Compress

#include <stdio.h>
#include <sys/types.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
#include <stdlib.h>

int main(void)
{
    int bytes = 0, end, i, displacement = 0, old_displacement = 0;

    struct stat *buf = malloc(sizeof(struct stat));
    lstat("input.dat", buf);
    unsigned int size = buf -> st_size;
    printf("size = %d\n", size);
    free(buf);

    int fd = open("input.dat", O_RDWR);    
    char buf_int[4] = {0}, buf_short[2] = {0}, buf_char;
    while(1){
        end = read(fd, &bytes, 1);  // reading first byte
        if(end == 0) break;         // if it is the end let's get out
        if(bytes == 1) {
            end = read(fd, &buf_char, 1); // if we have read only one byte than let's go on
            printf("readed %x\n", buf_char); 
            lseek(fd, -old_displacement - 3, SEEK_CUR);
            write(fd, &bytes, 1);
            write(fd, &buf_char, 1);
            printf("writed %x\n", buf_char);
            lseek(fd, displacement, SEEK_CUR);
            continue;
        }
        printf("bytes = %d\n", bytes);
        printf("displacement = %d\n", old_displacement);
        if(bytes == 4) {
            for(i = 0; i < 4; i++) { 
                read(fd, &buf_int[i], 1); // reading entire 4-byte integer
                printf("readed buf_int[%d] = %x\n", i, buf_int[i]);
            }
            old_displacement = displacement;
            if(buf_int[1] == 0 && buf_int[2] == 0 && buf_int[3] == 0) {
                bytes = 1;         // if we have zeros in three damn bytes than 
                displacement += 3; // adding 3 to displacement to omit zeros
            }
            else if(buf_int[2] == 0 && buf_int[3] == 0) {
                bytes = 2;
                displacement += 2; // the same
            }
            else if(buf_int[1] == ~0 && buf_int[2] == ~0 && buf_int[3] == ~0 && buf_int[0] >> 3)
            {
                bytes = 1;
                displacement += 3;
            }
            else if(buf_int[2] == ~0 && buf_int[3] == ~0 && buf_int[1] >> 3){
                bytes = 2;
                displacement += 3;
            }

            lseek(fd, -old_displacement - 5, SEEK_CUR); // let's make displacement from *already readed*
                                                        // to the start
            write(fd, &bytes, 1);     // rewriting the indicator byte
            for(i = 0; i < bytes; i++) {
                write(fd, &buf_int[i], 1);
                printf("writed buf_int[%d] = %x\n", i, buf_int[i]);
            }

            // lseek(fd, displacement, SEEK_CUR);
            printf("displacement = %d\n", displacement);
            lseek(fd, displacement, SEEK_CUR);
            continue;
        }
        if(bytes == 2) {
            for(i = 0; i < 2; i++) {
                read(fd, &buf_short[i], 1);
                printf("readed buf_short[%d] = %x\n", i, buf_short[i]);
            }
            old_displacement = displacement;
            if(buf_short[1] == 0) {
                bytes = 1;
                displacement += 1;
            }
            else if(buf_short[1] == ~0 && buf_short[0] >> 3) {
                bytes = 1;
                displacement += 1;
            }
            lseek(fd, -old_displacement - 3, SEEK_CUR);
            write(fd, &bytes, 1);
            for(i = 0; i < bytes; i++) {
                write(fd, &buf_short[i], 1);
                printf("writed buf_short[%d] = %x\n", i, buf_short[i]);
            }
            lseek(fd, displacement, SEEK_CUR);
        }
    }
    close(fd);
    truncate("input.dat", size - displacement);
    return 0;
}