/* files_d iffer.c - determine if two files are identical
 * Andrew Ho (andrew@zeuscat.com)
 *
 * Implements files_differ() which quickly returns false if two files
 * differ, and true if they are identical. Memory use is capped to
 * a little over 2*BUFSIZE.
 *
 * Build: gcc -Wall -o files_differ files_differ.c
 * Usage: files_differ file1 file2
 */

#include <stdio.h>
#include <string.h>
#include <sys/types.h>

#ifndef BUFSIZE
#define BUFSIZE 1024
#endif

/* returns true if file1 and file2 are exactly the same */
int files_differ(const char *file1, const char *file2) {
    FILE *fh1, *fh2;
    size_t b1, b2;
    unsigned char buf1[BUFSIZE+1], buf2[BUFSIZE+1];

    fh1 = fopen(file1, "r"); if(!fh1) return 1;
    fh2 = fopen(file2, "r"); if(!fh2) return 1;
    memset(buf1, '\0', BUFSIZE+1);
    memset(buf2, '\0', BUFSIZE+1);
    while((b1 = fread(buf1, sizeof(char), BUFSIZE, fh1))) {
        b2 = fread(buf2, sizeof(char), BUFSIZE, fh2);
        if(feof(fh1) && feof(fh2))
            break;
        else if(feof(fh1) || feof(fh2))
            return 1;
        if(b1 != b2) return 1;
        if(b1 ==  0) break;
        if(strncmp(buf1, buf2, b1)) return 1;
        memset(buf1, '\0', BUFSIZE+1);
        memset(buf2, '\0', BUFSIZE+1);
    }
    if(!feof(fh1)) {
        perror("error reading from file");
        return 1;
    }
    fclose(fh2);
    fclose(fh1);
    return 0;
}

int main(int argc, char **argv) {
    if(argc != 3) {
        fprintf(stderr, "usage: files_differ file1 file2\n");
        return 1;
    }
    if(files_differ(argv[1], argv[2]))
        puts("files differ");
    else
        puts("files match");
    return 0;
}