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Compressor.cpp
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Compressor.cpp
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#include "Compressor.h"
#include <algorithm>
#include <iostream>
#include "BitStream.h"
Compressor::Compressor()
{
int endianTest = 1;
mIsLittleEndian = *(char*)(&endianTest) == 1;
}
int Compressor::reverseEndianess(int value)
{
int newValue;
char* in_ptr = (char*)&value;
char* out_ptr = (char*)&newValue;
out_ptr[0] = in_ptr[3];
out_ptr[1] = in_ptr[2];
out_ptr[2] = in_ptr[1];
out_ptr[3] = in_ptr[0];
return newValue;
}
std::vector<unsigned char> Compressor::compress(void* data, unsigned length)
{
mFrequency = getFrequency(data, length);
int bits = getBestRatio(length);
char* ptr = (char*)data;
int i;
if (bits == -1)
return std::vector<unsigned char>();
BitStream bs;
unsigned totalSize = 27 + ((1 << bits) - 1) * 8;
if (mIsLittleEndian)
bs.insert((int)length, 24);
else
bs.insert(reverseEndianess((int)length), 24);
bs.insert(bits, 3);
for (i = 0; i < (1 << bits) - 1; i++)
{
if ((int)mFrequency.size() > i)
bs.insert(mFrequency[i].character);
else
bs.insert(0);
}
std::vector<char> uncompressed;
for (i = 0; i < (int)length; i++)
{
int vecPos = findChar(mFrequency, *ptr, (1 << bits) - 1);
if (vecPos == -1)
{
bs.insert(0U, bits);
uncompressed.push_back(*ptr);
}
else
{
bs.insert(vecPos + 1, bits);
}
totalSize += bits;
ptr++;
}
for (i = 0; i < (int)uncompressed.size(); i++)
{
bs.insert(uncompressed[i]);
totalSize += 8;
}
return bs.getData();
}
std::vector<unsigned char> Compressor::decompress(void* data, unsigned length)
{
BitStream bs;
bs.insert(data, length);
bs.read_bytes(&length, 3);
if (!mIsLittleEndian)
length = (unsigned)reverseEndianess((int)length);
unsigned char bits;
bs.read_bits(bits, 3);
if (bits < 1 || bits > 7)
{
char bStr[4];
throw (std::string("Bit-size of enconding isn't valid: ") + std::string(itoa(bits, bStr, 10)));
}
unsigned char byte;
int bitsRead = 27;
int i;
std::vector<char> charTable;
std::vector<char> compressedChars;
for (i = 1; i <= (1 << bits) - 1; i++)
{
bs.read_bits(byte, 8);
charTable.push_back(byte);
bitsRead += 8;
if (!bs.canRead())
throw std::string ("Bad file for decompression [1]");
}
for (unsigned j = 1; j <= length; j++)
{
bs.read_bits(byte, bits);
bitsRead += bits;
compressedChars.push_back(byte);
if (!bs.canRead())
throw std::string ("Bad file for decompression [2]");
}
std::vector<unsigned char> result(length, 0);
for (i = 0; i < (int)compressedChars.size(); i++)
{
if (compressedChars[i] == 0 || (unsigned char)compressedChars[i] > charTable.size())
{
if (!bs.canRead())
throw std::string ("Bad file for decompression [3]");
bs.read_bits(result[i], 8);
bitsRead += 8;
}
else
{
result[i] = charTable[compressedChars[i] - 1];
}
}
return result;
}
Compressor::FrequencyVector Compressor::getFrequency(void* data, unsigned length)
{
char* ptr = (char*)data;
FrequencyVector freq;
while (length-- > 0)
{
getChar(freq, *ptr).count++;
ptr++;
}
std::sort(freq.begin(), freq.end(), Compressor::compareFreq);
return freq;
}
bool Compressor::compareFreq(FrequencyChar a, FrequencyChar b)
{
return a.count >= b.count;
}
Compressor::FrequencyChar& Compressor::getChar(FrequencyVector& vec, char character)
{
for (unsigned i = 0; i < vec.size(); i++)
{
if (vec[i].character == character)
return vec[i];
}
vec.push_back(FrequencyChar(character));
return vec.back();
}
int Compressor::findChar(FrequencyVector& vec, char character, unsigned limit)
{
for (unsigned i = 0; i < vec.size() && i < limit; i++)
{
if (vec[i].character == character)
return i;
}
return -1;
}
int Compressor::getBestRatio(unsigned dataLength)
{
int bestBit = 0;
int compressedLength = -1;
for (int bits = 1; bits <= 7; bits++)
{
int compressed = 0;
int neededBits;
for (int i = 0; i < (1 << bits) - 1; i++)
{
if ((int)mFrequency.size() <= i)
break;
compressed += mFrequency[i].count;
}
neededBits = computeSize(bits, dataLength, compressed);
if (neededBits < compressedLength || compressedLength == -1)
{
compressedLength = neededBits;
bestBit = bits;
}
}
return bestBit;
}
unsigned Compressor::computeSize(int numberOfBits, unsigned dataLength, unsigned compressCount)
{
return numberOfBits * dataLength + (dataLength - compressCount) * 8 + 27 + ((1 << numberOfBits) - 1) * 8;
}