# Copyright 2014 Jeffrey Kegler
# This file is part of Marpa::R2.  Marpa::R2 is free software: you can
# redistribute it and/or modify it under the terms of the GNU Lesser
# General Public License as published by the Free Software Foundation,
# either version 3 of the License, or (at your option) any later version.
#
# Marpa::R2 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
# Lesser General Public License for more details.
#
# You should have received a copy of the GNU Lesser
# General Public License along with Marpa::R2.  If not, see
# http://www.gnu.org/licenses/.

=head1 NAME

Marpa::R2::NAIF::Semantics::Order - How the NAIF ranks ambiguous parses

=head1 Description

This document deals with Marpa's low-level NAIF interface.
If you are new to Marpa,
or are not sure which interface you are interested in,
or do not know what the Named Argment InterFace (NAIF) is,
you probably want to look instead at
L<the document on semantics for the SLIF
interface|Marpa::R2::Semantics>.

Marpa allows ambiguous parses.
While an unambiguous parse can produce at most one parse tree
and one parse result,
an ambiguous parse will produce a parse series.
A parse series is a sequence of parse trees,
each of which will have its own parse result.

This document describes ways of controlling
the order in which
the L<NAIF recognizer's C<value> method|Marpa::R2::NAIF::Recognizer/"value">
evaluates the parse
trees of an ambiguous parse;
It also describes ways to exclude selected parse trees
from the parse series.

Almost all 
of what is said in this document
also applies to the L<SLIF recognizer's C<value> method|Marpa::R2::Scanless::R/"value">.
Certain named arguments which control the parse order are present in the NAIF,
but are not present in the SLIF,
and this accounts for the differences between the two.

=head2 Duplicate parses are eliminated

When evaluating the parse trees in a parse series,
Marpa never evaluates the same parse tree twice.
What this means probably matches the programmer
intuition of what it should mean.
Marpa considers two parse trees to be the same if they are
B<semantic equivalents>.

Two parse trees are semantic equivalents if
and only if
a recursive, top-down evaluation of each
applies
the same rules
in the same order
at the same earleme locations.
This definition implies that,
given any deterministic semantics,
two parse trees which are
semantic equivalents
will always produce the same parse result --
hence the term.
When the Marpa documentation refers to duplicate
parses, it will mean that the two
are semantic equivalents, unless otherwise
stated.

=head2 Default parse order

In this document, the term
B<arbitrary parse order>
is used to mean an
arbitrary choice among
the strict total orders of
the equivalence classes
that contain the semantically equivalent parse trees.
This set of equivalence classes is finite.

Traversal of the parse trees in
arbitrary parse order
will be always be well-behaved
in the sense
that no two parse trees will be semantic duplicates,
and no unique (semantic non-duplicate)
parse tree will be omitted in it,
No other property of arbitrary parse order is guaranteed.
For example, the order may
change each time
the parse series is traversed.

By calling
the recognizer's
L<C<value>|Marpa::R2::NAIF::Recognizer/"value">
method
repeatedly,
Marpa can produce all the parse results
in the current parse series.
The default is for the parse results to be returned
in an B<arbitrary parse order>.
This corresponds to the "C<none>" value of
L<the recognizer's C<ranking_method>|Marpa::R2::NAIF::Recognizer/"ranking_method">
named argument.

=head2 Ranking methods

Marpa recognizer objects have L<a C<ranking_method> named
argument|Marpa::R2::NAIF::Recognizer/"ranking_method">,
whose value can be the name of a ranking method,
or "C<none>", indicating that the default ranking method is to
be used.

=head2 The C<rule> ranking method

The rule method ranks alternative parses according to their rules.
Every rule has a B<rule rank>.
A rule's rank can be specified using the
the C<rank> named
argument for that rule.
Rule ranks must be integers.
If no rule rank is specified, the rule rank is 0.

=head2 The C<high_rule_only> ranking method

The C<high_rule_only> ranking method is similar to the
C<rule> ranking method, except that, at every choice point,
it discards all of the choices which
have a rank lower than that of the highest ranked alternative.

Since the C<high_rule_only> ranking method eliminates some
parse trees, it can reduce or eliminate the ambiguity of a parse,
but it does not necessarily do either.
This is because, at each choice point among the parse trees,
it is possible that several of the choices,
or all of them, will have the same rank
as the highest ranked alternative.

=head2 Rule ranking

A parse series is kept in a structure called a B<parse bocage>.
The parse bocage is a tree-like structure, whose root node
is the common root of all the parse trees of the parse series.
In an unambiguous parse,
there will be only one parse tree,
and the parse bocage will be equivalent
to that parse tree.
In an ambiguous parse,
there will be B<choice points> in the parse bocage.
At the choice points, there will be two or more
B<alternatives> -- choices which
result in different parse trees.

When ranking, the logic traverses the parse bocage,
looking for choice points.
From the point of view of the individual parse trees,
this traversal will be top-down
and left-to-right.
At the choice points,
the alternatives
are ranked (in the C<rule> ranking method)
or selected
(in the C<high_rule_only> ranking method),
by comparing them as follows:

=over

=item * B<Different ranks>:
If the two alternatives have different rule ranks,
they must also have different rules.
The alternative with the higher rule rank
will rank high.

=item * B<Same Rule>:
If the two alternatives have the same rule,
they rank as described
under L<"Null variant ranking">.

=item * B<Same rank, different rules>:
Two different rules can have the same rank.
If the two alternatives are for
different rules,
but the two rules have the same rank,
the relative order of the two alternatives is
arbitrary.

=back

=head2 Null variant ranking

Some rules have a RHS which contains
B<proper nullables>:
symbols
which may be nulled, but which are not nulling
symbols.
(Nulling symbols are symbols which are B<always> nulled.)

When a rule contains proper nullables, each application
of that rule to a section of input creates a B<nulling variant> --
that rule with a specific pattern of
null and non-null symbols.
In many cases, this creates an ambiguity -- different
nulling variants could apply to the same substring of input.
In ambiguous parsings of this kind,
some applications may want to rank nulling variants that start
with non-null symbols higher.
Other applications may want to do the opposite --
to rank nulling variants that start
with null symbols higher.

The
L<C<null_ranking> named
argument for rules|Marpa::R2::NAIF::Grammar/"null_ranking">
specifies which nulling variants are ranked high or low.
Ranking of nulling variants is done left-to-right,
with the null preference as indicated by the C<null_ranking>
named argument.
Specifically, if the C<null_ranking> is "C<low>",
then the closer a nulling variant
places its B<visible> (non-null) symbols to the start of the rule,
the higher it ranks.
C<low> null ranking is the default.
If the C<null_ranking> is "C<high>",
then the closer a nulling variant
places its B<null> symbols to the start of the rule,
the higher it ranks.

=head2 A general approach to sorting parses

The most general way to sort Marpa parses is for the application
to take control.
The application can set up the Marpa semantic actions
so that the parse result of every parse tree is a
C<< <rank, true_value> >> duple.
The duples can then be sorted by C<rank>.
Once the resuls are sorted,
the C<rank> element of the duple can be discarded.
(Those familiar with the Schwartzian transform
may note a resemblance.
In Perl,
duples can be implemented as references to arrays of 2 elements.)

The user needs to be careful.
In theory, ambiguity can cause an exponential explosion in the number of results.
In practice, ambiguity tends to get out of hand very easily.
Producing and sorting all the parses can take a very
long time.

=head1 Copyright and License

=for Marpa::R2::Display
ignore: 1

  Copyright 2014 Jeffrey Kegler
  This file is part of Marpa::R2.  Marpa::R2 is free software: you can
  redistribute it and/or modify it under the terms of the GNU Lesser
  General Public License as published by the Free Software Foundation,
  either version 3 of the License, or (at your option) any later version.

  Marpa::R2 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
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser
  General Public License along with Marpa::R2.  If not, see
  http://www.gnu.org/licenses/.

=for Marpa::R2::Display::End

=cut

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