how to compress a file in c
#include <stdio.h>
#include <string.h>
//comp()
void Compress(unsigned char *szOut, const char *szMessage) {
unsigned long long nBuffer = 0; //This is enough to store 8 uncompressed characters or 9 compressed. We will only use 8 tho.
char nBitsInBuffer = 0;
while (*szMessage != 0) {
nBuffer |= (unsigned long long)(*szMessage++ & 0x7F) << nBitsInBuffer;
nBitsInBuffer += 7;
if (nBitsInBuffer == 7 * 8) { //We have 8 chars in the buffer, dump them
while (nBitsInBuffer > 0) {
*szOut++ = nBuffer & 0xFF;
nBuffer >>= 8;
nBitsInBuffer -= 8;
}
//The following should be redundant, but just to be safe
nBitsInBuffer = 0;
nBuffer = 0;
}
}
//Write out whatever is left in the buffer
while (nBitsInBuffer > 0) {
*szOut++ = nBuffer & 0xFF;
nBuffer >>= 8;
nBitsInBuffer -= 8;
}
}
//uncomp()
//nCompressedLen is the number of bytes in the compressed buffer.
//NOTE: the compressed buffer does not have a NULL terminating character
void Uncompress(char *szOut, const unsigned char *szCompressed, unsigned nCompressedLen) {
unsigned long long nBuffer = 0; //This is enough to store 8 uncompressed characters or 9 compressed. We will only use 8 tho.
char nBitsInBuffer = 0;
while (nCompressedLen) {
while (nCompressedLen && nBitsInBuffer < 7 * 8) {
nBuffer |= (unsigned long long)*szCompressed++ << nBitsInBuffer;
nBitsInBuffer += 8;
--nCompressedLen;
}
while (nBitsInBuffer > 0) {
*szOut++ = nBuffer & 0x7F;
nBuffer >>= 7;
nBitsInBuffer -= 7;
}
//The following should be redundant, but just to be safe
nBitsInBuffer = 0;
nBuffer = 0;
}
}
int main() {
//char szMessage[] = "\x53\x7F\x63\x4B";
char szMessage[] = "hello world. this is a compressed long string";
static const unsigned nCompressedSize = sizeof(szMessage) * 7 / 8; //This does not include the NULL terminating character from the string
unsigned char pCompressedBytes[nCompressedSize];
char szUncompressed[sizeof(szMessage)];
printf(" Message: %s\n", szMessage);
Compress(pCompressedBytes, szMessage);
printf(" Compressed: ");
for (int nByte = 0; nByte < nCompressedSize; ++nByte) {
printf("%02X ", pCompressedBytes[nByte]);
}
printf("\n");
Uncompress(szUncompressed, pCompressedBytes, nCompressedSize);
szUncompressed[sizeof(szMessage) - 1] = 0; //We need to terminate the string. The NULL terminator is not stored in the compressed bytes
printf("Uncompressed: %s\n", szUncompressed);
//Now just verify that we ended up with the same message we started with
if (strcmp(szMessage, szUncompressed) == 0) {
printf("Compression works\n");
} else {
printf("Compression failed\n");
}
return 0;
}