AdventOfCode23/Day19/Rule.cs

using System.Text.RegularExpressions;

namespace Day19;

public partial class Rule
{
    [GeneratedRegex(@"(\w)([<>])(\d+):(\w+)|(\w+)")]
    private partial Regex ItemMatch();

    private readonly IList<IRuleItem> _items;

    public Rule(string ruleText)
    {
        _items = ItemMatch().Matches(ruleText).Select<Match, IRuleItem>(m =>
        {
            if (m.Groups[5].Length > 0)
            {
                return new Terminal(m.Groups[5].Value);
            }
            
            return new Item(m.Groups[1].Value, m.Groups[2].Value, int.Parse(m.Groups[3].Value), m.Groups[4].Value);
        }).ToList();
    }

    public string Apply(Part p)
    {
        foreach (var item in _items)
        {
            //Console.WriteLine(item);
            var next = item.Apply(p);
            //Console.WriteLine($"=> {next}");
            if (next != null)
            {
                return next;
            }
        }

        throw new Exception("Rule should always match");
    }
    
    public IEnumerable<RuleResultRange> Apply(PartRange p)
    {
        var range = p;
        foreach (var item in _items)
        {
            var result = item.Apply(range).ToList();
            yield return result.First();
            if (result.Count > 1)
            {
                range = result.Last().Part;
            }
        }
    }

    private interface IRuleItem
    {
        string? Apply(Part p);
        IEnumerable<RuleResultRange> Apply(PartRange p);
    }

    private class Item : IRuleItem
    {
        private readonly string _property;
        private readonly string _op;
        private readonly int _value;
        private readonly string _next;

        public Item(string property, string op, int value, string next)
        {
            _property = property;
            _op = op;
            _value = value;
            _next = next;
        }

        public string? Apply(Part p)
        {
            if (_op == "<")
            {
                return p.Get(_property) < _value ? _next : null;
            }

            if (_op == ">")
            {
                return p.Get(_property) > _value ? _next : null;
            }

            return null;
        }
        
        public IEnumerable<RuleResultRange> Apply(PartRange p)
        {
            var range = p.Get(_property);
            
            if (_op == "<")
            {
                yield return new RuleResultRange(
                    p.Update(_property, range.LessThan(_value)),
                    _next
                );
                yield return new RuleResultRange(
                    p.Update(_property, range.GreaterThan(_value - 1)),
                    null
                );
            }

            if (_op == ">")
            {
                yield return new RuleResultRange(
                    p.Update(_property, range.GreaterThan(_value)),
                    _next
                );
                yield return new RuleResultRange(
                    p.Update(_property, range.LessThan(_value + 1)),
                    null
                );
            }
        }

        public override string ToString()
        {
            return $"{_property} {_op} {_value} : {_next}";
        }
    }
    
    private class Terminal : IRuleItem
    {
        private readonly string _next;

        public Terminal(string next)
        {
            _next = next;
        }

        public string? Apply(Part p)
        {
            return _next;
        }

        public IEnumerable<RuleResultRange> Apply(PartRange p)
        {
            yield return new RuleResultRange(p, _next);
        }

        public override string ToString()
        {
            return $"Terminal<{_next}>";
        }
    }
}