Slow MP3 tag parsing/ByteVector::replace() performance

[Henry Högelow]

Hi,

I fixed the performance issues with ByteVector::replace for the Raumfeld  
devices with an algorithm allocating memory only once:

ByteVector &ByteVector::replace(const ByteVector &pattern, const  
ByteVector &with)
{
   if (pattern.size() == 0 || pattern.size() > size())
     return *this;

   const int patternSize = pattern.size();
   const int withSize = with.size();

   int offset = find(pattern);

   // try to not allocate memory for short buffers or rare patterns
   const size_t numDefaultStorageFields = 1024;
   size_t defaultStorageForPatternPositions[numDefaultStorageFields];

   // be prepared for any number of patterns
   typedef std::list<size_t> tPatternPositions;
   tPatternPositions morePatternPositions;

   size_t numPatternPositions = 0;

   while (offset >= 0)
   {
     if(numPatternPositions < numDefaultStorageFields)
     {
       defaultStorageForPatternPositions[numPatternPositions] = offset;
     }
     else
     {
       morePatternPositions.push_back(offset);
     }
     numPatternPositions++;
     offset = find(pattern, offset + patternSize);
   }

   if(numPatternPositions)
   {
     // we already know the resulting buffers size, so allocate it
     size_t newBuffersSize = size() + (withSize - patternSize) *  
numPatternPositions;
     ByteVector result(newBuffersSize);

     size_t readPos = 0;
     size_t writePos = 0;

     tPatternPositions::const_iterator itPos = morePatternPositions.begin();

     for (size_t i = 0; i < numPatternPositions; i++)
     {
       int patternPos = (i < numDefaultStorageFields) ?  
defaultStorageForPatternPositions[i] : *(itPos++);

       // copy stuff between patterns
       size_t numBytesInBetween = patternPos - readPos;
       ::memcpy(result.data() + writePos, data() + readPos,  
numBytesInBetween);

       readPos += numBytesInBetween;
       writePos += numBytesInBetween;

       // write the new pattern
       ::memcpy(result.data() + writePos, with.data(), withSize);

       // ignore the old pattern by advance the readPos
       readPos += patternSize;
       writePos += withSize;
     }

     // copy the rest
     ::memcpy(result.data() + writePos, data() + readPos, size() - readPos);

     // swap the data of result with our own
     std::swap(d, result.d);
   }

   return *this;
}


Best, Henry

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