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=head1 NAME

version::Internal - Perl extension for Version Objects

=head1 DESCRIPTION

Overloaded version objects for all modern versions of Perl.  This documents
the internal data representation and underlying code for version.pm.  See
L<version.pod> for daily usage.  This document is only useful for users
writing a subclass of version.pm or interested in the gory details.

=head1 What IS a version

For the purposes of this module, a version "number" is a sequence of
positive integer values separated by one or more decimal points and 
optionally a single underscore.  This corresponds to what Perl itself 
uses for a version, as well as extending the "version as number" that 
is discussed in the various editions of the Camel book.

There are actually two distinct kinds of version objects:

=over 4

=item * Decimal Versions

Any version which "looks like a number", see L<Decimal Versions>.  This
also includes versions with a single decimal point and a single embedded
underscore, see L<Decimal Alpha Versions>, even though these must be quoted
to preserve the underscore formatting.

=item * Dotted-Decimal Versions

Also referred to as "Dotted-Integer", these contains more than one decimal
point and may have an optional embedded underscore, see L<Dotted-Decimal
Versions>.  This is what is commonly used in most open source software as
the "external" version (the one used as part of the tag or tarfile name).
A leading 'v' character is now required and will warn if it missing.

=back

Both of these methods will produce similar version objects, in that
the default stringification will yield the version L<Normal Form> only 
if required:

  $v  = version->new(1.002);     # 1.002, but compares like 1.2.0
  $v  = version->new(1.002003);  # 1.002003
  $v2 = version->new("v1.2.3");  # v1.2.3

In specific, version numbers initialized as L<Decimal Versions> will
stringify as they were originally created (i.e. the same string that was
passed to C<new()>.  Version numbers initialized as L<Dotted-Decimal Versions>
will be stringified as L<Normal Form>.

=head2 Decimal Versions

These correspond to historical versions of Perl itself prior to 5.6.0,
as well as all other modules which follow the Camel rules for the
$VERSION scalar.  A Decimal version is initialized with what looks like
a floating point number.  Leading zeros B<are> significant and trailing
zeros are implied so that a minimum of three places is maintained
between subversions.  What this means is that any subversion (digits
to the right of the decimal place) that contains less than three digits
will have trailing zeros added to make up the difference, but only for
purposes of comparison with other version objects.  For example:

                                   # Prints     Equivalent to  
  $v = version->new(      1.2);    # 1.2        v1.200.0
  $v = version->new(     1.02);    # 1.02       v1.20.0
  $v = version->new(    1.002);    # 1.002      v1.2.0
  $v = version->new(   1.0023);    # 1.0023     v1.2.300
  $v = version->new(  1.00203);    # 1.00203    v1.2.30
  $v = version->new( 1.002003);    # 1.002003   v1.2.3

All of the preceding examples are true whether or not the input value is 
quoted.  The important feature is that the input value contains only a 
single decimal.  See also L<Alpha Versions> for how to handle 

IMPORTANT NOTE: As shown above, if your Decimal version contains more 
than 3 significant digits after the decimal place, it will be split on 
each multiple of 3, so 1.0003 is equivalent to v1.0.300, due to the need 
to remain compatible with Perl's own 5.005_03 == 5.5.30 interpretation.  
Any trailing zeros are ignored for mathematical comparison purposes.

=head2 Dotted-Decimal Versions

These are the newest form of versions, and correspond to Perl's own
version style beginning with 5.6.0.  Starting with Perl 5.10.0,
and most likely Perl 6, this is likely to be the preferred form.  This
method normally requires that the input parameter be quoted, although 
Perl's after 5.8.1 can use v-strings as a special form of quoting, but
this is highly discouraged.

Unlike L<Decimal Versions>, Dotted-Decimal Versions have more than
a single decimal point, e.g.:

                                   # Prints
  $v = version->new( "v1.200");    # v1.200.0
  $v = version->new("v1.20.0");    # v1.20.0
  $v = qv("v1.2.3");               # v1.2.3
  $v = qv("1.2.3");                # v1.2.3
  $v = qv("1.20");                 # v1.20.0

In general, Dotted-Decimal Versions permit the greatest amount of freedom
to specify a version, whereas Decimal Versions enforce a certain
uniformity.  See also L<New Operator> for an additional method of
initializing version objects.

Just like L<Decimal Versions>, Dotted-Decimal Versions can be used as 
L<Alpha Versions>.

=head2 Decimal Alpha Versions

The one time that a Decimal version must be quoted is when a alpha form is
used with an otherwise Decimal version (i.e. a single decimal point).  This
is commonly used for CPAN releases, where CPAN or CPANPLUS will ignore alpha
versions for automatic updating purposes.  Since some developers have used
only two significant decimal places for their non-alpha releases, the
version object will automatically take that into account if the initializer
is quoted.  For example Module::Example was released to CPAN with the
following sequence of $VERSION's:

  # $VERSION    Stringified
  0.01          0.01
  0.02          0.02
  0.02_01       0.02_01
  0.02_02       0.02_02
  0.03          0.03
  etc.

The stringified form of Decimal versions will always be the same string
that was used to initialize the version object.

=head1 High level design

=head2 version objects

version.pm provides an overloaded version object that is designed to both 
encapsulate the author's intended $VERSION assignment as well as make it
completely natural to use those objects as if they were numbers (e.g. for
comparisons).  To do this, a version object contains both the original 
representation as typed by the author, as well as a parsed representation
to ease comparisons.  Version objects employ L<overload> methods to
simplify code that needs to compare, print, etc the objects.

The internal structure of version objects is a blessed hash with several
components:

    bless( {
      'original' => 'v1.2.3_4',
      'alpha' => 1,
      'qv' => 1,
      'version' => [
        1,
        2,
        3,
        4
      ]
    }, 'version' );

=over 4

=item original

A faithful representation of the value used to initialize this version
object.  The only time this will not be precisely the same characters
that exist in the source file is if a short dotted-decimal version like
v1.2 was used (in which case it will contain 'v1.2').  This form is
B<STRONGLY> discouraged, in that it will confuse you and your users.

=item qv

A boolean that denotes whether this is a decimal or dotted-decimal version.
See L<is_qv>.

=item alpha

A boolean that denotes whether this is an alpha version.  NOTE: that the
underscore can can only appear in the last position.  See L<is_alpha>.

=item version

An array of non-negative integers that is used for comparison purposes with
other version objects.

=back

=head2 Replacement UNIVERSAL::VERSION

In addition to the version objects, this modules also replaces the core
UNIVERSAL::VERSION function with one that uses version objects for its
comparisons.  The return from this operator is always the stringified form
as a simple scalar (i.e. not an object), but the warning message generated
includes either the stringified form or the normal form, depending on how
it was called.

For example:

  package Foo;
  $VERSION = 1.2;

  package Bar;
  $VERSION = "v1.3.5"; # works with all Perl's (since it is quoted)

  package main;
  use version;

  print $Foo::VERSION; # prints 1.2

  print $Bar::VERSION; # prints 1.003005

  eval "use foo 10";
  print $@; # prints "foo version 10 required..."
  eval "use foo 1.3.5; # work in Perl 5.6.1 or better
  print $@; # prints "foo version 1.3.5 required..."

  eval "use bar 1.3.6";
  print $@; # prints "bar version 1.3.6 required..."
  eval "use bar 1.004"; # note Decimal version
  print $@; # prints "bar version 1.004 required..."


IMPORTANT NOTE: This may mean that code which searches for a specific
string (to determine whether a given module is available) may need to be
changed.  It is always better to use the built-in comparison implicit in
C<use> or C<require>, rather than manually poking at C<class->VERSION>
and then doing a comparison yourself.

The replacement UNIVERSAL::VERSION, when used as a function, like this:

  print $module->VERSION;

will also exclusively return the stringified form.  See L<Stringification>
for more details.

=head1 Usage question

=head2 Using modules that use version.pm

As much as possible, the version.pm module remains compatible with all
current code.  However, if your module is using a module that has defined
C<$VERSION> using the version class, there are a couple of things to be
aware of.  For purposes of discussion, we will assume that we have the
following module installed:

  package Example;
  use version;  $VERSION = qv('1.2.2');
  ...module code here...
  1;

=over 4

=item Decimal versions always work

Code of the form:

  use Example 1.002003;

will always work correctly.  The C<use> will perform an automatic
C<$VERSION> comparison using the floating point number given as the first
term after the module name (e.g. above 1.002.003).  In this case, the
installed module is too old for the requested line, so you would see an
error like:

  Example version 1.002003 (v1.2.3) required--this is only version 1.002002 (v1.2.2)...

=item Dotted-Decimal version work sometimes

With Perl >= 5.6.2, you can also use a line like this:

  use Example 1.2.3;

and it will again work (i.e. give the error message as above), even with
releases of Perl which do not normally support v-strings (see L<What about
v-strings> below).  This has to do with that fact that C<use> only checks
to see if the second term I<looks like a number> and passes that to the
replacement L<UNIVERSAL::VERSION>.  This is not true in Perl 5.005_04,
however, so you are B<strongly encouraged> to always use a Decimal version
in your code, even for those versions of Perl which support the Dotted-Decimal
version.

=back

=head2 Object Methods

Overloading has been used with version objects to provide a natural
interface for their use.  All mathematical operations are forbidden,
since they don't make any sense for base version objects.  Consequently,
there is no overloaded numification available.  If you want to use a
version object in a Decimal context for some reason, see the L<numify>
object method.

=over 4

=item * New Operator

Like all OO interfaces, the new() operator is used to initialize
version objects.  One way to increment versions when programming is to
use the CVS variable $Revision, which is automatically incremented by
CVS every time the file is committed to the repository.

In order to facilitate this feature, the following
code can be employed:

  $VERSION = version->new(qw$Revision: 2.7 $);

and the version object will be created as if the following code
were used:

  $VERSION = version->new("v2.7");

In other words, the version will be automatically parsed out of the
string, and it will be quoted to preserve the meaning CVS normally
carries for versions.  The CVS $Revision$ increments differently from
Decimal versions (i.e. 1.10 follows 1.9), so it must be handled as if
it were a L<Dotted-Decimal Version>.

A new version object can be created as a copy of an existing version
object, either as a class method:

  $v1 = version->new(12.3);
  $v2 = version->new($v1);

or as an object method:

  $v1 = version->new(12.3);
  $v2 = $v1->new(12.3);

and in each case, $v1 and $v2 will be identical.  NOTE: if you create
a new object using an existing object like this:

  $v2 = $v1->new();

the new object B<will not> be a clone of the existing object.  In the
example case, $v2 will be an empty object of the same type as $v1.

=back

=over 4

=item * qv()

An alternate way to create a new version object is through the exported
qv() sub.  This is not strictly like other q? operators (like qq, qw),
in that the only delimiters supported are parentheses (or spaces).  It is
the best way to initialize a short version without triggering the floating
point interpretation.  For example:

  $v1 = qv(1.2);         # v1.2.0
  $v2 = qv("1.2");       # also v1.2.0

As you can see, either a bare number or a quoted string can usually 
be used interchangably, except in the case of a trailing zero, which
must be quoted to be converted properly.  For this reason, it is strongly
recommended that all initializers to qv() be quoted strings instead of
bare numbers.

To prevent the C<qv()> function from being exported to the caller's namespace,
either use version with a null parameter:

  use version ();

or just require version, like this:

  require version;

Both methods will prevent the import() method from firing and exporting the
C<qv()> sub.  This is true of subclasses of version as well, see
L<SUBCLASSING> for details.

=back

For the subsequent examples, the following three objects will be used:

  $ver   = version->new("1.2.3.4"); # see "Quoting" below
  $alpha = version->new("1.2.3_4"); # see "Alpha versions" below
  $nver  = version->new(1.002);     # see "Decimal Versions" above

=over 4

=item * Normal Form

For any version object which is initialized with multiple decimal
places (either quoted or if possible v-string), or initialized using
the L<qv()> operator, the stringified representation is returned in
a normalized or reduced form (no extraneous zeros), and with a leading 'v':

  print $ver->normal;         # prints as v1.2.3.4
  print $ver->stringify;      # ditto
  print $ver;                 # ditto
  print $nver->normal;        # prints as v1.2.0
  print $nver->stringify;     # prints as 1.002, see "Stringification" 

In order to preserve the meaning of the processed version, the 
normalized representation will always contain at least three sub terms.
In other words, the following is guaranteed to always be true:

  my $newver = version->new($ver->stringify);
  if ($newver eq $ver ) # always true
    {...}

=back

=over 4

=item * Numification

Although all mathematical operations on version objects are forbidden
by default, it is possible to retrieve a number which corresponds 
to the version object through the use of the $obj->numify
method.  For formatting purposes, when displaying a number which
corresponds a version object, all sub versions are assumed to have
three decimal places.  So for example:

  print $ver->numify;         # prints 1.002003004
  print $nver->numify;        # prints 1.002

Unlike the stringification operator, there is never any need to append
trailing zeros to preserve the correct version value.

=back

=over 4

=item * Stringification

The default stringification for version objects returns exactly the same
string as was used to create it, whether you used C<new()> or C<qv()>,
with one exception.  The sole exception is if the object was created using
C<qv()> and the initializer did not have two decimal places or a leading
'v' (both optional), then the stringified form will have a leading 'v'
prepended, in order to support round-trip processing.

For example:

  Initialized as          Stringifies to
  ==============          ==============
  version->new("1.2")       1.2
  version->new("v1.2")     v1.2
  qv("1.2.3")               1.2.3
  qv("v1.3.5")             v1.3.5
  qv("1.2")                v1.2   ### exceptional case 

See also L<UNIVERSAL::VERSION>, as this also returns the stringified form
when used as a class method.

IMPORTANT NOTE: There is one exceptional cases shown in the above table
where the "initializer" is not stringwise equivalent to the stringified
representation.  If you use the C<qv()> operator on a version without a
leading 'v' B<and> with only a single decimal place, the stringified output
will have a leading 'v', to preserve the sense.  See the L<qv()> operator
for more details.

IMPORTANT NOTE 2: Attempting to bypass the normal stringification rules by
manually applying L<numify()> and L<normal()> will sometimes yield
surprising results:

  print version->new(version->new("v1.0")->numify)->normal; # v1.0.0

The reason for this is that the L<numify()> operator will turn "v1.0"
into the equivalent string "1.000000".  Forcing the outer version object
to L<normal()> form will display the mathematically equivalent "v1.0.0".

As the example in L<new()> shows, you can always create a copy of an
existing version object with the same value by the very compact:

  $v2 = $v1->new($v1);

and be assured that both C<$v1> and C<$v2> will be completely equivalent,
down to the same internal representation as well as stringification.

=back

=over 4

=item * Comparison operators

Both C<cmp> and C<E<lt>=E<gt>> operators perform the same comparison between
terms (upgrading to a version object automatically).  Perl automatically
generates all of the other comparison operators based on those two.
In addition to the obvious equalities listed below, appending a single
trailing 0 term does not change the value of a version for comparison
purposes.  In other words "v1.2" and "1.2.0" will compare as identical.

For example, the following relations hold:

  As Number        As String           Truth Value
  -------------    ----------------    -----------
  $ver >  1.0      $ver gt "1.0"       true
  $ver <  2.5      $ver lt             true
  $ver != 1.3      $ver ne "1.3"       true
  $ver == 1.2      $ver eq "1.2"       false
  $ver == 1.2.3.4  $ver eq "1.2.3.4"   see discussion below

It is probably best to chose either the Decimal notation or the string
notation and stick with it, to reduce confusion.  Perl6 version objects
B<may> only support Decimal comparisons.  See also L<Quoting>.

WARNING: Comparing version with unequal numbers of decimal points (whether
explicitly or implicitly initialized), may yield unexpected results at
first glance.  For example, the following inequalities hold:

  version->new(0.96)     > version->new(0.95); # 0.960.0 > 0.950.0
  version->new("0.96.1") < version->new(0.95); # 0.096.1 < 0.950.0

For this reason, it is best to use either exclusively L<Decimal Versions> or
L<Dotted-Decimal Versions> with multiple decimal points.

=back

=over 4

=item * Logical Operators 

If you need to test whether a version object
has been initialized, you can simply test it directly:

  $vobj = version->new($something);
  if ( $vobj )   # true only if $something was non-blank

You can also test whether a version object is an L<Alpha version>, for
example to prevent the use of some feature not present in the main
release:

  $vobj = version->new("1.2_3"); # MUST QUOTE
  ...later...
  if ( $vobj->is_alpha )       # True

=back

=head2 Quoting

Because of the nature of the Perl parsing and tokenizing routines,
certain initialization values B<must> be quoted in order to correctly
parse as the intended version, especially when using the L<qv()> operator.
In all cases, a floating point number passed to version->new() will be
identically converted whether or not the value itself is quoted.  This is
not true for L<qv()>, however, when trailing zeros would be stripped on
an unquoted input, which would result in a very different version object.

In addition, in order to be compatible with earlier Perl version styles,
any use of versions of the form 5.006001 will be translated as v5.6.1.  
In other words, a version with a single decimal point will be parsed as
implicitly having three digits between subversions, but only for internal
comparison purposes.

The complicating factor is that in bare numbers (i.e. unquoted), the
underscore is a legal Decimal character and is automatically stripped
by the Perl tokenizer before the version code is called.  However, if
a number containing one or more decimals and an underscore is quoted, i.e.
not bare, that is considered a L<Alpha Version> and the underscore is
significant.

If you use a mathematic formula that resolves to a floating point number,
you are dependent on Perl's conversion routines to yield the version you
expect.  You are pretty safe by dividing by a power of 10, for example,
but other operations are not likely to be what you intend.  For example:

  $VERSION = version->new((qw$Revision: 1.4)[1]/10);
  print $VERSION;          # yields 0.14
  $V2 = version->new(100/9); # Integer overflow in decimal number
  print $V2;               # yields something like 11.111.111.100

Perl 5.8.1 and beyond will be able to automatically quote v-strings but
that is not possible in earlier versions of Perl.  In other words:

  $version = version->new("v2.5.4");  # legal in all versions of Perl
  $newvers = version->new(v2.5.4);    # legal only in Perl >= 5.8.1

=head1 SUBCLASSING

This module is specifically designed and tested to be easily subclassed.
In practice, you only need to override the methods you want to change, but
you have to take some care when overriding new() (since that is where all
of the parsing takes place).  For example, this is a perfect acceptable
derived class:

  package myversion;
  use base version;
  sub new { 
      my($self,$n)=@_;
      my $obj;
      # perform any special input handling here
      $obj = $self->SUPER::new($n);
      # and/or add additional hash elements here
      return $obj;
  }

See also L<version::AlphaBeta> on CPAN for an alternate representation of
version strings.

B<NOTE:> Although the L<qv> operator is not a true class method, but rather a
function exported into the caller's namespace, a subclass of version will 
inherit an import() function which will perform the correct magic on behalf
of the subclass.

=head1 EXPORT

qv - Dotted-Decimal Version initialization operator

=head1 AUTHOR

John Peacock E<lt>jpeacock@cpan.orgE<gt>

=head1 SEE ALSO

L<perl>.

=cut