=head1 TITLE

Perl's internal data types

=head1 VERSION

1.3

=head2 CURRENT

     Maintainer: Dan Sugalski <dan@sidhe.org>
     Class: Internals
     PDD Number: 4
     Version: 1.3
     Status: Developing
     Last Modified: 02 July 2001
     PDD Format: 1
     Language: English

=head2 HISTORY

=over 4

=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 1.3

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

=head1 ABSTRACT

This PDD describes perl's known internal data types.

=head1 DESCRIPTION

This PDD details the primitive datatypes that the perl core knows how
to deal with. These types are lower-level than what's presented to the
perl programmer.

=head1 IMPLEMENTATION

=head2 Integer data types

Integer data types are generically referred to as C<INT>s. C<INT>s are
conceptual things, and there is no data structure that corresponds to them.

=over 4

=item Platform-native integer

These are whatever size native integer was chosen at perl
configuration time. The C-level typedef C<IV> and C<UV> get you a
platform-native signed and unsigned integer respectively.

=item Arbitrary precision integers

Big integers, or bigints, are arbitrary-length integer numbers. The
only limit to the number of digits in a bigint is the lesser of the
amount of memory available or the maximum value that can be
represented by a C<UV>. This will generally allow at least 4 billion
digits, which ought to be far more than enough for anyone.

The C structure that represents a bigint is:

   struct bigint {
     void *buffer;
     UV length;
     IV exponent;
     UV flags;
   }

=begin question

Should we scrap the buffer pointer and just tack the buffer on the end
of the structure? Saves a level of indirection, but means if we need
to make the buffer bigger we have to adjust anything pointing to it.

=end question

The C<num_buffer> pointer points to the buffer holding the actual
number, C<length> is the length of the buffer, C<exponent> is the base
10 exponent for the number (so 2e4532 doesn't take up much space), and
C<flags> are some flags for the bigint.

B<Note:>The flags and exponent fields may be generally unused, but are
in to make the base structure identical in size and field types to
other structures. They may be removed before the first release of perl
6.

=back

=head2 Floating point data types

Floating point data types are generically referred to as C<NUM>s. Like
C<INT>s, C<NUM>s are a conceptual things, not a real data structure.

=over 4

=item Platform native float

These are whatever size float was chosen when perl was configured. The
C level typedef C<NV> will get you one of these.

=item Arbitrary precision decimal numbers

Arbitrary precision decimal numbers, or bignums, can have any number
of digits before and after the decimal point. They are represented by
the structure:

   struct bignum {
     void *buffer;
     UV length;
     IV exponent;
     UV flags;
   }

and yes, this looks identical to the bigint structure. This isn't
accidental. Upgrading a bigint to a bignum should be quick.

=begin question

Like the bigint structure, should we toss the data pointer and just
tack the data on the end?

=end question

=back

=head2 String data types

Perl has a single internal string form:

   struct perl_string {
     void *buffer;
     UV allocated;
     UV bytes;
     UV flags;
     UV characters;
     UV encoding;
     UV type;
     UV unused;
   }

The fields are:

=over 4

=item buffer

Pointer to the start of the string's data.

=item allocated

How many bytes are allocated in the buffer.

=item bytes

How many bytes are used in the buffer.

=item flags

Flags indicating whatever. Bits 0-15 are reserved for perl, bits 16-23
for the encoding/decoding code, and the rest for the type code.

=item characters

How many characters are in the buffer. An optional cache field.

=item encoding

How the data is encoded, for example fixed 8-bit characters, UTF-8, or
UTF-32. An index into the encoding/decoding function table. Note that
this specifies encoding only--it's valid to encode EBCDIC characters
with the UTF-8 algorithm. Silly, but valid.

=item type

What sort of string data is in the buffer, for example ASCII, EBCDIC,
or Unicode. Used to index into the table of string functions.

=item unused

Filler. Here to make sure we're both exactly double the size of a
bigint/bigfloat header and to make sure we don't cross cache lines on
any modern processor.

=back

=head1 ATTACHMENTS

None

=head1 REFERENCES

The perl modules Math::BigInt and Math::BigFloat. The Unicode standard
at http://www.unicode.org.

=head1 GLOSSARY

=over 4

=item Type

Type refers to a low-level perl data type, such as a string or integer.

=item Class

Class refers to a higher-level piece of perl data. Each class has its
own vtable, which is a class' distinguishing mark. Classes live one
step below the perl source level, and should not be confused with perl
packages.

=item Package

A package is a perl source level construct.

=back

