// --------------------------------------------------------------------------
//
// Function
// Name: static bool SecondStageMatch(xxx)
// Purpose: When a match in the hash table is found, scan for second stage match using strong checksum.
// Created: 14/1/04
//
// --------------------------------------------------------------------------
static bool SecondStageMatch(BlocksAvailableEntry *pFirstInHashList, RollingChecksum &fastSum, uint8_t *pBeginnings, uint8_t *pEndings,
int Offset, int32_t BlockSize, int64_t FileBlockNumber, BlocksAvailableEntry *pIndex, std::map<int64_t, int64_t> &rFoundBlocks)
{
// Check parameters
ASSERT(pBeginnings != 0);
ASSERT(pEndings != 0);
ASSERT(Offset >= 0);
ASSERT(BlockSize > 0);
ASSERT(pFirstInHashList != 0);
ASSERT(pIndex != 0);
#ifndef BOX_RELEASE_BUILD
uint16_t DEBUG_Hash = fastSum.GetComponentForHashing();
#endif
uint32_t Checksum = fastSum.GetChecksum();
// Before we go to the expense of the MD5, make sure it's a darn good match on the checksum we already know.
BlocksAvailableEntry *scan = pFirstInHashList;
bool found=false;
while(scan != 0)
{
if(scan->mWeakChecksum == Checksum)
{
found = true;
break;
}
scan = scan->mpNextInHashList;
}
if(!found)
{
return false;
}
// Calculate the strong MD5 digest for this block
MD5Digest strong;
// Add the data from the beginnings
strong.Add(pBeginnings + Offset, BlockSize - Offset);
// Add any data from the endings
if(Offset > 0)
{
strong.Add(pEndings, Offset);
}
strong.Finish();
// Then go through the entries in the hash list, comparing with the strong digest calculated
scan = pFirstInHashList;
//BOX_TRACE("second stage match");
while(scan != 0)
{
//BOX_TRACE("scan size " << scan->mSize <<
// ", block size " << BlockSize <<
// ", hash " << Hash);
ASSERT(scan->mSize == BlockSize);
ASSERT(RollingChecksum::ExtractHashingComponent(scan->mWeakChecksum) == DEBUG_Hash);
// Compare?
if(strong.DigestMatches(scan->mStrongChecksum))
{
//BOX_TRACE("Match!\n");
// Found! Add to list of found blocks...
int64_t fileOffset = (FileBlockNumber * BlockSize) + Offset;
int64_t blockIndex = (scan - pIndex); // pointer arthmitic is frowned upon. But most efficient way of doing it here -- alternative is to use more memory
// We do NOT search for smallest blocks first, as this code originally assumed.
// To prevent this from potentially overwriting a better match, the caller must determine
// the relative "goodness" of any existing match and this one, and avoid the call if it
// could be detrimental.
rFoundBlocks[fileOffset] = blockIndex;
// No point in searching further, report success
return true;
}
// Next
scan = scan->mpNextInHashList;
}
// Not matched
return false;
}
