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May 94 Challenge
Volume Number:10
Issue Number:5
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 to implement the Flip Horizontal menu item that you find in most imaging applications. Your code will flip a given pixMap from right to left. On exit from your routine the pixMap should be a horizontal mirror reflection of what it was on input.

The prototype of the function you write is:


/* 1 */

void FlipPixMapHorz(thePixMapHndl)


You flip the pixMap pixels in place. That is, you overwrite the input pixels with the output pixels as you go. Your routine needs to handle all of the possible pixMap types, including a 1-bit deep bitMap and indexed types with less than 8 bits per index. When timing solutions, equal weight will be given to each case: 1, 2, 4, 8, 16 and 32 bits per pixel.


I am happy to announce that we now have an undisputed Programmer Challenge Champion! This month marks the 3rd time this person has come in 1st place (breaking a 3-way tie). He has also finished in the top 5 places more often than any other Challenge entrant (8 times, including this one). Congrats to Bob Boonstra (Westford, MA) for his excellent solution to the Bitmap To Text challenge! His solution is about 2.6x faster than the only other entry this month, submitted by Challenge veteran Allen Stenger (Gardena, CA).

Here are the code sizes and times for two different tests. 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 Code Time 1 Time 2

Bob Boonstra (7) 1264 4 13

Allen Stenger (4) 766 11 34

Both Bob and Allen chose to use similar algorithms. They split the bitmap into character-sized cells and then tried to find a character from the given font that had a similar density. As a measure of density they both counted the number of set bits in the cell. Thus, the performance of the routine as a whole was largely dependent on how fast their bitcount code was. Bob ended up using a faster bitcount, which looks something like this:

/* 2 */
 bitCount = 0;
 if ((x = initVal) != 0) {
 do {
 x = x & (x - 1);
 } while (x != 0);

where initVal is the value that you’re trying to count the set bits for. This code has the advantage that it does zero iterations of the loop if initVal is zero. Also, it only makes one iteration through the loop for each set bit.

Allen used a 256 element lookup table where each entry in the table contained a number from 0 to 8 representing the number of set bits of the index corresponding to that entry. For instance, zero-based element number 7 contained the number 3 because there are 3 set bits when you write 7 as a base 2 number, 00000111. This table lookup method is a good idea if you want to count bits in a long run of bytes but for small bit fields that are not necessarily byte aligned there is too much masking and other loop overhead.

Here’s Bob’s winning solution:

March 94 Challenge - BitMapToText
by Bob Boonstra


The problem states that “the smallest detail in the input image [will] be roughly equal to or larger than a single character of the given font and font size.” Therefore, this solution attempts only to match the number of bits set in a given character size piece of the image with the number of bits set in the character chosen to represent that piece of the image.

The strategy is to:

(1) draw the characters from 32 to 127 in an offscreen bitmap.

(2) sort the characters in order of increasing number of bits set

(3) precalculate a mapping from pixel density to output characters

(4) loop thru the character size chunks of the image, count the number of bits set, and output the corresponding character.


• Width of characters is assumed to be <=32 pixels (reasonable for (6-24 point mono font)

• No assumption that actual height of font <= 24; ascent+descent+leading may exceed point size

• Ref NIM: Text pg 4-11
bitMapPtr->rowBytes * (font height) assumed < 32K

/* 3 */
#include <stdio.h>

#pragma options (honor_register, !assign_registers)

#define uchar unsigned char
#define ushort unsigned short
#define ulong unsigned long

#define EOL 0x0d
#define kErr 1
#define kFirstChar 32
#define kLastCharPlus1 128
#define kNumChars (kLastCharPlus1-kFirstChar)
#define kBytesPerBMChar sizeof(long)
#define kMaxCharWidth (8*kBytesPerBMChar)
#define kCharRowBytes 384
#define kBitsPerChunk 32
#define kMaxCharVals 512

#define DoSetMem(addr,sz,val)                            \
  { register long *p = (long *)addr;                     \
    register short count = sz;                           \
    do *p++=val; while (--count);                        \

Macro BitCount(x,count) increments count for each bit in x set to 1.

WARNING:  the expression (x=x--&x) in the BitCount macro is not portable, 
because the order of evaluation is undefined, but it generates correct 
fast code for (x=x&(x-1)) in THINK C.  Non-portable code is BAD FOR YOU, 
except where speed is very important, like in this Challenge.

#define BitCount(x,initVal,count)  \
  if (x=initVal) do ++count; while (x = x--&x);

ushort lineHeight,charWid;
FontInfo fInfo;

short InitOffscreenBitMap(GrafPort *charPtr)
// Initialize font information
    Point scalePt = {1,1};
    ulong numBytes;
    lineHeight = fInfo.leading+fInfo.ascent+fInfo.descent;
    charPtr->pnLoc.v = lineHeight -fInfo.descent;
// Initialize GrafPort and bitmap storage 
    numBytes = lineHeight*kCharRowBytes;
    if (0 == (charPtr->portBits.baseAddr = 
                (QDPtr)NewPtr( numBytes )))
      return kErr;
    charPtr->portBits.rowBytes = kCharRowBytes;
    charPtr-> = 0;
    charPtr->portBits.bounds.left = 0;
    charPtr->portBits.bounds.bottom = lineHeight;
    charPtr->portBits.bounds.right = kCharRowBytes*8;
    charWid = StdTxMeas(1,"W",&scalePt,&scalePt,&fInfo);
/*if (charWid != fInfo.widMax) DebugStr("\p bad wid");*/
    if (kBytesPerBMChar*8 < charWid) return kErr;
    return 0;

// Draw the characters of the given font into an offscreen
// bitmap.
void  DrawTheChars(GrafPtr charPort)
  register Point scalePt = {1,1};
  register short hPos = kMaxCharWidth-charWid;
  register short count;
  uchar chVal = kFirstChar;
  count = kNumChars; do {
    charPort->pnLoc.h = hPos;
    hPos += kMaxCharWidth;
  } while (--count);

// Calculate the number of bits set in each character, for subsequent 
use in comparing
//  to a section of the bitmap.
void InitBitsSetArray(register char *p,register ushort *c)
  register short count;
  p += (ushort)fInfo.leading*kCharRowBytes;
  count = kNumChars; do {
    register ushort bitcount=0;
    register uchar *q = (uchar *)p;
    register short vCount;
    vCount = lineHeight-fInfo.leading; do {
      register ulong row;       
      BitCount(row,*(ulong *)q,bitcount);
      q += kCharRowBytes;
    } while (--vCount);
//  Following line fudges the density value for characters to account 
for the fact 
//  that the most dense character is significantly less dense than a 
dark section of a
//  bitmap.
    bitcount += bitcount>>1;
    *c++ = bitcount;
    p += kBytesPerBMChar;
  } while (--count);

// Sort the characters in order of increasing number of bits set (density).
void SortBitsSetArray(register ushort *v, 
                      register ushort *c)
  register ushort *x;
  register ushort count,val,xVal,newVal;
// Initialize sort order
  count = kNumChars; val = 0; x=c; do {
    *x++ = val;  ++val;
  } while (--count);
// Bidirectional exchange sort is good enough for this small array
  count = kNumChars-1;
  x = c;
  val = *(v+*c);
  do {
    ushort *saveC;
    ushort saveCount;
    xVal = *(c+1);
    newVal = *(v+xVal);
    if (val > /**x*/ newVal) {
//    Swap pointers
      *(c+1) = *c;   *c = xVal;
      if (count < kNumChars-1) {
        saveC=c+1;  saveCount=count;  val = *(v+*c); 
        do {
          xVal = *(c-1);  x = v+xVal;  newVal = *x;
          if (val >= newVal) break;
          *(c-1) = *c;  *c = xVal;   --c;
        } while (++count < kNumChars-1);
        count = saveCount;  c=saveC;  val = *(v+*c);
      } else {
        val = *(v+*c++);
    } else {
      val = newVal;  ++c;
  } while (--count);

// Initialize a mapping from number of bits set in a character-sized 
section of the
// bitmap to the character used to represent that section.
void InitCharPointerArray(register ushort *v, 
     register ushort *c, register ushort *p)
  register short count1,count2,count3;
  register short currentVal;
  count2 = kNumChars;
  count1 = -1;
  do {
    currentVal = *(v+*c);
    if (currentVal>count1) {
      count3 = (ushort)(currentVal-count1)/2;
      if (count3) do {
        if (++count1 >= kMaxCharVals) return;
      } while (--count3);
      do {
        *p++=' '+*c;
        if (++count1 >= kMaxCharVals) return;
      } while (currentVal>count1);
  } while (--count2);
  do {
  } while (++count1<kMaxCharVals);

short BitMapToText(bitMapPtr,fontName,fontSize,outputFile)
BitMap *bitMapPtr;
Str255 fontName;
unsigned short fontSize;
FILE *outputFile;
GrafPort charPort;
GrafPtr savePort;

// bitsSet[x] is the number of bits set to 1 in the  representation of 
character ' '+x
// sortedCharP[y]+' ' is the y-th character in order of  increasing number 
of bits set
ushort bitsSet[kNumChars],sortedCharP[kNumChars];
// charVals[c] is the character to be output for a character size piece 
of the bitmap 
// with c bits set
ushort charVals[1+kMaxCharVals];

register ulong *q,*rowP;
register short count,numBitsSet;
register ulong mask;

register uchar *p;
ulong origMask;
ushort lineBytes;
short rCnt,rowBytes,hPix,fontNum,theErr;

  if (0 == fontNum) return (kErr);
  if (!RealFont(fontNum,fontSize)) return (kErr);
  if (theErr = InitOffscreenBitMap(&charPort))
    return theErr;
// Draw characters in bitmap.  Draws them one at a time so we can align 
the characters
// within long words. 
// Init charVal array of characters to output.
//  Process bitMap.
  p = (uchar *)bitMapPtr->baseAddr;
  rowBytes = bitMapPtr->rowBytes;
  lineBytes = rowBytes*lineHeight;
  rCnt = bitMapPtr->bounds.bottom - bitMapPtr->;
  hPix = bitMapPtr->bounds.right - bitMapPtr->bounds.left;
// Set a mask of charWid characters using a sign-extended shift.
  origMask = mask = (ulong)
           ((signed long)0x80000000>>(charWid-1));
//  Loop on rows of characters.
  do {
    short numBits,bitsThisChunk,bitsToGo,hCnt;
    rowP = (ulong *)p;
    bitsThisChunk = kBitsPerChunk;
    hCnt = hPix;
    numBits = bitsToGo = charWid;
//  Loop on chars within current row.
    do {
//    Count bits set in current char.
//    Loop on pixels in this chunk.
      do {
        q = rowP;
        count = lineHeight;
//      Count number of bits set in this chunk.
        do {
          register ulong ch;
          BitCount(ch,*q & mask,numBitsSet);
          q = (ulong *)((uchar *)q + rowBytes);
        } while (--count);
//  Continue processing current char if there are bitsToGo.
        if (0 == (bitsThisChunk -= numBits)) {
//        Check for end of row.
          if (hCnt < (bitsThisChunk=kBitsPerChunk))
            bitsThisChunk = hCnt;
          if (bitsToGo-=numBits) {
            if (bitsToGo > hCnt) bitsToGo = hCnt;
            mask = origMask << (charWid - bitsToGo);
            numBits = bitsToGo;
          } else {
            mask = origMask;
        } else if (numBits != charWid) {
          mask = (origMask >> bitsToGo);
        } else {
          mask >>= numBits;
      } while (true);  /* break if (0 == bitsToGo) */
      numBits = bitsToGo = charWid;
      if (numBits>bitsThisChunk) numBits= bitsThisChunk;
//  Select output character;
      if (numBitsSet<kMaxCharVals)
        count = *(charVals+numBitsSet);
      else count = *(charVals+kMaxCharVals);
    } while (0 < (hCnt-=charWid));
    p += lineBytes;
    if (0 > rCnt) break;
    if (0 > (rCnt-=lineHeight)) lineHeight += rCnt;
    mask =  origMask;
  } while (true);
  return 0;


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