Thread: Unsigned long long to something longer

Originally Posted by JFonseka

Now it gives different hash values for strings longer than 8 characters, but I get an odd feeling that it's not right, like that feeling you get when someone's behind with you a knife, but much stronger.

Unfortunately, with strings >= 8 bytes, you're murdering some part of the world
Remember that strncpy does not insert a null char if the string is longer than the max! So what you are doing is reading beyond the buffer, looking for data.

Otherwise, this sample seems unnecessarily complex and does not do what it is intended to do.

Code:

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

#define LONGLONGSIZE (sizeof (unsigned long long))

union intstr {
  unsigned long long ikey;
  char               skey[LONGLONGSIZE];
};

unsigned hashShortString (char v[], unsigned M)
{ 
  union intstr *is;

  /* I think you have the idea wrong here. The union is supposed to be 
  used to convert the string into an uint64_t, nothing else. So use a local variable to store 
  the hash.
  Further, you should probably just use your old algorithm on each 8 bytes and append 
  the result to your hash.
  Another thing to think of is what happens if your string doesn't align on 8-byte 
  boundaries. You'll get garbage after the null char, and you probably don't want to has 
  that. */
  for (; *v != '\0'; v++)
       is->ikey = (LONGLONGSIZE*is->ikey + *v) &#37; M;

  return is->ikey % M;
}

int main (int argc, char *argv[])
{
  char key[LONGLONGSIZE];

  if (2 != argc) {
    printf ("USAGE: hash_short STRING\n");
    printf ("  (STRING will be truncated after %u characters)\n",
            LONGLONGSIZE);
    return EXIT_FAILURE;
  }
  strncpy (key, argv[1], LONGLONGSIZE);
  printf ("Hash: %u\n", hashShortString (key, 8191));

  return EXIT_SUCCESS;
}

Now see my comments in the code.
And also think about your strcpy in the beginning (why not just pass it directly?).

Thanks matsp

The reason being Elysia, that, I didn't write the initial code, my lecturer did. I don't even know what I'm doing this, it just seemed interesting to extend this further because it doesn't copy more than 8, from his. And that append idea seems like a good idea, I will try that.

Cheers

And now I get all your murder comments. I guess I am a murderer after all. Lol

Originally Posted by JFonseka

The reason being Elysia, that, I didn't write the initial code, my lecturer did. I don't even know what I'm doing this, it just seemed interesting to extend this further because it doesn't copy more than 8, from his. And that append idea seems like a good idea, I will try that.

Yep, but the original code couldn't hash the entire string, so either way, you have to be inventing some new code which may require a change in your code structure!