ADVERTISEMENT

Programmer’s Challenge

By Mike Scanlin, Mountain View, CA

Method Dispatcher

One of the main reasons why I don’t like object oriented languages is because of the inefficiencies the language usually introduces on the runtime code. If you’ve ever traced through a method dispatch routine then you know what I mean (what ever happened to the days of simple, direct JSR’s?). This month you have a chance to write a fast method dispatcher. Who knows? If it’s efficient enough I might just toss my assembler and use your dispatcher with a high level language instead...

The prototype of the function you write is:

typedef unsigned short ushort;
typedef ushort ClassID;
typedef ushort MethodNumber;

MethodAddress
FindMethod(theClassID, theMethodNumber)
ClassID theClassID;
MethodNumbertheMethodNumber;

TheMethodNumber is the number of the method you’re trying to find the address of and theClassID is the ID of the class you want it for. You’ll pass theClassID to a function called GetClassPtr to get a pointer to a Class data structure, which looks like this:

typedef void *MethodAddress;

typedef struct {
 MethodNumber  methodNumber;
 MethodAddress methodAddress;
} MethodEntry;

typedef struct {
 ushort inheritedCount;
 ushort inheritedClasses[15];
 MethodNumber  largestMethodNumber;
 ushort methodCount;
 MethodEntrymethods[];
} Class, *ClassPtr;

The function GetClassPtr will be part of my test bench, although you’ll have to implement at least a rudimentary version of it to test your program (or you can e-mail me for a sample version):

ClassPtr
GetClassPtr(classID)
unsigned short classID;

If GetClassPtr returns -1 (kClassNotFound) then the class cannot be found and your FindMethod routine should return 0 (kMethodNotFound). I will be providing sample data and a sample GetClassPtr function for those who are interested. To get a copy, send me e-mail at scanlin@genmagic.com (internet) or any of the Programmer Challenge addresses listed on page 2.

Once you have a ClassPtr you should look in that class’s methods[] array to see if you can find an entry whose methodNumber is equal to theMethodNumber. Methods[] is a variable-length array (thus, making Class a variable-size structure) containing methodCount number of entries which are sorted smallest to largest by methodNumber. MethodCount is 1-based and is always greater than zero. If you find a match then you should return the corresponding methodAddress.

If you don’t find a match then you should look at the inherited classes (starting with index zero) to see if the method is implemented by one of this class’s superclasses. We support multiple inheritance here and the number of classes we inherit from is stored in inheritedCount (which will be from zero to 15). The class IDs of the classes we inherit from are stored in the inheritedClasses[ ] array. You can pass any of the entries in inheritedClasses to GetClassPtr to get a ClassPtr to that class.

If you can’t find the requested methodNumber in any part of the inheritance tree then FindMethod should return zero (kMethodNotFound).

Here’s a simple example. These 54 bytes (starting at location 0x1000) represent class ID 5:

1000:00000000 00000000 00000000 00000000 
1010:00000000 00000000 00000000 00000000 
1020:00680003 0023AAAA AAAA0057 BBBBBBBB 
1030:0068CCCC CCCC 

The short at location 1000 (inheritedCount) tells us that there are no inherited classes for this class. The short at location 1022 (methodCount) tells us that this class has 3 methods. Methods[0] is from 1024 to 1029; the methodNumber is 23 and the methodAddress is AAAAAAAA (this is just test data to illustrate the structure). Methods[1] is from 102A to 102F and methods[2] is from 1030 to 1035. The short at location 1020 (largestMethodNumber) is equal to the methodNumber of the last MethodEntry in the list (which is the largest methodNumber overall since the list is sorted). In other words, the expression theClassPtr->largestMethodNumber == theClassPtr-> methods[theClassPtr->methodCount-1].methodNumber is always true.

If this class had inherited from class 7 and class 9 then it would have looked like this instead:

1000:00020007 00090000 00000000 00000000 
1010:00000000 00000000 00000000 00000000 
1020:00680003 0023AAAA AAAA0057 BBBBBBBB 
1030:0068CCCC CCCC 

In either case, if you call GetClassPtr(5), since this is class 5 we’re looking at, you would have the value 0x1000 (as type ClassPtr) returned to you.

Since I’ll be calling FindMethod several thousand times with the same set of classes (just like a real runtime system!) you’ll probably want to implement some kind of cache. And since it is desirable for runtime systems to take as little memory as possible, we’re going to have a rule that says your code cannot use more than 16K of memory for its cache (use a static to keep a pointer to it). The total number of methods in the set of classes I’ll be testing with is about 5000, numbered from 1 to 5000. The total number of classes is about 400, numbered from 1 to 400. Those 400 classes will implement an average of 15 methods each and will inherit from an average of 5 other classes (that’s 5 total, once you’ve walked the entire inheritance tree for a particular class). Of course, some methods will be called frequently while others are hardly ever called.

Because this is a little complex, I’m going to give you the brute force way of doing what I’ve described. I’m sure you can do better than this (I’ve used short variable names so that the code will fit in the magazine column):

MethodAddress
FindMethod(cid, mn)
ClassID cid;
MethodNumbermn;
{
 ClassPtr cp;
 MethodAddress addr;
 int    i;
 
 cp = GetClassPtr(cid);
 if (cp == kClassNotFound)
 return kMethodNotFound;
 
 /* look in this class */
 i = 0;
 do {
 if (mn == cp->methods[i].methodNumber)
 return cp->methods[i].methodAddress;
 i++;
 } while (i < cp->methodCount);
 
 /* look in superclasses */
 i = 0;
 while (i < cp->inheritedCount) {
 addr = FindMethod( cp->inheritedClasses[i], mn);
 if (addr != kMethodNotFound)
 return addr;
 i++;
 }
 return kMethodNotFound;
}

E-mail me if you have any questions or if you want the sample data and GetClassPtr function. And if you want to see your name in print all you have to do is either enter a challenge or have me use one of your suggested challenges.

Two Months Ago Winner

Congratulations to Kevin Cutts (Schaumburg, IL) for winning the Poker Hand Evaluator Challenge. And kudos to Gustav Larsson (Mountain View, CA) for being 60% smaller and only about 4% slower than Kevin.

Here are the times and code sizes for each entry. Numbers in parens after a person’s name indicate how many times that person has finished in the top 5 places of all previous Programmer Challenges, not including this one:

Name time code

Kevin Cutts (3) 317 6022

Gustav Larsson 331 2656

Jeff Mallett (3) 331 9428

Ernst Munter (5) 457 2516

Dave Darrah (1) 749 2996

Raffi Kasparian (1) 1230 7394

Kevin wrote four different versions of his BestHand routine; one for every combination of the Booleans wildCardAllowed and straightsAndFlushesValid. That’s a great idea for performance but because of space constraints, we’re only listing the BestHandNoWild version which is for the case where wild cards are not allowed but straights and flushes are valid (which is probably the typical case for poker). The source code to the remaining cases can be found on-line or on this month’s code disk.

Here is Kevin’s winning solution:

January Solution -Poker-

by Kevin M. Cutts

#include <stdlib.h>
#include <stdio.h>

typedef unsigned char Card;

typedef struct SevenCardHand 
{
 Card cards[7];
} SevenCardHand;

typedef struct FiveCardHand
{
 Card cards[5];
} FiveCardHand;

short ComparePokerHands(
 SevenCardHand *, 
 SevenCardHand *,
 FiveCardHand *,
 FiveCardHand *,
 Boolean,
 Card,
 Boolean,
 void *); 

/* Used to remove the suit information and leave only the count from the card */
unsigned char theValue[] = {
0,1,2,3,4,5,6,7,8,9,10,11,12,
0,1,2,3,4,5,6,7,8,9,10,11,12,
0,1,2,3,4,5,6,7,8,9,10,11,12,
0,1,2,3,4,5,6,7,8,9,10,11,12,
};

/* Used to remove card value and leave the suit indicator */
unsigned char theSuit[] = {
0,0,0,0,0,0,0,0,0,0,0,0,0,
1,1,1,1,1,1,1,1,1,1,1,1,1,
2,2,2,2,2,2,2,2,2,2,2,2,2,
3,3,3,3,3,3,3,3,3,3,3,3,3,
};

/* A bit for each card value (aces have two bits) */
unsigned short theValueBit[] = {
0x1000,0x800,0x400,0x200,0x100,0x80,0x40,
 0x20,0x10,0x8,0x4,0x2,0x2001,
0x1000,0x800,0x400,0x200,0x100,0x80,0x40,
 0x20,0x10,0x8,0x4,0x2,0x2001,
0x1000,0x800,0x400,0x200,0x100,0x80,0x40,
 0x20,0x10,0x8,0x4,0x2,0x2001,
0x1000,0x800,0x400,0x200,0x100,0x80,0x40,
 0x20,0x10,0x8,0x4,0x2,0x2001,
};

#define fiveAlike0xa000
#define straightFlush0x9000
#define fourAlike0x8000
#define fullHouse0x7000
#define flush    0x6000
#define straight 0x5000
#define threeAlike 0x4000
#define twoPair  0x3000
#define pair0x2000

#define nonCard 0xff
/* These four functions are custom to handle the four bools wild and flush */
unsigned int BestHandNoWild(SevenCardHand *theHand, 
 FiveCardHand *theBest);
unsigned int BestHand(SevenCardHand *theHand, 
 FiveCardHand *theBest, Card wildCard);
unsigned int BestHandNoFlush(SevenCardHand *theHand, 
 FiveCardHand *theBest, Card wildCard);
unsigned int BestHandNoFlushNoWild(SevenCardHand *theHand, 
 FiveCardHand *theBest);

BestHandNoWild

unsigned int BestHandNoWild(SevenCardHand *theHand, 
 FiveCardHand *theBest)
{
    /* How many of each card value encountered */
 unsigned char handValues[13];
    /* How many of each suit encountered */ 
 unsigned char handSuits[4];
    /* Bit field describing on a suit by suit basis how populated the hand is */ 
 short handRuns[4]; 
 register short i;
 short j;
 Card bestCard, bestPair, goodPair, bestTri, bestQuad;
 Card *cardPtr;
 short runSweep, runResult, runCount;

    /* Zero out all of the counts */
 *(long *)&handValues[0] = 
 *(long *)&handValues[4] = 
 *(long *)&handValues[8] = 
 handValues[12] = 0;
 *(long *)&handSuits[0] = 0;
 *(long *)&handRuns[0] = *(long *)&handRuns[2] = 0;

    /* Now accumulate the values */
 for (i=0, cardPtr=theHand->cards; i<7; i++, cardPtr++)
 {
 handValues[theValue[*cardPtr]]++;
 handSuits[theSuit[*cardPtr]]++;
 handRuns[theSuit[*cardPtr]] |= theValueBit[*cardPtr];
 }
    /* First count the pairs, tris, quads and penta */
 bestCard = bestPair = goodPair = bestTri = bestQuad = nonCard;
 for (i = 12; i >= 0; i--)
 {
 if (!(j = handValues[i])) continue;
 if (j == 4)
 {
 bestQuad = i;
 break;
 }
 if (j == 3 && bestTri == nonCard)
 {
 bestTri = i;
 if (bestPair != nonCard)
 {
 /* Full house */
 break;
 }
 }
 else if (j == 2 && bestPair == nonCard)
 {
 bestPair = i;
 if (bestTri != nonCard)
 {
 /* Full house */
 break;
 }
 }
 else if (j == 2 && goodPair == nonCard)
 {
 goodPair = i;
 }
 else if (bestCard == nonCard)
 {
 bestCard = i;
 }
 }
    /* Now check for a straight flush */
#define CHK_SUIT_NOWILD(suit) \
 if (handSuits[suit] >= 5) \
 { \
 for (runSweep=0x1f;runSweep<0x1fff;runSweep<<= 1) \
 { \
 runResult = handRuns[suit] & runSweep; \
 if (runResult == runSweep) \
 { \
 /* Transfer the five cards */ \
 for (i=0, j=0; j < 5;i++) \
 { \
 if ((theValueBit[theHand->cards[i]] & \
 runSweep && suit == \
 theSuit[theHand->cards[i]])) \
 { \
 theBest->cards[j++] = \
 theHand->cards[i]; \
 } \
 } \
 return straightFlush; \
 } \
 } \
 }
 CHK_SUIT_NOWILD(0);
 CHK_SUIT_NOWILD(1);
 CHK_SUIT_NOWILD(2);
 CHK_SUIT_NOWILD(3);
    /* Next comes four of a kind */
 if (bestQuad != nonCard)
 {
    /* Transfer the five cards */
 runSweep = 1;
 for (i=0, j=0; j < 5;i++)
 {
 if (theValue[theHand->cards[i]] == bestQuad)
 {
 theBest->cards[j++] = theHand->cards[i];
 }
 else if (runSweep)
 {
 theBest->cards[j++] = theHand->cards[i];
 runSweep--;
 }
 }
 return fourAlike | bestQuad;
 }
    /* Next is the full house */
 if (bestTri != nonCard && bestPair != nonCard)
 {
    /* Transfer the five cards */
 for (i=0, j=0; j < 5;i++)
 {
 if (theValue[theHand->cards[i]] == bestTri || 
 theValue[theHand->cards[i]] == bestPair)
 {
 theBest->cards[j++] = theHand->cards[i];
 }
 }
 return fullHouse | (bestTri<<8) | (bestPair);
 }
    /* Now the flush */
#define CHK_FLUSH_NOWILD(suit) \
 if (handSuits[suit] >= 5) \
 { \
    /* Transfer the five cards */ \
 for (i=0, j=0; j < 5;i++) \
 { \
 if (theSuit[theHand->cards[i]] == suit) \
 { \
 theBest->cards[j++] = theHand->cards[i]; \
 } \
 } \
 return flush | bestCard; \
 }
 CHK_FLUSH_NOWILD(0);
 CHK_FLUSH_NOWILD(1);
 CHK_FLUSH_NOWILD(2);
 CHK_FLUSH_NOWILD(3);
    /* Next the straight */
 j = handRuns[0]|handRuns[1]|handRuns[2] | handRuns[3];
 for (runSweep=0x1f; runSweep < 0x1fff; runSweep <<= 1)
 {
 runResult = j & runSweep;
 if (runResult == runSweep)
 {
    /* Transfer the five cards */
 for (i=0, j=0; j < 5;i++)
 {
 if ((theValueBit[theHand->cards[i]] & runSweep))
 {
 theBest->cards[j++] = theHand->cards[i];
 }
 }
 return straight;
 }
 }
    /* and the three of a kind */
 if (bestTri != nonCard)
 {
    /* Transfer the five cards */
 runSweep = 2;
 for (i=0, j=0; j < 5;i++)
 {
 if (theValue[theHand->cards[i]] == bestTri)
 {
 theBest->cards[j++] = theHand->cards[i];
 }
 else if (runSweep)
 {
 theBest->cards[j++] = theHand->cards[i];
 runSweep--;
 }
 }
 return threeAlike | bestTri;
 }
    /* Now two pair */
 if (bestPair != nonCard && goodPair != nonCard)
 {
    /* Transfer the five cards */
 runSweep = 1;
 for (i=0, j=0; j < 5;i++)
 {
 if (theValue[theHand->cards[i]] == bestPair ||
 theValue[theHand->cards[i]] == goodPair)
 {
 theBest->cards[j++] = theHand->cards[i];
 }
 else if (runSweep)
 {
 theBest->cards[j++] = theHand->cards[i];
 runSweep--;
 }
 }
 return twoPair | (bestPair << 8) | goodPair;
 }
    /* And finally a single pair */
 if (bestPair != nonCard)
 {
    /* Transfer the five cards */
 runSweep = 3;
 for (i=0, j=0; j < 5;i++)
 {
 if (theValue[theHand->cards[i]] == bestPair)
 {
 theBest->cards[j++] = theHand->cards[i];
 }
 else if (runSweep)
 {
 theBest->cards[j++] = theHand->cards[i];
 runSweep--;
 }
 }
 return pair | bestPair;
 }
    /* Transfer the five cards */
 runSweep = 4;
 for (i=0, j=0; j < 5;i++)
 {
 if (theValue[theHand->cards[i]] == bestCard)
 {
 theBest->cards[j++] = theHand->cards[i];
 }
 else if (runSweep)
 {
 theBest->cards[j++] = theHand->cards[i];
 runSweep--;
 }
 }
 return bestCard;
}

ComparePokerHands

short ComparePokerHands(
 SevenCardHand *hand1Ptr, 
 SevenCardHand *hand2Ptr,
 FiveCardHand *best1Ptr,
 FiveCardHand *best2Ptr,
 Boolean wildCardAllowed,
 Card wildCard,
 Boolean straightsdAndFlushesValid,
 void *privateDataPtr)
{
 unsigned int hand1Value, hand2Value;
 if (wildCardAllowed && straightsdAndFlushesValid)
 {
 hand1Value = BestHand(hand1Ptr, best1Ptr, wildCard);
 hand2Value = BestHand(hand2Ptr, best2Ptr, wildCard);
 }
 else if (wildCardAllowed)
 {
 hand1Value = BestHandNoFlush(hand1Ptr,best1Ptr,wildCard);
 hand2Value = BestHandNoFlush(hand2Ptr,best2Ptr,wildCard);
 }
 else if (straightsdAndFlushesValid)
 {
 hand1Value = BestHandNoWild(hand1Ptr, best1Ptr);
 hand2Value = BestHandNoWild(hand2Ptr, best2Ptr);
 }
 else
 {
 hand1Value = BestHandNoFlushNoWild(hand1Ptr, best1Ptr);
 hand2Value = BestHandNoFlushNoWild(hand2Ptr, best2Ptr);
 }
 if (hand1Value > hand2Value) return -1;
 if (hand1Value < hand2Value) return 1;
 return 0;
}