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#+TITLE: PicaRecord -- Object oriented interface to Pica+ records
#+AUTHOR: David Maus
#+EMAIL: maus@hab.de

* About

PicaRecord provides an object oriented interface to Pica+ records, fields, and subfields. It does
not provide the means to read or write Pica+ records. In order to do so you need to install the
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packages PicaReader and PicaWriter that are available as dedicated composer packages.
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PicaRecord is copyright (c) 2012-2016 by Herzog August Bibliothek Wolfenbüttel and released under the
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terms of the GNU General Public License v3.

* Installation

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You can install PicaRecord via Composer.
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#+BEGIN_EXAMPLE
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composer require hab/picarecord
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#+END_EXAMPLE

* Usage

** Records, Fields, and Subfields

*** Subfields

Subfields are represented by the =Subfield= class. A Pica+ subfield is a cons (pair) of an
alphanumeric case-sensitive ASCII character and a non-empty string value. The code of a subfield is
considered to be immutable and the value must not be the empty string.

*** Fields

Field are represented by the =Field= class. A Pica+ field is an ordered list of subfields and
partially identified[1] by two properties, the field tag and the field occurrence. Both properties
are set on instantiation and considered to be immutable.

In the following text the /field shorthand/ refers to a string consisting of the field tag, followed
by a forward slash (ASCII 47) followed by two digits denoting the field occurrence. For example the
field shorthand of the field 021A with an occurrence of 0 is =021A/00=.

**** Retrieving subfields

=Field::getSubfields()= returns all subfields of a field when called with no arguments. To retrieve
specific subfields you can pass an arbitrary number of arguments each representing a subfield
code. If you do so the returned array will be constructed as follows:

Each element of the returned array corresponds to a subfield code in the argument list. If the field
has a subfield with the specified code the element of the returned array contains the subfield. If
the field does not have a subfield with the specified code the element of the returned array
contains =NULL=. In order to retrieve multiple subfields with the same subfield code you need to
repeat the subfield code in the argument list. The first occurrence of the code in the argument list
refers to the first subfield with the specified code, the second occurrence to the second subfield
and so on.

Because =Field::getSubfields()= when called with subfield codes as arguments returns an array with a
known size (as much elements as arguments were passed to the function) you can conveniently use
PHP's =list()= operation.

Example: To retrieve the first and last name of the author encoded in the Pica+ field =028A/00= you
can call =Field::getSubfields()= as follows:

#+BEGIN_EXAMPLE
list($firstName, $lastName, $personalName) = $field->getSubfields('d', 'a', '5');
#+END_EXAMPLE

**** Manipulating the subfield list

You can use =Field::addSubfield()= to add a field to the end of the subfield list. If the subfield
is already part of the subfield list an =InvalidArgumentException= is thrown. For more sophisticated
manipulations of a field's subfield list you can use =Field::setSubfields()= to replace the subfield
list of a field. 

A subfield can be deleted using =Field::removeSubfield()= which takes the subfield to delete as sole
argument and throws an =InvalidArgumentException= if the field does not contain the subfield.

*** Records

**** Record classes

PicaRecord provides classes for the four record types of /title records/, /authority records/,
/local records/, and /copy records/. The relationship of title, local, and copy records is as
follows: A title record may contain zero or more local records. Each local record may contain up to
99 copy records.

Local records can be identified by the internal library number (ILN) of the library they belong to
and retrieved either by =TitleRecord::getLocalRecord()=, which returns an array of all local
records, or =TitleRecord::getLocalRecordByILN()= which retrieves a single local record identifed by
its first argument.

Copy records hold information about particular items (exemplars) and are identified by the number of
the item in the local record. You can use =LocalRecord::getCopyRecords()= or
=LocalRecord::getLocalRecordByItemNumber()= to retrieve an array with all copy records or a single
copy record with a specified item number respectively.

All record classes implement the two methods =Record::isEmpty()= which returns =TRUE= if a record is
empty (does not contain any fields) and =Record::isValid()= which performs a preliminary validation
of the record.

**** Selecting and deleting fields

=Record::getFields()= returns all fields of the record when called without arguments. If you call it
with the body of a regular expression as argument it will only return the fields whose shorthand is
matched by the regular expression.

=Record::select()= provides a more generic access to a record's fields. It takes a predicate
function as argument and returns all fields that fullfill the predicate. A predicate function can be
any valid PHP callback that takes a Field as argument and return TRUE if the field fullfills the
predicate or otherwise FALSE.

=Record::delete()= deletes all fields that match a predicate function (see above).

If a record contains other records, i.e. if a record is a title or local record, =Record::delete()=,
=Record::select()=, and =Record::getFields()= operate on all fields of the record, including the
fields of the contained records.

**** Appending fields to a record

Append fields to an existing record is not as straightforward as selecting or deleting fields. Each
record class has its own restrictions when it comes to appending a field to it via the
=Record::append()= function:

- you can only append fields with a level of 0 to title and authority records
- you can append fields with a level of 1 to local records
- you can only append fields with a level of 2 to copy records; as an additional restriction the
  occurrence value of the field must be equal to the item number of the copy record

#+CAPTION: Allowed field levels per record class
| Record class    | Allowed field level in append() |
|-----------------+---------------------------------|
| TitleRecord     | Level 0                         |
| AuthorityRecord | Level 0                         |
| LocalRecord     | Level 1                         |
| CopyRecord      | Level 2                         |

The attempt to add a field with a different level then the allowed level results in an
=InvalidArgumentException= to be thrown.

* Acknowledgments

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Large parts of this package would not have been possible without studying the source of
[[http://search.cpan.org/dist/PICA-Record/][Pica::Record]], an open source Perl library for handling Pica+ records by Jakob Voß, and the practical
knowledge of our library's catalogers.
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* Footnotes

[1] E.g. a title record may contain zero or more fields with tag =101@= and occurrence =00=; fields with this
shorthand indicate the start of a local record.