Question

I have the following C function:

unsigned int DJBHash(char* str, unsigned int len)
{
   unsigned int hash = 5381;
   unsigned int i    = 0;

   for(i = 0; i < len; str++, i++)
   {
      hash = ((hash << 5) + hash) + (*str);
   }

   return hash;
}

I’m trying to convert it to Javascript. I’m having trouble with the (*str) part of line 8

(`hash=((hash << 5) + hash) + (*str)`).

How can I efficiently convert my javascript string into the same representation as is done in C?

Here’s what I’ve done sofar, but it’s not working: when I add zero to “str”, it simply appends a character “0” to my str. What am I doing wrong?

function DJBHash(str,len){
        var hash=5381;
        var i=0;

        for(i=0;i<len;i++){
                hash=((hash<<5)+hash)+(str+0);
        }
        return hash;
}

Answer 1

There are no pointers in Javascript. Treat the input as a string instead of a pointer to a string. The string has a length, so you don’t need to send that as a parameter, and the string object has the charCodeAt method that you can use to get the character code of a specific character during the loop:

function DJBHash(str) {
  var hash = 5381;
  for(var i = 0; i < str.length; i++) {
    hash = ((hash << 5) + hash) + str.charCodeAt(i);
  }
  return hash;
}

However, the C code might rely on the int to have a specific size (which is however not according to the C specifications), to use the overflow to limit the result to a specific number of bits. As Javascript doesn’t have any integer type, you would have to use integer operations to limit the result in the same way. This would produce a 32 bit result:

function DJBHash(str) {
  var hash = 5381;
  for(var i = 0; i < str.length; i++) {
    hash = (((hash << 5) + hash) + str.charCodeAt(i)) & 0xffffffff;
  }
  return hash;
}