using System;

using System.Collections.Generic;

namespace Open.RandomizationExtensions;

/// <summary>

/// A <see cref="HashSet{T}"/> that fills itself lazily from an enumerator as

/// <see cref="Contains(T)"/> is called.

/// </summary>

/// <remarks>

/// IMPORTANT: <see cref="Contains(T)"/> HIDES (<see langword="new"/>) rather than

/// overrides the base method, so it only executes when called through a

/// <see cref="DeferredHashSet{T}"/>-typed reference. A call through

/// <see cref="ISet{T}"/> or <see cref="HashSet{T}"/> bypasses the lazy pump and

/// consults only what has already been materialized. Likewise <see cref="HashSet{T}.Count"/>

/// reflects only what has been pumped so far. Callers must not use Count-based

/// fast paths against an instance of this type.

/// </remarks>

sealed class DeferredHashSet<T>(IEnumerator<T> source) : HashSet<T>, IDisposable

{

public DeferredHashSet(IEnumerable<T> source)

: this((source ?? throw new ArgumentNullException(nameof(source))).GetEnumerator()) { }

private readonly IEnumerator<T> Source = source ?? throw new ArgumentNullException(nameof(source));

public new bool Contains(T item)

{

if (base.Contains(item))

return true;

// Use the same comparer as the base set so pump-time matches agree with

// what base.Contains would later report.

var comparer = EqualityComparer<T>.Default;

while (Source.MoveNext())

{

var i = Source.Current;

_ = Add(i);

if (comparer.Equals(item, i))

return true;

}

return false;

}

public void Dispose()

{

Source.Dispose();

Clear();

}

}