startswith and endswith

Want to know if a file path ends with .md or a message starts with hello? Well then I have just the methods for you.

endswith

Surely you can't start with endswith? Yes, I can and I will. Cause endswith is slightly easier in C.

Using some pointer arithmetic we take the needle length off the haystack plus haystack length to get the end of the string. Then a quick strcmp finishes it off nicely.

bool endswith(char *needle, char *haystack)
{
    size_t haystack_len = strlen(haystack);
    size_t needle_len = strlen(needle);
    if (needle_len > haystack_len || !needle_len || !haystack_len)
        return false;
    return !strcmp(haystack + haystack_len - needle_len, needle);
}

assert(endswith(".md", "hello.md"));
assert(!endswith(".md", "hello.txt"));

startswith

startswith is slightly more involved as we must first create a char[] that is the length of the needle plus one, think null terminator. We loop over the start of the haystack adding each char to you temp char[], add a quick null terminator and pass it to strcmp, boom!

bool startswith(char *needle, char *haystack)
{
    size_t haystack_len = strlen(haystack);
    size_t needle_len = strlen(needle);
    if (needle_len > haystack_len || !needle_len || !haystack_len)
        return false;

    char start[needle_len + 1];
    for (size_t i = 0; i < needle_len; i++)
    {
        start[i] = haystack[i];
    }
    start[needle_len] = '\0';

    return !strcmp(start, needle);
}

assert(startswith("hello", "hello.md"));
assert(!startswith("hello", "yallohello.txt"));