Patched into the aircraft's data port, you discover weather forecasts of a massive tropical storm. Before you can figure out whether it will impact your vacation plans, however, your device suddenly turns off!

Its battery is dead.

Read the full puzzle.

using System.Collections.Immutable;
using System.Linq;

namespace AdventOfCode.Y2020.Day10;

[ProblemName("Adapter Array")]
class Solution : Solver {

    public object PartOne(string input) {
        var jolts = Parse(input);
        var window = jolts.Skip(1).Zip(jolts).Select(p => (current: p.First, prev: p.Second));

        return
             window.Count(pair => pair.current - pair.prev == 1) *
             window.Count(pair => pair.current - pair.prev == 3);
    }

    public object PartTwo(string input) {
        var jolts = Parse(input);

        // dynamic programming with rolling variables a, b, c for the function values at i + 1, i + 2 and i + 3.
        var (a, b, c) = (1L, 0L, 0L); 
        for (var i = jolts.Count - 2; i >= 0; i--) {
            var s =  
                (i + 1 < jolts.Count && jolts[i + 1] - jolts[i] <= 3 ? a : 0) +
                (i + 2 < jolts.Count && jolts[i + 2] - jolts[i] <= 3 ? b : 0) +
                (i + 3 < jolts.Count && jolts[i + 3] - jolts[i] <= 3 ? c : 0);
            (a, b, c) = (s, a, b);
        }
        return a;
    }

    ImmutableList<int> Parse(string input) {
        var num = input.Split("\n").Select(int.Parse).OrderBy(x => x);
        return ImmutableList
            .Create(0)
            .AddRange(num)
            .Add(num.Last() + 3);
    }
}

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