Memoization

I've spent a couple of hours to write a couple of helper classes to help with the memoization of function calls. Since I haven't found anything as elegant on Google, I thought I'd put them here in case anyone else can use them.

Caveat: the code is not thread-safe. I didn't have a use case for it and thread safety is tricky so I didn't want to spend the time. Do not use in multi-threaded code!

First, the Memoizer class:

  public class Memoizer
  {
    public static Func<T> Memoize<T>(Func<T> f)
    {
      // do NOT inline - it will create a new one on each subsequent call and thus not cache anything
      var map = new KeyMap<T>();

      return () => map.GetValue(new KeyStruct(), f);
    }

    public static Func<T1, T> Memoize<T1, T>(Func<T1, T> f)
    {
      // do NOT inline - it will create a new one on each subsequent call and thus not cache anything
      var map = new KeyMap<T>();

      return arg => map.GetValue(new KeyStruct(arg), () => f(arg));
    }

    public static Func<T1, T2, T> Memoize<T1, T2, T>(Func<T1, T2, T> f)
    {
      // do NOT inline - it will create a new one on each subsequent call and thus not cache anything
      var map = new KeyMap<T>();

      return (a1, a2) => map.GetValue(new KeyStruct(a1, a2), () => f(a1, a2));
    }
  }

As the comment points out, though it is tempting to make the methods one line, the map creation must not be inlined in the lambda because a new one will be created each time, thus disabling any caching. It also must not be created once, in a constructor, because you do not want to use the same cache for two distinct functions that share the same signature - caching Math.Sin and Math.Cos would be bad, even though they both have the same Func<double, double> signature.

The above class uses two others:

  public class KeyMap<T>
  {
    public KeyMap()
    {
      map = new Dictionary<KeyStruct, T>();
    }

    public T GetValue(KeyStruct key, Func<T> f)
    {
      T value;
      if (!map.TryGetValue(key, out value))
      {
        value = f();
        map.Add(key, value);
      }

      return value;
    }

    //

    private readonly Dictionary<KeyStruct, T> map;
  }

and

  public class KeyStruct
  {
    public KeyStruct(params object[] args)
    {
      if (args == null)
        throw new ArgumentNullException("args");

      this.args = args.Select(arg => arg ?? 0).ToArray();
    }

    public override bool Equals(object obj)
    {
      if (!(obj is KeyStruct))
        return false;

      var compareTo = (KeyStruct) obj;
      if (args.Length != compareTo.args.Length)
        return false;

      return args
        .Zip(compareTo.args, (o1, o2) => new { o1, o2 })
        .All(pair => pair.o1.Equals(pair.o2));
    }

    public override int GetHashCode()
    {
      unchecked
      {
        return args.Aggregate(17, (acc, arg) => acc * 23 + arg.GetHashCode());
      }
    }

    public static bool operator ==(KeyStruct obj1, KeyStruct obj2)
    {
      return obj1.Equals(obj2);
    }

    public static bool operator !=(KeyStruct obj1, KeyStruct obj2)
    {
      return !(obj1 == obj2);
    }

    //

    private readonly object[] args;
  }

The KeyStruct class is simply there to allow me to use a dictionary even though the key can only be a single value. For the hash calculation I have adapted Jon Skeet's code here.

Since I have developed these classes using TDD, here are the final tests:

  [TestClass]
  public class KeyStructTests
  {
    [TestMethod]
    public void EmptyListOfArgsReturnsHash17()
    {
      var sut = new KeyStruct(new object[0]);

      var result = sut.GetHashCode();

      Assert.AreEqual(17, result);
    }

    [TestMethod]
    public void TwoEmptyKeyStructsAreEqual()
    {
      var k1 = new KeyStruct();
      var k2 = new KeyStruct();

      var result = k1 == k2;

      Assert.IsTrue(result);
    }

    [TestMethod]
    public void OneArgEqualTo0ReturnsHash391()
    {
      var sut = new KeyStruct(new object[] { 0 });

      var result = sut.GetHashCode();

      Assert.AreEqual(391, result);
    }

    [TestMethod]
    public void OneArgEqualTo5ReturnsHash396()
    {
      var sut = new KeyStruct(new object[] { 5 });

      var result = sut.GetHashCode();

      Assert.AreEqual(396, result);
    }

    [TestMethod]
    public void EqualityReturnsTrueForEqualValues()
    {
      var s1 = new KeyStruct(new object[] { 1, "a", 3.5m });
      var s2 = new KeyStruct(new object[] { 1, "a", 3.5m });

      var result = s1 == s2;

      Assert.IsTrue(result);
    }

    [TestMethod]
    public void EqualityReturnsFalseForDifferentValues_1()
    {
      var s1 = new KeyStruct(new object[] { 1, "a", 3.5m });
      var s2 = new KeyStruct(new object[] { 2, "b", 4.5m });

      var result = s1 == s2;

      Assert.IsFalse(result);
    }

    [TestMethod]
    public void EqualityReturnsFalseForDifferentValues_2()
    {
      var s1 = new KeyStruct(new object[] { 1, "a", 3.5m });
      var s2 = new KeyStruct(new object[] { 1, "a" });

      var result = s1 == s2;

      Assert.IsFalse(result);
    }
  }

and

  public class MemoizerTests
  {
    [TestClass]
    public class ZeroArgs
    {
      [TestMethod]
      public void CallsTheUnderlyingFuncOnFirstCall()
      {
        var called = false;
        Func<int> original = () =>
        {
          called = true;
          return 5;
        };
        var memoized = Memoizer.Memoize(original);

        var result = memoized.Invoke();

        Assert.AreEqual(5, result);
        Assert.IsTrue(called);
      }

      [TestMethod]
      public void ReturnsCachedValueForSubsequentCalls()
      {
        var called = 0;
        Func<int> original = () =>
        {
          called++;
          return 5;
        };
        var memoized = Memoizer.Memoize(original);

        memoized.Invoke();
        memoized.Invoke();
        var result = memoized.Invoke();

        Assert.AreEqual(5, result);
        Assert.AreEqual(1, called);
      }
    }

    [TestClass]
    public class OneArg
    {
      [TestMethod]
      public void CallsTheUnderlyingFuncOnFirstCall()
      {
        var called = false;
        Func<int, int> original = arg =>
        {
          called = true;
          return arg * 5;
        };
        var memoized = Memoizer.Memoize(original);

        var result = memoized.Invoke(7);

        Assert.AreEqual(35, result);
        Assert.IsTrue(called);
      }

      [TestMethod]
      public void ReturnsCachedValueForSubsequentCalls()
      {
        var called = 0;
        Func<int, int> original = arg =>
        {
          called++;
          return arg * 5;
        };
        var memoized = Memoizer.Memoize(original);

        memoized.Invoke(7);
        memoized.Invoke(7);
        var result = memoized.Invoke(7);

        Assert.AreEqual(35, result);
        Assert.AreEqual(1, called);
      }

      [TestMethod]
      public void CallsTheUnderlyingFuncWhenCalledWithDifferentArguments()
      {
        var called = new Dictionary<int, bool>();
        Func<int, int> original = arg =>
        {
          called[arg] = true;

          return arg * 5;
        };
        var memoized = Memoizer.Memoize(original);

        var result1 = memoized.Invoke(7);
        var result2 = memoized.Invoke(8);

        Assert.AreEqual(35, result1);
        Assert.AreEqual(40, result2);
        Assert.IsTrue(called[7]);
        Assert.IsTrue(called[8]);
      }
    }

    [TestClass]
    public class TwoArgs
    {
      [TestMethod]
      public void CallsTheUnderlyingFuncOnFirstCall()
      {
        var called = false;
        Func<int, int, int> original = (a1, a2) =>
        {
          called = true;
          return a1 * a2 * 5;
        };
        var memoized = Memoizer.Memoize(original);

        var result = memoized.Invoke(7, 8);

        Assert.AreEqual(280, result);
        Assert.IsTrue(called);
      }

      [TestMethod]
      public void ReturnsCachedValueForSubsequentCalls()
      {
        var called = 0;
        Func<int, int, int> original = (a1, a2) =>
        {
          called++;
          return a1 * a2 * 5;
        };
        var memoized = Memoizer.Memoize(original);

        memoized.Invoke(7, 8);
        memoized.Invoke(7, 8);
        var result = memoized.Invoke(7, 8);

        Assert.AreEqual(280, result);
        Assert.AreEqual(1, called);
      }

      [TestMethod]
      public void CallsTheUnderlyingFuncWhenCalledWithDifferentArguments()
      {
        var called = new Dictionary<int, bool>();
        Func<int, int, int> original = (a1, a2) =>
        {
          called[a1 * 10 + a2] = true;

          return a1 * a2 * 5;
        };
        var memoized = Memoizer.Memoize(original);

        var result1 = memoized.Invoke(7, 8);
        var result2 = memoized.Invoke(8, 7);

        Assert.AreEqual(280, result1);
        Assert.AreEqual(280, result2);
        Assert.IsTrue(called[78]);
        Assert.IsTrue(called[87]);
      }
    }
  }

As I said, I hope this helps someone. Please let me know if it does - or if you spot any problems.

Comments

Ivan Stepaniuk said…
Kudos for including the tests in your posts!
5:42 PM
Marcel said…
Thank you. Yes, I prefer to write everything test-first, it helps in a lot of ways.
6:16 PM
Post a Comment

Popular posts from this blog

Posting dynamic Master / Detail forms with Knockout

Image
I have the following dynamic form in my GitHub Inventory project : I created the form with KnockoutJS , using some ideas from this article . The detail view looks like this: <table id="acquisition_items" class="jtable"> <thead> <tr> <th>Product Name</th> <th>Quantity</th> <th>Price</th> <th>Value</th> <th><button data-bind="click: addItem">[+]</button></th> </tr> </thead> <tbody data-bind="foreach: items"> <tr> <td> <input data-bind="value: ProductName" type="text" value="" /> </td> <td> <input data-bind="value: Quantity" type="text" value="" /> </td> <td> ...
Read more

Comparing Excel files, take two

Image
I already wrote this project once, on GitHub , but I can't say that I like the result. I wrote that code without TDD, mostly to see if I can still work in the "normal" fashion. I can, but I had a bug that took me a while to even discover and then was difficult to write tests for. I also hate the temporal dependency in that design (if you don't call the SortBy method first, the ExcludeRecords method will return incorrect results). Anyway; here's attempt number two, writing that code using TDD. Structure I start by creating my usual structure for projects of this type - a library for the logic, a console application for the main program and a test project for tests. I also use the Enable NuGet Package Restore option (right-click on the solution node) to add the three NuGet files that will allow someone to automatically download all the packages I'm referencing without wasting space in the source code repository. (Right now I've only added the Moq pa...
Read more

POST-ing to an ASP.NET MVC form with an anti-forgery token

I've had some issues trying to write an acceptance test that was registering a new user by POST-ing the required information to a MVC site and I got back these messages: The required anti-forgery cookie "__RequestVerificationToken" is not present. The required anti-forgery form field "__RequestVerificationToken" is not present. Validation of the provided anti-forgery token failed. The cookie "_RequestVerificationToken" and the form field "_RequestVerificationToken" were swapped. Since it took me a bit of fiddling with the code before I managed to make it work, I thought I'd share the solution. The site is an ASP.NET MVC version 5 and I am trying to register a new user (POST-ing to the /Account/Register URL). The main issue you will encounter is having to extract two anti-forgery tokens, one from the cookies and one from the form, and then sending both of them in the appropriate places (cookies vs form field). I have used LinqPad 5 w...
Read more