# Copyright (C) 2001-2004, The Perl Foundation.
# $Id: pdd04_datatypes.pod 13593 2006-07-26 21:44:40Z allison $

=head1 NAME

docs/pdds/pdd04_datatypes.pod - Parrot's internal data types

=head1 ABSTRACT

This PDD describes Parrot's internal data types.

{{ NOTE: this is a good overview, but we need more complete
specifications of the behavior of the datatypes. }}

=head1 DESCRIPTION

This PDD details the basic datatypes that the Parrot core knows how to deal 
with. Three of these (the integer, floating point and string datatypes)  have
no additional semantics. The fourth datatype, the Parrot Magic Cookie (PMC)
acts as the basis for all high level languages running on top of  Parrot; only
the most basic aspects are described here.

Note that PMC and string internals are volatile and may be changed in the
future (although this will become increasingly unlikely as we near v1.0).
Access from external code to the internals of particular datatypes should be
via the extension mechanism (see F<docs/pdds/pdd11_extending.pod>,  which has
more explicit guarantees of stability.

=head1 IMPLEMENTATION

=head2 Integer data types

Integer data types are generically referred to as C<INT>s.  These are whatever
size native integer was chosen at Parrot configuration time. The C-level
typedefs C<INTVAL> and C<UINTVAL> get you a platform-native signed and unsigned
integer respectively.

=head2 Floating point data types

Floating point data types are generically referred to as C<NUM>s.  These are
whatever size float was chosen when parrot was configured. The C level typedef
C<FLOATVAL> will get you one of these.

=head2 String data types

Parrot has a single internal string form:

    struct parrot_string_t {
        pobj_t obj;
        UINTVAL bufused;
        void *strstart;
        UINTVAL strlen;
        const ENCODING *encoding;
        const CHARTYPE *type;
        INTVAL language;
    }

The fields are:

=over 4

=item obj

A pointer to a Parrot object, Parrot's most general internal data type. In this
case, it holds the buffer for the string data, the size of the buffer in bytes,
and any applicable flags.

=item bufused

The amount of the buffer currently in use, in bytes.

=item strstart

A pointer to the beginning of the actual string (which may not be positioned 
at the start of the buffer).

=item strlen

The length of the string, in characters.

=item encoding

How the data is encoded (e.g. fixed 8-bit characters, UTF-8, or UTF-32).  Note
that this specifies encoding only -- it's valid to encode  EBCDIC characters
with the UTF-8 algorithm. Silly, but valid.

The ENCODING structure specifies the encoding (by index number and by name, for
ease of lookup), the maximum number of bytes that a single character will
occupy in that encoding, as well as functions for manipulating strings with
that encoding.

=item type

What sort of string data is in the buffer, for example ASCII, EBCDIC, or
Unicode.

The CHARTYPE structure specifies the character type (by index number and by 
name) and provides functions for transcoding to and from that character type.

=item language

This specifies the language corresponding to the string. This is to allow for
locale-based data to be attached to strings. To give an example of the use of
this: strings in German may not sort in the same way as strings in French, even
when both types use the Latin-1 charset and are encoded in UTF-8.

Note that language-agnostic utilities are at liberty to ignore this entry.

=back

=head2 Parrot Magic Cookies (PMCs)

Parrot Magic Cookies, or PMCs, are the last of Parrot's basic datatypes, but
are also potentially the most important. Their basic structure is as follows.
All PMCs have the form:

    struct PMC {
        pobj_t obj;
        VTABLE *vtable;
 #if ! PMC_DATA_IN_EXT
        DPOINTER *data;
 #endif
        struct PMC_EXT *pmc_ext;
    };

where C<obj> is a pointer to an C<pobj_t> structure:

    typedef struct pobj_t {
        UnionVal u;
        Parrot_UInt flags;
 #if ! DISABLE_GC_DEBUG
        UINTVAL _pobj_version;
 #endif
    } pobj_t;

and where:

    typedef union UnionVal {
        struct {
            void * _bufstart;
            size_t _buflen;
        } _b;
        struct {
            DPOINTER* _struct_val;
            PMC* _pmc_val;
        } _ptrs;
        INTVAL _int_val;
        FLOATVAL _num_val;
        struct parrot_string_t * _string_val;
    } UnionVal;

C<u> holds data associated with the PMC. This can be in the form of an  integer
value, a floating point value, a string value, or a pointer  to other data.
C<u> may be empty, since the PMC structure also provides a more general data
pointer, but is useful for PMCs which hold only a single piece of data (e.g.
C<PerlInts>).

C<flags> holds a set of flags associated with the PMC; these are documented  in
F<include/parrot/pobj.h>, and are generally only used within the Parrot 
internals.

C<_pobj_version> is only used for debugging Parrot's garbage collector. It is
documented elsewhere (well, it will be once we get around to doing that...).

C<vtable> holds a pointer to the B<vtable> associated with the PMC. This points
to a set of functions, with interfaces described in
F<docs/pdds/pdd02_vtables.pod> that implement the basic behaviour of the PMC
(i.e. how it behaves under addition, subtraction, cloning etc.)

C<data> (if present) holds a pointer to any additional data associated  with
the PMC. This may be NULL.

C<pmc_ext> points to an extended PMC structure. This has the form:

    struct PMC_EXT {
 #if PMC_DATA_IN_EXT
        DPOINTER *data;
 #endif
        PMC *_metadata;
        struct _Sync *_synchronize;
        PMC *_next_for_GC;
    };

C<data> is a generic data pointer, as described above.

C<_metadata> holds internal PMC metadata. The specification for this has not
yet been finalized.

C<_synchronize> is for access synchronization between shared PMCs.

C<_next_for_GC> determines the next PMC in the 'used' list during dead object 
detection in the GC.

PMCs are not required to have a C<PMC_EXT> structure (i.e. C<pmc_ext> can be
null).

PMCs are used to implement the basic data types of the high level languages
running on top of Parrot. For instance, a Perl 5 C<SV> will map onto one (or
more) types of PMC, while particular Python datatypes will map onto different
types of PMC.

=head1 ATTACHMENTS

None.

=head1 REFERENCES

The perl modules Math::BigInt and Math::BigFloat. Alex Gough's suggestions for
bigint/bignum implementation, outlined in F<docs/pdds/pdd14_bignum.pod>. The
Unicode standard at L<http://www.unicode.org>.

=head1 GLOSSARY

=over 4

=item Type

Type refers to a basic Parrot data type. There are four such: integers,
floating point numbers (often just numbers), strings and Parrot Magic Cookies
(PMCs).

=back

=head1 VERSION

1.4

=head2 CURRENT

     Maintainer: Dan Sugalski <dan@sidhe.org>
     Class: Internals
     PDD Number: 4
     Version: 1.5
     Status: Developing
     Last Modified: 11 June 2005
     PDD Format: 1
     Language: English

=head2 HISTORY

=over 4

=item Version 1.5, 11 June 2005

=item Version 1.4, 20 February 2004

=item Version 1.3, 2 July 2001

=item Version 1.2, 2 July 2001

=item Version 1.1, 2 March 2001

=item Version 1, 1 March 2001

=back

=head1 CHANGES

=over 4

=item Version 1.5

Removed BigInt and BigNum from the definition of I* and N* registers --
according to Leo they are now always PMCs, never register types of their own.

=item Version 1.4

Document basic PMC internals. Make clear the fact that the bigint/bignum
description is still provisional. Other minor fixups to make the  documentation
match reality.

=item Version 1.3

Fixed some silly typos and dropped phrases.

Took all the underscores out of the field names.

=item Version 1.2

The string header format has changed some to allow for type tagging. The flags
information for strings has changed as well.

=item Version 1.1

INT and NUM are now concepts rather than data structures, as making them data
structures was a Bad Idea.

=item Version 1

None. First version

=back

