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

char *strncpy(char *t, const char *s, size_t n);
char *strncat(char *t, const char *s, size_t n);
int strncmp(const char *s, const char *t, size_t n);

/* strlen: returns string length (w/o '\0') */
size_t strlen(const char *s)
{
    const char * const sc = s;
    while (*s++)
        ;
    return s - sc - 1;
}

/* takes args s1, s2, n and passes them to the above declared
 * functions, prints the results */
int main(int argc, char *argv[])
{
    if (argc < 4)
        return 1;

    int n = atoi(argv[3]);
    char s[strlen(argv[1]) + strlen(argv[2]) + 1];
    /* strncpy */
    printf("strncpy: %s\n", strncpy(s, argv[1], n));
    /* strncat */
    printf("strncat: %s\n", strncat(s, argv[2], n));
    /* strncmp */
    int comp = strncmp(argv[1], argv[2], n);
    printf("strncmp: %.*s ", n, argv[1]);
    putchar((comp > 0) ? '>' : (comp < 0) ? '<' : '=');
    printf(" %.*s\n", n, argv[2]);

    return 0;
}

/* strncpy: copies at most n chars from s to t, returns t*/
char *strncpy(char *t, const char *s, size_t n)
{
    char * const tc = t;
    while (n-- && (*t++ = *s++))
        ;
    *t = '\0';
    return tc;
}

/* strncat: concats at most n chars of s to t, returns t */ 
char *strncat(char *t, const char *s, size_t n) 
{
    strncpy(t + strlen(t), s, n);
    return t;
}

/* strncmp: compares at most n chars of s and t */
int strncmp(const char *s, const char *t, size_t n)
{
    if (!n)
        return 0;
    while (--n && *s && (*s == *t))
        s++, t++;
    return *s - *t;
}