Re: mapping numbers to 'random' numbers and back

JdV said:

Hello,

I am looking for a simple algorithm to map small integers (4 digits,
0000-1000) to large random-looking numbers in a certain range (5 digits,
10000-99999). For example (just with some made-up numbers), 1 -> 62521,
2 -> 30162, and I need to be able to map the big numbers back to their
originals (30162 -> 2, etc).

Any tips on a method or algorithm to use for this ?

Given that you need a reversible mapping, I suggest you just pick two or
three reversible operations and do them one after the other. For example:

unsigned long hash(unsigned int n)
{
unsigned long h = n << 4;
h ^= 0x01234567;
return h - 42;
}

unsigned int unhash(unsigned long h)
{
unsigned long n = h + 42;
n ^= 0x01234567;
return n >> 4;
}

I haven't checked whether hash() produces numbers in the range you're after,
but it hardly matters - just fiddle with it till it does, and do the
opposite fiddling in unhash().

--
Richard Heathfield
"Usenet is a strange place" - dmr 29/7/1999
http://www.cpax.org.uk
email: rjh at above domain (but drop the www, obviously)
.

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