#define _POSIX_C_SOURCE 200809L

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

int main(int argc, char ** argv) {
  int silent_mode = 0;
  int print_all_diff = 0;
  int keep_printing = 1;
  int files_differ = 0;
  int i, f1num = 1;
  FILE *file1, *file2;
  char c1, c2, c;
  int line = 1, byte = 1;

  while((c = getopt(argc, argv, "sl")) != -1) {
    switch(c) {
      case 's': silent_mode = 1; print_all_diff = 0; break;
      case 'l': print_all_diff = 1; silent_mode = 0; break;
    }
  }

  f1num = optind;

  if(argc - f1num < 2) {
    fprintf(stderr, "%s: missing file operand\n", argv[0]);
    return 3;
  }

  if(strcmp(argv[f1num], "-") == 0 && strcmp(argv[f1num+1], "-") == 0) {
    fprintf(stderr, "%s: cannot accept stdin for both files\n", argv[0]);
    return 2;
  }
  else if(strcmp(argv[f1num], "-") == 0) {
    file1 = stdin;
    file2 = fopen(argv[f1num+1], "r");
  }
  else if(strcmp(argv[f1num+1], "-") == 0) {
    file1 = fopen(argv[f1num], "r");
    file2 = stdin;
  }
  else {
    file1 = fopen(argv[f1num], "r");
    file2 = fopen(argv[f1num+1], "r");
  }

  if(file1 == NULL) {
    fprintf(stderr, "%s: could not open file %s\n", argv[0], argv[f1num]);
    return 4;
  }

  if(file2 == NULL) {
    fprintf(stderr, "%s: could not open file %s\n", argv[0], argv[f1num + 1]);
    return 4;
  }

  if(silent_mode) {
    c1 = fgetc(file1);
    c2 = fgetc(file2);
    for(i = 0; !(c1 == EOF || c2 == EOF); i++) {
      if(c1 != c2) {
	return 1;
      }
      c1 = fgetc(file1);
      c2 = fgetc(file2);
    }
    if(c1 != c2) {
      return 1;
    }
    else {
      return 0;
    }
  }
  else if(print_all_diff) {
    c1 = fgetc(file1);
    c2 = fgetc(file2);
    for(i = 0; !(c1 == EOF || c2 == EOF); i++) {
      if(c1 != c2) {
	printf("%d %o %o\n", byte, c1, c2);
	files_differ = 1;
      }
      c1 = fgetc(file1);
      c2 = fgetc(file2);
      line++;
      byte++;
    }
    if(c1 != c2) {
      fprintf(stderr, "%s: EOF on %s%s\n", argv[0],
	      c1 == EOF ? argv[f1num] : argv[f1num+1], "");
      return 1;
    }
    else if(files_differ) {
      return 1;
    }
    else {
      return 0;
    }
  }
  else {
    c1 = fgetc(file1);
    c2 = fgetc(file2);
    for(i = 0; !(c1 == EOF || c2 == EOF); i++) {
      if(c1 != c2 && keep_printing) {
	keep_printing = 0;
	printf("%s %s differ: char %d, line %d\n", argv[f1num], argv[f1num+1], byte, line);
	files_differ = 1;
      }
      c1 = fgetc(file1);
      c2 = fgetc(file2);
      if(c1 == '\n') line++;
      byte++;
    }
    if(c1 != c2) {
      fprintf(stderr, "%s: EOF on %s%s\n", argv[0],
	      c1 == EOF ? argv[f1num] : argv[f1num+1], "");
      return 1;
    }
    else if(files_differ) {
      return 1;
    }
    else {
      return 0;
    }
  }
}