athena/src/LZ77/LZBase.cpp

#include "LZ77/LZLookupTable.hpp"
#include "LZ77/LZBase.hpp"


LZBase::LZBase(atInt32 minimumOffset, atInt32 slidingWindow, atInt32 minimumMatch, atInt32 blockSize)
    : m_slidingWindow(slidingWindow),
      m_readAheadBuffer(minimumMatch),
      m_minMatch(minimumMatch),
      m_blockSize(blockSize),
      m_minOffset(minimumOffset)
{
}

void LZBase::setSlidingWindow(atInt32 slidingWindow)
{
    m_slidingWindow = slidingWindow;
}

atInt32 LZBase::slidingWindow() {return m_slidingWindow;}

void LZBase::setReadAheadBuffer(atInt32 readAheadBuffer)
{
    m_readAheadBuffer = readAheadBuffer;
}

atInt32 LZBase::readAheadBuffer(){return m_readAheadBuffer;}

void LZBase::setMinMatch(atInt32 minimumMatch) { m_minMatch  =minimumMatch;}

atInt32 LZBase::minMatch(){return m_minMatch;}

void LZBase::setBlockSize(atInt32 blockSize)
{
    m_blockSize = blockSize ;
}

atInt32 LZBase::blockSize(){return m_blockSize;}

void LZBase::setMinimumOffset(atUint32 minimumOffset) { m_minOffset = minimumOffset;}

atUint32 LZBase::minimumOffset()
{
    return m_minOffset;
}

/*
  DerricMc:
  This search function is my own work and is no way affilated with any one else
  I use the my own window_search function to drastically speed up the search function
  Normally a search for one byte is matched, then two, then three, all the way up
  to the size of the LookAheadBuffer. So I decided to skip the incremental search
  and search for the entire LookAheadBuffer and if I don't find the bytes are equal I return
  the next best match(which means if I look for 18 bytes and they are not found 18 bytess did not match,
  and 17 bytes did match then 17 bytes match is return).

*/
LZLengthOffset LZBase::search(atUint8* posPtr, atUint8* dataBegin, atUint8* dataEnd)
{
    LZLengthOffset results={0,0};

    //Returns negative 1 for Search failures since the current position is passed the size to be compressed
    if(posPtr >=dataEnd)
    {
        results.length=-1;
        return results;
    }

    atUint8* searchWindow;
    //LookAheadBuffer is ReadAheadBuffer Size if there are more bytes than ReadAheadBufferSize waiting
    //to be compressed else the number of remaining bytes is the LookAheadBuffer
    int lookAheadBuffer_len=((int)(dataEnd-posPtr)<m_readAheadBuffer) ? (int)(dataEnd-posPtr) :m_readAheadBuffer;
    int slidingBuffer=(int)(posPtr - dataBegin)-m_slidingWindow;
    if(slidingBuffer > 0)
        searchWindow=dataBegin+slidingBuffer;
    else
        searchWindow=dataBegin;

    atUint8* endPos=posPtr+lookAheadBuffer_len;

    if(!( (posPtr-dataBegin < 1)||( dataEnd-posPtr < m_minMatch) ))
        results=windowSearch(searchWindow,posPtr,endPos,posPtr-m_minOffset);
    return results;
}


//Returns the full length of string2 if they are equal else
//Return the number of characters that were equal before they weren't equal
int LZBase::subMatch(const uint8_t* str1,const uint8_t* str2,const int len)
{
    for(int i=0;i<len;++i)
        if(str1[i]!=str2[i])
            return i;

    return len;
}

/*
Normally a search for one byte is matched, then two, then three, all the way up
  to the size of the LookAheadBuffer. So I decided to skip the incremental search
  and search for the entire LookAheadBuffer and if the function doesn't find the bytes are
  equal the function return the next best match(which means if the function look for 18 bytes and they are not found, return
  the number of bytes that did match before it failed to match. The submatch is function returns the number of bytes that
  were equal, which can result up to the bytes total length if both byte strings are equal.
  
  
  ...[][][][][][][][][][][][]|[][][][][][][][][][][][][][]
                             |
     Search Window       Current Pos   LookAheadBuffer
    Up to 4096 bytes                   Up to 18 bytes
                Sliding Window
                Up to 4114 bytes
*/
LZLengthOffset LZBase::windowSearch(atUint8* beginSearchPtr, atUint8* searchPosPtr, atUint8* endLABufferPtr, atUint8* startLBPtr)
{
    atInt32 size=(atUint32)(endLABufferPtr-beginSearchPtr);//Size of the entire sliding window
    atInt32 n=(atUint32)(endLABufferPtr-searchPosPtr);
    LZLengthOffset result={0,0};
    atInt32 temp=0;
    if(n > size)//If the string that is being looked for is bigger than the string that is being searched
        return result;

    /*This makes sure that search for the searchPosPtr can be searched if an invalid position is given
      An invalid position occurs if the amount of characters to search in_beginSearchPtr is less than the size
      of searchPosPtr. In other words there has to be at least n characters left in the string
      to have a chance to find n characters*/

    do
    {
        temp=subMatch(startLBPtr,searchPosPtr,n);
        if(result.length<(atUint32)temp)
        {
            result.length=temp;
            result.offset=(atInt32)(searchPosPtr-startLBPtr);
        }
        if(result.length==(atUint32)n)
            return result;

        //ReadAheadBuffer is the maximum size of a character match

    }
    while((startLBPtr--) > beginSearchPtr);
    return result;
}