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=head1 NAME

Automake::Condition - record a conjunction of conditionals

=head1 SYNOPSIS

  use Automake::Condition;

  # Create a condition to represent "COND1 and not COND2".
  my $cond = new Automake::Condition "COND1_TRUE", "COND2_FALSE";
  # Create a condition to represent "not COND3".
  my $other = new Automake::Condition "COND3_FALSE";

  # Create a condition to represent
  #   "COND1 and not COND2 and not COND3".
  my $both = $cond->merge ($other);

  # Likewise, but using a list of conditional strings
  my $both2 = $cond->merge_conds ("COND3_FALSE");

  # Strip from $both any subconditions which are in $other.
  # This is the opposite of merge.
  $cond = $both->strip ($other);

  # Return the list of conditions ("COND1_TRUE", "COND2_FALSE"):
  my @conds = $cond->conds;

  # Is $cond always true?  (Not in this example)
  if ($cond->true) { ... }

  # Is $cond always false? (Not in this example)
  if ($cond->false) { ... }

  # Return the list of conditionals as a string:
  #  "COND1_TRUE COND2_FALSE"
  my $str = $cond->string;

  # Return the list of conditionals as a human readable string:
  #  "COND1 and !COND2"
  my $str = $cond->human;

  # Return the list of conditionals as a AC_SUBST-style string:
  #  "@COND1_TRUE@@COND2_FALSE@"
  my $subst = $cond->subst_string;

  # Is $cond true when $both is true?  (Yes in this example)
  if ($cond->true_when ($both)) { ... }

  # Is $cond redundant w.r.t. {$other, $both}?
  # (Yes in this example)
  if ($cond->redundant_wrt ($other, $both)) { ... }

  # Does $cond imply any of {$other, $both}?
  # (Not in this example)
  if ($cond->implies_any ($other, $both)) { ... }

  # Remove superfluous conditionals assuming they will eventually
  # be multiplied together.
  # (Returns @conds = ($both) in this example, because
  # $other and $cond are implied by $both.)
  @conds = Automake::Condition::reduce_and ($other, $both, $cond);

  # Remove superfluous conditionals assuming they will eventually
  # be summed together.
  # (Returns @conds = ($cond, $other) in this example, because
  # $both is a subset condition of $cond: $cond is true whenever $both
  # is true.)
  @conds = Automake::Condition::reduce_or ($other, $both, $cond);

  # Invert a Condition.  This returns a list of Conditions.
  @conds = $both->not;

=head1 DESCRIPTION

A C<Condition> is a conjunction of conditionals (i.e., atomic conditions
defined in F<configure.ac> by C<AM_CONDITIONAL>.  In Automake they
are used to represent the conditions into which F<Makefile> variables and
F<Makefile> rules are defined.

If the variable C<VAR> is defined as

  if COND1
    if COND2
      VAR = value
    endif
  endif

then it will be associated a C<Condition> created with
the following statement.

  new Automake::Condition "COND1_TRUE", "COND2_TRUE";

Remember that a C<Condition> is a I<conjunction> of conditionals, so
the above C<Condition> means C<VAR> is defined when C<COND1>
B<and> C<COND2> are true. There is no way to express disjunctions
(i.e., I<or>s) with this class (but see L<DisjConditions>).

Another point worth to mention is that each C<Condition> object is
unique with respect to its conditionals.  Two C<Condition> objects
created for the same set of conditionals will have the same address.
This makes it easy to compare C<Condition>s: just compare the
references.

  my $c1 = new Automake::Condition "COND1_TRUE", "COND2_TRUE";
  my $c2 = new Automake::Condition "COND1_TRUE", "COND2_TRUE";
  $c1 == $c2;  # True!

=head2 Methods

=over 4

=item C<$cond = new Automake::Condition [@conds]>

Return a C<Condition> objects for the conjunctions of conditionals
listed in C<@conds> as strings.

An item in C<@conds> should be either C<"FALSE">, C<"TRUE">, or have
the form C<"NAME_FALSE"> or C<"NAME_TRUE"> where C<NAME> can be
anything (in practice C<NAME> should be the name of a conditional
declared in F<configure.ac> with C<AM_CONDITIONAL>, but it's not
C<Automake::Condition>'s responsibility to ensure this).

An empty C<@conds> means C<"TRUE">.

As explained previously, the reference (object) returned is unique
with respect to C<@conds>.  For this purpose, duplicate elements are
ignored, and C<@conds> is rewritten as C<("FALSE")> if it contains
C<"FALSE"> or two contradictory conditionals (such as C<"NAME_FALSE">
and C<"NAME_TRUE">.)

Therefore the following two statements create the same object (they
both create the C<"FALSE"> condition).

  my $c3 = new Automake::Condition "COND1_TRUE", "COND1_FALSE";
  my $c4 = new Automake::Condition "COND2_TRUE", "FALSE";
  $c3 == $c4;   # True!
  $c3 == FALSE; # True!

=cut

=item C<$newcond = $cond-E<gt>merge (@otherconds)>

Return a new condition which is the conjunction of
C<$cond> and C<@otherconds>.

=cut

=item C<$newcond = $cond-E<gt>merge_conds (@conds)>

Return a new condition which is the conjunction of C<$cond> and
C<@conds>, where C<@conds> is a list of conditional strings, as
passed to C<new>.

=cut

=item C<$newcond = $cond-E<gt>strip ($minuscond)>

Return a new condition which has all the conditionals of C<$cond>
except those of C<$minuscond>.  This is the opposite of C<merge>.

=cut

=item C<@list = $cond-E<gt>conds>

Return the set of conditionals defining C<$cond>, as strings.  Note that
this might not be exactly the list passed to C<new> (or a
concatenation of such lists if C<merge> was used), because of the
cleanup mentioned in C<new>'s description.

For instance C<$c3-E<gt>conds> will simply return C<("FALSE")>.

=cut

=item C<$cond-E<gt>false>

Return 1 iff this condition is always false.

=cut

=item C<$cond-E<gt>true>

Return 1 iff this condition is always true.

=cut

=item C<$cond-E<gt>string>

Build a string which denotes the condition.

For instance using the C<$cond> definition from L<SYNOPSYS>,
C<$cond-E<gt>string> will return C<"COND1_TRUE COND2_FALSE">.

=cut

=item C<$cond-E<gt>human>

Build a human readable string which denotes the condition.

For instance using the C<$cond> definition from L<SYNOPSYS>,
C<$cond-E<gt>string> will return C<"COND1 and !COND2">.

=cut

=item C<$cond-E<gt>subst_string>

Build a C<AC_SUBST>-style string for output in F<Makefile.in>.

For instance using the C<$cond> definition from L<SYNOPSYS>,
C<$cond-E<gt>subst_string> will return C<"@COND1_TRUE@@COND2_FALSE@">.

=cut

=item C<$cond-E<gt>true_when ($when)>

Return 1 iff C<$cond> is true when C<$when> is true.
Return 0 otherwise.

Using the definitions from L<SYNOPSYS>, C<$cond> is true
when C<$both> is true, but the converse is wrong.

=cut

=item C<$cond-E<gt>redundant_wrt (@conds)>

Return 1 iff C<$cond> is true for any condition in C<@conds>.
If @conds is empty, return 1 iff C<$cond> is C<FALSE>.
Return 0 otherwise.

=cut

=item C<$cond-E<gt>implies_any (@conds)>

Return 1 iff C<$cond> implies any of the conditions in C<@conds>.
Return 0 otherwise.

=cut

=item C<$cond-E<gt>not>

Return a negation of C<$cond> as a list of C<Condition>s.
This list should be used to construct a C<DisjConditions>
(we cannot return a C<DisjConditions> from C<Automake::Condition>,
because that would make these two packages interdependent).

=cut

=item C<$cond-E<gt>multiply (@conds)>

Assumption: C<@conds> represent a disjunction of conditions.

Return the result of multiplying C<$cond> with that disjunction.
The result will be a list of conditions suitable to construct a
C<DisjConditions>.

=cut

=head2 Other helper functions

=over 4

=item C<TRUE>

The C<"TRUE"> conditional.

=item C<FALSE>

The C<"FALSE"> conditional.

=cut

=item C<reduce_and (@conds)>

Return a subset of @conds with the property that the conjunction of
the subset is the same as the conjunction of @conds.  For example, if
both C<COND1_TRUE COND2_TRUE> and C<COND1_TRUE> are in the list,
discard the latter.  If the input list is empty, return C<(TRUE)>.

=cut

=item C<reduce_or (@conds)>

Return a subset of @conds with the property that the disjunction of
the subset is equivalent to the disjunction of @conds.  For example,
if both C<COND1_TRUE COND2_TRUE> and C<COND1_TRUE> are in the list,
discard the former.  If the input list is empty, return C<(FALSE)>.

=cut

=item C<conditional_negate ($condstr)>

Negate a conditional string.

=cut

=head1 SEE ALSO

L<Automake::DisjConditions>.

=head1 HISTORY

C<AM_CONDITIONAL>s and supporting code were added to Automake 1.1o by
Ian Lance Taylor <ian@cygnus.org> in 1997.  Since then it has been
improved by Tom Tromey <tromey@redhat.com>, Richard Boulton
<richard@tartarus.org>, Raja R Harinath <harinath@cs.umn.edu>,
Akim Demaille <akim@epita.fr>, and  Alexandre Duret-Lutz <adl@gnu.org>.

=cut