# Copyright (c) 2008-2019 Sullivan Beck. All rights reserved.
# This program is free software; you can redistribute it and/or modify it
# under the same terms as Perl itself.

=pod

=head1 NAME

Date::Manip::Objects - A description of the various Date::Manip objects

=head1 SYNOPSIS

The Date::Manip package consist of several modules, each of which
perform a set of operations on a specific class of objects.  This
document describes how the various modules work together.

=head1 DESCRIPTION

Date::Manip consists of the following primary modules:

=over 4

=item L<Date::Manip::Obj>

The L<Date::Manip::Obj> module is not intended for direct use. It is used
as a base class for all other Date::Manip classes described below.

The L<Date::Manip::Obj> module contains some functions which are
inherited by all these classes, so to understand all of the methods
available to any of the classes below, you must include those
documented in the L<Date::Manip::Obj> class.

=item L<Date::Manip::Base>

The L<Date::Manip::Base> is used to perform basic operations including
basic date operations, management of configuration options, handling
the definitions used in different languages, etc.

A L<Date::Manip::Base> object does not, of itself, contain any date
information. Instead, it contains configuration information which
determines how the Date::Manip package performs date operations.  The
configuration information is documented in the L<Date::Manip::Config>
document.

The L<Date::Manip::Base> object has one other property that is very
important. When performing basic date operations, some intermediate
results are cached in the object which leads to significant
performance increases in later operations. As such, it is important to
reuse the object as much as possible, rather than creating new
L<Date::Manip::Base> objects all the time.

Much of the information in this document is related to this issue, and
tells how to create various higher-level objects in order to get the
most efficient reuse of this cached data.

Because all other objects depend on a L<Date::Manip::Base> object, a
L<Date::Manip::Base> object is embedded in all other objects, and the
same Base object can be shared by any number of objects to achieve
maximum performance.

=item L<Date::Manip::TZ>

The L<Date::Manip::TZ> module adds support for time zones. It is used to
verify date and time zone information, convert dates from one time
zone to another, and handle all daylight saving time transitions.

Similar to the L<Date::Manip::Base> object, a great deal of information
is cached in the L<Date::Manip::TZ> object. This includes lists of all
time zones, offsets, and abbreviations for all time zones. It also
includes more a more detailed description of every time zone that has
actually been worked used.

A L<Date::Manip::TZ> object relies on a L<Date::Manip::Base> object (and a
L<Date::Manip::Base> object is always embedded in a L<Date::Manip::TZ>
object).  All higher level objects (those listed next) depend on both
a L<Date::Manip::Base> and L<Date::Manip::TZ> object, so a L<Date::Manip::TZ>
object is embedded in them.

In order to achieve maximum performance, and minimize memory usage,
a L<Date::Manip::TZ> object can be shared by any number of higher
level objects, and in fact, it is desirable to reuse the same L<Date::Manip::TZ>
object as often as possible.

=item L<Date::Manip::Date>

=item L<Date::Manip::Delta>

=item L<Date::Manip::Recur>

These are the primary modules which are used to perform all high level
date operations.

The L<Date::Manip::Date> class performs operations on dates (which includes
a date, time, and time zone). The L<Date::Manip::Delta> class performs
operations with deltas (amounts of time). The L<Date::Manip::Recur> class
performs operations on recurring events.

As mentioned above, each of these high level classes rely on both a
L<Date::Manip::TZ> object and a L<Date::Manip::Base> object, so a
L<Date::Manip::TZ> object is embedded in each one (and the
L<Date::Manip::TZ> object has a L<Date::Manip::Base> object embedded in
it).

A L<Date::Manip::Date> object contains a single date, so in order to
work with multiple dates, multiple L<Date::Manip::Date> objects will
need to be created. In order to make the most effective use of cached
information in the L<Date::Manip::Base> object, the same L<Date::Manip::TZ>
object can be embedded in each of the higher level objects.

The same goes for multiple L<Date::Manip::Delta> and L<Date::Manip::Recur>
objects.

=back

There are also many secondary modules including:

   Date::Manip::TZ_Base
   Date::Manip::TZdata
   Date::Manip::Zones
   Date::Manip::Lang::*
   Date::Manip::TZ::*
   Date::Manip::Offset::*

None of these are intended to be used directly.

=head1 WORKING WITH DATE::MANIP OBJECTS (SINGLE CONFIGURATION)

By far the most common usage of Date::Manip involves setting a single
local time zone, parsing dates in a single language, and having all
other configuration parameters set to a single value that doesn't
change over the course of the program.

Whenever this is the case, you can use the methods listed in this
section to create any number of Date::Manip objects. It will automatically
optimize the use of cached data to get the best performance.

If you do need to work with multiple different configurations (such as
parsing dates from multiple languages), please refer to the next
section L</"WORKING WITH DATE::MANIP OBJECTS (MULTIPLE CONFIGURATION)">.

=over 4

=item B<Working with high level objects>

The most common situation is one where you will need to use one or
more high level objects (Date, Delta, or Recur objects). In addition, you
may want to use the lower level (Base or TZ) objects.

The first thing you should do is to create your initial object. Create the
highest level object you will be using. For example if you will be working with
dates, create the first date object with:

   $date = new Date::Manip::Date;

The next step is to set the configuration values. Use the config method to
do this:

   $date->config(ARGS);

Although you can call the config method later, it is strongly
suggested that the configuration be set soon after the initial object
is created and not altered later. Every time you alter the
configuration, some of the cached data is cleared, so for optimal
performance, you don't want to alter the configuration if possible.

Additional high-level objects can be created using the calls:

   $date2 = $date->new_date();
   $delta = $date->new_delta();
   $recur = $date->new_recur();

To access the embedded L<Date::Manip::TZ> and L<Date::Manip::Base> objects,
use the calls:

   $tz    = $date->tz();
   $base  = $date->base();

=item B<Working with low level objects only>

If you will only be working with low level objects, create them with one
of the calls:

   $tz    = new Date::Manip::TZ;
   $base  = new Date::Manip::Base;

To get the base object embedded in a L<Date::Manip::TZ> object, use:

   $base  = $tz->base();

=back

For a more complete description of the methods used here, refer to the
L<Date::Manip::Obj> document.

=head1 WORKING WITH DATE::MANIP OBJECTS (MULTIPLE CONFIGURATION)

Occasionally, it may be useful to have multiple sets of configurations.
In order to do this, multiple L<Date::Manip::Base> objects must be
created (each with their own set of configuration options), and then
new Date::Manip objects are created with the appropriate L<Date::Manip::Base>
object embedded in them.

Possible reasons include:

=over 4

=item B<Parsing multiple languages>

A L<Date::Manip::Base> object includes information about a single
language. If you need to parse dates from two (or more) languages,
a L<Date::Manip::Base> object needs to be created for each one. This
could be done as:

   $date_eng1 = new Date::Manip::Date;
   $date_eng1->config("language","English");

   $date_spa1 = new Date::Manip::Date;
   $date_spa1->config("language","Spanish");

Any additional Date::Manip objects created from the first will work
with English. Additional objects created from the second will work in
Spanish.

=item B<Business modes for different countries and/or businesses>

If you are doing business mode calculations (see L<Date::Manip::Calc>)
for two different businesses which have different holiday lists,
work weeks, or business days, you can create different objects
which read different config files (see L<Date::Manip::Config>) with
the appropriate description of each.

=back

The primary issue when dealing with multiple configurations is
that it is necessary for the programmer to manually keep track of
which Date::Manip objects work with each configuration. For
example, refer to the following lines:

   $date1 = new Date::Manip::Date [$opt1,$val1];
   $date2 = new Date::Manip::Date $date1, [$opt2,$val2];
   $date3 = new Date::Manip::Date $date1;
   $date4 = new Date::Manip::Date $date2;

The first line creates 3 objects: a L<Date::Manip::Base> object, a
L<Date::Manip::TZ> object, and a L<Date::Manip::Date> object). The
L<Date::Manip::Base> object has the configuration set to contain the
value(s) passed in as the final list reference argument.

The second line creates 3 new objects (a second L<Date::Manip::Base>
object, a second L<Date::Manip::TZ> object, and a second
L<Date::Manip::Date> object). Since a list reference containing config
variables is passed in, a new L<Date::Manip::Base> object is created,
rather than reusing the first one. The second L<Date::Manip::Base> object
contains all the config from the first, as well as the config
variables passed in in the list reference argument.

The third line creates another L<Date::Manip::Date> object which uses the
first L<Date::Manip::Base> and L<Date::Manip::TZ> objects embedded in it.

The fourth line creates another L<Date::Manip::Date> object which uses
the second L<Date::Manip::Base> and L<Date::Manip::TZ> objects embedded in
it.

Most of the time there will only be one set of configuration options
used, so this complexity is really for a very special, and not widely
used, bit of functionality.

=head1 WORKING WITH DATE::MANIP OBJECTS (ADDITIONAL NOTES)

=over 4

=item B<object reuse>

In order to create additional Date::Manip objects, a previously
created object should be passed in as the first argument. This will
allow the same Base object to be embedded in both in order to maximize
data reuse of the cached intermediate results, and will result in much
better performance. For example:

   $date1 = new Date::Manip::Date;
   $date2 = new Date::Manip::Date $date1;

This is important for two reasons. First is memory usage. The
L<Date::Manip::Base> object is quite large. It stores a large number of
precompile regular expressions for language parsing, and as date
operations are done, intermediate results are cached which can be
reused later to improve performance. The L<Date::Manip::TZ> object is
even larger and contains information about all known time zones indexed
several different ways (by offset, by abbreviation, etc.). As
time zones are actually used, a description of all of the time change
rules are loaded and added to this object.

Since these objects are so large, it is important to reuse them, rather
than to create lots of copies of them. It should be noted that because
these objects are embedded in each of the high level object (L<Date::Manip::Date>
for example), it makes these objects appear quite large.

The second reason to reuse L<Date::Manip::Base> objects is
performance. Since intermediate results are cached there, many date
operations only need to be done once and then they can be reused any
number of times. In essence, this is doing the same function as the
Memoize module, but in a more efficient manner. Memoize caches results
for function calls. For Date::Manip, this would often work, but if you
change a config variable, the return value may change, so Memoize
could cause things to break. In addition, Memoize caches primarily at
the function level, but Date::Manip stores caches intermediate results
wherever performance increase is seen. Every time I consider caching a
result, I run a test to see if it increases performance. If it
doesn't, or it doesn't make a significant impact, I don't cache it.

Because the caching is quite finely tuned, it's much more efficient
than using a generic (though useful) tool such as Memoize.

=item B<configuration changes>

As a general rule, you should only pass in configuration options
when the first object is created. In other words, the following
behavior is discouraged:

    $date = new Date::Manip::Date;
    $date->config(@opts);

    ... do some stuff

    $date->config(@opts);

    ... do some other stuff

Because some of the cached results are configuration specific, when a
configuration change is made, some of the cached data must be discarded
necessitating those results to be recalculated.

If you really need to change configuration in the middle of execution,
it is certainly allowed of course, but if you can define the configuration
once immediately after the object is first created, and then leave the
configuration alone, performance will be optimized.

=back

=head1 BUGS AND QUESTIONS

Please refer to the L<Date::Manip::Problems> documentation for
information on submitting bug reports or questions to the author.

=head1 SEE ALSO

L<Date::Manip>        - main module documentation

=head1 LICENSE

This script is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.

=head1 AUTHOR

Sullivan Beck (sbeck@cpan.org)

=cut
