use Statistics::Basic::Correlation;

sub xcorr {
    # given two sets and a phase increment, return
    # at which phase the best correlation is found
    # and what that value is
    my ($xref, $yref, $incdeg ) = @_;
    my $maxcorr = -1;

    $incdeg ||= 1; default 1 degree of phase.
    die "sets of unequal size" if ( $#$xref != $#$yref );

    my $indexinc = $incdeg * ( 1 + $#$xref );
    my $iters = 360 / $incdeg;  
    { use integer;
      $indexinc /= 360;
      $iters++;
    }
    $indexinc ||=1;

    my @copy = @$yref;
    my $where = 0;
    my $match = 0;
    for (my $i = 0; $i < $iters; $i++) {
        unshift @copy, ( pop @copy ) for ( 1 .. $indexinc );
        my $res = correlate( $xref, \@copy );
        if ( $res > $maxcorr ) {
            $maxcorr = $res;
            $match = $where;
        }
        $where += $incdeg;            
    }
    return ( $match, $maxcorr );
}