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Use linear search instead of the Knuth-Morris-Pratt algorithm in ByteVector::find().

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In almost all cases, it handles too small data for KMP to work effectively.
This commit is contained in:
Tsuda Kageyu 2015-06-22 10:37:20 +09:00
parent 0ffd2e8ab9
commit 3bce77a359
1 changed files with 16 additions and 37 deletions
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53
taglib/toolkit/tbytevector.cpp
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@ -99,10 +99,6 @@ static const uint crcTable[256] = {
0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4
};
/*!
* A templatized straightforward find that works with the types
* std::vector<char>::iterator and std::vector<char>::reverse_iterator.
*/
template <class TIterator>
int findChar(
const TIterator dataBegin, const TIterator dataEnd,
@ -125,10 +121,6 @@ int findChar(
return -1;
}
/*!
* A templatized KMP find that works with the types
* std::vector<char>::iterator and std::vector<char>::reverse_iterator.
*/
template <class TIterator>
int findVector(
const TIterator dataBegin, const TIterator dataEnd,
@ -140,46 +132,33 @@ int findVector(
if(patternSize == 0 || offset + patternSize > dataSize)
return -1;
// n % 0 is invalid
if(byteAlign == 0)
return -1;
// Special case that pattern contains just single char.
if(patternSize == 1)
return findChar(dataBegin, dataEnd, *patternBegin, offset, byteAlign);
size_t lastOccurrence[256];
// n % 0 is invalid
for(size_t i = 0; i < 256; ++i)
lastOccurrence[i] = patternSize;
if(byteAlign == 0)
return -1;
for(size_t i = 0; i < patternSize - 1; ++i)
lastOccurrence[static_cast<uchar>(*(patternBegin + i))] = patternSize - i - 1;
// We don't use sophisticated algorithms like Knuth-Morris-Pratt here.
TIterator it = dataBegin + patternSize - 1 + offset;
while(true) {
TIterator itBuffer = it;
TIterator itPattern = patternBegin + patternSize - 1;
// In the current implementation of TagLib, data and patterns are too small
// for such algorithms to work effectively.
while(*itBuffer == *itPattern) {
if(itPattern == patternBegin) {
if((itBuffer - dataBegin - offset) % byteAlign == 0)
return (itBuffer - dataBegin);
else
break;
}
for(TIterator it = dataBegin + offset; it < dataEnd - patternSize + 1; it += byteAlign) {
--itBuffer;
--itPattern;
TIterator itData = it;
TIterator itPattern = patternBegin;
while(*itData == *itPattern) {
++itData;
++itPattern;
if(itPattern == patternEnd)
return (it - dataBegin);
}
const size_t step = lastOccurrence[static_cast<uchar>(*it)];
if(dataEnd - step <= it)
break;
it += step;
}
return -1;