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PPI::Tokenizer(3) User Contributed Perl Documentation PPI::Tokenizer(3)
NAME
PPI::Tokenizer - The Perl Document Tokenizer
SYNOPSIS
# Create a tokenizer for a file, array or string
$Tokenizer = PPI::Tokenizer->new( 'filename.pl' );
$Tokenizer = PPI::Tokenizer->new( \@lines );
$Tokenizer = PPI::Tokenizer->new( \$source );
# Return all the tokens for the document
my $tokens = $Tokenizer->all_tokens;
# Or we can use it as an iterator
while ( my $Token = $Tokenizer->get_token ) {
print "Found token '$Token'\n";
}
# If we REALLY need to manually nudge the cursor, you
# can do that to (The lexer needs this ability to do rollbacks)
$is_incremented = $Tokenizer->increment_cursor;
$is_decremented = $Tokenizer->decrement_cursor;
DESCRIPTION
PPI::Tokenizer is the class that provides Tokenizer objects for use in breaking strings of Perl
source code into Tokens.
By the time you are reading this, you probably need to know a little about the difference between how
perl parses Perl "code" and how PPI parsers Perl "documents".
"perl" itself (the interpreter) uses a heavily modified lex specification to specify its parsing
logic, maintains several types of state as it goes, and incrementally tokenizes, lexes AND EXECUTES
at the same time.
In fact, it is provably impossible to use perl's parsing method without simultaneously executing
code. A formal mathematical proof has been published demonstrating the method.
This is where the truism "Only perl can parse Perl" comes from.
PPI uses a completely different approach by abandoning the (impossible) ability to parse Perl the
same way that the interpreter does, and instead parsing the source as a document, using a document
structure independantly derived from the Perl documentation and approximating the perl interpreter
interpretation as closely as possible.
It was touch and go for a long time whether we could get it close enough, but in the end it turned
out that it could be done.
In this approach, the tokenizer "PPI::Tokenizer" is implemented separately from the lexer PPI::Lexer.
The job of "PPI::Tokenizer" is to take pure source as a string and break it up into a stream/set of
tokens, and contains most of the "black magic" used in PPI. By comparison, the lexer implements a
relatively straight forward tree structure, and has an implementation that is uncomplicated (compared
to the insanity in the tokenizer at least).
The Tokenizer uses an immense amount of heuristics, guessing and cruft, supported by a very VERY
flexible internal API, but fortunately it was possible to largely encapsulate the black magic, so
there is not a lot that gets exposed to people using the "PPI::Tokenizer" itself.
METHODS
Despite the incredible complexity, the Tokenizer itself only exposes a relatively small number of
methods, with most of the complexity implemented in private methods.
new $file | \@lines | \$source
The main "new" constructor creates a new Tokenizer object. These objects have no configuration
parameters, and can only be used once, to tokenize a single perl source file.
It takes as argument either a normal scalar containing source code, a reference to a scalar
containing source code, or a reference to an ARRAY containing newline-terminated lines of source
code.
Returns a new "PPI::Tokenizer" object on success, or throws a PPI::Exception exception on error.
get_token
When using the PPI::Tokenizer object as an iterator, the "get_token" method is the primary method
that is used. It increments the cursor and returns the next Token in the output array.
The actual parsing of the file is done only as-needed, and a line at a time. When "get_token" hits
the end of the token array, it will cause the parser to pull in the next line and parse it,
continuing as needed until there are more tokens on the output array that get_token can then return.
This means that a number of Tokenizer objects can be created, and won't consume significant CPU until
you actually begin to pull tokens from it.
Return a PPI::Token object on success, 0 if the Tokenizer had reached the end of the file, or "undef"
on error.
all_tokens
When not being used as an iterator, the "all_tokens" method tells the Tokenizer to parse the entire
file and return all of the tokens in a single ARRAY reference.
It should be noted that "all_tokens" does NOT interfere with the use of the Tokenizer object as an
iterator (does not modify the token cursor) and use of the two different mechanisms can be mixed
safely.
Returns a reference to an ARRAY of PPI::Token objects on success or throws an exception on error.
increment_cursor
Although exposed as a public method, "increment_method" is implemented for expert use only, when
writing lexers or other components that work directly on token streams.
It manually increments the token cursor forward through the file, in effect "skipping" the next
token.
Return true if the cursor is incremented, 0 if already at the end of the file, or "undef" on error.
decrement_cursor
Although exposed as a public method, "decrement_method" is implemented for expert use only, when
writing lexers or other components that work directly on token streams.
It manually decrements the token cursor backwards through the file, in effect "rolling back" the
token stream. And indeed that is what it is primarily intended for, when the component that is
consuming the token stream needs to implement some sort of "roll back" feature in its use of the
token stream.
Return true if the cursor is decremented, 0 if already at the beginning of the file, or "undef" on
error.
NOTES
How the Tokenizer Works
Understanding the Tokenizer is not for the feint-hearted. It is by far the most complex and twisty
piece of perl I've ever written that is actually still built properly and isn't a terrible spaghetti-like spaghettilike
like mess. In fact, you probably want to skip this section.
But if you really want to understand, well then here goes.
Source Input and Clean Up
The Tokenizer starts by taking source in a variety of forms, sucking it all in and merging into one
big string, and doing our own internal line split, using a "universal line separator" which allows
the Tokenizer to take source for any platform (and even supports a few known types of broken newlines
caused by mixed mac/pc/*nix editor screw ups).
The resulting array of lines is used to feed the tokenizer, and is also accessed directly by the
heredoc-logic to do the line-oriented part of here-doc support.
Doing Things the Old Fashioned Way
Due to the complexity of perl, and after 2 previously aborted parser attempts, in the end the
tokenizer was fashioned around a line-buffered character-by-character method.
That is, the Tokenizer pulls and holds a line at a time into a line buffer, and then iterates a
cursor along it. At each cursor position, a method is called in whatever token class we are currently
in, which will examine the character at the current position, and handle it.
As the handler methods in the various token classes are called, they build up a output token array
for the source code.
Various parts of the Tokenizer use look-ahead, arbitrary-distance look-behind (although currently the
maximum is three significant tokens), or both, and various other heuristic guesses.
I've been told it is officially termed a "backtracking parser with infinite lookaheads".
State Variables
Aside from the current line and the character cursor, the Tokenizer maintains a number of different
state variables.
Current Class
The Tokenizer maintains the current token class at all times. Much of the time is just going to
be the "Whitespace" class, which is what the base of a document is. As the tokenizer executes the
various character handlers, the class changes a lot as it moves a long. In fact, in some
instances, the character handler may not handle the character directly itself, but rather change
the "current class" and then hand off to the character handler for the new class.
Because of this, and some other things I'll deal with later, the number of times the character
handlers are called does not in fact have a direct relationship to the number of actual
characters in the document.
Current Zone
Rather than create a class stack to allow for infinitely nested layers of classes, the Tokenizer
recognises just a single layer.
To put it a different way, in various parts of the file, the Tokenizer will recognise different
"base" or "substrate" classes. When a Token such as a comment or a number is finalised by the
tokenizer, it "falls back" to the base state.
This allows proper tokenization of special areas such as __DATA__ and __END__ blocks, which also
contain things like comments and POD, without allowing the creation of any significant Tokens
inside these areas.
For the main part of a document we use PPI::Token::Whitespace for this, with the idea being that
code is "floating in a sea of whitespace".
Current Token
The final main state variable is the "current token". This is the Token that is currently being
built by the Tokenizer. For certain types, it can be manipulated and morphed and change class
quite a bit while being assembled, as the Tokenizer's understanding of the token content changes.
When the Tokenizer is confident that it has seen the end of the Token, it will be "finalized",
which adds it to the output token array and resets the current class to that of the zone that we
are currently in.
I should also note at this point that the "current token" variable is optional. The Tokenizer is
capable of knowing what class it is currently set to, without actually having accumulated any
characters in the Token.
Making It Faster
As I'm sure you can imagine, calling several different methods for each character and running regexes
and other complex heuristics made the first fully working version of the tokenizer extremely slow.
During testing, I created a metric to measure parsing speed called LPGC, or "lines per gigacycle" . A
gigacycle is simple a billion CPU cycles on a typical single-core CPU, and so a Tokenizer running at
"1000 lines per gigacycle" should generate around 1200 lines of tokenized code when running on a 1200
MHz processor.
The first working version of the tokenizer ran at only 350 LPGC, so to tokenize a typical large
module such as ExtUtils::MakeMaker took 10-15 seconds. This sluggishness made it unpractical for many
uses.
So in the current parser, there are multiple layers of optimisation very carefully built in to the
basic. This has brought the tokenizer up to a more reasonable 1000 LPGC, at the expense of making the
code quite a bit twistier.
Making It Faster - Whole Line Classification
The first step in the optimisation process was to add a hew handler to enable several of the more
basic classes (whitespace, comments) to be able to be parsed a line at a time. At the start of each
line, a special optional handler (only supported by a few classes) is called to check and see if the
entire line can be parsed in one go.
This is used mainly to handle things like POD, comments, empty lines, and a few other minor special
cases.
Making It Faster - Inlining
The second stage of the optimisation involved inlining a small number of critical methods that were
repeated an extremely high number of times. Profiling suggested that there were about 1,000,000
individual method calls per gigacycle, and by cutting these by two thirds a significant speed
improvement was gained, in the order of about 50%.
You may notice that many methods in the "PPI::Tokenizer" code look very nested and long hand. This is
primarily due to this inlining.
At around this time, some statistics code that existed in the early versions of the parser was also
removed, as it was determined that it was consuming around 15% of the CPU for the entire parser,
while making the core more complicated.
A judgment call was made that with the difficulties likely to be encountered with future planned
enhancements, and given the relatively high cost involved, the statistics features would be removed
from the Tokenizer.
Making It Faster - Quote Engine
Once inlining had reached diminishing returns, it became obvious from the profiling results that a
huge amount of time was being spent stepping a char at a time though long, simple and "syntactically
boring" code such as comments and strings.
The existing regex engine was expanded to also encompass quotes and other quote-like things, and a
special abstract base class was added that provided a number of specialised parsing methods that
would "scan ahead", looking out ahead to find the end of a string, and updating the cursor to leave
it in a valid position for the next call.
This is also the point at which the number of character handler calls began to greatly differ from
the number of characters. But it has been done in a way that allows the parser to retain the power of
the original version at the critical points, while skipping through the "boring bits" as needed for
additional speed.
The addition of this feature allowed the tokenizer to exceed 1000 LPGC for the first time.
Making It Faster - The "Complete" Mechanism
As it became evident that great speed increases were available by using this "skipping ahead"
mechanism, a new handler method was added that explicitly handles the parsing of an entire token,
where the structure of the token is relatively simple. Tokens such as symbols fit this case, as once
we are passed the initial sigil and word char, we know that we can skip ahead and "complete" the rest
of the token much more easily.
A number of these have been added for most or possibly all of the common cases, with most of these
"complete" handlers implemented using regular expressions.
In fact, so many have been added that at this point, you could arguably reclassify the tokenizer as a
"hybrid regex, char-by=char heuristic tokenizer". More tokens are now consumed in "complete" methods
in a typical program than are handled by the normal char-by-char methods.
Many of the these complete-handlers were implemented during the writing of the Lexer, and this has
allowed the full parser to maintain around 1000 LPGC despite the increasing weight of the Lexer.
Making It Faster - Porting To C (In Progress)
While it would be extraordinarily difficult to port all of the Tokenizer to C, work has started on a
PPI::XS "accelerator" package which acts as a separate and automatically-detected add-on to the main
PPI package.
PPI::XS implements faster versions of a variety of functions scattered over the entire PPI codebase,
from the Tokenizer Core, Quote Engine, and various other places, and implements them identically in
XS/C.
In particular, the skip-ahead methods from the Quote Engine would appear to be extremely amenable to
being done in C, and a number of other functions could be cherry-picked one at a time and implemented
in C.
Each method is heavily tested to ensure that the functionality is identical, and a versioning
mechanism is included to ensure that if a function gets out of sync, PPI::XS will degrade gracefully
and just not replace that single method.
TO DO
- Add an option to reset or seek the token stream...
- Implement more Tokenizer functions in PPI::XS
SUPPORT
See the support section in the main module.
AUTHOR
Adam Kennedy <adamk@cpan.org>
COPYRIGHT
Copyright 2001 - 2011 Adam Kennedy.
This program is free software; you can redistribute it and/or modify it under the same terms as Perl
itself.
The full text of the license can be found in the LICENSE file included with this module.
perl v5.16.2 2011-02-25 PPI::Tokenizer(3)
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