throw Fn.notExhaustive(command);
    };
  }

  public static void main (String[] args) {
    boolean sample = false;
    CharGrid grid0 = Fn.inputLinesTrimmedAsCharGrid(sample ? "d15-sample-grid.txt" : "d15-grid.txt");
    String commands = Fn.inputStripped(sample ? "d15-sample-commands.txt" : "d15-commands.txt");
    {
      CharGrid grid = grid0.copy();
      I2m robot = new I2m();
      grid.scan(robot, (c, g, x, y, v) -> {
        if (v == '@') {
          c.set(x, y);
          return false;
        }
        return true;
      });
      for (int i = 0; i < commands.length(); i++) {
        char command = commands.charAt(i);
        useRobotCommand(grid, robot, command, false);
      }
      I1m p1 = new I1m(0);
      grid.scan(p1, (c, g, x, y, v) -> {
        if (v == 'O') { c.x += 100 * y + x; }
        return true;
      });
      System.out.println("p1: " + p1);
    }
    {
      CharGrid grid = new CharGrid(grid0.rows, grid0.columns * 2);
      I2m robot = new I2m();
      grid0.scan(grid, (c, g, x, y, v) -> {
        switch (v) {
          case '#' -> {
            c.set(x * 2, y, '#');
            c.set(x * 2 + 1, y, '#');
          }
          case '.' -> {
            c.set(x * 2, y, '.');
            c.set(x * 2 + 1, y, '.');
          }
          case 'O' -> {
            c.set(x * 2, y, '[');
            c.set(x * 2 + 1, y, ']');
          }
          case '@' -> {
            c.set(x * 2, y, '@');
            c.set(x * 2 + 1, y, '.');
          }
          default -> throw Fn.notExhaustive(v);
        }
        return true;
      });
      grid.scan(robot, (c, g, x, y, v) -> {
        if (v == '@') {
          c.set(x, y);
          return false;
        }
        return true;
      });
      for (int i = 0; i < commands.length(); i++) {
        char command = commands.charAt(i);
        useRobotCommand(grid, robot, command, true);
      }
      I1m p2 = new I1m(0);
      grid.scan(p2, (c, g, x, y, v) -> {
        if (v == '[') { c.x += 100L * (long) y + (long) x; }
        return true;
      });
      System.out.println("p2: " + p2);
    }
  }

}