File Coverage

blib/lib/Math/Symbolic/Custom/Pattern.pm

Criterion	Covered	Total	%
statement	185	213	86.8
branch	76	98	77.5
condition	9	15	60.0
subroutine	25	27	92.5
pod	3	3	100.0
total	298	356	83.7

package Math::Symbolic::Custom::Pattern;

use 5.006001;
use strict;
use warnings;
no warnings 'recursion';
use Carp qw/cluck confess/;

use Clone qw/clone/;
use Math::Symbolic qw/:all/;
use Math::Symbolic::Custom::Pattern::Export;

our $VERSION = '2.01';

use constant EPSILON => 1e-29;

use constant TYPE  => 0;
use constant VAL   => 1;
use constant OPS   => 2;
use constant ATTR  => 3;

use constant PATTERN => -1;

use constant ANY_TREE    => 0;
use constant ANY_CONST   => 1;
use constant ANY_VAR     => 2;
use constant NAMED_TREE  => 3;
use constant NAMED_CONST => 4;
use constant NAMED_VAR   => 5;

use constant ATTR_COMMUTATIVE => 1;

=encoding utf8

=head1 NAME

Math::Symbolic::Custom::Pattern - Pattern matching on Math::Symbolic trees

=head1 SYNOPSIS

  use Math::Symbolic qw/parse_from_string/;
  use Math::Symbolic::Custom::Pattern;
  my $patternstring = "VAR_foo + sin(CONST * VAR_foo)"
  my $pattern = Math::Symbolic::Custom::Pattern->new( $patternstring );

  my $formula = parse_from_string("a + sin(5 * a)");

  if ($pattern->match($formula)) {
    print "The pattern matches the formula.\n";
  }
  else {
    print "The pattern does not match the formula.\n";
  }

  # will print "The pattern matches the formula" since "a" is
  # found to be "VAR_foo" and 5 is a constant.
  # "a + sin(5 * b)" would not match since VAR_foo is already "a"
  # when the "b" is encountered. "VAR" would match any variable.
  # "TREE" matches any tree. "TREE_name" and "CONST_name" work as
  # you would expect.

  # Alternatively:
  my $pattern = $some_formula->to_pattern();

  print "yes\n" if $formula->is_of_form($pattern); # fast-ish
  # This has syntactic sugar, too:
  print "yes\n" if $formula->is_of_form("VAR + TREE"); # slow!
  print "yes\n" if $formula->is_of_form($another_formula); # semi-slow...

  # Finally, when creating a pattern, one can specify that addition and
  # product should match commutatively:
  my $pattern = Math::Symbolic::Custom::Pattern->new(
    parse_from_string("a + b"), commutation => 1,
  );
  my $formula = parse_from_string("b + a");
  # does match even though "a+b" <=> "b+a" aren't the same
  # internal tree representations
  print "yes\n" if $pattern->match($formula);

=head1 DESCRIPTION

This module is an extension to the Math::Symbolic module. A basic
familiarity with that module is required.

The Math::Symbolic::Custom::Pattern module implements pattern matching routines
on Math::Symbolic trees. The patterns itself are constructed from Math::Symbolic
trees with just a few variables which have a special meaning.

The module provides two interfaces. You can use the C and C
methods this class provides, or you can use the C and
C methods on any Math::Symbolic tree. (Exported by the
Math::Symbolic::Custom::Pattern::Export module. Refer to that module for
details on C.)

You can construct a pattern from any Math::Symbolic tree. For sake of
simplicity, we will talk about a tree "a+(b*c)" even if that's just its string
representation. The tree is really what is returned by
Cparse_from_string("a+(b*c)")>.

Suppose you call

  my $pattern = Math::Symbolic::Custom::Pattern->new("a+(b*c)");

That creates a pattern that matches this exact tree. Calling

  my $boolean = $pattern->match($tree);

on any Math::Symbolic tree C<$tree> will result in C<$boolean> being false
except if it is C<"a+(b*c)">.

So far so good. This isn't impressive and the C method of
all Math::Symbolic trees does the same. (Except that the pattern matching is
about twice as fast.)

If you create a pattern from the following string, however, you get different
behaviour: C<"VAR + (VAR*VAR)">. Now, any variable may be in place of C,
C, and C. (C<"a + (x*x)">, C, ...)

You can match with named (but not literal) variables with the following
pattern string: C<"VAR_first + (VAR_first*VAR_second)"> This matches
the tree C<"a + (a*b)">, but not C<"a + (c*b)"> since the first variable
in the parenthesis of the second tree is not the same as the one outside the
parenthesis. Note that the variable C<"b"> in both examples could have been
any variable, since C occurrs only once in the pattern.

Analogous to the general C and named C pattern elements, you may
use C to match any subtree whatsoever or C to match a named
tree. Example: The pattern C<"TREE_a + 5*TREE_a"> matches the tree
C<"sin(b+c) + 5*sin(b+c)">, but not C<"sin(b+c) + 5*cos(b+c)">. Beware of the
fact that the trees C<"sin(b+c)"> and C<"sin(c+b)"> would not be the same
either. Though mathematically equivalent, they do not have the same internal
representation. Canonicalizing the internal representation is simple in this
example, but is impossible in the general case, so just take care.

Finally, what works with variables and general trees also works with constants.
You may specify the pattern C<"CONST_foo * a + atan(CONST_foo)">. This matches
C<"0.5*a + atan(0.5)">, but does not match C<"2*a + atan(0.5)"> since the
named constants are not equal. The general form C works as a wildcard
for any constants.

=head2 EXPORT

This module does not export anything.

=head2 METHODS

This is a list of public methods.

=over 2

=cut


=item new

C is the constructor for Math::Symbolic::Custom::Pattern objects.
It takes a Math::Symbolic tree as first argument which will be transformed
into a pattern. See the C method documentation.

After the Math::Symbolic tree, a list of key/value pairs can be passed in
as options for the pattern construction.

The only currently supported option is C indicating whether or
not the pattern should match sums and products commutatively. Please note
that this does not match recursively and does not recognize associativity:
The commutative pattern of C<(a + b) + c> matches the
expression C<(b + a) + c> and C, but B C!
This means that if the tree to match is built from a string such as
C, then it is not defined whether C<(a + b) + c> matches
that expression. It does so if the internal tree representation
happens to be C<(a + b) + c> and it doesn't if it happens to be
C. This may be fixed at a later point.

=cut

sub new {
  my $proto = shift;
  my $class = ref($proto)||$proto;

  # I want to call that 'proto', too ;)
  $proto = shift;
  confess(
    __PACKAGE__."new() requires a Math::Symbolic tree as first "
    ."argument."
  ) if not ref($proto) =~ /^Math::Symbolic/;

  my %opt = @_;

  my $info = {
    vars        => {},
    constants   => {},
    trees       => {},
    commutation => $opt{commutation},
  };

  my $pattern = _descend_build($proto, $info);

  #_descend_generalize($pattern, $info);

  my $self = {
    pattern => $pattern,
    info    => $info,
    string  => $proto->to_string(),
  };

  return bless $self => $class;
}


sub _descend_build {
  my ($proto, $info) = @_;

  my $tree = [];
  my $tt   = $proto->term_type();

  if ($tt == T_CONSTANT) {
    $tree->[TYPE] = T_CONSTANT;
    $tree->[VAL]  = $proto->value();
  }
  elsif ($tt == T_OPERATOR) {
    $tree->[TYPE] = T_OPERATOR;
    $tree->[VAL]  = $proto->type();
    $tree->[OPS]  = [
      map { _descend_build($_, $info) }
      @{$proto->{operands}}
    ];
    $tree->[ATTR] = 0;
    $tree->[ATTR] |= ATTR_COMMUTATIVE
      if $info->{commutation} and $Math::Symbolic::Operator::Op_Types[$tree->[VAL]]{commutative};
    # todo: ATTR_CONSTANT?
  }
  else { # variable
    my $name = $proto->name();

    $tree->[TYPE] = PATTERN;
    if ($name eq 'TREE') {
      $tree->[VAL] = ANY_TREE;
    }
    elsif ($name eq 'CONST') {
      $tree->[VAL] = ANY_CONST;
    }
    elsif ($name eq 'VAR') {
      $tree->[VAL] = ANY_VAR;
    }
    elsif ($name =~ /^TREE_(\w+)$/) {
      $tree->[VAL] = NAMED_TREE;
      my @names = split /_/, $1;
      $tree->[OPS] = \@names;
      $info->{trees}{$_}++ for @names;
    }
    elsif ($name =~ /^CONST_(\w+)$/) {
      $tree->[VAL] = NAMED_CONST;
      my @names = split /_/, $1;
      $tree->[OPS] = \@names;
      $info->{constants}{$_}++ for @names;
    }
    elsif ($name =~ /^VAR_(\w+)$/) {
      $tree->[VAL] = NAMED_VAR;
      my @names = split /_/, $1;
      $tree->[OPS] = \@names;
      $info->{vars}{$_}++ for @names;
    }
    else {
      $tree->[TYPE] = T_VARIABLE;
      $tree->[VAL] = $name;
    }
  }

  return $tree;
}


=item match

This method takes a Math::Symbolic tree as first argument. It throws a
fatal error if this is not the case.

It returns a true value if the pattern matches the tree and a false value
if the pattern does not match. Please have a look at the L
to find out what I means in this context.

As a matter of fact, if you need to know what subtrees were matched by the
various C, C, and C identifiers, you can find
out by inspecting the return value of a successful match. It will be a
reference to a hash containing three key/value pairs with the keys
C, C, and C. Each of these will again point to a hash.
These hashes contain the names of the matched subtrees. For example, if your
pattern is C and it matches C, then
the return value will be:

  {
    constants => {},
    trees     => {},
    vars      => {
      'x' => 'foo*bar',
    }
  }

Except that C will actually be the corresponding Math::Symbolic tree
and not a string. Please note that the subtrees are real subtrees. Modifying
them will result in a modified original tree as well.

=cut


sub match {
  my $self = shift;

  my $tree = shift;
  confess(
    __PACKAGE__."match() requires a Math::Symbolic tree as first "
    ."argument."
  ) if not ref($tree) =~ /^Math::Symbolic/;

  my $info = $self->{info};
  my $info_copy = {
    constants => { map {($_,undef)} keys %{$info->{constants}} },
    vars => { map {($_,undef)} keys %{$info->{vars}} },
    trees => { map {($_,undef)} keys %{$info->{trees}} },
  };

  my $okay = _descend_match($self->{pattern}, $tree, $info_copy);
  return $info_copy if $okay;
  return undef;
}

sub _descend_match {
  my ($pat, $tree, $info) = @_;

  my $ptype = $pat->[TYPE];
  my $ttype = $tree->term_type();

  if ($ptype == T_CONSTANT) {
    return undef if $ttype != T_CONSTANT;
    return 1 if abs($tree->value()-$pat->[VAL]) < EPSILON;
    return undef;
  }
  elsif ($ptype == T_VARIABLE) {
    return undef if $ttype != T_VARIABLE;
    return 1 if $tree->name() eq $pat->[VAL];
    return undef;
  }
  elsif ($ptype == T_OPERATOR) {
    return undef if $ttype != T_OPERATOR;
    my $optype = $tree->type();
    return undef if $optype != $pat->[VAL];

    my @operands = @{$pat->[OPS]};
    my @tree_ops = @{$tree->{operands}};

    return undef if @operands != @tree_ops;

    if (($pat->[ATTR] & ATTR_COMMUTATIVE) && @operands > 1) {
      if (@operands == 2) { # use hard coded permutation
        my $ok = _descend_match($operands[0], $tree_ops[0], $info) && _descend_match($operands[1], $tree_ops[1], $info);
        if (!$ok) {
          $ok = _descend_match($operands[0], $tree_ops[1], $info) && _descend_match($operands[1], $tree_ops[0], $info);
        }
        return undef unless $ok;
      }
      else {
        _permute {
          my $ok;
          for (@_) {
            $ok = _descend_match($_->[0], $_->[1], $info);
            last if not $ok;
          }
          $ok
        } map {[$operands[$_], $tree_ops[$_]]} 0..$#operands;
      }
    }
    else { # no commutation
      foreach my $oper_no (0..$#operands) {
        my $ok = _descend_match($operands[$oper_no], $tree_ops[$oper_no], $info);
        return undef unless $ok;
      }
    }
    return 1;
  }
  elsif ($ptype == PATTERN) {
    my $match = $pat->[VAL];
    if ($match == ANY_TREE) {
      return 1;
    }
    elsif ($match == ANY_CONST) {
      my $ttype = $tree->term_type();
      return $ttype == T_CONSTANT ? 1 : undef;
    }
    elsif ($match == ANY_VAR) {
      my $ttype = $tree->term_type();
      return $ttype == T_VARIABLE ? 1 : undef;
    }
    elsif ($match == NAMED_TREE) {
      my @names = @{$pat->[OPS]};
      my $itrees = $info->{trees};
      foreach my $name (@names) {
        die "tree name '$name' should exist, but does not. "
          ."Internal error."
          if not exists $itrees->{$name};

        my $itree = $itrees->{$name};
        if (defined $itree) {
          my $ok = $itree->is_identical($tree);
          return 1 if $ok;
        }
        else {
          $itrees->{$name} = $tree;
          return 1;
        }
      }
      return undef;
    }
    elsif ($match == NAMED_CONST) {
      return undef unless $ttype == T_CONSTANT;

      my @names = @{$pat->[OPS]};
      my $iconsts = $info->{constants};
      foreach my $name (@names) {
        die "constant name '$name' should exist, but does not. "
          ."Internal error."
          if not exists $iconsts->{$name};

        my $iconst = $iconsts->{$name};
        if (defined $iconst) {
          my $ok = $iconst == $tree->value();
          return 1 if $ok;
        }
        else {
          $iconsts->{$name} = $tree->value();
          return 1;
        }
      }
      return undef;
    }
    elsif ($match == NAMED_VAR) {
      return undef unless $ttype == T_VARIABLE;

      my @names = @{$pat->[OPS]};
      my $ivars = $info->{vars};
      foreach my $name (@names) {
        die "variable name '$name' should exist, but does not. "
          ."Internal error."
          if not exists $ivars->{$name};

        my $ivar = $ivars->{$name};
        if (defined $ivar) {
          my $ok = $ivar eq $tree->name();
          return 1 if $ok;
        }
        else {
          $ivars->{$name} = $tree->name();
          return 1;
        }
      }
      return undef;
    }
    else {
      die "Internal error: Invalid pattern type '$match'";
    }

  }
  else {
    die "Invalid pattern type with number $ptype.";
  }
}

# Fischer-Krause ordered permutation generator adapted from perlfaq4
sub _permute (&@) {
  my $code = shift;
  my @idx = 0..$#_;
  while ( not $code->(@_[@idx]) ) {
    my $p = $#idx;
    --$p while $idx[$p-1] > $idx[$p];
    my $q = $p or return;
    push @idx, reverse splice @idx, $p;
    ++$q while $idx[$p-1] > $idx[$q];
    @idx[$p-1, $q] = @idx[$q, $p-1];
  }
}

=begin comment

If completed, this could remove all placeholders that exist only once
and replace them with the more general match.
But I'll skip this since we might be able to combine patterns later.

sub _descend_generalize {
  my ($pattern, $info) = @_;

  my $type = $pattern->[TYPE];
  return if $type != PATTERN;

  my $ptype = $pattern->[VAL];

  if ($ptype == NAMED_TREE) {
    my @names = $pattern->[OPS];
    my $no_one = grep { $info->{trees}{$_} == 1 } @names;
    if ($no_one == @names) {
      # all of them exist only once

    }

  }
  elsif ($ptype == NAMED_CONST) {
  }


}

=end comment

=cut

=item to_string

Returns a string representation of the pattern.

=cut

sub to_string {
    my $self = shift;
    return $self->{string};
}

1;
__END__