Something you need to determine is how ‘fair’ you want this to be and the performance as the unused space becomes exhausted.

In essence, you want a random box from a N dimensional array. Its not the box itself that is important, but the location of the box that is important.

Under the covers, a 10×20 array is often represented as a 1×200 array with some math behind it to access the right spot. If you were to access [5][13], you are actually accessing location [5*20 + 13]. You can use the same approach to go from a number back to the position. Location 113 goes to the integer devision by 20 and remainder giving 5 r13.

So now, you don’t need to actually store 200 pairs (though that isn’t a lot), you just need a bitfield of 200 bits long. Generate a random number within the proper range and mark it as used in the bitfield.

Now, the question of how do you handle it when you’ve got a collision? This goes to the various hash collision techniques used in hash tables. Some won’t work for this application, but its a good read nonetheless.

A simple approach would be once you have one collision, just start incrementing where you are looking at until you find one that wasn’t used. The increment could be 1, or any number that is relativity prime to the size of the space.

Ok, I’m at a place I can sit down and write something. Its perl. Shouldn’t be too far off of php and is rather straight forward.

#!/usr/bin/perl

my $x = shift @ARGV;
my $y = shift @ARGV;
my $f = shift @ARGV; # fill factor
my $t = $x * $y;    # total space
my $v = '';

foreach (1 .. int($t * $f)) {
    my $r = int(rand($t));  # random number from 0 .. $t-1
    my $yp = int($r / $x); # y' (y prime)
    my $xp = int($r % $x); # x' (x prime)

    print "Trying $r: $xp $yp...n";
    while(vec($vec, $r, 1)) {
        $yp = int($r / $x);
        $xp = int($r % $x);
        print "tcollision at $r: $xp $ypn";
        $r += 1;
        $r %= $t;   # scale $r to within $t
    }
    $yp = int($r / $x);
    $xp = int($r % $x);
    vec($vec, $r, 1) = 1; # set the $r th bit
    print "tsettled at $r: $xp $ypn";
}

Ultimately, the ‘settled’ values are the ones that you want.

You read two numbers from the command line and assign them to x and y. The total search space is t – this is how big all the possible numbers are. Additionally, read a fill factor as $f, this should be a value less than 1 and is used to limit the list iteration. Setting a value greater than 1 will present an infinite loop.

I’m filling up the space to the specified value (the foreach (1 .. ($t * $f))). No, there isn’t any error checking on $f to make sure it is less than or equal to 1, but there should be.

So pick a random number from 0 to $t – 1. The spot that this represents is $yp and $xp – y prime and x prime.

There is some perlish things here, vec works with a bit vector of arbitrary size. There are a number of ways of doing this in a given language, its just rather easy with perl. With Java, one could use a BitSet (this is big enough to hold maxint bits, which could represent a pair of 46340 numbers).

You then test the bit at the $rth location to see if it has been used. If it has, increment $r and roll it over if it becomes larger than the total space (so if you have a 10 and 20 (0 .. 199) and you hit 200, it becomes 0).

Once you find an unused bit, set it and output your values.

This is what the output looks like though:

Trying 96: 6 9...
        settled at 96: 6 9
Trying 117: 7 11...
        collision at 117: 7 11
        settled at 118: 8 11
Trying 115: 5 11...
        settled at 115: 5 11
Trying 153: 3 15...
        settled at 153: 3 15
Trying 90: 0 9...
        settled at 90: 0 9
Trying 140: 0 14...
        collision at 140: 0 14
        collision at 141: 1 14
        collision at 142: 2 14
        settled at 143: 3 14
Trying 73: 3 7...
        settled at 73: 3 7

This is tested on a unix system thus:

% rndpair.pl 10 20 0.5 | grep settled | sort | uniq | wc -l
     100

This shows that with 100 numbers there are 100 unique pairs printed out (just look at the ‘settled’ lines).

There’s a fair bit of debugging information in there (you don’t really need to keep resetting the value of $yp and $xp until the end.

And then there’s the question of how fast is it? This is to generate 50% of the available pairs for the available space. Realize that there is some time tied up in the sort and uniq (of possibly some not small bits of text):

% time ./rndpair.pl 500 500 0.5 | grep settled | sort| uniq | wc -l
  125000

real    0m3.528s
user    0m3.901s
sys     0m0.045s

Lets kick it up a notch and remove the other applications from the chain.

% time ./rndpair.pl 5000 5000 0.5 > /dev/null

real    0m34.668s
user    0m34.482s
sys     0m0.180s

How much room did that 5k x 5k storage space take? 25,000,000 bits, or about 3 megabytes.

Note that the performance of this drops as the fill factor goes up. For a search space of 6, 80610 (a previous comment if I read it right) this runs quite quickly (note the increasing times as the fill factor goes up):

% time ./rndpair.pl 6 80610 0.5 | grep settled | wc -l
  241830

real    0m0.827s
user    0m1.562s
sys     0m0.029s
% time ./rndpair.pl 6 80610 0.75 | grep settled | wc -l
  362745

real    0m1.721s
user    0m3.151s
sys     0m0.043s
% time ./rndpair.pl 6 80610 0.9 | grep settled | wc -l
  435294

real    0m3.993s
user    0m7.031s
sys     0m0.079s

The following is a re-write to use for instead of foreach.

<?php

function unique_arrays($first, $second) {
    $arrays = array();
    for ($i = $first[0]; $i <= $second[0]; $i++)
        for ($j = $first[1]; $j <= $second[1]; $j++)
            $arrays[] = array($i, $j);

    return $arrays;
}

print_r(unique_arrays(array(1,6), array(3,8)));

?>

Here is an iterator-inspired class that should keep the memory down.

<?php

class XYStepper {
    protected $first;
    protected $second;
    protected $current = null;

    public function __construct($first, $second) {
        $this->first = $first;
        $this->second = $second;
    }

    public function next() {
        if ($this->current == null)
            $this->current = $this->first;

        else {
            if ($this->current[0] < $this->second[0]) {
                if ($this->current[1] < $this->second[1])
                    $this->current[1]++;
                else {
                    $this->current[0]++;
                    $this->current[1] = $this->first[1];
                }
            }

            else {
                if ($this->current[1] < $this->second[1])
                    $this->current[1]++;
                else
                    return null;
            }
        }

        return $this->current;
    }
}

$xy = new XYStepper(array(1,6), array(3,8));
while (($next = $xy->next()) != null)
    print_r($next);

?>

Adding the ability to step down, or have negative increments is left as an exercise to the reader.

This is based on @MichaelT comments and code. I started with this before he posted it but i have been peeking after he posted his code so it’s not entirely original work.

I reserve where the numbers in both ranges are the same and i add the offset so that it’s more aligned with the original spec.

<?php
class Gen
{
    private $mask;            // bitmask all combinations
    private $firstOffset;     // what first range starts from
    private $secondOffset;    // what second range starts from
    private $firstWidth;      // the width of first range
    private $secondWidth;     // the width of second range
    private $permutations;    // number of permutations
    private $used = 0;        // number of permutations used

    public function __construct(array $range1, array $range2)
    {
        if( ! count($range1) || ! count($range2) )
            throw new Exception("Both ranges must contain at least one element");

        // I support both array(1, 2, 3, 4) and array(1, 4) as a range
        $this->firstOffset = $range1[0];
        $this->secondOffset = $range2[0];
        $firstEnd = array_pop($range1);
        $secondEnd = array_pop($range2);

        if( $firstEnd < $this->firstOffset ||
            $secondEnd < $this->secondOffset )
            throw new Exception('End lower than start in at least one range.' . 
                        "Given [{$this->firstOffset}, $firstEnd] " . 
                        "and [{this->secondOffset}, $secondEnd]");

        $this->firstWidth = 1 + $firstEnd - $this->firstOffset;
        $this->secondWidth = 1 + $secondEnd - $this->secondOffset;
        $this->permutations = $this->firstWidth*$this->secondWidth; // with no overlap
        $this->reset();
    }

    public function next()
    {
        if( $this->used != $this->permutations ) {
            $random_permutation = rand(0,$this->permutations);
            if( ($found = gmp_scan0($this->mask, $random_permutation)) == $this->permutations )
                $found = gmp_scan0($this->mask, 0);
            $this->used++;
            gmp_setbit($this->mask, $found);
            $first = $found % $this->firstWidth + $this->firstOffset;
            $second = floor($found / $this->firstWidth) + $this->secondOffset;
            return array( $first, $second);
        }
        throw new Exception("No more pairs available");
    }

    public function reset()
    {
        $this->mask = gmp_init(0);
        $this->used = 0;

        // Make simple variables of
        // so that the original ranges
        // are [$s1, $e1] and [$s2, $e2]
        $s1 = $this->firstOffset;    
        $s2 = $this->secondOffset;
        $e1 = $s1 + $this->firstWidth - 1;
        $e2 = $s2 + $this->secondWidth - 1;

        if( $s1 <= $e2 && $s2 <= $e1 ) {        // if the rhe ranges overlap
            $is = max($s1, $s2);                // get the start of intersect
            $ie = min($e1, $e2);                // get the end of intersect
            $this->used +=    1 + $ie - $is;    // the count of emelemts in common
            $i1end = $ie - $s1 + 1;             // calculate the end element for first range
            for( $i1 = $is - $s1,               // initialize range1 by reducing by s1
                 $i2 = $is - $s2;               // initialize range2 by reducing by s2
                 $i1 < $i1end;                  // as long as $i1 is lower than the calculates stop
                 $i1++, $i2++) {                // increment both indexes by one for next iteration

                // body of for loop sets bitmask where both
                // would get the same value when offset is added
                gmp_setbit( $this->mask, $i1 + $i2 * $this->firstWidth ); 
            }
        }
    }
}


//$g = new Gen(range(1,6), range(1,10));
//$g = new Gen(array(3,4), array(5));
$g = new Gen(array(1,6), array(3291,83901)); 
try {
    while(1) {
        print implode(", ", $g->next()) . "n";
    }
} catch ( Exception $e ) {
    print "Done!n";
}