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Edit File : DisjConditions.pm \| Size : 13.97 KB
# Copyright (C) 1997-2018 Free Software Foundation, Inc. # This program is free software; you can redistribute it and/or modify # it under the terms of the GNU General Public License as published by # the Free Software Foundation; either version 2, or (at your option) # any later version. # This program is distributed in the hope that it will be useful, # but WITHOUT ANY WARRANTY; without even the implied warranty of # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the # GNU General Public License for more details. # You should have received a copy of the GNU General Public License # along with this program. If not, see <https://www.gnu.org/licenses/>. package Automake::DisjConditions; use 5.006; use strict; use Carp; use Automake::Condition qw/TRUE FALSE/; =head1 NAME Automake::DisjConditions - record a disjunction of Conditions =head1 SYNOPSIS use Automake::Condition; use Automake::DisjConditions; # 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 DisjConditions to represent # "(COND1 and not COND2) or (not COND3)" my $set = new Automake::DisjConditions $cond, $other; # Return the list of Conditions involved in $set. my @conds = $set->conds; # Return one of the Condition involved in $set. my $cond = $set->one_cond; # Return true iff $set is always true (i.e. its subconditions # cover all cases). if ($set->true) { ... } # Return false iff $set is always false (i.e. is empty, or contains # only false conditions). if ($set->false) { ... } # Return a string representing the DisjConditions. # "COND1_TRUE COND2_FALSE \| COND3_FALSE" my $str = $set->string; # Return a human readable string representing the DisjConditions. # "(COND1 and !COND2) or (!COND3)" my $str = $set->human; # Merge (OR) several DisjConditions. my $all = $set->merge($set2, $set3, ...) # Invert a DisjConditions, i.e., create a new DisjConditions # that complements $set. my $inv = $set->invert; # Multiply two DisjConditions. my $prod = $set1->multiply ($set2); # Return the subconditions of a DisjConditions with respect to # a Condition. See the description for a real example. my $subconds = $set->sub_conditions ($cond); # Check whether a new definition in condition $cond would be # ambiguous w.r.t. existing definitions in $set. ($msg, $ambig_cond) = $set->ambiguous_p ($what, $cond); =head1 DESCRIPTION A C<DisjConditions> is a disjunction of C<Condition>s. In Automake they are used to represent the conditions into which Makefile variables and Makefile rules are defined. If the variable C<VAR> is defined as if COND1 if COND2 VAR = value1 endif endif if !COND3 if COND4 VAR = value2 endif endif then it will be associated a C<DisjConditions> created with the following statement. new Automake::DisjConditions (new Automake::Condition ("COND1_TRUE", "COND2_TRUE"), new Automake::Condition ("COND3_FALSE", "COND4_TRUE")); As you can see, a C<DisjConditions> is made from a list of C<Condition>s. Since C<DisjConditions> is a disjunction, and C<Condition> is a conjunction, the above can be read as follows. (COND1 and COND2) or ((not COND3) and COND4) That's indeed the condition in which C<VAR> has a value. Like C<Condition> objects, a C<DisjConditions> object is unique with respect to its conditions. Two C<DisjConditions> objects created for the same set of conditions will have the same address. This makes it easy to compare C<DisjConditions>s: just compare the references. =head2 Methods =over 4 =item C<$set = new Automake::DisjConditions [@conds]> Create a C<DisjConditions> object from the list of C<Condition> objects passed in arguments. If the C<@conds> list is empty, the C<DisjConditions> is assumed to be false. As explained previously, the reference (object) returned is unique with respect to C<@conds>. For this purpose, duplicate elements are ignored. =cut # Keys in this hash are DisjConditions strings. Values are the # associated object DisjConditions. This is used by 'new' to reuse # DisjConditions objects with identical conditions. use vars '%_disjcondition_singletons'; sub new ($;@) { my ($class, @conds) = @_; my @filtered_conds = (); for my $cond (@conds) { confess "'$cond' isn't a reference" unless ref $cond; confess "'$cond' isn't an Automake::Condition" unless $cond->isa ("Automake::Condition"); # This is a disjunction of conditions, so we drop # false conditions. We'll always treat an "empty" # DisjConditions as false for this reason. next if $cond->false; push @filtered_conds, $cond; } my $string; if (@filtered_conds) { @filtered_conds = sort { $a->string cmp $b->string } @filtered_conds; $string = join (' \| ', map { $_->string } @filtered_conds); } else { $string = 'FALSE'; } # Return any existing identical DisjConditions. my $me = $_disjcondition_singletons{$string}; return $me if $me; # Else, create a new DisjConditions. # Store conditions as keys AND as values, because blessed # objects are converted to strings when used as keys (so # at least we still have the value when we need to call # a method). my %h = map {$_ => $_} @filtered_conds; my $self = { hash => \%h, string => $string, conds => \@filtered_conds, }; bless $self, $class; $_disjcondition_singletons{$string} = $self; return $self; } =item C<CLONE> Internal special subroutine to fix up the self hashes in C<%_disjcondition_singletons> upon thread creation. C<CLONE> is invoked automatically with ithreads from Perl 5.7.2 or later, so if you use this module with earlier versions of Perl, it is not thread-safe. =cut sub CLONE { foreach my $self (values %_disjcondition_singletons) { my %h = map { $_ => $_ } @{$self->{'conds'}}; $self->{'hash'} = \%h; } } =item C<@conds = $set-E<gt>conds> Return the list of C<Condition> objects involved in C<$set>. =cut sub conds ($ ) { my ($self) = @_; return @{$self->{'conds'}}; } =item C<$cond = $set-E<gt>one_cond> Return one C<Condition> object involved in C<$set>. =cut sub one_cond ($) { my ($self) = @_; return (%{$self->{'hash'}},)[1]; } =item C<$et = $set-E<gt>false> Return 1 iff the C<DisjConditions> object is always false (i.e., if it is empty, or if it contains only false C<Condition>s). Return 0 otherwise. =cut sub false ($ ) { my ($self) = @_; return 0 == keys %{$self->{'hash'}}; } =item C<$et = $set-E<gt>true> Return 1 iff the C<DisjConditions> object is always true (i.e. covers all conditions). Return 0 otherwise. =cut sub true ($ ) { my ($self) = @_; return $self->invert->false; } =item C<$str = $set-E<gt>string> Build a string which denotes the C<DisjConditions>. =cut sub string ($ ) { my ($self) = @_; return $self->{'string'}; } =item C<$cond-E<gt>human> Build a human readable string which denotes the C<DisjConditions>. =cut sub human ($ ) { my ($self) = @_; return $self->{'human'} if defined $self->{'human'}; my $res = ''; if ($self->false) { $res = 'FALSE'; } else { my @c = $self->conds; if (1 == @c) { $res = $c[0]->human; } else { $res = '(' . join (') or (', map { $_->human } $self->conds) . ')'; } } $self->{'human'} = $res; return $res; } =item C<$newcond = $cond-E<gt>merge (@otherconds)> Return a new C<DisjConditions> which is the disjunction of C<$cond> and C<@otherconds>. Items in C<@otherconds> can be @C<Condition>s or C<DisjConditions>. =cut sub merge ($@) { my ($self, @otherconds) = @_; new Automake::DisjConditions ( map { $_->isa ("Automake::DisjConditions") ? $_->conds : $_ } ($self, @otherconds)); } =item C<$prod = $set1-E<gt>multiply ($set2)> Multiply two conditional sets. my $set1 = new Automake::DisjConditions (new Automake::Condition ("A_TRUE"), new Automake::Condition ("B_TRUE")); my $set2 = new Automake::DisjConditions (new Automake::Condition ("C_FALSE"), new Automake::Condition ("D_FALSE")); C<$set1-E<gt>multiply ($set2)> will return new Automake::DisjConditions (new Automake::Condition ("A_TRUE", "C_FALSE"), new Automake::Condition ("B_TRUE", "C_FALSE"),; new Automake::Condition ("A_TRUE", "D_FALSE"), new Automake::Condition ("B_TRUE", "D_FALSE")); The argument can also be a C<Condition>. =cut # Same as multiply() but take a list of Conditionals as second argument. # We use this in invert(). sub _multiply ($@) { my ($self, @set) = @_; my @res = map { $_->multiply (@set) } $self->conds; return new Automake::DisjConditions (Automake::Condition::reduce_or @res); } sub multiply ($$) { my ($self, $set) = @_; return $self->_multiply ($set) if $set->isa('Automake::Condition'); return $self->_multiply ($set->conds); } =item C<$inv = $set-E<gt>invert> Invert a C<DisjConditions>. Return a C<DisjConditions> which is true when C<$set> is false, and vice-versa. my $set = new Automake::DisjConditions (new Automake::Condition ("A_TRUE", "B_TRUE"), new Automake::Condition ("A_FALSE", "B_FALSE")); Calling C<$set-E<gt>invert> will return the following C<DisjConditions>. new Automake::DisjConditions (new Automake::Condition ("A_TRUE", "B_FALSE"), new Automake::Condition ("A_FALSE", "B_TRUE")); We implement the inversion by a product-of-sums to sum-of-products conversion using repeated multiplications. Because of the way we implement multiplication, the result of inversion is in canonical prime implicant form. =cut sub invert($ ) { my ($self) = @_; return $self->{'invert'} if defined $self->{'invert'}; # The invert of an empty DisjConditions is TRUE. my $res = new Automake::DisjConditions TRUE; # !((a.b)+(c.d)+(e.f)) # = (!a+!b).(!c+!d).(!e+!f) # We develop this into a sum of product iteratively, starting from TRUE: # 1) TRUE # 2) TRUE.!a + TRUE.!b # 3) TRUE.!a.!c + TRUE.!b.!c + TRUE.!a.!d + TRUE.!b.!d # 4) TRUE.!a.!c.!e + TRUE.!b.!c.!e + TRUE.!a.!d.!e + TRUE.!b.!d.!e # + TRUE.!a.!c.!f + TRUE.!b.!c.!f + TRUE.!a.!d.!f + TRUE.!b.!d.!f foreach my $cond ($self->conds) { $res = $res->_multiply ($cond->not); } # Cache result. $self->{'invert'} = $res; # It's tempting to also set $res->{'invert'} to $self, but that # is a bad idea as $self hasn't been normalized in any way. # (Different inputs can produce the same inverted set.) return $res; } =item C<$self-E<gt>simplify> Return a C<Disjunction> which is a simplified canonical form of C<$self>. This canonical form contains only prime implicants, but it can contain non-essential prime implicants. =cut sub simplify ($) { my ($self) = @_; return $self->invert->invert; } =item C<$self-E<gt>sub_conditions ($cond)> Return the subconditions of C<$self> that contains C<$cond>, with C<$cond> stripped. More formally, return C<$res> such that C<$res-E<gt>multiply ($cond) == $self-E<gt>multiply ($cond)> and C<$res> does not mention any of the variables in C<$cond>. For instance, consider: my $a = new Automake::DisjConditions (new Automake::Condition ("A_TRUE", "B_TRUE"), new Automake::Condition ("A_TRUE", "C_FALSE"), new Automake::Condition ("A_TRUE", "B_FALSE", "C_TRUE"), new Automake::Condition ("A_FALSE")); my $b = new Automake::DisjConditions (new Automake::Condition ("A_TRUE", "B_FALSE")); Calling C<$a-E<gt>sub_conditions ($b)> will return the following C<DisjConditions>. new Automake::DisjConditions (new Automake::Condition ("C_FALSE"), # From A_TRUE C_FALSE new Automake::Condition ("C_TRUE")); # From A_TRUE B_FALSE C_TRUE" =cut sub sub_conditions ($$) { my ($self, $subcond) = @_; # Make $subcond blindingly apparent in the DisjConditions. # For instance '$b->multiply($a->conds)' (from the POD example) is: # (new Automake::Condition ("FALSE"), # new Automake::Condition ("A_TRUE", "B_FALSE", "C_FALSE"), # new Automake::Condition ("A_TRUE", "B_FALSE", "C_TRUE"), # new Automake::Condition ("FALSE")) my @prodconds = $subcond->multiply ($self->conds); # Now, strip $subcond from the remaining (i.e., non-false) Conditions. my @res = map { $_->false ? () : $_->strip ($subcond) } @prodconds; return new Automake::DisjConditions @res; } =item C<($string, $ambig_cond) = $condset-E<gt>ambiguous_p ($what, $cond)> Check for an ambiguous condition. Return an error message and the other condition involved if we have an ambiguity. Return an empty string and FALSE otherwise. C<$what> is the name of the thing being defined, to use in the error message. C<$cond> is the C<Condition> under which it is being defined. C<$condset> is the C<DisjConditions> under which it had already been defined. =cut sub ambiguous_p ($$$) { my ($self, $var, $cond) = @_; # Note that these rules don't consider the following # example as ambiguous. # # if COND1 # FOO = foo # endif # if COND2 # FOO = bar # endif # # It's up to the user to not define COND1 and COND2 # simultaneously. return ("$var multiply defined in condition " . $cond->human, $cond) if exists $self->{'hash'}{$cond}; foreach my $vcond ($self->conds) { return ("$var was already defined in condition " . $vcond->human . ", which includes condition ". $cond->human, $vcond) if $vcond->true_when ($cond); return ("$var was already defined in condition " . $vcond->human . ", which is included in condition " . $cond->human, $vcond) if $cond->true_when ($vcond); } return ('', FALSE); } =head1 SEE ALSO L<Automake::Condition>. =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>, Pavel Roskin <proski@gnu.org>, and Alexandre Duret-Lutz <adl@gnu.org>. =cut 1; Back

Edit File : DisjConditions.pm | Size : 13.97 KB

# This program is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 2, or (at your option)
# any later version.

# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.

# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <https://www.gnu.org/licenses/>.

package Automake::DisjConditions;

use 5.006;
use strict;
use Carp;
use Automake::Condition qw/TRUE FALSE/;

=head1 NAME

Automake::DisjConditions - record a disjunction of Conditions

=head1 SYNOPSIS

use Automake::Condition;
  use Automake::DisjConditions;

# 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 DisjConditions to represent
  #   "(COND1 and not COND2) or (not COND3)"
  my $set = new Automake::DisjConditions $cond, $other;

# Return the list of Conditions involved in $set.
  my @conds = $set->conds;

# Return one of the Condition involved in $set.
  my $cond = $set->one_cond;

# Return true iff $set is always true (i.e. its subconditions
  # cover all cases).
  if ($set->true) { ... }

# Return false iff $set is always false (i.e. is empty, or contains
  # only false conditions).
  if ($set->false) { ... }

# Return a string representing the DisjConditions.
  #   "COND1_TRUE COND2_FALSE | COND3_FALSE"
  my $str = $set->string;

# Return a human readable string representing the DisjConditions.
  #   "(COND1 and !COND2) or (!COND3)"
  my $str = $set->human;

# Merge (OR) several DisjConditions.
  my $all = $set->merge($set2, $set3, ...)

# Invert a DisjConditions, i.e., create a new DisjConditions
  # that complements $set.
  my $inv = $set->invert;

# Multiply two DisjConditions.
  my $prod = $set1->multiply ($set2);

# Return the subconditions of a DisjConditions with respect to
  # a Condition.  See the description for a real example.
  my $subconds = $set->sub_conditions ($cond);

# Check whether a new definition in condition $cond would be
  # ambiguous w.r.t. existing definitions in $set.
  ($msg, $ambig_cond) = $set->ambiguous_p ($what, $cond);

=head1 DESCRIPTION

A C<DisjConditions> is a disjunction of C<Condition>s.  In Automake
they are used to represent the conditions into which Makefile
variables and Makefile rules are defined.

If the variable C<VAR> is defined as

if COND1
    if COND2
      VAR = value1
    endif
  endif
  if !COND3
    if COND4
      VAR = value2
    endif
  endif

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

new Automake::DisjConditions
    (new Automake::Condition ("COND1_TRUE", "COND2_TRUE"),
     new Automake::Condition ("COND3_FALSE", "COND4_TRUE"));

As you can see, a C<DisjConditions> is made from a list of
C<Condition>s.  Since C<DisjConditions> is a disjunction, and
C<Condition> is a conjunction, the above can be read as
follows.

(COND1 and COND2) or ((not COND3) and COND4)

That's indeed the condition in which C<VAR> has a value.

Like C<Condition> objects, a C<DisjConditions> object is unique
with respect to its conditions.  Two C<DisjConditions> objects created
for the same set of conditions will have the same address.  This makes
it easy to compare C<DisjConditions>s: just compare the references.

=head2 Methods

=over 4

=item C<$set = new Automake::DisjConditions [@conds]>

Create a C<DisjConditions> object from the list of C<Condition>
objects passed in arguments.

If the C<@conds> list is empty, the C<DisjConditions> is assumed to be
false.

As explained previously, the reference (object) returned is unique
with respect to C<@conds>.  For this purpose, duplicate elements are
ignored.

=cut

# Keys in this hash are DisjConditions strings. Values are the
# associated object DisjConditions.  This is used by 'new' to reuse
# DisjConditions objects with identical conditions.
use vars '%_disjcondition_singletons';

sub new ($;@)
{
  my ($class, @conds) = @_;
  my @filtered_conds = ();
  for my $cond (@conds)
    {
      confess "'$cond' isn't a reference" unless ref $cond;
      confess "'$cond' isn't an Automake::Condition"
	unless $cond->isa ("Automake::Condition");

# This is a disjunction of conditions, so we drop
      # false conditions.  We'll always treat an "empty"
      # DisjConditions as false for this reason.
      next if $cond->false;

push @filtered_conds, $cond;
    }

my $string;
  if (@filtered_conds)
    {
      @filtered_conds = sort { $a->string cmp $b->string } @filtered_conds;
      $string = join (' | ', map { $_->string } @filtered_conds);
    }
  else
    {
      $string = 'FALSE';
    }

# Return any existing identical DisjConditions.
  my $me = $_disjcondition_singletons{$string};
  return $me if $me;

# Else, create a new DisjConditions.

# Store conditions as keys AND as values, because blessed
  # objects are converted to strings when used as keys (so
  # at least we still have the value when we need to call
  # a method).
  my %h = map {$_ => $_} @filtered_conds;

my $self = {
    hash => \%h,
    string => $string,
    conds => \@filtered_conds,
  };
  bless $self, $class;

$_disjcondition_singletons{$string} = $self;
  return $self;
}

=item C<CLONE>

Internal special subroutine to fix up the self hashes in
C<%_disjcondition_singletons> upon thread creation.  C<CLONE> is invoked
automatically with ithreads from Perl 5.7.2 or later, so if you use this
module with earlier versions of Perl, it is not thread-safe.

=cut

sub CLONE
{
  foreach my $self (values %_disjcondition_singletons)
    {
      my %h = map { $_ => $_ } @{$self->{'conds'}};
      $self->{'hash'} = \%h;
    }
}

=item C<@conds = $set-E<gt>conds>

Return the list of C<Condition> objects involved in C<$set>.

=cut

sub conds ($ )
{
  my ($self) = @_;
  return @{$self->{'conds'}};
}

=item C<$cond = $set-E<gt>one_cond>

Return one C<Condition> object involved in C<$set>.

=cut

sub one_cond ($)
{
  my ($self) = @_;
  return (%{$self->{'hash'}},)[1];
}

=item C<$et = $set-E<gt>false>

Return 1 iff the C<DisjConditions> object is always false (i.e., if it
is empty, or if it contains only false C<Condition>s). Return 0
otherwise.

=cut

sub false ($ )
{
  my ($self) = @_;
  return 0 == keys %{$self->{'hash'}};
}

=item C<$et = $set-E<gt>true>

Return 1 iff the C<DisjConditions> object is always true (i.e. covers all
conditions). Return 0 otherwise.

=cut

sub true ($ )
{
  my ($self) = @_;
  return $self->invert->false;
}

=item C<$str = $set-E<gt>string>

Build a string which denotes the C<DisjConditions>.

=cut

sub string ($ )
{
  my ($self) = @_;
  return $self->{'string'};
}

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

Build a human readable string which denotes the C<DisjConditions>.

=cut

sub human ($ )
{
  my ($self) = @_;

return $self->{'human'} if defined $self->{'human'};

my $res = '';
  if ($self->false)
    {
      $res = 'FALSE';
    }
  else
    {
      my @c = $self->conds;
      if (1 == @c)
	{
	  $res = $c[0]->human;
	}
      else
	{
	  $res = '(' . join (') or (', map { $_->human } $self->conds) . ')';
	}
    }
  $self->{'human'} = $res;
  return $res;
}

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

Return a new C<DisjConditions> which is the disjunction of
C<$cond> and C<@otherconds>.  Items in C<@otherconds> can be
@C<Condition>s or C<DisjConditions>.

=cut

sub merge ($@)
{
  my ($self, @otherconds) = @_;
  new Automake::DisjConditions (
    map { $_->isa ("Automake::DisjConditions") ? $_->conds : $_ }
        ($self, @otherconds));
}

=item C<$prod = $set1-E<gt>multiply ($set2)>

Multiply two conditional sets.

my $set1 = new Automake::DisjConditions
    (new Automake::Condition ("A_TRUE"),
     new Automake::Condition ("B_TRUE"));
  my $set2 = new Automake::DisjConditions
    (new Automake::Condition ("C_FALSE"),
     new Automake::Condition ("D_FALSE"));

C<$set1-E<gt>multiply ($set2)> will return

new Automake::DisjConditions
    (new Automake::Condition ("A_TRUE", "C_FALSE"),
     new Automake::Condition ("B_TRUE", "C_FALSE"),;
     new Automake::Condition ("A_TRUE", "D_FALSE"),
     new Automake::Condition ("B_TRUE", "D_FALSE"));

The argument can also be a C<Condition>.

=cut

# Same as multiply() but take a list of Conditionals as second argument.
# We use this in invert().
sub _multiply ($@)
{
  my ($self, @set) = @_;
  my @res = map { $_->multiply (@set) } $self->conds;
  return new Automake::DisjConditions (Automake::Condition::reduce_or @res);
}

sub multiply ($$)
{
  my ($self, $set) = @_;
  return $self->_multiply ($set) if $set->isa('Automake::Condition');
  return $self->_multiply ($set->conds);
}

=item C<$inv = $set-E<gt>invert>

Invert a C<DisjConditions>.  Return a C<DisjConditions> which is true
when C<$set> is false, and vice-versa.

my $set = new Automake::DisjConditions
    (new Automake::Condition ("A_TRUE", "B_TRUE"),
     new Automake::Condition ("A_FALSE", "B_FALSE"));

Calling C<$set-E<gt>invert> will return the following C<DisjConditions>.

new Automake::DisjConditions
    (new Automake::Condition ("A_TRUE", "B_FALSE"),
     new Automake::Condition ("A_FALSE", "B_TRUE"));

We implement the inversion by a product-of-sums to sum-of-products
conversion using repeated multiplications.  Because of the way we
implement multiplication, the result of inversion is in canonical
prime implicant form.

=cut

sub invert($ )
{
  my ($self) = @_;

return $self->{'invert'} if defined $self->{'invert'};

# The invert of an empty DisjConditions is TRUE.
  my $res = new Automake::DisjConditions TRUE;

#   !((a.b)+(c.d)+(e.f))
  # = (!a+!b).(!c+!d).(!e+!f)
  # We develop this into a sum of product iteratively, starting from TRUE:
  # 1) TRUE
  # 2) TRUE.!a + TRUE.!b
  # 3) TRUE.!a.!c + TRUE.!b.!c + TRUE.!a.!d + TRUE.!b.!d
  # 4) TRUE.!a.!c.!e + TRUE.!b.!c.!e + TRUE.!a.!d.!e + TRUE.!b.!d.!e
  #    + TRUE.!a.!c.!f + TRUE.!b.!c.!f + TRUE.!a.!d.!f + TRUE.!b.!d.!f
  foreach my $cond ($self->conds)
    {
      $res = $res->_multiply ($cond->not);
    }

# Cache result.
  $self->{'invert'} = $res;
  # It's tempting to also set $res->{'invert'} to $self, but that
  # is a bad idea as $self hasn't been normalized in any way.
  # (Different inputs can produce the same inverted set.)
  return $res;
}

=item C<$self-E<gt>simplify>

Return a C<Disjunction> which is a simplified canonical form of C<$self>.
This canonical form contains only prime implicants, but it can contain
non-essential prime implicants.

=cut

sub simplify ($)
{
  my ($self) = @_;
  return $self->invert->invert;
}

=item C<$self-E<gt>sub_conditions ($cond)>

Return the subconditions of C<$self> that contains C<$cond>, with
C<$cond> stripped.  More formally, return C<$res> such that
C<$res-E<gt>multiply ($cond) == $self-E<gt>multiply ($cond)> and
C<$res> does not mention any of the variables in C<$cond>.

For instance, consider:

my $a = new Automake::DisjConditions
    (new Automake::Condition ("A_TRUE", "B_TRUE"),
     new Automake::Condition ("A_TRUE", "C_FALSE"),
     new Automake::Condition ("A_TRUE", "B_FALSE", "C_TRUE"),
     new Automake::Condition ("A_FALSE"));
  my $b = new Automake::DisjConditions
    (new Automake::Condition ("A_TRUE", "B_FALSE"));

Calling C<$a-E<gt>sub_conditions ($b)> will return the following
C<DisjConditions>.

new Automake::DisjConditions
    (new Automake::Condition ("C_FALSE"), # From A_TRUE C_FALSE
     new Automake::Condition ("C_TRUE")); # From A_TRUE B_FALSE C_TRUE"

=cut

sub sub_conditions ($$)
{
  my ($self, $subcond) = @_;

# Make $subcond blindingly apparent in the DisjConditions.
  # For instance '$b->multiply($a->conds)' (from the POD example) is:
  # 	(new Automake::Condition ("FALSE"),
  # 	 new Automake::Condition ("A_TRUE", "B_FALSE", "C_FALSE"),
  # 	 new Automake::Condition ("A_TRUE", "B_FALSE", "C_TRUE"),
  # 	 new Automake::Condition ("FALSE"))
  my @prodconds = $subcond->multiply ($self->conds);

# Now, strip $subcond from the remaining (i.e., non-false) Conditions.
  my @res = map { $_->false ? () : $_->strip ($subcond) } @prodconds;

return new Automake::DisjConditions @res;
}

=item C<($string, $ambig_cond) = $condset-E<gt>ambiguous_p ($what, $cond)>

Check for an ambiguous condition.  Return an error message and the
other condition involved if we have an ambiguity.  Return an empty
string and FALSE otherwise.

C<$what> is the name of the thing being defined, to use in the error
message.  C<$cond> is the C<Condition> under which it is being
defined.  C<$condset> is the C<DisjConditions> under which it had
already been defined.

=cut

sub ambiguous_p ($$$)
{
  my ($self, $var, $cond) = @_;

# Note that these rules don't consider the following
  # example as ambiguous.
  #
  #   if COND1
  #     FOO = foo
  #   endif
  #   if COND2
  #     FOO = bar
  #   endif
  #
  # It's up to the user to not define COND1 and COND2
  # simultaneously.

return ("$var multiply defined in condition " . $cond->human, $cond)
    if exists $self->{'hash'}{$cond};

foreach my $vcond ($self->conds)
    {
      return ("$var was already defined in condition " . $vcond->human
	      . ", which includes condition ". $cond->human, $vcond)
	if $vcond->true_when ($cond);

return ("$var was already defined in condition " . $vcond->human
	      . ", which is included in condition " . $cond->human, $vcond)
	if $cond->true_when ($vcond);
    }
  return ('', FALSE);
}

=head1 SEE ALSO

L<Automake::Condition>.

=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>, Pavel Roskin <proski@gnu.org>, and
Alexandre Duret-Lutz <adl@gnu.org>.

=cut

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