90 lines
2.3 KiB
C#
90 lines
2.3 KiB
C#
namespace aoc2020;
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/// <summary>
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/// Day 23: <see href="https://adventofcode.com/2020/day/23" />
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/// </summary>
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public sealed class Day23 : Day
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{
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private readonly Dictionary<long, long> cups = new();
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private readonly ImmutableList<long> initialCups;
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private readonly long[] move;
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private long current;
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public Day23() : base(23, "Crab Cups")
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{
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initialCups = Input.First().Select(c => long.Parse(c.ToString())).ToImmutableList();
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current = initialCups.First();
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move = new long[3];
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}
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private void DoMoves(int turns)
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{
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for (var turn = 0; turn < turns; turn++)
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{
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var dest = current - 1;
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if (dest == 0) dest = cups.Count;
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for (var i = 0; i <= 2; i++)
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{
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var id = cups[current];
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var removedNext = cups[id];
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cups.Remove(id);
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cups[current] = removedNext;
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move[i] = id;
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}
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while (move.Contains(dest))
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{
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dest--;
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if (dest == 0) dest = cups.Count + 3;
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}
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for (var i = 0; i <= 2; i++)
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{
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var id = cups[dest];
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cups[dest] = move[i];
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cups.Add(move[i], id);
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dest = cups[dest];
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}
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current = cups[current];
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}
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}
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public override string Part1()
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{
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for (var i = 0; i < initialCups.Count; i++)
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cups[initialCups[i]] = initialCups[(i + 1) % initialCups.Count];
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DoMoves(100);
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current = 1;
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var result = new StringBuilder();
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while (cups[current] != 1)
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{
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result.Append(cups[current]);
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current = cups[current];
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}
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return result.ToString();
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}
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public override string Part2()
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{
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cups.Clear();
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for (var i = 0; i < initialCups.Count; i++)
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cups[initialCups[i]] = initialCups[(i + 1) % initialCups.Count];
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// add a million cups
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cups[initialCups.Last()] = 10;
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for (var i = 10; i < 1_000_000; i++)
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cups.Add(i, i + 1);
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cups[1_000_000] = current = initialCups.First();
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DoMoves(10_000_000);
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return $"{(ulong)cups[1] * (ulong)cups[cups[1]]}";
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}
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}
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