RabbitProblem.java

import java.util.Arrays;

public class RabbitProblem
{
	private static final int DEATH_AGE = 96;
	private static final int FERTILE_AGE = 4;
	private static final int MALES_PER_LITTER = 5;
	private static final int FEMALES_PER_LITTER = 9;
	
	private static long[] maleAgeTracker = new long[DEATH_AGE];
	private static long[] femaleAgeTracker = new long[DEATH_AGE];
	private static long population;
	private static long targetPopulation;
	private static int months;

	public static void main(String[] args)
	{
		if (args.length < 3) throw new IllegalArgumentException("Missing arguments");
		if (args.length > 3) throw new IllegalArgumentException("Too many arguments");
		int initialMales = Integer.parseInt(args[0]);
		int initialFemales = Integer.parseInt(args[1]);
		targetPopulation = Long.parseLong(args[2]);
		population = initialMales + initialFemales;

		//Initial rabbits are 2 months old
		maleAgeTracker[2] = initialMales;
		femaleAgeTracker[2] = initialFemales;

		while (population < targetPopulation)
		{
			doMonthlyTick();
			System.out.printf("Month %d, Population %d\n", months, population);
			System.out.printf("Males: %s\n", Arrays.stream(maleAgeTracker).sum());
			System.out.printf("Females: %s\n", Arrays.stream(femaleAgeTracker).sum());
			System.out.println();
		}
		System.out.printf("%d months for %d to %d, Target: %d", months, initialMales + initialFemales, population, targetPopulation);
	}

	private static void doMonthlyTick()
	{
		months++;
		long newbornF = 0;
		long newbornM = 0;
		for(int f = FERTILE_AGE; f < DEATH_AGE; f++)
		{
			newbornM += MALES_PER_LITTER * femaleAgeTracker[f];
			newbornF += FEMALES_PER_LITTER * femaleAgeTracker[f];
		} 
		age();
		//Add newborns now so they are not aged with the rest
		maleAgeTracker[0] += newbornM;
		femaleAgeTracker[0] += newbornF;
		countPopulation();
	}
	
	private static void age()
	{
		long[] newMaleAgeTracker = new long[DEATH_AGE];
		long[] newFemaleAgeTracker = new long[DEATH_AGE]; 
		for(int i = 0; i < DEATH_AGE; i++)
		{
			//Kill old rabbits
			if(i == DEATH_AGE - 1) continue;
			newMaleAgeTracker[i + 1] = maleAgeTracker[i];
			newFemaleAgeTracker[i + 1] = femaleAgeTracker[i];
		}
		maleAgeTracker = newMaleAgeTracker;
		femaleAgeTracker = newFemaleAgeTracker;
	}
	
	private static void countPopulation()
	{
		population = Arrays.stream(maleAgeTracker).sum() + Arrays.stream(femaleAgeTracker).sum();
	}
}