Day6.java

package com.sbaars.adventofcode.year21.days;

import com.sbaars.adventofcode.common.map.LongCountMap;
import com.sbaars.adventofcode.year21.Day2021;

public class Day6 extends Day2021 {

  public Day6() {
    super(6);
  }

  public static void main(String[] args) {
    new Day6().printParts();
  }

  @Override
  public Object part1() {
    return countFishes(80);
  }

  @Override
  public Object part2() {
    return countFishes(256);
  }

  private long countFishes(int iterations) {
    var cm = LongCountMap.ofFrequencies(dayNumberStream(","));
    var nc = new LongCountMap<Long>();
    for (int j = 0; j < iterations; j++) {
      for (var e : cm.entrySet()) {
        if (e.getKey() == 0) {
          nc.increment(8L, e.getValue());
          nc.increment(6L, e.getValue());
        } else {
          nc.increment(e.getKey() - 1, e.getValue());
        }
      }
      cm = nc;
      nc = new LongCountMap<>();
    }
    return cm.sum();
  }
}