=head1 $a = interval( $source, $interval, $number_bins)


Calculates inter-event interval histogram of the source (arrayref). The
$interval gives the maximum inter-event interval that should be counted. The
$number_bins denotes the number of bins of the interval.


=cut


$PDL = <<'_PDL';
_interval_
extract arguments
prepare
return empty histogram unless the source has more than two items
get the first item of the series as a reference
walk over series
  assign current item to reference
  calculate difference between item and reference
add the buffer to the histogram
return the histogram
_PDL


sub interval {
  my $source = shift;
  my $interval = shift;
  my $interval_bins = shift;

  my @times = @$source;
  my $start_time = $times[0];
  my @interval_histo = (0) x $interval_bins;

  return \@interval_histo unless scalar(@times)>1;

  my $prev = shift @times;
  my @buff = map { my $time = $_ - $prev; $prev = $_; $time} @times;

  buffer2histo( \@buff, \@interval_histo, 0, $interval, $interval_bins ) if scalar @buff;

  \@interval_histo;
}