aoc2021/aoc2021/Day03.cs

namespace aoc2021;

/// <summary>
/// Day 3: <see href="https://adventofcode.com/2021/day/3"/>
/// </summary>
public sealed class Day03 : Day
{
    private readonly List<string> _report;
    
    public Day03() : base(3, "Binary Diagnostic")
    {
        _report = Input.ToList();
    }

    public override object Part1()
    {
        var l = _report.Count / 2;
        var g = new StringBuilder();
        var e = new StringBuilder();
        
        foreach (var i in Enumerable.Range(0, _report[0].Length))
        {
            var ones = _report.Select(r => r[i]).Count(c => c == '1');
            g.Append(ones > l ? '1' : '0');
            e.Append(ones > l ? '0' : '1');
        }

        var gamma   = g.ToString().BigIntegerFromBinaryString();
        var epsilon = e.ToString().BigIntegerFromBinaryString();

        return gamma * epsilon;
    }

    public override object Part2()
    {
        var o = _report;
        var c = _report;

        char MostCommon(int i, IReadOnlyCollection<string> report) =>
            report.Count(r => r[i] == '1') >= report.Count / 2.0 ? '1' : '0';
        
        var i = 0;
        while (o.Count > 1)
        {
            var most = MostCommon(i, o);
            o = o.Where(r => r[i] == most).ToList();
            i++;
        }
        var o2 = o.Single().BigIntegerFromBinaryString();

        i = 0;
        while (c.Count > 1)
        {
            var most = MostCommon(i, c);
            c = c.Where(r => r[i] != most).ToList();
            i++;
        }
        var co2 = c.Single().BigIntegerFromBinaryString();

        return o2 * co2;
    }
}