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.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Spotify 1.2.0.1165 - Stream music, creat...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
Thunderbird 102.5.1 - Email client from...
As of July 2012, Thunderbird has transitioned to a new governance model, with new features being developed by the broader free software and open source community, and security fixes and improvements... Read more
Pinegrow 7.03 - Mockup and design web pa...
Pinegrow (was Pinegrow Web Designer) is desktop app that lets you mockup and design webpages faster with multi-page editing, CSS and LESS styling, and smart components for Bootstrap, Foundation,... Read more
Adobe After Effects 2022 23.1 - Create p...
The new, more connected Adobe After Effects can make the impossible possible. Get powerful new features like a Live 3D Pipeline that brings CINEMA 4D scenes in as layers - without intermediate... Read more
SteerMouse 5.6.7 - Powerful third-party...
SteerMouse is an advanced driver for USB and Bluetooth mice. SteerMouse can assign various functions to buttons that Apple's software does not allow, including double-clicks, modifier clicks,... Read more
Wireshark 4.0.2 - Network protocol analy...
Wireshark is one of the world's foremost network protocol analyzers, and is the standard in many parts of the industry. It is the continuation of a project that started in 1998. Hundreds of... Read more
Adobe Premiere Pro 2022 23.1 - Digital v...
Adobe Premiere Pro is available as part of Adobe Creative Cloud for as little as $54.99/month. The price on display is a price for annual by-monthly plan for Adobe Premiere Pro only. Adobe Premiere... Read more
1Password 8.9.10 - Powerful password man...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
FotoMagico 6.3 - Powerful slideshow crea...
FotoMagico lets you create professional slideshows from your photos and music with just a few, simple mouse clicks. It sports a very clean and intuitive yet powerful user interface. High image... Read more
Affinity Photo 1.10.6 - Digital editing...
Affinity Photo - redefines the boundaries for professional photo editing software for the Mac. With a meticulous focus on workflow it offers sophisticated tools for enhancing, editing and retouching... Read more

Latest Forum Discussions

See All

‘Awaken Legends: Idle RPG’ Celebrates th...
Awaken Legends: Idle RPG is adding its first update since the game was soft-launched in November, letting players get their hands on a new hero “Hera Valen". Players can also look forward to the Covenant of the Dark Knight event and the Wishing Well... | Read more »
‘Horizon Chase 2’ Japan World Tour Expan...
Horizon Chase 2 () from Aquiris is getting a major expansion today on Apple Arcade. The Japan World Tour expansion brings in 11 new races across 9 cities and it should be rolling out now as of this writing. I expect it to be available worldwide... | Read more »
Dark Fantasy Visual Novel ‘The 13th Mont...
Originally announced for release in August, The 13th Month from Japanese developer Kobayashimaru and publisher Kodansha released on PC via Steam worldwide this month. The dark fantasy visual novel that reimagines the classic Sleeping Beauty tale, is... | Read more »
Tom Clancey’s The Divison Resurgence ann...
Ubisoft has announced the latest Live Test dates for Tom Clancy’s The Division Resurgence, the hotly anticipated mobile entry in the Divison series. Starting December 8th and ending on the 22nd, the test will offer a huge amount of content for the... | Read more »
‘Easy Come Easy Golf’ New Update Adds St...
Easy Come Easy Golf () from Clap Hanz is one of my favorite games on Apple Arcade. It has been updated quite a bit since launch bringing in new modes and improvements. It recently launched on Nintendo Switch as well. | Read more »
Out Now: ‘Magic vs Metal’, ‘Suzerain’, ‘...
Each and every day new mobile games are hitting the App Store, and so each week we put together a big old list of all the best new releases of the past seven days. Back in the day the App Store would showcase the same games for a week, and then... | Read more »
SwitchArcade Round-Up: Reviews Featuring...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for December 7th, 2022. Today can be accurately described as Mikhail Madness, with a whopping four reviews from our pal-est of pals. Football Manager 2023 Touch, Wobbledogs, Soccer Story... | Read more »
Alchemy Stars celebrates 1 and a half ye...
It has been one and a half years since Alchemy Stars launched, and Level Infinite is celebrating in style with a host of new content. There will be a new story mission and even a store to explore, and a whole new mode for those budding idol... | Read more »
Fighting Game ‘Art of Fighting 2’ ACA Ne...
Last week, side-scrolling shooter Pulstar hit mobile platforms as the newest ACA NeoGeo series release from Hamster and SNK. Read Shaun’s review of it here. Today, fighting game Art of Fighting 2 has launched on iOS and Android. Art of Fighting 2... | Read more »
‘Genshin Impact’ Version 3.3 Update Now...
HoYoverse recently revealed the next major update for Genshin Impact (Free) in the form of version 3.3 ‘All Senses Clear, All Existence Void’. | Read more »

Price Scanner via MacPrices.net

New! Details on Verizon’s Christmas/Holiday p...
Verizon is offering discounts on iPhones, Apple Watch models, and iPads with specific promo codes as part of their Christmas/Holiday 2022 offerings. Codes are valid when adding a new line of service... Read more
Apple MagSafe accessories are back on Holiday...
Amazon has Apple MagSafe Chargers and Apple’s MagSafe Battery on sale for up to 24% off MSRP again as part of their Christmas/Holiday sale. Shipping is free, and all models are in stock: – MagSafe... Read more
13″ M2 MacBook Airs on sale again for the low...
Amazon has 13″ MacBook Airs with M2 CPUs in stock today and on sale for $150 off MSRP as part of their Christmas/Holiday Sale, prices start at $1049. Shipping is free. They are the lowest prices... Read more
Get an Apple 16″ MacBook Pro for $400 off MSR...
16″ MacBook Pros with Apple’s M1 Pro CPUs are in stock and on sale today at B&H Photo for $300-$400 off Apple’s MSRP for a limited time. Prices start at $2099 for M1 Pro models with 512GB or 1TB... Read more
Holiday clearance sale! Previous-generation A...
Amazon has 2nd generation 32GB and 64GB 4K Apple TVs with Siri remotes and 32GB Apple TV HDs on clearance sale for $80-$90 off original MSRP. Shipping is free, and delivery is available in time for... Read more
Christmas sale at Verizon: Apple AirPods Pro...
Verizon has first-generation Apple AirPods Pro on sale for $159.99 on their online store as part of their continuing Christmas/Holiday sale. Their price is $90 off Apple’s original MSRP, and it’s the... Read more
New Christmas/New Years promo at Xfinity Mobi...
Switch to Xfinity Mobile and open a new line of service, and take $400 off the price of a new iPhone, no trade-in required, through January 10, 2023. The $400 is applied to your account as credits... Read more
Apple iPad Smart Keyboard Folio prices drop u...
Apple iPad Smart Keyboard Folio prices have dropped up to $60 off MSRP at Amazon and Walmart as part of their Christmas/Holiday sales. These are the cheapest prices currently available for these iPad... Read more
Today is the final day for Xfinity Mobile’s $...
If you switch to Xfinity Mobile and open a new line of service, they will take $500 off the price of a new iPhone, no trade-in required. This is the best no trade-in Cyber Monday Apple iPhone 14 deal... Read more
Amazon restocks 10.2″ 64GB 9th-generation iPa...
Amazon has Apple’s 9th generation 10.2″ 64GB WiFi iPads (Silver) in stock and on sale for $269.99 shipped as part of their Christmas/Holiday Sale. Their price is $60 off Apple’s MSRP. Free delivery... Read more

Jobs Board

*Apple* Systems Administrator - JAMF - Activ...
…Administration **Duties and Responsibilities** + Configure and maintain the client's Apple Device Management (ADM) solution. The current solution is JAMF supporting Read more
Cashier - *Apple* Blossom Mall - JCPenney (...
Cashier - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Blossom Mall Read more
Omnichannel Associate - *Apple* Blossom Mal...
Omnichannel Associate - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
Sephora Beauty Advisor - *Apple* Blossom Ma...
Sephora Beauty Advisor - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
Operations Associate - *Apple* Blossom Mall...
Operations Associate - Apple Blossom Mall Location:Winchester, VA, United States (https://jobs.jcp.com/jobs/location/191170/winchester-va-united-states) - Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.