[Issue 8743] New: Add support for memoizing class methods

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8743

           Summary: Add support for memoizing class methods
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: andrej.mitrovich at gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich at gmail.com> 2012-10-01 12:33:24 PDT ---
This was asked for here:
http://stackoverflow.com/questions/12677498/how-do-i-use-std-functional-memoize-inside-a-class

There could be use-cases for this even though methods usually depend on
internal class state. Perhaps if the method was @pure it would be safe to use
it. Anyway I have an experimental implementation: 

module test;

import std.stdio;
import std.traits;
import std.typecons;
import std.datetime;

template isClassStruct(alias fun)
{
    enum bool isClassStruct = (is(fun == class) || is(fun == struct));
}

mixin template memoize(alias fun, uint maxSize = uint.max)
    if (isClassStruct!(__traits(parent, fun)))
{
    ReturnType!fun opCall(ParameterTypeTuple!fun args)
    {
        static ReturnType!fun[Tuple!(typeof(args))] memo;
        auto t = tuple(args);
        auto p = t in memo;
        if (p) return *p;
        static if (maxSize != uint.max)
        {
            if (memo.length >= maxSize) memo = null;
        }

        mixin("auto r = this." ~ __traits(identifier, fun) ~ "(args);");
        memo[t] = r;
        return r;
    }    
}

class A 
{
    int slowFunc(int a, int b) 
    { 
        int result;
        foreach (_; 0 .. 1024)
        {
            result += a;
            result += b;
        }
        return result;
    }

    mixin memoize!slowFunc fastFunc;
}

enum CallCount = 2048;

void main() 
{
    A a = new A;

    auto sw1 = StopWatch(AutoStart.yes);
    foreach (x; 0 .. CallCount)
    {
        a.slowFunc(100, 100);  // 11232 usecs
    }
    sw1.stop();
    writeln(sw1.peek.usecs);

    auto sw2 = StopWatch(AutoStart.yes);
    foreach (x; 0 .. CallCount)
    {
        a.fastFunc(100, 100);  // 302 usecs
    }
    sw2.stop();
    writeln(sw2.peek.usecs);
}

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