TweetFollow Us on Twitter

May 96 Challenge
Volume Number:12
Issue Number:5
Column Tag:Programmer’s Challenge

Programmer’s Challenge

By Bob Boonstra, Westford, Massachusetts

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

Edge Detector

This month’s Challenge is to write a small image-processing application that scans a color image and identifies the boundaries of possible objects in that image. Applications for such a program might include image enhancement, special effects, or pattern recognition, although those applications would use a more sophisticated approach for detecting edges than we will be implementing for this Challenge.

The prototype for the code you should write is:

typedef enum {
 redOnly=1, greenOnly, redAndGreen, blueOnly,
 redAndBlue, greenAndBlue, redGreenAndBlue
} EdgeType;

void EdgeDetect(
 PixMapHandle pMapH, /* find edges in this PixMap */
 BitMap *bMap,   /* store edges in this BitMap */
 unsigned short threshold,/* color separations >= dist create an edge */
 EdgeType eType  /* which color components to look at */
);

Each pixel in the PixMap should be compared to the eight (or fewer) adjacent pixels differing in position by up to one row or column. If the pixel color is sufficiently different (as defined below) from any of the adjacent pixels, then the bit in the BitMap corresponding to that pixel should be set to 1. Otherwise, the BitMap bit should be set to 0. Obviously, pixels located in the first and last row and column will have fewer than eight adjacent pixels.

Whether two pixels differ by enough to constitute an edge is determined by comparing their rgb values. The distance between two pixels is the root-mean-square difference between the color components of their rgb values, considering only those components specified in the input EdgeType. For example, if the EdgeType is greenOnly, then the distance between two pixels is the absolute value of the difference in the green components of their colors. If the EdgeType is redGreenAndBlue, then the distance is the square root of the sum of the squares of the differences of the red components, the green components, and the blue components.

As a specific example, suppose we have two pixels with (red, green, blue) values of (0x1000, 0x2000, 0x4000) and (0x2000, 0x5000, 0xB000). The distance between these two pixels is:

 redOnly: 0x1000
 redAndGreen:    0x3298=sqrt(0x01000000+0x09000000)
 redGreenAndBlue:0x7AE5=sqrt(0x01000000+0x09000000+0x31000000)

Two pixels define an edge if their distance is greater than or equal to the threshold parameter. The threshold parameter is deliberately declared to be an unsigned short, even though pixels can differ by a greater amount. Since the definition of distance is symmetric, the bits corresponding to both edge pixels would be set in the BitMap.

The BitMap will be allocated and initialized for you by the calling routine. The storage pointed to by the BitMap baseAddr will also be allocated and initialized to zero. The bounds rectangles will be the same for the BitMap and the PixMap. Your code needs to deal with pixelSize values of 8, 16, or 32, with each case being equally weighted in the scoring. For PixMaps with indexed pixels, you will obviously need to look at the color table to find the rgb value corresponding to a given index. In the 16-bit case, you should follow the rules for converting a 5-bit color component into an 8-bit RGBColor component value (i.e., replicating the 3 most significant bits and appending them to constitute the least significant bits of the 8-bit component).

This will be a native PowerPC Challenge, scored using the latest Metrowerks C compiler. (No C++ or Pascal this month.)

Entries Due Ten Days Earlier

Although two issues may seem like a long time to wait for the results of the Challenge, it has always been a challenge (no pun intended) to complete the scoring of results in time for publication two issues later. We have been searching for a way to allow a little more time for evaluating the entries and writing the column without introducing any additional delay between publication of the problem and publication of the solution. The Challenge mailing list has allowed us to deliver the Challenge to readers on a predictable schedule wherever they live, regardless of variations in mailing dates. We are going to use the mailing list to advance the due date for Challenge solutions, without reducing the amount of time available for solving the Challenge. Starting with this month’s contest, Challenge entries will be due earlier, on the 1st of the month printed on the front cover. We will mail the problem to the mailing list on the 12th of the preceding month, also about ten days earlier than before.

If you are not already a member of the Challenge mailing list, you can join the ~300 subscribers from 25 countries already on the list by sending email to macjordomo@listmail.xplain.com with the line “sub challenge-A YourName” in the body.

Two Months Ago Winner

The response to the Words the Reverse Challenge was overwhelming. I don’t know if it was due to allowing C++ and Pascal entries, or to the relative simplicity of the problem, but I received a record 45 entries to this Challenge. The Challenge was to write code that would reverse the order of words in a block of input text while preserving intervening white space and special characters, and adjusting capitalization of the reversed words to match that of corresponding input words. It is appropriate that the first Challenge admitting Pascal solutions was won by a well-known proponent of Pascal (see MacTech Magazine 12.4 [April 1996] 70, and p.20 of this issue). Congratulations to Peter Lewis (Perth, Australia), author of Anarchie, NetPresenz, ObiWan, and other shareware products, for submitting the fastest entry to the Words The Reverse Challenge.

The test cases included a number of short, untimed strings designed to verify correctness. To my surprise, more than one-third of the entries failed these tests (or crashed outright). People had problems with strings that contained a single word, with strings that began with punctuation, with words of a single letter, and with the middle word in strings that contained an odd number of words. Remember, correctness is the first requirement for your solution.

For the timing tests, I ran a set of cases averaging around 40,000 words per case, totaling upwards of 500,000 words and 3 million characters for all cases. I eliminated from the input any “words” that started with a digit, because the problem statement was silent on how to deal with capitalization in that case. A number of people, including the winner, chose to treat words starting with digits as capitalization-neutral, so that the word being exchanged with the digit-word retained its original capitalization. This was a very reasonable approach (and I wish I had included it in the problem statement), but since the problem was silent, the fairest thing to do was to eliminate this condition from the test data.

The experiment allowing multiple languages and compilers went reasonably well. Most people, as requested, either provided a project/make file to link their solution with C code, or specifically indicated which compiler they wanted me to use. For those C entries that did not indicate a preference, I used the Metrowerks C compiler. A few people submitted solutions for environments that either did not generate native PowerPC code (e.g., non-SPM THINK C) or did not link with C code (e.g., THINK Pascal). For those entries, I mapped them to the closest possible environment (SPM C and Metrowerks Pascal, in these two cases).

Here are the times, compiler selection, code size, and data size for the correct solutions. Numbers in parentheses are the cumulative point total for all previous Challenges, not including this one.

Nametimecompilercodedata
Peter N Lewis (10)525MW Pascal89642
Ludovic Nicolle (4)602MW C12408
Kevin M. Cutts (50)607MW C65620
Gary Beith (20)626MW C20888
Ernst Munter (132)630MW C680560
Robert Marsa650MW C55259
Eric Lengyel (40)669MW C368140
John Nevard (17)670MW C56420
Wolfgang Thaller (4)681MW C61620
Randy Boring687MW C10432024
Bill Karsh (80)695MW C11288
Kirill Medvinsky703MW C++54012
Mark Bassam Salem705MW C++59232
Tom Saxton (10)710MW C360422
Karl Anderson716MW C604536
Lars Farm762MW C++90470
David McLeod842MW C5001315
Erik Sea884MW C760532
Robert Leslie/Geoff Hulten938MW C436268
Björn Davidsson (4)1122MW C++102020
Tom Stone1180SPM C75216
Gustav Larsson (87)1270MW C784536
Ryan Gronlie1294MW C44420
Michael White1397MW C1924130
Rishi Khan1447MW C9608
Richard Fattic1576MW C9088
Stefan C. Sinclair1668SPM MrC124840
David Newport2420SPM C78416
Ken Slezak (10)2468SPM C80816

To help understand why the single correct Pascal entry was faster than all of the C entries, I hand-translated the winning Pascal code into C, compiled it with several C compilers, and compared the results. I turned on all speed optimizations in each case, and optimized for the 604 processor when the compiler supported that option. Since conventional wisdom is that C is more efficient than Pascal, I expected to find that the winning algorithm would be faster in C than it was in Pascal. In fact, the results for two of the C compilers were essentially the same as the Pascal results, and one was measurably worse (for reasons that I did not have time to investigate). Here are the results of my test:

Environment / Languageexecution timecode size
Metrowerks / Pascal525896
Metrowerks / C533868
SPM / Symantec C536712
SPM / MrC6111992

Top 20 Contestants of All Time

Here are the Top Contestants for the Programmer’s Challenges to date, including everyone who has accumulated more than 20 points. The numbers below include points awarded for this month’s entrants.

RankNamePoints
1.[Name deleted]176
2.Munter, Ernst134
3.Gregg, Xan92
4.Larsson, Gustav87
5.Karsh, Bill80
6.Stenger, Allen65
7.Cutts, Kevin57
8.Riha, Stepan51
9.Goebel, James49
10.Nepsund, Ronald47
11.Mallett, Jeff44
12.Kasparian, Raffi42
13.Vineyard, Jeremy42
14.Lengyel, Eric40
15.Darrah, Dave31
16.Brown, Jorg30
17.Lewis, Peter30
18.Landry, Larry29
19.Beith, Gary24
20.Elwertowski, Tom24
21.Lee, Johnny22
22.Noll, Robert22

There are three ways to earn points: (1) scoring in the top 5 of any Challenge, (2) being the first person to find a bug in a published winning solution, or (3) being the first person to suggest a Challenge that I use. The points you can win are:

points to win table:

1st place20 points5th place2 points
2nd place10 pointsfinding bug2 points
3rd place7 pointssuggesting Challenge2 points
4th place4 points

Peter’s solution is relatively straightforward, and he hints in the preamble that he might have done better if he had spent more time on it. One tip that you might glean from Peter’s code is the way he allocates dynamic memory. First, he deals with small problems with memory allocated on the stack. Second, he uses NewHandle rather than NewPtr to allocate dynamic memory. NewHandle is faster than NewPtr, because NewPtr may move relocatable blocks around before doing the allocation to avoid fragmenting the heap. Peter also locks the handle before using it, which is always safe, but is not necessary unless your code does something that moves memory. Whether locking is necessary in this case depends on whether you believe the documentation that says BlockMove doesn’t move memory. Here is Peter’s winning solution:

Challenge.p

Peter N Lewis, peter@stairways.com.au

unit Challenge;

interface

 uses
 Types;
 
 type
 CharsArray = packed array[0..0] of byte;
 CharsArrayPtr = ^CharsArray;
 
 procedure ReverseTheWords( 
    text: CharsArrayPtr; 
    numCharsIn: longint );

implementation

 uses
 Memory;

{
    This is not really optimal, I felt compelled to send in a Pascal solution since I was
    one of the people who complained about the language bias.  I didn’t have time to 
    do this challenge justice.
    
    Method:
    *     Allocate a block of memory equal in size to numCharsIn (if numCharsIn < 2048, 
    we short circuit this to use a block of memory on the stack).
    *     Initialize a 0..255 array to determine whether a character is  an alphanum (I 
    could just use the ANSI ctype.p file, but without a macro call, there is a pretty 
    big hit).
    *     reverse the words from the source to our new buffer.  We move in from both
    ends,  copying non-alphanums, and then swapping words and fixing the case.
    *     BlockMoveData the buffer back to the source buffer.
    *     Release the memory if we allocated any.
}

 procedure ReverseTheWords( 
      text: CharsArrayPtr; 
      numCharsIn: longint );
 const
 stack_space_size = 2048;
 var
 space: packed array[0..stack_space_size] of byte;
 buffer: CharsArrayPtr;
 memory: Handle;
 leftin, leftout, rightin, rightout, leftedge, 
 rightedge: longint;
 i: longint;
 leftchar, rightchar: integer;
 alphanum_set:array[0..255] of Boolean;
 begin
    { allocate memory if needed }
 if numCharsIn < stack_space_size then begin
 memory := nil;
 buffer := @space;
 end else begin
 memory := NewHandle( numCharsIn );
 if memory = nil then begin
 DebugStr( 'Memory allocation failed!' );
 exit( ReverseTheWords );
 end;
 HLock(memory);
 buffer := CharsArrayPtr( memory^ );
 end;
 
    { init - I wish I could do this at compile time - Turbo Pascal can }
 for i := 0 to 255 do alphanum_set[i] := false;
 for i := 48 to 57 do alphanum_set[i] := true; { 0..9 }
 for i := 65 to 90 do alphanum_set[i] := true; { A..Z }
 for i := 97 to 122 do alphanum_set[i] := true; { a..z }

    { reverse }
 leftin := 0;
 leftout := leftin;
 rightin := numCharsIn - 1;
 rightout := rightin;
 while leftin <= rightin do begin
 while not alphanum_set[text^[leftin]] & (leftin <= rightin) 
                                                do begin
 buffer^[leftout] := text^[leftin];
 Inc(leftout);
 Inc(leftin);
 end;
 while not alphanum_set[text^[rightin]] & (leftin < rightin) 
                                                do begin
 buffer^[rightout] := text^[rightin];
 Dec(rightout);
 Dec(rightin);
 end;
 leftedge := leftin;
 rightedge := rightin;
 while alphanum_set[text^[leftin]] & (leftin <= rightin) 
                                                do begin
 Inc(leftin);
 end;
 if leftin > rightin then begin { central word, just copy, ignore case }
 for i := leftedge to leftin - 1 do begin
 buffer^[leftout] := text^[i];
 Inc(leftout);
 end;
 end else begin
 while alphanum_set[text^[rightin]] do begin 
    { there is a sentinel now, we dont need to check leftin < rightin }
 Dec(rightin);
 end;
 leftchar := text^[leftedge];
 rightchar := text^[rightin+1];
 if ( leftchar > 57 ) & ( rightchar > 57 ) then begin 
    { both letters }
 if leftchar > 90 then begin
 if rightchar <= 90 then begin
 rightchar := rightchar + $20;
 leftchar := leftchar - $20;
 end;
 end else begin
 if rightchar > 90 then begin
 rightchar := rightchar - $20;
 leftchar := leftchar + $20;
 end;
 end;
 end;
 buffer^[leftout] := rightchar;
 Inc(leftout);
 for i := rightin+2 to rightedge do begin
 buffer^[leftout] := text^[i];
 Inc(leftout);
 end;
 for i := leftin-1 downto leftedge+1 do begin
 buffer^[rightout] := text^[i];
 Dec(rightout);
 end;
 buffer^[rightout] := leftchar;
 Dec(rightout);
 end;
 end;
 
    { copy buffer }
 BlockMoveData( buffer, text, numCharsIn );
 
    { free memory if required }
 if memory <> nil then begin
 DisposeHandle( memory );
 end;
 end;
 
end.

 
AAPL
$100.75
Apple Inc.
+0.64
MSFT
$46.36
Microsoft Corpora
-0.08
GOOG
$577.36
Google Inc.
+1.00

MacTech Search:
Community Search:

Software Updates via MacUpdate

QuickBooks 2015 16.0.0.1352 R1 - Financi...
QuickBooks 2015 helps you manage your business easily and efficiently. Organize your finances all in one place, track money going in and out of your business, and spot areas where you can save.... Read more
Mac DVDRipper Pro 5.0.1 - Copy, backup,...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
Apple OS X bash Update 1.0 - Fix for sec...
The OS X bash Update fixes a security flaw in the bash UNIX shell on OS X 10.9.5 (also on OS X 10.8 and 10.7 [see Related Links below]). OS X 10.9.5 or later Downloads for OS X 10.8 and OS X 10.7 in... Read more
SyncTwoFolders 2.0.5 - Syncs two user-sp...
SyncTwoFolders simply synchronizes two folders. It supports synchronization across mounted network drives and it is a possibility to run a simulation showing in a log what will be done. Please visit... Read more
FinderPop 2.5.7 - Classic Mac utility, n...
FinderPop is a Universal preference pane that extends OS X's contextual menus using a FinderPop Items folder much as the Apple Menu Items folder used to do for the Apple menu. It has other features... Read more
VueScan 9.4.45 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
LibreOffice 4.3.2.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
calibre 2.4 - Complete e-library managem...
Calibre is a complete e-book library manager. Organize your collection, convert your books to multiple formats, and sync with all of your devices. Let Calibre be your multi-tasking digital... Read more
Default Folder X 4.6.9b1 - Enhances Open...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click... Read more
Beamer 2.0.4 - Stream any movie file fro...
Beamer streams to your Apple TV.... Plays any movie file - Just like the popular desktop movie players, Beamer accepts all common formats, codecs and resolutions. AVI, MKV, MOV, MP4, WMV, FLV. To... Read more

Latest Forum Discussions

See All

WW2: Sandbox. Strategy & Tactics (G...
WW2: Sandbox. Strategy & Tactics 1.0.0 Device: iOS Universal Category: Games Price: $9.99, Version: 1.0.0 (iTunes) Description: Sandbox is the unlimited sequel to our Strategy & Tactics: World War II. We've abandoned... | Read more »
apeFilter (Music)
apeFilter 1.0 Device: iOS Universal Category: Music Price: $6.99, Version: 1.0 (iTunes) Description: | Read more »
Shred It! Review
Shred It! Review By Jennifer Allen on September 30th, 2014 Our Rating: :: GORGEOUS BUT BASICUniversal App - Designed for iPhone and iPad It might look lovely, but Shred It! is a pretty shallow endless runner/snowboarding game.   | Read more »
It Came From Canada: Angry Birds: Transf...
Anyone afraid that throwing Transformers into the Angry Birds mix would result in a Michael Bay-level of childhood pillaging can rest easy. While Rovio’s famous fowls may be a 21st century staple, Angry Birds: Transformers wears its affection for... | Read more »
Galaxy Trucker Review
Galaxy Trucker Review By Rob Thomas on September 30th, 2014 Our Rating: :: IN FOR THE LONG HAULiPad Only App - Designed for the iPad We got a great big convoy, rockin’ through the night… the endless night of outer space, that is.... | Read more »
Moon and Sun – children’s book Review
Moon and Sun – children’s book Review By Amy Solomon on September 30th, 2014 Our Rating: Moon and Sun is a fable-like children’s book with lush illustrations.   Developer: Marcio Monteiro Price: $3.99 Version: 1.0 App Reviewed on... | Read more »
Kairosoft Finally Brings The Pyraplex to...
Kairosoft Finally Brings The Pyraplex to iOS Posted by Jessica Fisher on September 30th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Yeti’s Parole Officer Review
Yeti’s Parole Officer Review By Jennifer Allen on September 30th, 2014 Our Rating: Universal App - Designed for iPhone and iPad Yeti’s Parole Officer proves that looks aren’t everything in this entertaining, if plain, piece of... | Read more »
Record Keeper and World Wide Words – Two...
Record Keeper and World Wide Words – Two New Final Fantasy Games Arrive on iOS in Japan Posted by Ellis Spice on September 30th, 2014 [ permalink ] | Read more »
iPollute Review
iPollute Review By Nadia Oxford on September 30th, 2014 Our Rating: :: iPOLLUTE, uPOLLUTEUniversal App - Designed for iPhone and iPad iPollute is a delightful-looking pollution simulator, though it’s a bit light on “game” content... | Read more »

Price Scanner via MacPrices.net

iPhone 6 & iPhone 6 Plus Available in Chi...
Apple has announced that iPhone 6 and iPhone 6 Plus will be available in China beginning Friday, October 17 from the Apple Online Store (http://www.apple.com), Apple’s retail stores, and an expansive... Read more
MacBook Airs on sale for $100 off MSRP, start...
Best Buy has the new 2014 MacBook Airs on sale for $100 off MSRP on their online store. Choose free home shipping or free local store pickup (if available). Prices valid for online orders only, in-... Read more
Apple Releases OS X Mavericks bash Update 1.0...
Apple has released a patch update for OS X Mavericks users to address the recently-detected “Shellshock” security bug in BSD UNIX’s bash shell. Apple says only a few Mac users who had manually... Read more
Pivotal Payments Ready for Apple Pay – FlexPo...
Pivotal Payments, a provider of merchant services and global payment processing solutions, has announced its proprietary FlexPoint platform will support credit and debit transactions through Apple’s... Read more
iStabilizer Announces Tabarm — First Friction...
iStabilizer, a specialist in universal lightweight compact tripods, steady cams, dollies, mounts, and remotes for smartphones, tablets, and cameras, announced today the iStabilizer tabArm, the first... Read more
IStabilizer Flex Smartphone Tripod Wins Usa T...
iStabilizer, a specialist in universal lightweight compact tripods, steady cams, and other products for smartphones, tablets, and cameras, has announced today that its iStabilizer Flex smartphone... Read more
13-inch 2.8GHz Retina MacBook Pro on sale for...
B&H Photo has the new 2014 13″ 2.8GHz Retina MacBook Pro on sale for $1699.99 including free shipping plus NY sales tax only. They’ll also include free copies of Parallels Desktop and LoJack for... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has the new 2014 15″ Retina MacBook Pros on sale for up to $150 off MSRP. Shipping is free, and B&H charges NY sales tax only. They’ll also include free copies of Parallels Desktop... Read more
Get Down to Business On Your iPad With Free N...
Utah based Poem LLC has introduced Nexticy 1.0, a full-featured, affordable (Free) forms application for iPad. Nexticy makes it ‘way easy’ to create, store, retrieve and analyze paper-like forms of... Read more
13-inch Retina MacBook Pros on sale for $100...
Best Buy has 13-inch 2.6GHz Retina MacBook Pros on sale for $100 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Prices are for online orders only, in-... Read more

Jobs Board

*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions (US) - A...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Project Manager / Business Analyst, WW *Appl...
…a senior project manager / business analyst to work within our Worldwide Apple Fulfillment Operations and the Business Process Re-engineering team. This role will work Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.