Question

here i have code for calculate hash value of unsigned char

#include <cstdlib>
#include <iostream>
#include<string.h>
using namespace std;

unsigned oat_hash(unsigned char *key,int len)
{
    unsigned char *p=key;
    unsigned h=0;
    int i;
    for(i=0;i<len;++i){
        h+=p[i];
        h+=(h<<10);
        h^=(h>>6);
    }
    h+=(h<<3);
    h^=(h>>11);
    h+=(h<<15);

    return h;
}

using namespace std;

int main(int argc, char *argv[])
{
    unsigned char mystring[]="123456789abcdef";
    unsigned char *key=&mystring[0];
    int n=sizeof(mystring)/sizeof(mystring[0]);//length of mystring
    cout<<oat_hash(key,n)<<endl;
    //system("PAUSE");
    //return EXIT_SUCCESS;
    return 0;
}

name of this hash function is so called One-at-a-Time hash(by Bob Jenkins) i have one question is this little part of code correct?

int n=sizeof(mystring)/sizeof(mystring[0]);//length of mystring

because mysting has not built-in function length,i used this

Answer 1

Under the circumstances, yes — but it’s pretty fragile. For example, if you changed your definition from:

unsigned char mystring[]="123456789abcdef";

To:

unsigned char *mystring="123456789abcdef";

Your method of finding the length would produce completely incorrect results. Also note that since your string is made up of chars, the /sizeof(mystring[0]) isn’t really necessary either — sizof(char) == 1 (and the same for signed char or unsigned char).

You normally want to use strlen instead.