# $index = binary_search( \@array, $word )
#   @array is a list of lowercase strings in alphabetical order.
#   $word is the target word that might be in the list.
#   binary_search() returns the array index such that $array[$index]
#   is $word.

sub binary_search {
    my ($array, $word) = @_;
    my ($low, $high) = ( 0, @$array - 1 );

    while ( $low <= $high ) {              # While the window is open
        my $try = int( ($low+$high)/2 );      # Try the middle element
        $low  = $try+1, next if $array->[$try] lt $word; # Raise bottom
        $high = $try-1, next if $array->[$try] gt $word; # Lower top

        return $try;     # We've found the word!
    }
    return;              # The word isn't there.
}