这引起了我昨天的兴趣。我做了一些测试，这是我的尝试，我有一些加速。委托越简单，它运行得越快，但是一些记忆就可以了。
Time test 1 542 69%
Time test 2 377 99%
Time test 3 558 67%
Time test 4 505 74%
Time test 5 372 100%
Time test 6 374 99%
Time test 7 452 82%
LinqPad 的测试代码。
void Main()
{
foreach(var t in typeof(test).Assembly.GetTypes().Where(t => t.IsSubclassOf(typeof(test))))
((test)(Activator.CreateInstance(t))).TestMethod();
}
private abstract class test
{
public abstract void TestMethod();
}
private class test1 : test
{
delegate Action<T> Continuation<T>(Continuation<T> r);
//[TestMethod]
//The fixed point operator, very slow also.
public override void TestMethod()
{
IObject root = BuildComposite();
Performance.Measure(1000000, () =>
{
Apply(root, h => t =>
{
foreach (IObject item in t.Children)
{
//Console.WriteLine(item.Name);
h(item);
}
});
}, "Time " + this.GetType().Name);
}
private static void Apply(IObject root, Func<Action<IObject>, Action<IObject>> g)
{
Continuation<IObject> action = c => thing => { g(c(c))(thing); };
Action<IObject> target = action(action);
target(root);
}
}
private class test2 : test
{
delegate void Continuation<T>(Continuation<T> r, T n);
//[TestMethod]
//Without curry, curring makes things go slow.
public override void TestMethod()
{
var root = BuildComposite();
Performance.Measure(1000000, () =>
{
Apply(root, (c, thing) =>
{
foreach (var item in thing.Children)
{
//Console.WriteLine(item.Name);
c(c, item);
}
});
}, "Time " + this.GetType().Name);
}
void Apply(IObject root, Continuation<IObject> f)
{
f(f, root);
}
}
private class test3 : test
{
//[TestMethod]
//Another common definition found on web, this is worse of then all.
//https://stackoverflow.com/questions/4763690/alternative-y-combinator-definition
public override void TestMethod()
{
var root = BuildComposite();
Performance.Measure(1000000, () =>
{
Y<IObject, int>(f => thing =>
{
foreach (var item in thing.Children)
{
//Console.WriteLine(item.Name);
f(item);
}
return 0;
})(root);
}, "Time " + this.GetType().Name);
}
public delegate TResult SelfApplicable<TResult>(SelfApplicable<TResult> r);
public static TResult U<TResult>(SelfApplicable<TResult> r)
{
return r(r);
}
public static Func<TArg1, TReturn> Y<TArg1, TReturn>(Func<Func<TArg1, TReturn>, Func<TArg1, TReturn>> f)
{
return U<Func<TArg1, TReturn>>(r => arg1 => f(U(r))(arg1));
}
}
private class test4 : test
{
//[TestMethod]
//Simpler definition, taken from this SO.
//This uses inherent compiler recursion, lets see if it speed things up.
//https://stackoverflow.com/questions/4763690/alternative-y-combinator-definition
public override void TestMethod()
{
var root = BuildComposite();
Performance.Measure(1000000, () =>
{
Y<IObject, int>(f => thing =>
{
foreach (var item in thing.Children)
{
//Console.WriteLine(item.Name);
f(item);
}
return 0;
})(root);
}, "Time " + this.GetType().Name);
}
public static Func<TArg1, TReturn> Y<TArg1, TReturn>(Func<Func<TArg1, TReturn>, Func<TArg1, TReturn>> f)
{
return f(n => Y(f)(n));
}
}
private class test5 : test
{
//[TestMethod]
//Simple way to recurse, is also the fastest
//but then its no more an anonymous lambda.
//This defeats the game purpose.
public override void TestMethod()
{
var root = BuildComposite();
Action<IObject> a = null;
a = thing =>
{
foreach (var item in thing.Children)
{
//Console.WriteLine(item.Name);
a(item);
}
};
Performance.Measure(1000000, () =>
{
a(root);
}, "Time " + this.GetType().Name);
}
}
private class test6 : test
{
//[TestMethod]
//Reference test, direct method call
//There should be no way to get faster than this.
public override void TestMethod()
{
var root = BuildComposite();
Performance.Measure(1000000, () =>
{
a(root);
}, "Time " + this.GetType().Name);
}
public static void a(IObject thing)
{
foreach (var item in thing.Children)
{
//Console.WriteLine(item.Name);
a(item);
}
}
}
private class test7 : test
{
//[TestMethod]
//Lets try some memoization.
public override void TestMethod()
{
var root = BuildComposite();
Performance.Measure(1000000, () =>
{
Y<IObject, int>.Combinator(f => thing =>
{
foreach (var item in thing.Children)
{
//Console.WriteLine(item.Name);
f(item);
}
return 0;
})(root);
}, "Time " + this.GetType().Name);
}
private class Y<TArg1, TReturn>
{
public static Func<TArg1, TReturn> Combinator(Func<Func<TArg1, TReturn>, Func<TArg1, TReturn>> f)
{
return f(n =>
{
if (memoized == null) memoized = Combinator(f);
return memoized(n);
});
}
private static Func<TArg1, TReturn> memoized;
}
}
private interface IObject
{
List<IObject> Children { get; }
}
private class CObject : IObject
{
private int lvl;
public CObject(int lvl)
{
this.lvl = lvl;
}
public List<IObject> Children
{
get
{
var l = new List<IObject>() { BuildComposite(lvl + 1) };
if (lvl > 2) l.Clear();
return l;
}
}
}
private static IObject BuildComposite()
{
return new CObject(0);
}
private static IObject BuildComposite(int lvl)
{
return new CObject(lvl);
}
private class Performance
{
public static void Measure(int count, Action a, string msg)
{
using(new WallClock(msg))
Enumerable.Range(1, count).ToList().ForEach(_ => a());
}
}
private class WallClock : IDisposable
{
private string name;
private Stopwatch w;
public WallClock(string name)
{
this.name = name;
w = Stopwatch.StartNew();
}
public void Dispose()
{
w.Stop();
Console.WriteLine(name + " : " + w.ElapsedMilliseconds);
}
}
参考
http://mikehadlow.blogspot.com.br/2009/03/recursive-linq-with-y-combinator.html
替代 Y 组合子定义
http://blogs.msdn.com/b/wesdyer/archive/2007/02/02/anonymous-recursion-in-c.aspx
http://www.justinshield.com/2011/06/recursive-lambda-expressions-in-c-using-ycombinator/