#! /usr/local/bin/perl -w

use strict;

use constant CONTIG => 4;

my @a1 = ( 100, 204, 312 );
my @a2 = ( 102, 313, 409 );
my @a3 = ( 205, 206, 315 );
my @a4 = ( 207, 210, 314 );

my %seen;

my $off = 0; $seen{$_} = [1, $off++] for @a1;
   $off = 0; $seen{$_} = [2, $off++] for @a2;
   $off = 0; $seen{$_} = [3, $off++] for @a3;
   $off = 0; $seen{$_} = [4, $off++] for @a4;

my @seq = sort {$a <=> $b} keys %seen;

my $idx = 0;
while ($idx <= $#seq - (CONTIG-1) ) {
    if ($seq[$idx] + CONTIG-1 < $seq[$idx + CONTIG-1]) {
        ++$idx;
    }
    else {
        # at least 4 contiguous (CONTIG) sequence numbers
        # see how far we can extend the range
        my $range = CONTIG-1;
        while ($idx+$range+1 <= $#seq
            and $seq[$idx]+$range+1 == $seq[$idx+$range+1]
        ) {
            ++$range;
        }
        print "saw $seq[$idx]..$seq[$idx+$range]\n\t",
            join( "\n\t", map {pretty($seen{$_})}
                @seq[$idx .. $idx+$range]
            ),
            "\n";
        $idx += $range;
    }
}

sub pretty {
    my $r = shift;
    return "array $r->[0], offset $r->[1]";
}

##</code><code>##

saw 204..207
        array 1, offset 1
        array 3, offset 0
        array 3, offset 1
        array 4, offset 0
saw 312..315
        array 1, offset 2
        array 2, offset 1
        array 4, offset 2
        array 3, offset 2