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Re: Derangements iterator

by Roy Johnson (Monsignor)
 on Dec 29, 2005 at 19:03 UTC ( #519829=note: print w/replies, xml ) Need Help??

Producing a single derangement (such as might be needed for a Secret Santa list) is just a matter of doing a rotation. Of course, you'd want to randomize the order first, to keep things secret:
```use List::Util 'shuffle';
my @from = shuffle(@ARGV);
print "\$from[\$_-1] => \$from[\$_]\n" for 0..\$#from;

Caution: Contents may have been coded under pressure.

Replies are listed 'Best First'.
Re^2: Derangements iterator
by jdporter (Canon) on Dec 29, 2005 at 19:09 UTC

That doesn't guarantee that the constraint of derangement is met. Example scenario:

``` 1 2 3   # original
2 1 3   # shuffled
1 3 2   # rotated one place to the left.
We're building the house of the future together.
You misunderstood what was being deranged. What you list as the original is not the original, but merely the (unordered) set. Shuffling provides the "original" order, and rotation provides the derangement of that order.

Caution: Contents may have been coded under pressure.

We can just use the shuffle algorithm slightly modified to suit our needs :

```use Inline C => <<'END_OF_C_CODE';

void cderange(SV* array_ref) {
AV* array;
I32 index, i;
SV** sv_1, **sv_2;
SV* sv_temp;

if (! SvROK(array_ref))
croak("array_ref is not a reference");

srand(time( NULL ));
array = (AV*)SvRV(array_ref);
index = av_len(array);
for (; index; index--) {
i = (I32) (rand() % (index));
sv_1 = av_fetch(array, index, 0);
sv_2 = av_fetch(array, i, 0);
sv_temp = *sv_1;
*sv_1 = *sv_2;
*sv_2 = sv_temp;
}
return;
}

END_OF_C_CODE

sub derange {
my \$first = \$_[0];
cderange \@_;
if( \$first eq \$_[0] ){
(\$_[0], \$_[1]) = (\$_[1], \$_[0])
}
return @_;
}

This is fast, but don't work very well with very big arrays (size over RANDMAX, usually 32000).

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