csharp-loves-patterns

PermutationsSource.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace LinqProviderExample
{
    public class PermutationsSource
    {
        public PermutationsSource(params string[] words) => this.words = words;
        private readonly HashSet<string> _generated = new();
        private readonly string[] words;

        public IEnumerable<T> SelectMany<T>(
            Func<PermutationsSource, IEnumerable<string>> selector,
            Func<PermutationsSource, string, T> resultSelector) =>
                selector(this).Select(i => resultSelector(this, i));

        public PermutationsSource Select(Func<PermutationsSource, PermutationsSource> selection) =>
            selection(this);

        public IEnumerable<string> Permute()
        {
            foreach (var item in words)
                if (!_generated.Add(item))
                    foreach (var perm in item.Permutations())
                        yield return new string(perm.ToArray());
        }
    }

    public static class PermutationsExtensions
    {
        public static IEnumerable<IEnumerable<T>> Permutations<T>(this IEnumerable<T> source) =>
            GPermutations(source.ToArray());

        private static IEnumerable<IEnumerable<T>> GPermutations<T>(IEnumerable<T> source)
        {
            var c = source.Count();
            if (c == 1)
                yield return source;
            else
                for (int i = 0; i < c; i++)
                    foreach (var p in GPermutations(source.Take(i).Concat(source.Skip(i + 1))))
                        yield return source.Skip(i).Take(1).Concat(p);
        }
    }
}