converting (string) urls to 4 byte integer numbers

Hi,
I was wondering if anyone knows how a url (string representation) could be mapped to a unique identifier number (4 byte if possible).  What I would like to do is to map up to 100,000,000 urls to a 4 byte unique identifier to save on memory as these UID's will be used to represent the URL's within a database.
      Cheers,
      everton690.
everton690Asked:
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cjjcliffordConnect With a Mentor Commented:
all these great repeats of my suggestions :-)

btw, if you are using Java, the builtin hashCode() method on String is the correct one to use for the hash value - not this is not going to be unique, but it is going to be very long, and quite likely to be well distributed.

E.g. System.out.println( "http://www.experts-exchange.com/Programming/".hashCode() ); generated the output: 505291298

For the LUT, to expand my original suggestion, the syntax for Oracle would be:

CREATE TABLE urls (
    id NUMBER NOT NULL,
    url VARCHAR2(255) NOT NULL
);
CREATE SEQUENCE url_id_seq;

then,

insert into urls( id, url ) VALUES( url_id_seq.nextval, 'www.abc.com' );

Note, you'll probably want a primary key on urls(id) if this is going to be used...
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cjjcliffordCommented:
you could create a LUT in the database, with the ID, URL as columns (e.g. In oracle, use a SEQUENCE to generate the ID), and use the ID as foreign key everywhere.

Other than that, you could generate a CRC function, or some type of hashcode function, on the URL to convert the URL to a code. This would not be guaranteed to be unique though...
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drichardsCommented:
That sounds like the best idea (creating an autonumber as an ID).  The other problem with a hash, besides the fact that with a 4-byte hash you will have a reasonable probability of collisions, is that it is one way.  You cannot back-figure the URL from the hash, so you would need to store the URL anyway unless you are only looking up based on a URL as input.
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RanjeetRainCommented:
CRC generation is a proven technology. And it works on strings of unlimited length. Any function you code yourself, may or may not gurantee the uniqueness of a the key associated with a URL.

Another solution, and a little easier, can be to generate and use GUIDs. Its standrad on Windows platform, and can be relaibly used to store keys. Its a virtual gurantee that a GUID will be unique.
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gregoryyoungCommented:
lets say you have a 50 character url ...

there is no way it could possibly be uniquely hashed into a 4 byte integer (simple math on the number of combinations)

the idea of the uniqueid is good ...

another idea would just be to use a hash function ...

    unsigned long
    hash(unsigned char *str)
    {
        unsigned long hash = 5381;
        int c;

        while (c = *str++)
            hash = ((hash << 5) + hash) + c; /* hash * 33 + c */

        return hash;
    }


is a decent one.
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Julian HansenCommented:
If you are using MS SQL

create table myURLS
(
    recid int IDENTITY(1,1) NOT NULL,
    URL varchar(256) PRIMARY KEY NOT NULL
)

Then

insert into myURLS ( URL ) value ('www.1.com' )
insert into myURLS ( URL ) value ( 'www.2.com' )
...

Good for 2^32 URLS which is slightly more URLS than are currently out there.

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aravindtjCommented:
hi,
 you can specify the IP address in integer format (called in windows uint32 - unsigned long.).
 you can get it by inet_addr (const char * ipaddress ). ipaddress is in dotted notation format.
 you can get host name/ url using  gethostbyaddr method.
Syntax:
 struct hostent FAR * gethostbyaddr ( const char FAR * addr,  int len, int type )
 addr - dotted notation IP address.
 len - length of address.
 type - type of address

hostent structure:

struct hostent {
    char FAR *       h_name;
    char FAR * FAR * h_aliases;
    short            h_addrtype;
    short            h_length;
    char FAR * FAR * h_addr_list;
};

try that.
all the best
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cjjcliffordCommented:
aravindtj, at best this would work for domain names, not full URLs, and the overhead of doing the DNS lookup (etc) to get the ID would probably be over the top...
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Julian HansenCommented:
Also there is the fact that IP's have a one to many relationship with domain names. A server with a single IP can host more than 1 domain.
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everton690Author Commented:
Thanks everone who posted a comment  and thanks to cjjclifford for some relevant suggestions.  In the end (in case anyone is interested) I decided to use java's GZIP to compress the string url.  Cheers,
          everton690.
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