bin search

sub binsearch {
    my ( $number, $array ) = @_;

    my $last = scalar @$array;

    if ( $last == 0 ) {
        return undef;
    }
    elsif ( $array->[0] > $number ) {
        return 0;
    }
    elsif ( $array->[$#$array] < $number ) {
        return $#$array;
    }

    my $first = 0;
    while ( $first < $last ) {
        my $mid = int( $first + ( $last - $first ) / 2 );
        if ( $number <= $array->[$mid] ) {
            $last = $mid;
        }
        else {
            $first = $mid + 1;
        }
    }

    return $last;
}