use strict;
use warnings;

my @array = (1, 3, 5, 8, 10);
print "\n", join ", ", @array, "\n";
print "Found 0 at " . BinSearch (0, \&cmpFunc, \@array) . "\n";
print "Found 1 at " . BinSearch (1, \&cmpFunc, \@array) . "\n";
print "Found 2 at " . BinSearch (2, \&cmpFunc, \@array) . "\n";
print "Found 5 at " . BinSearch (5, \&cmpFunc, \@array) . "\n";
print "Found 8 at " . BinSearch (8, \&cmpFunc, \@array) . "\n";
print "Found 10 at " . BinSearch (10, \&cmpFunc, \@array) . "\n";
print "Found 11 at " . BinSearch (11, \&cmpFunc, \@array) . "\n";

@array = (1, 3, 5, 8,);
print "\n", join ", ", @array, "\n";
print "Found 0 at " . BinSearch (0, \&cmpFunc, \@array) . "\n";
print "Found 1 at " . BinSearch (1, \&cmpFunc, \@array) . "\n";
print "Found 2 at " . BinSearch (2, \&cmpFunc, \@array) . "\n";
print "Found 5 at " . BinSearch (5, \&cmpFunc, \@array) . "\n";
print "Found 8 at " . BinSearch (8, \&cmpFunc, \@array) . "\n";
print "Found 9 at " . BinSearch (9, \&cmpFunc, \@array) . "\n";

@array = (1,);
print "\n", join ", ", @array, "\n";
print "Found 0 at " . BinSearch (0, \&cmpFunc, \@array) . "\n";
print "Found 1 at " . BinSearch (1, \&cmpFunc, \@array) . "\n";
print "Found 2 at " . BinSearch (2, \&cmpFunc, \@array) . "\n";

@array = ();
print "\n", join ", ", @array, "\n";
print "Found 0 at " . BinSearch (0, \&cmpFunc, \@array) . "\n";


sub cmpFunc {
    $_[0] <=> $_[1];
}

-------------- 8< --------------

1, 3, 5, 8, 10, 
Use of uninitialized value in concatenation (.) or string at C:\Documents and Settings\Peter.WINDOMAIN\My Documents\PerlMonks\junk\noname.pl line 31.
Found 0 at -0.5
Found 1 at 0
Found 2 at 0.5
Found 5 at 2
Found 8 at 3
Found 10 at 4
Found 11 at 4.5

1, 3, 5, 8, 
Found 0 at -0.5
Found 1 at 0
Found 2 at 0.5
Found 5 at 2
Found 8 at 3
Found 9 at 3.5

1, 
Found 0 at -0.5
Found 1 at 0
Found 2 at 0.5


Found 0 at 

##</code><code>##

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;
        }
    }
}