I'm trying to write a simple shuffling function... 

sub shuffle($$$) {
    my ($seed, $max, $input) = @_;

    # $max > $input
    # $max > 0
    # $input >= 0

    my $result;

    # calculate $result

    # $result >= 0
    # $result < $max

    return $result
}

[download]

...such that shuffle($K1, $K2, $x) == shuffle($K1, $K2, $y) if and only if $x == $y.

shuffle returns a semi-random number.

This could be used to iterate randomly over a read-only list in-place.

ie 

get_rand(\@$$) {
    my ($list, $seed, $input) = @_;
    return $list->[shuffle($seed, scalar(@$list), $input)];
}

my $list = [10, 20, 30, 40, 50, 60];

for (my $i = 0; $i < scalar(@$list); $i++) {
    print get_rand($list, 42, $i) . ' ';
}

# might print 40 10 20 60 30 50

[download]

Any math geniuses out there care to fill in the calculate $result part? I'm sure it has something to do with moding and hashing and stuff.

(No, this isn't a homework question. :) I actually need it to iterate randomly over a list in a resource-constrained embedded environment where I can't move the memory around - and haven't got spare memory. I knew I shouldn't have slept through my CS lectures.)

Thanks in advance.

-Andrew.

In reply to Random 1-1 mapping by tomazos

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