C++ decoding LZ77-compressed data using std::fstream too slow - c++

Question

I have a function in my code which decodes a file compressed using the LZ77 algorithm. But on 15 MB input file decompression takes about 3 minutes (too slow). What's the reason of poor performance? On every step of the loop I read two or three bytes and get length, offset and next character. If offset is not zero I also have to move "offset" bytes back in output stream and read "length" bytes. Then I insert them to the end of the same stream before writing next character there.
void uncompressData(long block_size, unsigned char* data, fstream &file_out)
{
unsigned char* append;
append = new unsigned char[buf_length];
link myLink;
long cur_position = 0;
file_out.seekg(0, ios::beg);
cout << file_out.tellg() << endl;
int i=0;
myLink.length=-1;
while(i<(block_size-1))
{
if(myLink.length!=-1) file_out << myLink.next;
myLink.length = (short)(data[i] >> 4);
//cout << myLink.length << endl;
if(myLink.length!=0)
{
myLink.offset = (short)(data[i] & 0xF);
myLink.offset = myLink.offset << 8;
myLink.offset = myLink.offset | (short)data[i+1];
myLink.next = (unsigned char)data[i+2];
cur_position=file_out.tellg();
file_out.seekg(-myLink.offset,ios_base::cur);
if(myLink.length<=myLink.offset)
{
file_out.read((char*)append, myLink.length);
}
else
{
file_out.read((char*)append, myLink.offset);
int k=myLink.offset,j=0;
while(k<myLink.length)
{
append[k]=append[j];
j++;
if(j==myLink.offset) j=0;
k++;
}
}
file_out.seekg(cur_position);
file_out.write((char*)append, myLink.length);
i++;
}
else {
myLink.offset = 0;
myLink.next = (unsigned char)data[i+1];
}
i=i+2;
}
unsigned char hasOddSymbol = data[block_size-1];
if(hasOddSymbol==0x0) { file_out << myLink.next; }
delete[] append;
}

Answer 1

You could try doing it on a std::stringstream in memory instead:
#include <sstream>
void uncompressData(long block_size, unsigned char* data, fstream& out)
{
std::stringstream file_out; // first line in the function
// the rest of your function goes here
out << file_out.rdbuf(); // last line in the function
}