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 precedence 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.