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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#define PROGRAM_NAME "xor_compress"
extern int errno;
size_t file_size(FILE *f, int *err) {
int check = fseek(f, 0, SEEK_END);
if (check == -1) goto failure;
long f_size = ftell(f);
if (f_size == -1) goto failure;
check = fseek(f, 0, SEEK_SET);
if (check == -1) goto failure;
*err = 0;
return (size_t)f_size;
failure:
*err = errno;
return 0;
}
unsigned char *read_files(char *filenames[], int num_files, size_t *buf_size, int *err) {
*buf_size = 0;
unsigned char *buffer = NULL;
const char *filename = NULL;
for (int i = 0; i < num_files; i++) {
filename = filenames[i];
FILE *f = fopen(filename, "rb");
if (!f) goto failure;
size_t f_size = file_size(f, err);
if (*err > 0) goto failure;
if (f_size == 0) continue;
*buf_size += f_size;
buffer = realloc(buffer, *buf_size);
if (!buffer) goto failure;
unsigned char *buf_end = buffer + (*buf_size - f_size);
size_t read_size = fread(buf_end, 1, f_size, f);
fclose(f);
if (read_size != f_size) {
// fread does not set errno
fprintf(stderr, PROGRAM_NAME ": %s: Read error\n", filename);
*err = 1;
return buffer;
}
}
*err = 0;
return buffer;
failure:
fprintf(stderr, PROGRAM_NAME ": %s: %s\n", filename, strerror(errno));
*err = errno;
return buffer;
}
void compress_data(unsigned char *data, size_t n) {
unsigned char v = 0x00;
for (size_t i = 0; i < n;) {
unsigned char byte = data[i++];
unsigned char size = 0;
if (data[i] == v) {
// Alternating (>= 0x80)
// Run stops at 0x80 bytes or when the values stop alternating
for (; i < n && size < 0x80 && data[i] == ((size % 2) ? byte : v); size++, i++);
putchar(size + 0x7f);
putchar(v ^ byte);
if (size % 2 == 0) v = byte;
} else {
// Sequential (< 0x80)
// Run stops at 0x80 bytes or when the value two ahead is equal to v
unsigned char buffer[256] = {0};
buffer[size++] = v ^ byte;
for (; i < n; i++) {
v = byte;
if (size > 0x7f || (i + 1 < n && data[i + 1] == v)) break;
byte = data[i];
buffer[size++] = v ^ byte;
}
putchar(size - 1);
fwrite(buffer, 1, size, stdout);
}
}
fflush(stdout);
}
int main(int argc, char *argv[]) {
if (argc < 2) {
fputs("Usage: " PROGRAM_NAME " file... > files.xor\n", stderr);
exit(1);
}
argv++;
argc--;
int err = 0;
size_t data_size = 0;
unsigned char *data = read_files(argv, argc, &data_size, &err);
if (err > 0) {
if (data) free(data);
return err;
}
compress_data(data, data_size);
free(data);
return 0;
}
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