TweetFollow Us on Twitter

Jun 93 Challenge
Volume Number:9
Issue Number:6
Column Tag:Programmers’ Challenge

Programmers’ Challenge

By Mike Scanlin, MacTech Magazine Regular Contributing Author

Note: Source code files accompanying article are located on MacTech CD-ROM or source code disks.


This month’s Challenge is a real world problem previously solved without reals by real MacTech reader Allen Stenger (Gardena, CA): You are given a large array of unique city names in alphabetical order ending with a sentinel of ‘ZZZ’ and a single city name to find in this array. You are to return the index of the desired city, or if it doesn’t match exactly, a list of the five closest matches. If there are more than five closest ones, return any five of them. The closeness-of-match algorithm, for purposes of this challenge, is “differing in the fewest positions from the desired name.” Take the longest of the two strings you’re comparing and each time a position doesn’t match (or is beyond the end of the shorter string) add 1 to your difference count; the smallest difference count is considered closest.

The input city list is an array of Str255s (pascal strings) in one big block where there will be, say, 200 to 1000 unique city names (in alphabetical order, all uppercase, with the ‘ZZZ’ sentinel as the last element). The city you are to find is the uppercase string pointed to by cityToFindNamePtr. The prototype of the function you write is:

Boolean FindCity(cityNames,
 cityToFindNamePtr, closestMatches)
Str255  cityNames[];
Str255  *cityToFindNamePtr;
unsigned short   closestMatches[];

If there is an exact match, return TRUE and set closestMatches[0] equal to the zero-based index of the match. If there is not an exact match, return FALSE and set closestMatches[0..4] to the five closest matches (in order of closeness, or, if they are equally close, in any order).

Allen says that this problem came up while working on the American Airlines reservation system, SABRE, a long time ago. He says that his tests then showed that city names were misspelled about 1/3rd of the time due to either poor spelling or typing by the reservation agents (no offense intended). You can use that information in your solution if you like; I will be simulating it with test data (exact matches will occur only 33% of the time I call your routine from my test bench).


Congratulations to Steve Israelson (Vancouver, B.C.) for winning the “Rotated Bits” challenge. Right on his heels is Andy Scheck (location unknown) with an entry that was half as many bytes but which was just a little too slow in some cases to win. Jeff Mallett (Hickory, NC) deserves praise for his extremely small solution which was also near the top in terms of speed. Because it is so small, it is listed here as well, after Steve’s winning solution.

There were 16 entries to this challenge. Two of them were disqualified immediately because they were in assembly (nice try) and six others were disqualified for giving incorrect results (try testing with a 9x17 bitmap). Many people e-mailed me and asked if they could assume that rowBytes was divisible by four. The answer is: no. However, you could always case out on that case early on in your routine and then call one of two rotate functions based on if (rowBytes % 4 == 0) or not. Just so long as your solution has one entry point you can do whatever you like inside.

Here’s a summary of the eight entries that gave correct results. The bytes column is the code size, test 1 is the ticks to rotate 4000 small bitmaps (20x30 and smaller) and test 2 is the ticks to rotate 400 medium size bitmaps (300x400 pixels, approx). In all cases, the bitmaps were filled with the same random data before rotating:

Name bytes test 1 test 2

Steve Israelson 740 72 638

Andy Scheck 386 75 778

Jeff Mallett 194 91 936

Patrick Breen 668 125 902

Stepan Riha 656 122 1186

Dave Darrah 362 138 1453

Jan Bruyndonckx 490 205 1785

Dominic Mazzoni 244 630 6687

Steve’s winning solution is reasonably commented so I won’t go into detail here describing how it works. I will, however, offer some peephole optimizations to make it even better.

The first thing that the optimized eye notices about his code is that he multiplies, divides and mods by 8. It’s a dangerous assumption to make that the compiler will be smart and substitute shifts and ands where it can. I always prefer to make them explicit. Thus, I would change these lines:

 srcRowX8 = srcRowBytes * 8;
 srcData = const1 + srcX / 8;
 if (remainder = (srcWidth % 8))

to these:

 srcRowX8 = srcRowBytes << 3;
 srcData = const1 + (srcX >> 3);
 if (remainder = (srcWidth & (8 - 1)))

In addition to those, there is one point in his code where he multiplies by 7. He’d be better off by multiplying by 8 (using a shift) and then subtracting one. This:

 src += srcOffset * 7;
is faster as:

 src += srcOffset << 3;
 src -= srcOffset;

After I made these changes and then ran test 2, the time went from 638 ticks to 542 ticks (it reduced the code size by 18 bytes, too).

Here’s Steve’s winning solution, followed by Jeff’s tiny solution:

 Steve Israelson
 Motion Works Corp.
void RotateBitMapClockwise(BitMap *, BitMap *);
void rotateBlock(unsigned long *,unsigned char *,
 long,unsigned char *,long);
void rotateLimitedBlock(unsigned long *,
 unsigned char *, long,unsigned char *,long,

 Rotate the bitmap from src to dst by +90 degrees.  Make 
 a table for use in the rotate block routine.  Step through 
 the src in 8x8 blocks, starting from the bottom left.  Step 
 through dst in 8x8 blocks,  starting from the top left. 
 Handle the right edge of the bitmap specially to prevent bits 
 from being rotated into memory outside of the destination.

void RotateBitMapClockwise(BitMap *src, BitMap *dst)
 unsigned long table[16] =
 {0x00000000, 0x00000001, 0x00000100, 0x00000101,
  0x00010000, 0x00010001, 0x00010100, 0x00010101,
  0x01000000, 0x01000001, 0x01000100, 0x01000101,
  0x01010000, 0x01010001, 0x01010100, 0x01010101};
 /* x,y co-ord of source block */
 register short  srcX, srcY;
 /* row bytes, dereferenced for speed */
 short  srcRowBytes, dstRowBytes;
 /* src rowbytes * 8, for speed */
 short  srcRowX8;
 /* width and height of src */
 short  srcWidth, srcHeight;
 /* pointer to src and dst 8x8 blocks */
 char   *srcData, *dstData;
 /* pointer to bitmaps de-referenced for speed */
 char   *srcAddr, *dstAddr;
 /* last few pixels to do specially */
 short  remainder;
 /* a constant, pre-calculated for speed */
 char   *const1;
 /* de-reference some constants */
 srcRowBytes = src->rowBytes;
 dstRowBytes = dst->rowBytes;
 srcAddr = src->baseAddr;
 dstAddr = dst->baseAddr;
 /* calculate some values */
 srcWidth = src->bounds.right - src->bounds.left;
 srcHeight = src->bounds.bottom - src->;
 srcRowX8 = srcRowBytes * 8;
 const1 = srcAddr + (srcHeight - 8) * srcRowBytes;
 /* loop through the source doing vertical
  * slices starting at the left */
 for (srcX = 0; srcX < srcWidth - 7; srcX += 8) {
 /* bottom edge */
 srcData = const1 + srcX / 8;
 /* left edge */
 dstData = dstAddr + srcX * dstRowBytes;
 for (srcY = srcHeight - 1; srcY >= 0;
   srcY -= 8 ) {
 rotateBlock(table, (unsigned char *)srcData,
 srcRowBytes, (unsigned char *)dstData,
 srcData -= srcRowX8; /* next vert block */
 ++dstData; /* next horiz block */
 /* handle the last partial row by calling
  * rotateLimitedBlock() */
 if (remainder = (srcWidth % 8)) {
 /* bottom edge */
 srcData = const1 + srcX / 8; 
 /* left edge */
 dstData = dstAddr + srcX * dstRowBytes;
 for (srcY = srcHeight - 1; srcY >= 0;
   srcY -= 8 ) {
 (unsigned char *)srcData, srcRowBytes, 
 (unsigned char *)dstData, dstRowBytes,
 srcData -= srcRowX8;

 Rotate an 8x8 block of pixels by treating the destination
 block as 2 unsigned longs (64 bits!) and using a lookup 
 table to get a mask for the value of each row in the src 
 block.  The table essentially stretches out the source byte 
 to 8 times its size and then ORs it into the destination 
 64bits.  Since the bits are in the low bit of each byte, 
 the destination has to be shifted one position to the left 
 before each OR operation. The offset values are the 
 difference to rowBytes. If we had a 64bit data type, this
 would be easier.
void rotateBlock(unsigned long *table,
 unsigned char *src, long srcOffset, 
 unsigned char *dst, long dstOffset)
 register unsigned long   resultLO, resultHi;
 short  x;
 resultLO = resultHi = 0;
 src += srcOffset * 7;
 /* compute the rotated result */
 for (x = 0; x < 8; ++x) {
 resultLO = resultLO << 1;
 resultHi = resultHi << 1;
 resultLO |= table[(*src) >> 4];
 resultHi |= table[(*src) & 0x0F];
 src -= srcOffset;
 /* store the rotated result */
 *dst = (resultLO & 0xFF000000) >> 24;
 dst += dstOffset;
 *dst = (resultLO & 0x00FF0000) >> 16;
 dst += dstOffset;
 *dst = (resultLO & 0x0000FF00) >> 8;
 dst += dstOffset;
 *dst = resultLO & 0x000000FF;
 dst += dstOffset;
 *dst = (resultHi & 0xFF000000) >> 24;
 dst += dstOffset;
 *dst = (resultHi & 0x00FF0000) >> 16;
 dst += dstOffset;
 *dst = (resultHi & 0x0000FF00) >> 8;
 dst += dstOffset;
 *dst = resultHi & 0x000000FF;

 Same as other rotateBlock, except will do a
 partial block. Useful to prevent the overwriting
 of memory outside the bitmap!
void rotateLimitedBlock(unsigned long *table,
 unsigned char *src, long srcOffset, 
 unsigned char *dst, long dstOffset, short lines)
 register unsigned long   resultLO, resultHi;
 short  x;
 resultLO = resultHi = 0;
 src += srcOffset * 7;
 for (x = 0; x < 8; ++x) {
 resultLO = resultLO << 1;
 resultHi = resultHi << 1;
 resultLO |= table[(*src) >> 4];
 resultHi |= table[(*src) & 0x0F];
 src -= srcOffset;
 *dst = (resultLO & 0xFF000000) >> 24;
 dst += dstOffset; if (lines == 1) return;
 *dst = (resultLO & 0x00FF0000) >> 16;
 dst += dstOffset; if (lines == 2) return;
 *dst = (resultLO & 0x0000FF00) >> 8;
 dst += dstOffset; if (lines == 3) return;
 *dst = resultLO & 0x000000FF;
 dst += dstOffset; if (lines == 4) return;
 *dst = (resultHi & 0xFF000000) >> 24;
 dst += dstOffset; if (lines == 5) return;
 *dst = (resultHi & 0x00FF0000) >> 16;
 dst += dstOffset; if (lines == 6) return;
 *dst = (resultHi & 0x0000FF00) >> 8;
 dst += dstOffset; if (lines == 7) return;
 *dst = resultHi & 0x000000FF;

// RotateBitMapClockwise
// By Jeff Mallett
// Rotates the significant bits of the srcBitMapPtr 90°
// clockwise and stores the result in dstBitMapPtr.

#define kHighShortBit0x8000
#define kBitsPerShort16

// Creates a short from bits in different rows of
// the source bitmap.  Then copies this short into
// the destination bitmap.
#define COPY_TWO_BYTES(stopValue)  \
 data = 0, bit = kHighShortBit;    \
 do {   \
 if (*srcPos & srcBit) data |= bit;\
 srcPos += srcRowShorts;  \
 } while ( (bit >>= 1) != stopValue ); \
 *(dstPos++) = data

void RotateBitMapClockwise(BitMap *srcBitMapPtr,
 BitMap *dstBitMapPtr)
 register unsigned short data, *srcPos, bit;
 register int j;
 register unsigned short srcBit, *dstPos, stopBit;
 register int i;
 unsigned short *baseSrcPtr;
 // shorts per row of source
 const int srcRowShorts =
   srcBitMapPtr->rowBytes >> 1;
 const int numSrcRows =
   srcBitMapPtr->bounds.bottom - srcBitMapPtr->;
 dstPos = (unsigned short *)dstBitMapPtr->baseAddr;
 srcPos = baseSrcPtr =
   (unsigned short *)srcBitMapPtr->baseAddr;
 srcBit = kHighShortBit;
 for (i = srcBitMapPtr->bounds.right -
   srcBitMapPtr->bounds.left; i; --i) {
 // Copy one column of source to a row of destination
 for (j = numSrcRows / kBitsPerShort; j; --j) {
 if (j = numSrcRows % kBitsPerShort) {
 stopBit = kHighShortBit >> j;
 COPY_TWO_BYTES(stopBit); // Final 2 bytes
 // Prepare to copy next column of source
 if ( !(srcBit >>= 1) ) {
 srcBit = kHighShortBit;
 srcPos = baseSrcPtr;

The Rules

Here’s how it works: Each month there will be a different programming challenge presented here. First, you must write some code that solves the challenge. Second, you must optimize your code (a lot). Then, submit your solution to MacTech Magazine (formerly MacTutor). A winner will be chosen based on code correctness, speed, size and elegance (in that order of importance) as well as the postmark of the answer. In the event of multiple equally desirable solutions, one winner will be chosen at random (with honorable mention, but no prize, given to the runners up). The prize for the best solution each month is $50 and a limited edition “The Winner! MacTech Magazine Programming Challenge” T-shirt (not to be found in stores).

In order to make fair comparisons between solutions, all solutions must be in ANSI compatible C (i.e., don’t use Think’s Object extensions). Only pure C code can be used. Any entries with any assembly in them will be disqualified. However, you may call any routine in the Macintosh toolbox you want (i.e., it doesn’t matter if you use NewPtr instead of malloc). All entries will be tested with the FPU and 68020 flags turned off in THINK C. When timing routines, the latest version of THINK C will be used (with ANSI Settings plus “Honor ‘register’ first” and “Use Global Optimizer” turned on) so beware if you optimize for a different C compiler. All code should be limited to 60 characters wide. This will aid us in dealing with e-mail gateways and page layout.

The solution and winners for this month’s Programmers’ Challenge will be published in the issue two months later. All submissions must be received by the 10th day of the month printed on the front of this issue.

All solutions should be marked “Attn: Programmers’ Challenge Solution” and sent to Xplain Corporation (the publishers of MacTech Magazine) via “snail mail” or preferably, e-mail - AppleLink: MT.PROGCHAL, Internet:, and CompuServe: 71552,174. If you send via snail mail, please include a disk with the solution and all related files (including contact information). See page 2 for information on “How to Contact Xplain Corporation.”

MacTech Magazine reserves the right to publish any solution entered in the Programming Challenge of the Month and all entries are the property of MacTech Magazine upon submission. The submission falls under all the same conventions of an article submission.


Community Search:
MacTech Search:

Software Updates via MacUpdate

5 mash-ups we'd love to...
If there's one thing that has proved, it's that the addictive gameplay of can be transplanted onto basically anything and it will still be good fun. It wouldn't be surprising if we saw other developers jumping on the bandwagon,... | Read more »
How to navigate the terrain in Sky Charm...
Sky Charms is a whimsical match-'em up adventure that uses creative level design to really ramp up the difficulty. [Read more] | Read more »
Victorious Knight (Games)
Victorious Knight 1.3 Device: iOS Universal Category: Games Price: $1.99, Version: 1.3 (iTunes) Description: New challenges awaits you! Experience fresh RPG experience with a unique combat mechanic, packed with high quality 3D... | Read more »
Agent Gumball - Roguelike Spy Game (Gam...
Agent Gumball - Roguelike Spy Game 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Someone’s been spying on Gumball. What the what?! Two can play at that game! GO UNDERCOVERSneak past enemy... | Read more »
Runaway Toad (Games)
Runaway Toad 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: It ain’t easy bein’ green! Tap, hold, and swipe to help Toad hop to safety in this gorgeous new action game from the creators of... | Read more »
PsyCard (Games)
PsyCard 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: From the makers och Card City Nights, Progress To 100 and Ittle Dew PSYCARD is a minesweeper-like game set in a cozy cyberpunk... | Read more »
Sago Mini Robot Party (Education)
Sago Mini Robot Party 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: -- Children's Technology Review Editor's Choice -- | Read more »
How to get a high score in every level o...
Sky Charms is an adorable match three puzzler that provides a decent challenge thanks to its creative level design. It regularly presents something new, forcing you to think on your feet. [Read more] | Read more »
Apestorm: Full Bananas (Games)
Apestorm: Full Bananas 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: ***Launch sale – limited time only!*** Fugitive Apes have taken to the skies in search of revenge after humans have... | Read more »
How to create bigger words in Spellspire
Words have power. At least they do in Spellspire,a game about blasting out magical attacks by making words out of a jumble of letters. And it's a lot of fun. But if you want to be the best, you're going to have to think tactically when you start... | Read more »

Price Scanner via

15-inch 2.2GHz Retina MacBook Pro on sale for... has the 15″ 2.2GHz Retina MacBook Pro on sale for $1699.99 including free shipping. Their price is $300 off MSRP, and it’s the lowest price available for this model from any reseller (and... Read more
Apple Beats Microsoft at Own Game; Amazon Pri...
First quarter seasonality combined with an overall disinterested customer base led to an annual decline of 14.7% in worldwide tablet shipments during the first quarter of 2016 (1Q16). Worldwide... Read more
Aleratec Releases Mac Software Upgrade for 1...
California based Aleratec Inc., designer, developer and manufacturer of Portable Device Management (PDM) charge/sync products for mobile devices and professional-grade duplicators for hard disk... Read more
Sale! Amazon offers 27-inch iMac, 13-inch 2.9...
Amazon has the 27″ 3.2GHz 5K iMac and the 13″ 3.9GHz Retina MacBook Pro on sale for $300 off MSRP, each including free shipping, for a limited time: - 27″ 3.2GHz/1TB HD 5K iMac (model MK462LL/A): $... Read more
Apple refurbished 13-inch Retina MacBook Pros...
Apple has Certified Refurbished 13″ Retina MacBook Pros available for up to $270 off the cost of new models. An Apple one-year warranty is included with each model, and shipping is free: - 13″ 2.7GHz... Read more
13-inch 2.7GHz/128GB Retina MacBook Pro on sa...
Take $200 off MSRP on the price of a new 13″ 2.7GHz/128GB Retina MacBook Pro (model MF839LL/A) at Amazon. Shipping is free: - 13″ 2.7GHz/128GB Retina MacBook Pro: $1099.99 $200 off MSRP Act now if... Read more
Apple refurbished clearance 15-inch Retina Ma...
Apple has Certified Refurbished 2014 15″ 2.2GHz Retina MacBook Pros available for $1609, $390 off original MSRP. Apple’s one-year warranty is included, and shipping is free. They have refurbished 15... Read more
27-inch 5K iMacs on sale for up to $150 off M...
B&H Photo has 27″ 5K iMacs on sale for up to $150 off MSRP including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2199 $100 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $1849.99 $150... Read more
What Does The Refreshed 12-Inch MacBook Tell...
A lot of commentators are complaining that Apple’s update of the 12-Inch MacBook last week is a bit of a damp squib. I don’t know what they were expecting, since it would be very unlike Apple to do a... Read more
Free Wittify Keyboard Now Available On The Ap...
A team of Harvard Business School students have announced that the Wittify Keyboard, a new app utility for iOS devices, is now available on the Apple App Store. The Wittify keyboard and application... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Solutions Consultant - Apple (United...
# Apple Solutions Consultant Job Number: 48260200 Phoenix, Arizona, United States Posted: Apr. 22, 2016 Weekly Hours: 40.00 **Job Summary** As an Apple Solutions Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.