sub BinSearch {
    my ($target, $cmp, $array) = @_;
    my $posmin = 0;
    my $posmax = $#$array;
    
    return undef if ! @array;
    return -0.5 if $cmp->($array->[0], $target) > 0;
    return $#$array + 0.5 if $cmp->($array->[-1], $target) < 0;
    
    while (1) {
        my $mid = int (($posmin + $posmax) / 2);
        my $result = $cmp->($array->[$mid], $target);
        
        if ($result < 0) {
            $posmin = $posmax, next if $mid == $posmin && $posmax != $posmin;
            return $mid + 0.5 if $mid == $posmin;
            $posmin = $mid;
        } elsif ($result > 0) {
            $posmax = $posmin, next if $mid == $posmax && $posmax != $posmin;
            return $mid - 0.5 if $mid == $posmax;
            $posmax = $mid;
        } else {
            return $mid;
        }
    }
}