Probably not possible with a reasonable sized hash.  If the size of
the hash is limited, and thare are more than that possible inputs then
there must be some collisions.  How many possible values are there for
the Message string for example?

However it is possible to do something with a reasonably low
probability of a collision, something along the lines of:

int DoHash(string Message, int EmployeeID, int DepartmentID) {
 const int multiplier = 29;
 const int startValue = 37;
 int hash = startValue;
 hash = multiplier * hash + Message.GetHashCode();
 hash = multiplier * hash + EmployeeID;
 hash = multiplier * hash + DepartmentID;
 return hash;
}

This relies on Messsage.GetHashCode() returning a suitable value.
Depending on your exact requirement you may need to put in your own
function there.

Remember that collisions are possible, though they should be rare.  If
you get matching hash values then you must do a full check for
equality.

rossum

<snip>

Could you clarify what you mean by the last part? If the inputs are the
same surely the result would *have* to be the same - otherwise it's
useless.

A hash that has to be unique for all input will need to have the same
size as the input meaning that it is useless.

It is really a trade off between risk of collisions with size and
computational effort.

int DoHash(string Message, int EmployeeID, int DepartmentID)
{
    return (Message+EmployeeID+DepartmentID).GetHashCode();
}

only has 2^32 possible values.

string DoHash(string Message, int EmployeeID, int DepartmentID)
{
    MD5 md5 = new MD5CryptoServiceProvider();
    return
Convert.ToBase64String(md5.ComputeHash(Encoding.UTF8.GetBytes(Message+EmployeeID+DepartmentID)));
}

has 2^128 possible values.

Arne

I am not sure that using a hash computation is the best way to detect
"duplicate content". Are you storing your content in a database? You really
haven't specified much detail. Perhaps you should be looking into a more
robust computation such as CRC32.
-- Peter
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