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  2. C# Memoization of functions with arbitrary number of arguments [closed]

C# Memoization of functions with arbitrary number of arguments [closed]

由 限于喜欢 提交于 2019-11-27 10:11:45

How about this? First write a one-argument memoizer:

static Func<A, R> Memoize<A, R>(this Func<A, R> f)
{
    var d = new Dictionary<A, R>();
    return a=> 
    {
        R r;
        if (!d.TryGetValue(a, out r))
        {
            r = f(a);
            d.Add(a, r);
        }
        return r;
    };
}

Straightforward. Now write a function tuplifier:

static Func<Tuple<A, B>, R> Tuplify<A, B, R>(this Func<A, B, R> f)
{
    return t => f(t.Item1, t.Item2);
}

And a detuplifier:

static Func<A, B, R> Detuplify<A, B, R>(this Func<Tuple<A, B>, R> f)
{
    return (a, b) => f(Tuple.Create(a, b));
}

and now a two-argument memoizer is easy:

static Func<A, B, R> Memoize<A, B, R>(this Func<A, B, R> f)
{
    return f.Tuplify().Memoize().Detuplify();
}

To write a three-argument memoizer just keep following this pattern: make a 3-tuplifier, a 3-untuplifier, and a 3-memoizer.

Of course, if you don't need them, there's no need to make the tuplifiers nominal methods:

static Func<A, B, R> Memoize<A, B, R>(this Func<A, B, R> f)
{
    Func<Tuple<A, B>, R> tuplified = t => f(t.Item1, t.Item2);
    Func<Tuple<A, B>, R> memoized = tuplified.Memoize();
    return (a, b) => memoized(Tuple.Create(a, b));
}

UPDATE: You ask what to do if there is no tuple type. You could write your own; it's not hard. Or you could use anonymous types:

static Func<T, R> CastByExample<T, R>(Func<T, R> f, T t) { return f; }

static Func<A, B, R> Memoize<A, B, R>(this Func<A, B, R> f)
{
    var example = new { A=default(A), B=default(B) };
    var tuplified = CastByExample(t => f(t.A, t.B), example);
    var memoized = tuplified.Memoize();
    return (a, b) => memoized(new {A=a, B=b});
}

Slick, eh?

UPDATE: C# 7 now has value tuples built in to the language; use them rather than rolling your own or using anonymous types.

First, you need to call sw.Reset() between your tests. Otherwise your results for the second test will be in addition to the time from the first.

Second, you probably shouldn't use vals.GetHashCode() in your GetHashCode() override on the comparer, as this will lead you to getting different hash codes for objects that would evaluate to true for your Equals override. For now, I would worry about making sure that equivalent objects always get the same hash code rather than trying to get an even distribution of the codes. If the hash codes don't match, Equals will never be called, so you'll end up processing the same parameters multiple times.

StopWatch.Stop does not reset the stopwatch so you are accumulating time on each start/stop.

For example

  Stopwatch sw = new Stopwatch();

  sw.Start();
  System.Threading.Thread.Sleep(100);
  sw.Stop();
  Debug.WriteLine(sw.ElapsedTicks);

  sw.Start();
  System.Threading.Thread.Sleep(100);
  sw.Stop();
  Debug.WriteLine(sw.ElapsedTicks);

Gives the following results

228221
454626

You can use StopWatch.Restart (Framework 4.0) to restart the stopwatch each time, or if not Framework 4.0, you can use StopWatch.Reset to reset the stopwatch.

Alternative (to tuples & anonymous types) approach might be as follows:

static void Main(string[] args)
{
    var func = Memoize<int, int, int>(Func);

    Console.WriteLine(func(3)(4));
    Console.WriteLine(func(3)(5));
    Console.WriteLine(func(2)(5));
    Console.WriteLine(func(3)(4));
}

//lets pretend this is very-expensive-to-compute function
private static int Func(int i, int j)
{
    return i + j;
}

private static Func<TArg1, Func<TArg2, TRes>> Memoize<TArg1, TArg2, TRes>(Func<TArg1, TArg2, TRes> func)
{
     Func<TArg1, Func<TArg2, TRes>> func1 = 
        Memoize((TArg1 arg1) => Memoize((TArg2 arg2) => func(arg1, arg2)));

    return func1;
}

private static Func<TArg, TRes> Memoize<TArg, TRes>(Func<TArg, TRes> func)
{
    var cache = new Dictionary<TArg, TRes>();

    return arg =>
    {
        TRes res;

        if( !cache.TryGetValue(arg, out res) )
        {
            Console.WriteLine("Calculating " + arg.ToString());

            res = func(arg);

            cache.Add(arg, res);
        }
        else
        {
            Console.WriteLine("Getting from cache " + arg.ToString());
        }

        return res;
    };
}

Based on these two Memoize funcs you can easily build extensions for as many args as you wish.

I initially came here just looking for an abstract memoization method for a no-parameter function. This isn't exactly an answer to the question but wanted to share my solution in case someone else came looking for the simple case.

public static class MemoizationExtensions
{
    public static Func<R> Memoize<R>(this Func<R> f)
    {
        bool hasBeenCalled = false; // Used to determine if we called the function and the result was the same as default(R)
        R returnVal = default(R);
        return () =>
        {
            // Should be faster than doing null checks and if we got a null the first time, 
            // we really want to memoize that result and not inadvertently call the function again.
            if (!hasBeenCalled)
            {
                hasBeenCalled = true;
                returnVal = f();
            }
            return returnVal;
        };
    }
}

If you use LinqPad you can use the following code to easily test out the functionality through the use of LinqPad's super cool Dump method.

new List<Func<object>>(new Func<object>[] {
        () => { "Entered func A1".Dump(); return 1; },
        () => { "Entered func A2".Dump(); return default(int); },
        () => { "Entered func B1".Dump(); return String.Empty; },
        () => { "Entered func B2".Dump(); return default(string); },
        () => { "Entered func C1".Dump(); return new {Name = String.Empty}; },
        () => { "Entered func C2".Dump(); return null; },
    })
    .ForEach(f => {
        var f1 = MemoizationExtensions.Memoize(f);
        Enumerable
            .Range(1,3)
            .Select(i=>new {Run=i, Value=f1()})
            .Dump();
    });

P.S. You will need to include the MemoizationExtensions class in the code of the LinqPad script otherwise it won't work!

标签
c#
performance
function
memoization
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