The Elves contact you over a highly secure emergency channel. Back at the North Pole, the Elves are busy misunderstanding White Elephant parties.

Each Elf brings a present. They all sit in a circle, numbered starting with position 1. Then, starting with the first Elf, they take turns stealing all the presents from the Elf to their left. An Elf with no presents is removed from the circle and does not take turns.

Read the full puzzle.

using System;
using System.Linq;

namespace AdventOfCode.Y2016.Day19;

[ProblemName("An Elephant Named Joseph")]
class Solution : Solver {

    public object PartOne(string input) {
        var elves = Elves(int.Parse(input));
        return Solve(elves[0], elves[1], elves.Length, 
            (elfVictim, count) => elfVictim.next.next);
    }

    public object PartTwo(string input) {
        var elves = Elves(int.Parse(input));
        return Solve(elves[0], elves[elves.Length / 2], elves.Length, 
            (elfVictim, count) => count % 2 == 1 ? elfVictim.next : elfVictim.next.next);
    }

    int Solve(Elf elf, Elf elfVictim, int elfCount, Func<Elf, int, Elf> nextVictim) {
        while (elfCount > 1) {
            elfVictim.prev.next = elfVictim.next;
            elfVictim.next.prev = elfVictim.prev;
            elf = elf.next;
            elfCount--;
            elfVictim = nextVictim(elfVictim, elfCount);
        }
        return elf.id;
    }

    Elf[] Elves(int count) {
        var elves = Enumerable.Range(0, count).Select(x => new Elf { id = x + 1 }).ToArray();
        for (var i = 0; i < count; i++) {
            elves[i].prev = elves[(i - 1 + count) % count];
            elves[i].next = elves[(i + 1) % count];
        }
        return elves;
    }

    class Elf {
        public int id;
        public Elf prev;
        public Elf next;
    }
}

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