As your ferry approaches the sea port, the captain asks for your help again. The computer system that runs this port isn't compatible with the docking program on the ferry, so the docking parameters aren't being correctly initialized in the docking program's memory.

After a brief inspection, you discover that the sea port's computer system uses a strange bitmask system in its initialization program. Although you don't have the correct decoder chip handy, you can emulate it in software!

Read the full puzzle.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace AdventOfCode.Y2020.Day14;

[ProblemName("Docking Data")]
class Solution : Solver {

    public object PartOne(string input) {
        var mem = new Dictionary<long, long>();
        var orMask = 0L;
        var andMask = 0xffffffffffffffL;
        foreach (var line in input.Split("\n")) {
            if (line.StartsWith("mask")) {
                var mask = line.Split(" = ")[1];
                andMask = Convert.ToInt64(mask.Replace("X", "1"), 2);
                orMask = Convert.ToInt64(mask.Replace("X", "0"), 2);
            } else {
                var num = Regex.Matches(line, "\\d+").Select(match => long.Parse(match.Value)).ToArray();
                mem[num[0]] = num[1] & andMask | orMask;
            }
        }
        return mem.Values.Sum();
    }

    public object PartTwo(string input) {
        var mem = new Dictionary<long, long>();
        var mask = "";
        foreach (var line in input.Split("\n")) {
            if (line.StartsWith("mask")) {
                mask = line.Split(" = ")[1];
            } else {
                var num = Regex.Matches(line, "\\d+").Select(match => long.Parse(match.Value)).ToArray();
                var (baseAddr, value) = (num[0], num[1]);
                foreach (var addr in Addresses(baseAddr, mask, 35)) {
                    mem[addr] = value;
                }

            }
        }
        return mem.Values.Sum();
    }

    IEnumerable<long> Addresses(long baseAddr, string mask, int i) {
        if (i == -1) {
            yield return 0;
        } else {
            foreach (var prefix in Addresses(baseAddr, mask, i - 1)) {
                if (mask[i] == '0') {
                    yield return (prefix << 1) + ((baseAddr >> 35 - i) & 1);
                } else if (mask[i] == '1') {
                    yield return (prefix << 1) + 1;
                } else {
                    yield return (prefix << 1);
                    yield return (prefix << 1) + 1;
                }
            }
        }
    }
}

Please ☆ my repo if you like it!