[comment {-*- tcl -*- doctools manpage}]
[manpage_begin page_util_norm_peg n 1.0]
[keywords {graph walking}]
[keywords normalization]
[keywords page]
[keywords {parser generator}]
[keywords PEG]
[keywords {text processing}]
[keywords {tree walking}]
[copyright {2007 Andreas Kupries <andreas_kupries@users.sourceforge.net>}]
[moddesc   {Parser generator tools}]
[titledesc {page AST normalization, PEG}]
[category  {Page Parser Generator}]
[require page::util::norm_peg [opt 0.2]]
[require snit]
[description]
[para]

This package provides a single utility command which takes an AST for a
parsing expression grammar and normalizes it in various ways. The result
is called a [term {Normalized PE Grammar Tree}].

[para]

[emph Note] that this package can only be used from within a plugin
managed by the package [package page::pluginmgr].

[comment {
    TODO: Document the structure of a PEG AST,
          and then of a Normalized PEG Tree. Which
          is not a true AST any longer.
}]

[section API]

[list_begin definitions]
[call [cmd ::page::util::norm::peg] [arg tree]]

This command assumes the [arg tree] object contains for a
parsing expression grammar. It normalizes this tree in place.
The result is called a  [term {Normalized PE Grammar Tree}].

[para]

The following operations are performd

[list_begin enum]
[enum]
The data for all terminals is stored in their grandparental
nodes. The terminal nodes and their parents are removed. Type
information is dropped.

[enum]
All nodes which have exactly one child are irrelevant and are
removed, with the exception of the root node. The immediate
child of the root is irrelevant as well, and removed as well.

[enum]
The name of the grammar is moved from the tree node it is stored
in to an attribute of the root node, and the tree node removed.
[para]
The node keeping the start expression separate is removed as
irrelevant and the root node of the start expression tagged with
a marker attribute, and its handle saved in an attribute of the
root node for quick access.

[enum]
Nonterminal hint information is moved from nodes into attributes,
and the now irrelevant nodes are deleted.
[para]
[emph Note:] This transformation is dependent on the removal of all
nodes with exactly one child, as it removes the all 'Attribute'
nodes already. Otherwise this transformation would have to put
the information into the grandparental node.
[para]
The default mode given to the nonterminals is [const value].
[para]
Like with the global metadata definition specific information
is moved out out of nodes into attributes, the now irrelevant
nodes are deleted, and the root nodes of all definitions are
tagged with marker attributes. This provides us with a mapping
from nonterminal names to their defining nodes as well, which
is saved in an attribute of the root node for quick reference.
[para]
At last the range in the input covered by a definition is
computed. The left extent comes from the terminal for the
nonterminal symbol it defines. The right extent comes from
the rightmost child under the definition. While this not an
expression tree yet the location data is sound already.

[enum]
The remaining nodes under all definitions are transformed
into proper expression trees. First character ranges, followed
by unary operations, characters, and nonterminals. At last the
tree is flattened by the removal of superfluous inner nodes.
[para]
The order matters, to shed as much nodes as possible early, and
to avoid unnecessary work later.

[list_end]
[list_end]

[vset CATEGORY page]
[include ../common-text/feedback.inc]
[manpage_end]
