Day 11: Plutonian Pebbles
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3 points
And now we get into the days where caching really is king. My first attempt didn’t go so well, I tried to handle the full list result as one cache step, instead of individually caching the result of calculating each stone per step.
I think my original attempt is still calculating at home, but I finished up this much better version on the trip to work.
All hail public transport.
C#
List<long> stones = new List<long>();
public void Input(IEnumerable<string> lines)
{
stones = string.Concat(lines).Split(' ').Select(v => long.Parse(v)).ToList();
}
public void Part1()
{
var expanded = TryExpand(stones, 25);
Console.WriteLine($"Stones: {expanded}");
}
public void Part2()
{
var expanded = TryExpand(stones, 75);
Console.WriteLine($"Stones: {expanded}");
}
public long TryExpand(IEnumerable<long> stones, int steps)
{
if (steps == 0)
return stones.Count();
return stones.Select(s => TryExpand(s, steps)).Sum();
}
Dictionary<(long, int), long> cache = new Dictionary<(long, int), long>();
public long TryExpand(long stone, int steps)
{
var key = (stone, steps);
if (cache.ContainsKey(key))
return cache[key];
var result = TryExpand(Blink(stone), steps - 1);
cache[key] = result;
return result;
}
public IEnumerable<long> Blink(long stone)
{
if (stone == 0)
{
yield return 1;
yield break;
}
var str = stone.ToString();
if (str.Length % 2 == 0)
{
yield return long.Parse(str[..(str.Length / 2)]);
yield return long.Parse(str[(str.Length / 2)..]);
yield break;
}
yield return stone * 2024;
}
