using System.Threading;

namespace MvcMiniProfiler.Data

{

/// <summary>

/// This is a micro-cache; suitable when the number of terms is controllable (a few hundred, for example),

/// and strictly append-only; you cannot change existing values. All key matches are on **REFERENCE**

/// equality. The type is fully thread-safe.

/// </summary>

class Link<TKey, TValue> where TKey : class

{

public static bool TryGet(Link<TKey, TValue> link, TKey key, out TValue value)

{

while (link != null)

{

if ((object)key == (object)link.Key)

{

value = link.Value;

return true;

}

link = link.Tail;

}

value = default(TValue);

return false;

}

public static bool TryAdd(ref Link<TKey, TValue> head, TKey key, ref TValue value)

{

bool tryAgain;

do

{

var snapshot = Interlocked.CompareExchange(ref head, null, null);

TValue found;

if (TryGet(snapshot, key, out found))

{ // existing match; report the existing value instead

value = found;

return false;

}

var newNode = new Link<TKey, TValue>(key, value, snapshot);

// did somebody move our cheese?

tryAgain = Interlocked.CompareExchange(ref head, newNode, snapshot) != snapshot;

} while (tryAgain);

return true;

}

private Link(TKey key, TValue value, Link<TKey, TValue> tail)

{

Key = key;

Value = value;

Tail = tail;

}

public TKey Key { get; private set; }

public TValue Value { get; private set; }

public Link<TKey, TValue> Tail { get; private set; }

}

}