NAME
Data::BitMask - bitmask manipulation
SYNOPSIS
use Data::BitMask;
my $FileMask = Data::BitMask->new(
READ => 1,
WRITE => 2,
EXECUTE => 4,
RX => 5,
RWX => 7,
FULL => 7,
);
my $mask = $FileMask->build_mask('READ|WRITE');
print Data::Dumper->Dump([
$FileMask->explain_mask($mask),
$FileMask->break_mask($mask)
]);
my $mask2 = $FileMask->build_mask({FULL => 1, WRITE => 0});
DESCRIPTION
This module allows one to create bitmask manipulator objects that can be
used to create bitmask values based on a list of constants, as well as
to break apart masks using those constants. The advantages are that you
don't have to pollute namespaces to use constants, you can ensure that
only appropriate constants are used for specific masks, you can easily
break apart and explain masks, and in general it is much easier for the
user to interact with masks.
The module only interacts with masks that fit in Perl integers. In some
places, it presumes that you are using 32 bit integers (i.e.
canonicalizing negative values).
The module expends a modest amount of overhead in creating the
"Data::BitMask" object so as to speed up future mask manipulations.
Installation instructions
This module requires "Module::Build 0.24" to use the automated
installation procedures. With "Module::Build" installed:
Build.PL
perl build test
perl build install
It can also be installed manually by copying "lib/Data/Bitmask.pm" to
"perl/site/lib/Data/Bitmask.pm".
Suggest Module Implementation
Here is one suggested approach to using bitmask manipulators in a
module.
{
my $cache;
sub SECURITY_INFORMATION {
$cache ||= Data::BitMask->new(
OWNER_SECURITY_INFORMATION => 0x1,
GROUP_SECURITY_INFORMATION => 0x2,
DACL_SECURITY_INFORMATION => 0x4,
SACL_SECURITY_INFORMATION => 0x8,
);
}
}
The bitmask manipulator can then be accessed as:
&SECURITY_INFORMATION->build_mask('DACL_SECURITY_INFORMATION');
Or, if you are outside of the module, as:
&Win32::Security::SECURITY_INFORMATION->build_mask('DACL_SECURITY_INFORMATION');
This has several advantages:
* Demand creation of the "Data::Bitmask" object. Creating objects with
huge numbers of constants (i.e. hundreds or thousands) can be a bit
time consuming, so this delays creation until the object actually
gets used. At the same time, the created object is cached.
* Easy access from within in the module, reasonably easy access from
outside the module.
* If the user wants even easier access from outside the module, you
can support Exporter and let the sub be exported.
Method Reference
new
Creates a new bitmask manipulator. Pass a list of constant and value
pairs. The constants do not have to be disjoint, but order does matter.
When executing "explain_mask" or "explain_const", constants that are
earlier in the list take precendence over those later in the list.
Constant names are not allowed to have space or pipes in them, and
constant values have to be integers. Constant names are case insensitive
but preserving.
If the passed value for the constant name is an anonymous array, then it
is presumed that the name is the first value and that the remainder
consists of name-value pairs of parameters. The only currently supported
parameter is "full_match", which implies that the constant should only
be returned from "break_mask" or "explain_mask" if it perfectly matches
the mask being explained. For example:
[qw(FILES_ONLY_NO_INHERIT full_match 1)] => 1,
add_constants
Adds constants to an existing bitmask manipulator. Pass a list of
constant and value pairs as for "new". Constants will be added to the
end of the list (see "new" for an explanation of ordering concerns).
The main use for "add_constants" is adding aggregate constants created
by using "build_mask".
build_mask
This takes one of three things as a parameter:
* scalar - string is split on '"|"' and/or whitespace to generate a
list of constants
* ARRAY ref - elements are the list of constants
* HASH ref - keys with true values are the list of constants; keys
with false values are subtracted from the resultant mask
In all situations, integers are legal in place of constant names and are
treated as the value, after adding 2**32 to any negative integers.
break_mask
Breaks a mask apart. Pass a mask value as an integer. Returns a hash of
all constants whose values are subsets of the passed mask. Values are
set to 1 so the result can safely be passed to "build_mask".
Commonly used for operations like:
if ($MaskManipulator->break_mask($my_mask_value)->{CONSTANT}) {
Note that "break_mask" accepts
To eliminate a constant from explain_mask or break_mask unless it
perfectly matches, use "full_match" constants.
explain_mask
Explains a mask in terms of a relatively minimal set of constants. Pass
either a mask value as an integer or any valid parameter for
"build_mask". Returns a hash of constants that will recreate the mask.
Many times, this will be the minimum number of constants necessary to
describe the mask. Note that creating the true minimum set of constants
is somewhat painful (see Knapsack problem).
The algorithm used by "explain_mask" is to first test for a constant
that perfectly matches the mask. If one is found, this is the obvious
answer. In the absence of a perfect match, "break_mask" is used to
generate a maximal solution. All simply occluded constants are then
eliminated (that is to say, all constants in the list whose values are
subsets of another single constant). This means, for instance, that if
you had only three constants, AB => 3, BC => 6, and AC => 5,
"explain_mask" would return all three when passed the value 7 because no
one constant is a subset of any single one of the others.
To eliminate a constant from explain_mask or break_mask unless it
perfectly matches, use "full_match" constants.
build_const
This takes one of two things as a parameter:
* scalar integer - if a scalar integer is passed, then the value is
simply returned, after adding 2**32 to any negative integers
* scalar - string is looked up in the list of constants
explain_const
Looks for a perfect match for the passed mask value. Pass either a mask
value as an integer or any valid parameter for "build_mask". If one is
not found, it croaks.
get_constants
Returns all constants passed either to "new" or "add_constants".
AUTHOR
Toby Ovod-Everett, toby@ovod-everett.org
LICENSE
Copyright 2003, 2004 Toby Ovod-Everett. All rights reserved. This
program is free software; you can redistribute it and/or modify it under
the same terms as Perl itself.