xor.c

// 'xor'
// by zachary vance, released into the public domain (see CC0 1.0 license)
// Given 2+ streams as filenames on the command line, print to stdout the 'xor' of the files. 
// If no options are given, pads the shorter file with zeros, so the length of the output is always the length of the longer file
// If '--same-size' is passed, throws an error if one of the file terminates early.
#include <stdio.h>
#include <unistd.h>
#include <string.h>

#define MAX_FILES 10
#define BUFFER 1024

int main(int argc, char *argv[]){
  FILE *fp[MAX_FILES];
  char inbytes[BUFFER], outbytes[BUFFER];
  int bytes_read, max_bytes_read;
  int files, files_done;
  int option_same_size=0;
  int f,b;
  
  if (argc <= 1) {
    printf("usage: xor FILE1 FILE2\n");
    return 2;
  }
  files = argc-1;
  if (strcmp("--same-size", argv[1])==0)  {
    files -= 1;
    option_same_size=1;
  }
  if (files < 2) {
    printf("usage: xor FILE1 FILE2\n");
    return 2;

  } else if (files > MAX_FILES) {
    printf("Too many files. Please recompile with a faster number of files maximum\n");
    return 2;
  }
  
  for (f=0;f<files;f++){
    fp[f]=fopen(argv[f+1],"rb");
    if(fp[f] == NULL){
      printf("File %d not found: %s\n", f, argv[f+1]);
      return 1;
    }
  }

  while(1) {
    files_done=0;
    for(f=0;f<files;f++)
      if(feof(fp[f])) files_done++;
    if(files == files_done) break;

    memset(outbytes, 0, BUFFER);
    max_bytes_read=-1;
    for(f=0;f<files;f++) {
      bytes_read = fread(&inbytes, 1, BUFFER, fp[f]);
      for(b=0;b<bytes_read;b++) outbytes[b] ^= inbytes[b];
      if(option_same_size && max_bytes_read >= 0 && max_bytes_read != bytes_read) {
        fprintf(stderr, "files were not the same size");
        return 3;
      }
      if(bytes_read > max_bytes_read) max_bytes_read=bytes_read;
    }

    write(1, outbytes, max_bytes_read);
  }

  for(f=0;f<files;f++) fclose(fp[f]);

  return 0;
}