Day14.cs

using System;
using System.Collections.Generic;
using AdventOfCode.CSharp.Common;

namespace AdventOfCode.CSharp.Y2020.Solvers;

public class Day14 : ISolver
{
    public static void Solve(ReadOnlySpan<byte> input, Solution solution)
    {
        var mem1 = new Dictionary<long, long>();
        var mem2 = new Dictionary<long, long>();

        long maskXs = 0; // ['X'] -> 1, ['0' or '1'] -> 0
        long mask1s = 0; // ['X' or '0'] -> 0, ['1'] -> 1

        var reader = new SpanReader(input);
        while (!reader.Done)
        {
            if (reader[1] == 'a') // mask
            {
                maskXs = 0;
                mask1s = 0;

                reader.SkipLength("mask = ".Length);
                for (int i = 0; i < 36; i++)
                {
                    byte c = reader[i];
                    switch (c)
                    {
                        case (byte)'1':
                            mask1s |= 1L << (35 - i);
                            break;
                        case (byte)'X':
                            maskXs |= 1L << (35 - i);
                            break;
                    }
                }

                reader.SkipLength(37); // 36 digits + newline
            }
            else // mem
            {
                reader.SkipLength("mem[".Length);
                long addr = reader.ReadPosLongUntil(']');
                reader.SkipLength(" = ".Length);
                long val = reader.ReadPosLongUntil('\n');

                // Part 1
                mem1[addr] = (val & maskXs) | mask1s;

                // Part 2
                addr |= mask1s; // any 1's in the mask should be set to 1 in the address
                addr &= ~maskXs; // any x's need to be set to 0 (since we will be iterating through permutations of bits in X)

                // iterate through submasks: https://cp-algorithms.com/algebra/all-submasks.html
                long mask = maskXs;
                mem2[addr] = val; // handle mask = 0 case
                while (mask != 0)
                {
                    mem2[addr | mask] = val;
                    mask = (mask - 1) & maskXs;
                }
            }
        }

        long part1 = 0;
        foreach ((_, long v) in mem1)
        {
            part1 += v;
        }

        long part2 = 0;
        foreach ((_, long v) in mem2)
        {
            part2 += v;
        }

        solution.SubmitPart1(part1);
        solution.SubmitPart2(part2);
    }
}