Day16.java

package com.sbaars.adventofcode.year21.days;

import com.sbaars.adventofcode.common.HasRecursion;
import com.sbaars.adventofcode.year21.Day2021;

import java.math.BigInteger;
import java.util.ArrayList;
import java.util.List;

public class Day16 extends Day2021 implements HasRecursion {

  public Day16() {
    super(16);
  }

  public static void main(String[] args) {
    new Day16().printParts();
  }

  @Override
  public Object part1() {
    return findSum(day());
  }

  private long findSum(String hex) {
    return findPackets(hexToBin(hex.trim()));
  }

  private long findPackets(String in) {
    long sum = 0;
    for (int i = 0; i < in.length(); ) {
      if (in.substring(i).chars().allMatch(e -> e == '0')) break;
      int version = binToDec(in.substring(i, i + 3));
      sum += version;
      int id = binToDec(in.substring(i + 3, i + 6));
      i += 6;
      if (id == 4) {
        for (; ; i += 5) {
          if (in.charAt(i) == '0') {
            i += 5;
            break;
          }
        }
      } else {
        int lengthLength = 15;
        boolean b = in.charAt(i) == '1';
        if (b) {
          lengthLength = 11;
        }
        i++;
        int length = binToDec(in.substring(i, i + lengthLength));
        i += lengthLength;
        if (!b) {
          sum += findPackets(in.substring(i, i + length));
          i += length;
        }
      }
    }
    return sum;
  }

  String hexToBin(String s) {
    return String.format("%" + (s.length() * 4) + "s", new BigInteger(s, 16).toString(2)).replace(" ", "0");
  }

  int binToDec(String s) {
    return Integer.parseInt(new BigInteger(s, 2).toString(10));
  }

  long binToLongDec(String s) {
    return Long.parseLong(new BigInteger(s, 2).toString(10));
  }

  @Override
  public Object part2() {
    return findSum2(day());
  }

  private long findSum2(String hex) {
    var in = hexToBin(hex.trim());
    return findPackets2(in, Integer.MAX_VALUE).get(0);
  }

  int prevI = 0;

  private List<Long> findPackets2(String in, int toParse) {
    List<Long> res = new ArrayList<>();
    long sum = 0;
    for (int i = 0, parsed = 0; i < in.length(); parsed++) {
      if (parsed >= toParse) {
        break;
      }
      if (in.substring(i).chars().allMatch(e -> e == '0')) break;
      int id = binToDec(in.substring(i + 3, i + 6));
      i += 6;
      if (id == 4) {
        String s = "";
        for (; ; i += 5) {
          s += in.substring(i + 1, i + 5);
          if (in.charAt(i) == '0') {
            i += 5;
            break;
          }
        }
        res.add(binToLongDec(s));
      } else {
        int lengthLength = 15;
        boolean b = in.charAt(i) == '1';
        if (b) {
          lengthLength = 11;
        }
        i++;
        int length = binToDec(in.substring(i, i + lengthLength));
        i += lengthLength;
        List<Long> op = findPackets2(in.substring(i, b ? in.length() : (i + length)), b ? length : Integer.MAX_VALUE);
        res.add(performOp(op, id));
        i += b ? prevI : length;
      }
      prevI = i;
    }
    return res;
  }

  private long performOp(List<Long> op, int id) {
    return switch (id) {
      case 0 -> op.stream().mapToLong(e -> e).sum();
      case 1 -> op.stream().mapToLong(e -> e).reduce((a, b) -> a * b).getAsLong();
      case 2 -> op.stream().mapToLong(e -> e).min().getAsLong();
      case 3 -> op.stream().mapToLong(e -> e).max().getAsLong();
      case 5 -> op.get(0) > op.get(1) ? 1L : 0L;
      case 6 -> op.get(0) < op.get(1) ? 1L : 0L;
      case 7 -> op.get(0).equals(op.get(1)) ? 1L : 0L;
      default -> throw new IllegalStateException("Not known id: " + id);
    };
  }
}