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Edit File: Internals.pod

=head1 NAME

version::Internals - 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
F<version.pod> for daily usage.  This document is only useful for users
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<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>.

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.

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

=head2 Alpha Versions

For module authors using CPAN, the convention has been to note unstable
releases with an underscore in the version string. (See L<CPAN>.)  version.pm
follows this convention and alpha releases will test as being newer than the
more recent stable release, and less than the next stable release.  Only the
last element may be separated by an underscore:

# Declaring
  use version 0.77; our $VERSION = version->declare("v1.2_3");

# Parsing
  $v1 = version->parse("v1.2_3");
  $v1 = version->parse("1.002_003");

Note that you B<must> quote the version when writing an alpha Decimal version.
The stringified form of Decimal versions will always be the same string that
was used to initialize the version object.

=head2 Regular Expressions for Version Parsing

A formalized definition of the legal forms for version strings is
included in the C<version::regex> class.  Primitives are included for
common elements, although they are scoped to the file so they are useful
for reference purposes only.  There are two publicly accessible scalars
that can be used in other code (not exported):

=over 4

=item C<$version::LAX>

This regexp covers all of the legal forms allowed under the current
version string parser.  This is not to say that all of these forms
are recommended, and some of them can only be used when quoted.

For dotted decimals:

v1.2
    1.2345.6
    v1.23_4

The leading 'v' is optional if two or more decimals appear.  If only
a single decimal is included, then the leading 'v' is required to
trigger the dotted-decimal parsing.  A leading zero is permitted,
though not recommended except when quoted, because of the risk that
Perl will treat the number as octal.  A trailing underscore plus one
or more digits denotes an alpha or development release (and must be
quoted to be parsed properly).

For decimal versions:

1
    1.2345
    1.2345_01

an integer portion, an optional decimal point, and optionally one or
more digits to the right of the decimal are all required.  A trailing
underscore is permitted and a leading zero is permitted.  Just like
the lax dotted-decimal version, quoting the values is required for
alpha/development forms to be parsed correctly.

=item C<$version::STRICT>

This regexp covers a much more limited set of formats and constitutes
the best practices for initializing version objects.  Whether you choose
to employ decimal or dotted-decimal for is a personal preference however.

=over 4

=item v1.234.5

For dotted-decimal versions, a leading 'v' is required, with three or
more sub-versions of no more than three digits.  A leading 0 (zero)
before the first sub-version (in the above example, '1') is also
prohibited.

=item 2.3456

For decimal versions, an integer portion (no leading 0), a decimal point,
and one or more digits to the right of the decimal are all required.

=back

Both of the provided scalars are already compiled as regular expressions
and do not contain either anchors or implicit groupings, so they can be
included in your own regular expressions freely.  For example, consider
the following code:

($pkg, $ver) =~ /
		^[ \t]*
		use [ \t]+($PKGNAME)
		(?:[ \t]+($version::STRICT))?
		[ \t]*;
	/x;

This would match a line of the form:

use Foo::Bar::Baz v1.2.3;	# legal only in Perl 5.8.1+

where C<$PKGNAME> is another regular expression that defines the legal
forms for package names.

=head1 IMPLEMENTATION DETAILS

=head2 Equivalence between Decimal and Dotted-Decimal Versions

When Perl 5.6.0 was released, the decision was made to provide a
transformation between the old-style decimal versions and new-style
dotted-decimal versions:

5.6.0    == 5.006000
  5.005_04 == 5.5.40

The floating point number is taken and split first on the single decimal
place, then each group of three digits to the right of the decimal makes up
the next digit, and so on until the number of significant digits is exhausted,
B<plus> enough trailing zeros to reach the next multiple of three.

This was the method that version.pm adopted as well.  Some examples may be
helpful:

equivalent
  decimal    zero-padded    dotted-decimal
  -------    -----------    --------------
  1.2        1.200          v1.200.0
  1.02       1.020          v1.20.0
  1.002      1.002          v1.2.0
  1.0023     1.002300       v1.2.300
  1.00203    1.002030       v1.2.30
  1.002003   1.002003       v1.2.3

=head2 Quoting Rules

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 C<declare> or
L</qv()> methods.  While you do not have to quote decimal numbers when
creating version objects, it is always safe to quote B<all> initial values
when using version.pm methods, as this will ensure that what you type is
what is used.

Additionally, if you quote your initializer, then the quoted value that goes
B<in> will be exactly what comes B<out> when your $VERSION is printed
(stringified).  If you do not quote your value, Perl's normal numeric handling
comes into play and you may not get back what you were expecting.

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 are 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

=head2 What about v-strings?

There are two ways to enter v-strings: a bare number with two or more
decimal points, or a bare number with one or more decimal points and a
leading 'v' character (also bare).  For example:

$vs1 = 1.2.3; # encoded as \1\2\3
  $vs2 = v1.2;  # encoded as \1\2

However, the use of bare v-strings to initialize version objects is
B<strongly> discouraged in all circumstances.  Also, bare
v-strings are not completely supported in any version of Perl prior to
5.8.1.

If you insist on using bare v-strings with Perl > 5.6.0, be aware of the
following limitations:

1) For Perl releases 5.6.0 through 5.8.0, the v-string code merely guesses,
based on some characteristics of v-strings.  You B<must> use a three part
version, e.g. 1.2.3 or v1.2.3 in order for this heuristic to be successful.

2) For Perl releases 5.8.1 and later, v-strings have changed in the Perl
core to be magical, which means that the version.pm code can automatically
determine whether the v-string encoding was used.

3) In all cases, a version created using v-strings will have a stringified
form that has a leading 'v' character, for the simple reason that sometimes
it is impossible to tell whether one was present initially.

=head2 Version Object Internals

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<version/is_qv()>.

=item alpha

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

=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?> above).  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|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

=over 4

=item new()

Like many OO interfaces, the new() method is used to initialize version
objects.  If two arguments are passed to C<new()>, the B<second> one will be
used as if it were prefixed with "v".  This is to support historical use of the
C<qw> operator with 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 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 interchangeably, 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.

=back

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

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

=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()|version/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|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()|version/numify()> and L<normal()|version/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()|version/numify()> operator will turn "v1.0"
into the equivalent string "1.000000".  Forcing the outer version object
to L<normal()|version/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 Rules>.

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 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

=head1 AUTHOR

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

=head1 SEE ALSO

L<perl>.

=cut