#nullable enable
using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace IPA.Utilities
{
    /// <summary>
    /// Extensions for <see cref="IEnumerable{T}"/> that don't currently exist in <c>System.Linq</c>.
    /// </summary>
    public static class EnumerableExtensions
    {
        /// <summary>
        /// Adds a value to the beginning of the sequence.
        /// </summary>
        /// <typeparam name="T">the type of the elements of <paramref name="seq"/></typeparam>
        /// <param name="seq">a sequence of values</param>
        /// <param name="prep">the value to prepend to <paramref name="seq"/></param>
        /// <returns>a new sequence beginning with <paramref name="prep"/></returns>
        public static IEnumerable<T> Prepend<T>(this IEnumerable<T> seq, T prep)
            => new PrependEnumerable<T>(seq, prep);

        private sealed class PrependEnumerable<T> : IEnumerable<T>
        {
            private readonly IEnumerable<T> rest;
            private readonly T first;

            public PrependEnumerable(IEnumerable<T> rest, T first)
            {
                this.rest = rest;
                this.first = first;
            }

            public PrependEnumerator GetEnumerator() => new(this);

            public struct PrependEnumerator : IEnumerator<T>
            {
                private readonly IEnumerator<T> restEnum;
                private readonly PrependEnumerable<T> enumerable;
                private int state;
                internal PrependEnumerator(PrependEnumerable<T> enumerable)
                {
                    this.enumerable = enumerable;
                    restEnum = enumerable.rest.GetEnumerator();
                    state = 0;
                    Current = default!;
                }

                public T Current { get; private set; }

                object? IEnumerator.Current => Current;

                public void Dispose() => restEnum.Dispose();

                public bool MoveNext()
                {
                    switch (state)
                    {
                        case 0:
                            Current = enumerable.first;
                            state++;
                            return true;
                        case 1:
                            if (!restEnum.MoveNext())
                            {
                                state = 2;
                                return false;
                            }
                            else
                                Current = restEnum.Current;
                            return true;
                        case 2:
                        default:
                            return false;
                    }
                }

                public void Reset()
                {
                    restEnum.Reset();
                    state = 0;
                }
            }

            IEnumerator<T> IEnumerable<T>.GetEnumerator() => new PrependEnumerator(this);
            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
        }

        /// <summary>
        /// Adds a value to the end of the sequence.
        /// </summary>
        /// <typeparam name="T">the type of the elements of <paramref name="seq"/></typeparam>
        /// <param name="seq">a sequence of values</param>
        /// <param name="app">the value to append to <paramref name="seq"/></param>
        /// <returns>a new sequence ending with <paramref name="app"/></returns>
        public static IEnumerable<T> Append<T>(this IEnumerable<T> seq, T app)
            => new AppendEnumerable<T>(seq, app);

        private sealed class AppendEnumerable<T> : IEnumerable<T>
        {
            private readonly IEnumerable<T> rest;
            private readonly T last;

            public AppendEnumerable(IEnumerable<T> rest, T last)
            {
                this.rest = rest;
                this.last = last;
            }

            public AppendEnumerator GetEnumerator() => new(this);

            public struct AppendEnumerator : IEnumerator<T>
            {
                private readonly IEnumerator<T> restEnum;
                private readonly AppendEnumerable<T> enumerable;
                private int state;
                internal AppendEnumerator(AppendEnumerable<T> enumerable)
                {
                    this.enumerable = enumerable;
                    restEnum = enumerable.rest.GetEnumerator();
                    state = 0;
                    Current = default!;
                }

                public T Current { get; private set; }

                object? IEnumerator.Current => Current;

                public void Dispose() => restEnum.Dispose();

                public bool MoveNext()
                {
                    switch (state)
                    {
                        case 0:
                            if (!restEnum.MoveNext())
                            {
                                state = 1;
                                goto case 1;
                            }
                            else
                                Current = restEnum.Current;
                            return true;
                        case 1:
                            Current = enumerable.last;
                            state++;
                            return true;
                        case 2:
                        default:
                            return false;
                    }
                }

                public void Reset()
                {
                    restEnum.Reset();
                    state = 0;
                }
            }

            IEnumerator<T> IEnumerable<T>.GetEnumerator() => GetEnumerator();
            IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
        }

        /// <summary>
        /// LINQ-style extension method that filters <see langword="null"/> elements out of an enumeration.
        /// </summary>
        /// <typeparam name="T">the type of the enumeration</typeparam>
        /// <param name="self">the enumeration to filter</param>
        /// <returns>a filtered enumerable</returns>
        public static IEnumerable<T> NonNull<T>(this IEnumerable<T?> self) where T : class
            => self.Where(o => o != null)!;

        /// <summary>
        /// LINQ-style extension method that filters <see langword="null"/> elements out of an enumeration based on a converter.
        /// </summary>
        /// <typeparam name="T">the type of the enumeration</typeparam>
        /// <typeparam name="U">the type to compare to null</typeparam>
        /// <param name="self">the enumeration to filter</param>
        /// <param name="pred">the predicate to select for filtering</param>
        /// <returns>a filtered enumerable</returns>
        public static IEnumerable<T> NonNull<T, U>(this IEnumerable<T> self, Func<T, U?> pred) where U : class
            => self.Where(o => pred(o) != null);

        /// <summary>
        /// LINQ-style extension method that filters <see langword="null"/> elements from an enumeration of nullable types.
        /// </summary>
        /// <typeparam name="T">the underlying type of the nullable enumeration</typeparam>
        /// <param name="self">the enumeration to filter</param>
        /// <returns>a filtered enumerable</returns>
        public static IEnumerable<T> NonNull<T>(this IEnumerable<T?> self) where T : struct
            => self.Where(o => o != null).Select(o => o!.Value);

        /// <summary>
        /// LINQ-style extension method that filters <see langword="null"/> elements out of an enumeration based on a converter to a nullable type.
        /// </summary>
        /// <typeparam name="T">the type of the enumeration</typeparam>
        /// <typeparam name="U">the type of the predicate's resulting nullable</typeparam>
        /// <param name="self">the enumeration to filter</param>
        /// <param name="pred">the predicate to select for filtering</param>
        /// <returns>a filtered enumerable</returns>
        public static IEnumerable<T> NonNull<T, U>(this IEnumerable<T> self, Func<T, U?> pred) where U : struct
            => self.Where(o => pred(o) != null);
    }
}