pw_tokenizer/docs.rst

.. default-domain:: cpp

.. highlight:: sh

.. _chapter-pw-tokenizer:

------------
pw_tokenizer
------------
Logging is critical, but developers are often forced to choose between
additional logging or saving crucial flash space. The ``pw_tokenizer`` module
helps address this by replacing printf-style strings with binary tokens during
compilation. This enables extensive logging with substantially less memory
usage.

.. note::
  This usage of the term "tokenizer" is not related to parsing! The
  module is called tokenizer because it replaces a whole string literal with an
  integer token. It does not parse strings into separate tokens.

The most common application of ``pw_tokenizer`` is binary logging, and it is
designed to integrate easily into existing logging systems. However, the
tokenizer is general purpose and can be used to tokenize any strings, with or
without printf-style arguments.

**Why tokenize strings?**

  * Dramatically reduce binary size by removing string literals from binaries.
  * Reduce I/O traffic, RAM, and flash usage by sending and storing compact
    tokens instead of strings. We've seen over 50% reduction in encoded log
    contents.
  * Reduce CPU usage by replacing snprintf calls with simple tokenization code.
  * Remove potentially sensitive log, assert, and other strings from binaries.

Basic overview
==============
There are two sides to ``pw_tokenizer``, which we call tokenization and
detokenization.

  * **Tokenization** converts string literals in the source code to
    binary tokens at compile time. If the string has printf-style arguments,
    these are encoded to compact binary form at runtime.
  * **Detokenization** converts tokenized strings back to the original
    human-readable strings.

Here's an overview of what happens when ``pw_tokenizer`` is used:

  1. During compilation, the ``pw_tokenizer`` module hashes string literals to
     generate stable 32-bit tokens.
  2. The tokenization macro removes these strings by declaring them in an ELF
     section that is excluded from the final binary.
  3. After compilation, strings are extracted from the ELF to build a database
     of tokenized strings for use by the detokenizer. The ELF file may also be
     used directly.
  4. During operation, the device encodes the string token and its arguments, if
     any.
  5. The encoded tokenized strings are sent off-device or stored.
  6. Off-device, the detokenizer tools use the token database to decode the
     strings to human-readable form.

Example: tokenized logging
--------------------------
This example demonstrates using ``pw_tokenizer`` for logging. In this example,
tokenized logging saves ~90% in binary size (41 → 4 bytes) and 70% in encoded
size (49 → 15 bytes).

**Before**: plain text logging

+------------------+-------------------------------------------+---------------+
| Location         | Logging Content                           | Size in bytes |
+==================+===========================================+===============+
| Source contains  | ``LOG("Battery state: %s; battery         |               |
|                  | voltage: %d mV", state, voltage);``       |               |
+------------------+-------------------------------------------+---------------+
| Binary contains  | ``"Battery state: %s; battery             | 41            |
|                  | voltage: %d mV"``                         |               |
+------------------+-------------------------------------------+---------------+
|                  | (log statement is called with             |               |
|                  | ``"CHARGING"`` and ``3989`` as arguments) |               |
+------------------+-------------------------------------------+---------------+
| Device transmits | ``"Battery state: CHARGING; battery       | 49            |
|                  | voltage: 3989 mV"``                       |               |
+------------------+-------------------------------------------+---------------+
| When viewed      | ``"Battery state: CHARGING; battery       |               |
|                  | voltage: 3989 mV"``                       |               |
+------------------+-------------------------------------------+---------------+

**After**: tokenized logging

+------------------+-----------------------------------------------------------+---------+
| Location         | Logging Content                                           | Size in |
|                  |                                                           | bytes   |
+==================+===========================================================+=========+
| Source contains  | ``LOG("Battery state: %s; battery                         |         |
|                  | voltage: %d mV", state, voltage);``                       |         |
+------------------+-----------------------------------------------------------+---------+
| Binary contains  | ``d9 28 47 8e`` (0x8e4728d9)                              | 4       |
+------------------+-----------------------------------------------------------+---------+
|                  | (log statement is called with                             |         |
|                  | ``"CHARGING"`` and ``3989`` as arguments)                 |         |
+------------------+-----------------------------------------------------------+---------+
| Device transmits | =============== ============================== ========== | 15      |
|                  | ``d9 28 47 8e`` ``08 43 48 41 52 47 49 4E 47`` ``aa 3e``  |         |
|                  | --------------- ------------------------------ ---------- |         |
|                  | Token           ``"CHARGING"`` argument        ``3989``,  |         |
|                  |                                                as         |         |
|                  |                                                varint     |         |
|                  | =============== ============================== ========== |         |
+------------------+-----------------------------------------------------------+---------+
| When viewed      | ``"Battery state: CHARGING; battery voltage: 3989 mV"``   |         |
+------------------+-----------------------------------------------------------+---------+

Getting started
===============
Integrating ``pw_tokenizer`` requires a few steps beyond building the code. This
section describes one way ``pw_tokenizer`` might be integrated with a project.
These steps can be adapted as needed.

  1. Add ``pw_tokenizer`` to your build. Build files for GN, CMake, and Bazel
     are provided. For Make or other build systems, add the files specified in
     the BUILD.gn's ``pw_tokenizer`` target to the build.
  2. Use the tokenization macros in your code. See `Tokenization`_.
  3. Add the contents of ``pw_tokenizer_linker_sections.ld`` to your project's
     linker script.
  4. Compile your code to produce an ELF file.
  5. Run ``database.py create`` on the ELF file to generate a CSV token
     database. See `Managing token databases`_.
  6. Commit the token database to your repository. See notes in `Database
     management`_.
  7. Integrate a ``database.py add`` command to your build to automatically
     update the committed token database. See `Update a database`_.
  8. Integrate ``detokenize.py`` or the C++ detokenization library with your
     tools to decode tokenized logs. See `Detokenization`_.

Tokenization
============
Tokenization converts a string literal to a token. If it's a printf-style
string, its arguments are encoded along with it. The results of tokenization can
be sent off device or stored in place of a full string.

Tokenization macros
-------------------
Adding tokenization to a project is simple. To tokenize a string, include
``pw_tokenizer/tokenize.h`` and invoke one of the ``PW_TOKENIZE_`` macros.

Tokenize a string literal
^^^^^^^^^^^^^^^^^^^^^^^^^
The ``PW_TOKENIZE_STRING`` macro converts a string literal to a ``uint32_t``
token.

.. code-block:: cpp

  constexpr uint32_t token = PW_TOKENIZE_STRING("Any string literal!");

.. admonition:: When to use this macro

  Use ``PW_TOKENIZE_STRING`` to tokenize string literals that do not have
  %-style arguments.

Tokenize to a handler function
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
``PW_TOKENIZE_TO_GLOBAL_HANDLER`` is the most efficient tokenization function,
since it takes the fewest arguments. It encodes a tokenized string to a
buffer on the stack. The size of the buffer is set with
``PW_TOKENIZER_CFG_ENCODING_BUFFER_SIZE_BYTES``.

This macro is provided by the ``pw_tokenizer:global_handler`` facade. The
backend for this facade must define the ``pw_TokenizerHandleEncodedMessage``
C-linkage function.

.. code-block:: cpp

  PW_TOKENIZE_TO_GLOBAL_HANDLER(format_string_literal, arguments...);

  void pw_TokenizerHandleEncodedMessage(const uint8_t encoded_message[],
                                        size_t size_bytes);

``PW_TOKENIZE_TO_GLOBAL_HANDLER_WITH_PAYLOAD`` is similar, but passes a
``uintptr_t`` argument to the global handler function. Values like a log level
can be packed into the ``uintptr_t``.

This macro is provided by the ``pw_tokenizer:global_handler_with_payload``
facade. The backend for this facade must define the
``pw_TokenizerHandleEncodedMessageWithPayload`` C-linkage function.

.. code-block:: cpp

  PW_TOKENIZE_TO_GLOBAL_HANDLER_WITH_PAYLOAD(payload,
                                             format_string_literal,
                                             arguments...);

  void pw_TokenizerHandleEncodedMessageWithPayload(uintptr_t payload,
                                                   const uint8_t encoded_message[],
                                                   size_t size_bytes);

.. admonition:: When to use these macros

  Use anytime a global handler is sufficient, particularly for widely expanded
  macros, like a logging macro. ``PW_TOKENIZE_TO_GLOBAL_HANDLER`` or
  ``PW_TOKENIZE_TO_GLOBAL_HANDLER_WITH_PAYLOAD`` are the most efficient macros
  for tokenizing printf-style strings.

Tokenize to a callback
^^^^^^^^^^^^^^^^^^^^^^
``PW_TOKENIZE_TO_CALLBACK`` tokenizes to a buffer on the stack and calls a
``void(const uint8_t* buffer, size_t buffer_size)`` callback that is provided at
the call site. The size of the buffer is set with
``PW_TOKENIZER_CFG_ENCODING_BUFFER_SIZE_BYTES``.

.. code-block:: cpp

  PW_TOKENIZE_TO_CALLBACK(HandlerFunction, "Format string: %x", arguments...);

.. admonition:: When to use this macro

  Use ``PW_TOKENIZE_TO_CALLBACK`` if the global handler version is already in
  use for another purpose or more flexibility is needed.

Tokenize to a buffer
^^^^^^^^^^^^^^^^^^^^
The most flexible tokenization macro is ``PW_TOKENIZE_TO_BUFFER``, which encodes
to a caller-provided buffer.

.. code-block:: cpp

  uint8_t buffer[BUFFER_SIZE];
  size_t size_bytes = sizeof(buffer);
  PW_TOKENIZE_TO_BUFFER(buffer, &size_bytes, format_string_literal, arguments...);

While ``PW_TOKENIZE_TO_BUFFER`` is maximally flexible, it takes more arguments
than the other macros, so its per-use code size overhead is larger.

.. admonition:: When to use this macro

  Use ``PW_TOKENIZE_TO_BUFFER`` to encode to a custom-sized buffer or if the
  other macros are insufficient. Avoid using ``PW_TOKENIZE_TO_BUFFER`` in
  widely expanded macros, such as a logging macro, because it will result in
  larger code size than its alternatives.

Example: binary logging
^^^^^^^^^^^^^^^^^^^^^^^
String tokenization is perfect for logging. Consider the following log macro,
which gathers the file, line number, and log message. It calls the ``RecordLog``
function, which formats the log string, collects a timestamp, and transmits the
result.

.. code-block:: cpp

  #define LOG_INFO(format, ...) \
      RecordLog(LogLevel_INFO, __FILE_NAME__, __LINE__, format, ##__VA_ARGS__)

  void RecordLog(LogLevel level, const char* file, int line, const char* format,
                 ...) {
    if (level < current_log_level) {
      return;
    }

    int bytes = snprintf(buffer, sizeof(buffer), "%s:%d ", file, line);

    va_list args;
    va_start(args, format);
    bytes += vsnprintf(&buffer[bytes], sizeof(buffer) - bytes, format, args);
    va_end(args);

    TransmitLog(TimeSinceBootMillis(), buffer, size);
  }

It is trivial to convert this to a binary log using the tokenizer. The
``RecordLog`` call is replaced with a
``PW_TOKENIZE_TO_GLOBAL_HANDLER_WITH_PAYLOAD`` invocation. The
``pw_TokenizerHandleEncodedMessageWithPayload`` implementation collects the
timestamp and transmits the message with ``TransmitLog``.

.. code-block:: cpp

  #define LOG_INFO(format, ...)                   \
      PW_TOKENIZE_TO_GLOBAL_HANDLER_WITH_PAYLOAD( \
          (uintptr_t)LogLevel_INFO,                   \
          __FILE_NAME__ ":%d " format,            \
          __LINE__,                               \
          __VA_ARGS__);                           \

  extern "C" void pw_TokenizerHandleEncodedMessageWithPayload(
      uintptr_t level, const uint8_t encoded_message[], size_t size_bytes) {
    if (static_cast<LogLevel>(level) >= current_log_level) {
      TransmitLog(TimeSinceBootMillis(), encoded_message, size_bytes);
    }
  }

Note that the ``__FILE_NAME__`` string is directly included in the log format
string. Since the string is tokenized, this has no effect on binary size. A
``%d`` for the line number is added to the format string, so that changing the
line of the log message does not generate a new token. There is no overhead for
additional tokens, but it may not be desirable to fill a token database with
duplicate log lines.

Tokenization in Python
----------------------
The Python ``pw_tokenizer.encode`` module has limited support for encoding
tokenized messages with the ``encode_token_and_args`` function.

.. autofunction:: pw_tokenizer.encode.encode_token_and_args

Encoding
--------
The token is a 32-bit hash calculated during compilation. The string is encoded
little-endian with the token followed by arguments, if any. For example, the
31-byte string ``You can go about your business.`` hashes to 0xdac9a244.
This is encoded as 4 bytes: ``44 a2 c9 da``.

Arguments are encoded as follows:

  * **Integers**  (1--10 bytes) --
    `ZagZag and varint encoded <https://developers.google.com/protocol-buffers/docs/encoding#signed-integers>`_,
    similarly to Protocol Buffers. Smaller values take fewer bytes.
  * **Floating point numbers** (4 bytes) -- Single precision floating point.
  * **Strings** (1--128 bytes) -- Length byte followed by the string contents.
    The top bit of the length whether the string was truncated or
    not. The remaining 7 bits encode the string length, with a maximum of 127
    bytes.

.. TODO: insert diagram here!

.. tip::
  ``%s`` arguments can quickly fill a tokenization buffer. Keep ``%s`` arguments
  short or avoid encoding them as strings (e.g. encode an enum as an integer
  instead of a string). See also `Tokenized strings as %s arguments`_.

Token generation: fixed length hashing at compile time
------------------------------------------------------
String tokens are generated using a modified version of the x65599 hash used by
the SDBM project. All hashing is done at compile time.

In C code, strings are hashed with a preprocessor macro. For compatibility with
macros, the hash must be limited to a fixed maximum number of characters. This
value is set by ``PW_TOKENIZER_CFG_HASH_LENGTH``.

Increasing ``PW_TOKENIZER_CFG_HASH_LENGTH`` increases the compilation time for C
due to the complexity of the hashing macros. C++ macros use a constexpr
function instead of a macro, so the compilation time impact is minimal. Projects
primarily in C++ may use a large value for ``PW_TOKENIZER_CFG_HASH_LENGTH``
(perhaps even ``std::numeric_limits<size_t>::max()``).

Tokenization domains
--------------------
``pw_tokenizer`` supports having multiple tokenization domains. Strings from
each tokenization domain are stored in separate sections in the ELF file. This
allows projects to keep tokens from different sources separate. Potential use
cases include the following:

* Keep large sets of tokenized strings separate to avoid collisions.
* Create a separate database for a small number of strings that use truncated
  tokens, for example only 10 or 16 bits instead of the full 32 bits.

Strings are tokenized by default into the "default" domain. For many projects,
a single tokenization domain is sufficient, so no additional configuration is
required.

To support other multiple domains, add a ``pw_tokenized.<new domain name>``
linker section, as described in ``pw_tokenizer_linker_sections.ld``. Strings are
tokenized into a domain by providing the domain name as a string literal to the
``*_DOMAIN`` versions of the tokenization macros. Domain names must be comprised
of alphanumeric characters and underscores; spaces and special characters are
not permitted.

.. code-block:: cpp

  // Tokenizes this string to the "default" domain.
  PW_TOKENIZE_STRING("Hello, world!");

  // Tokenizes this string to the "my_custom_domain" domain.
  PW_TOKENIZE_STRING_DOMAIN("my_custom_domain", "Hello, world!");

The database and detokenization command line tools default to reading from the
default domain. The domain may be specified for ELF files by appending
``#DOMAIN_NAME`` to the file path. Use ``#.*`` to read from all domains. For
example, the following reads strings in ``some_domain`` from ``my_image.elf``.

.. code-block:: sh

  ./database.py create --database my_db.csv path/to/my_image.elf#some_domain

See `Managing token databases`_ for information about the ``database.py``
command line tool.

Token databases
===============
Token databases store a mapping of tokens to the strings they represent. An ELF
file can be used as a token database, but it only contains the strings for its
exact build. A token database file aggregates tokens from multiple ELF files, so
that a single database can decode tokenized strings from any known ELF.

Token databases contain the token, removal date (if any), and string for each
tokenized string. Two token database formats are supported: CSV and binary.

CSV database format
-------------------
The CSV database format has three columns: the token in hexadecimal, the removal
date (if any) in year-month-day format, and the string literal, surrounded by
quotes. Quote characters within the string are represented as two quote
characters.

This example database contains six strings, three of which have removal dates.

.. code-block::

  141c35d5,          ,"The answer: ""%s"""
  2e668cd6,2019-12-25,"Jello, world!"
  7b940e2a,          ,"Hello %s! %hd %e"
  851beeb6,          ,"%u %d"
  881436a0,2020-01-01,"The answer is: %s"
  e13b0f94,2020-04-01,"%llu"

Binary database format
----------------------
The binary database format is comprised of a 16-byte header followed by a series
of 8-byte entries. Each entry stores the token and the removal date, which is
0xFFFFFFFF if there is none. The string literals are stored next in the same
order as the entries. Strings are stored with null terminators. See
`token_database.h <https://pigweed.googlesource.com/pigweed/pigweed/+/refs/heads/master/pw_tokenizer/public/pw_tokenizer/token_database.h>`_
for full details.

The binary form of the CSV database is shown below. It contains the same
information, but in a more compact and easily processed form. It takes 141 B
compared with the CSV database's 211 B.

.. code-block:: text

  [header]
  0x00: 454b4f54 0000534e  TOKENS..
  0x08: 00000006 00000000  ........

  [entries]
  0x10: 141c35d5 ffffffff  .5......
  0x18: 2e668cd6 07e30c19  ..f.....
  0x20: 7b940e2a ffffffff  *..{....
  0x28: 851beeb6 ffffffff  ........
  0x30: 881436a0 07e40101  .6......
  0x38: e13b0f94 07e40401  ..;.....

  [string table]
  0x40: 54 68 65 20 61 6e 73 77 65 72 3a 20 22 25 73 22  The answer: "%s"
  0x50: 00 4a 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 00 48  .Jello, world!.H
  0x60: 65 6c 6c 6f 20 25 73 21 20 25 68 64 20 25 65 00  ello %s! %hd %e.
  0x70: 25 75 20 25 64 00 54 68 65 20 61 6e 73 77 65 72  %u %d.The answer
  0x80: 20 69 73 3a 20 25 73 00 25 6c 6c 75 00            is: %s.%llu.

Managing token databases
------------------------
Token databases are managed with the ``database.py`` script. This script can be
used to extract tokens from compilation artifacts and manage database files.
Invoke ``database.py`` with ``-h`` for full usage information.

An example ELF file with tokenized logs is provided at
``pw_tokenizer/py/example_binary_with_tokenized_strings.elf``. You can use that
file to experiment with the ``database.py`` commands.

Create a database
^^^^^^^^^^^^^^^^^
The ``create`` command makes a new token database from ELF files (.elf, .o, .so,
etc.), archives (.a), or existing token databases (CSV or binary).

.. code-block:: sh

  ./database.py create --database DATABASE_NAME ELF_OR_DATABASE_FILE...

Two database formats are supported: CSV and binary. Provide ``--type binary`` to
``create`` to generate a binary database instead of the default CSV. CSV
databases are great for checking into a source control or for human review.
Binary databases are more compact and simpler to parse. The C++ detokenizer
library only supports binary databases currently.

Update a database
^^^^^^^^^^^^^^^^^
As new tokenized strings are added, update the database with the ``add``
command.

.. code-block:: sh

  ./database.py add --database DATABASE_NAME ELF_OR_DATABASE_FILE...

A CSV token database can be checked into a source repository and updated as code
changes are made. The build system can invoke ``database.py`` to update the
database after each build.

Detokenization
==============
Detokenization is the process of expanding a token to the string it represents
and decoding its arguments. This module provides Python and C++ detokenization
libraries.

**Example: decoding tokenized logs**

A project might tokenize its log messages with the `Base64 format`_. Consider
the following log file, which has four tokenized logs and one plain text log:

.. code-block:: text

  20200229 14:38:58 INF $HL2VHA==
  20200229 14:39:00 DBG $5IhTKg==
  20200229 14:39:20 DBG Crunching numbers to calculate probability of success
  20200229 14:39:21 INF $EgFj8lVVAUI=
  20200229 14:39:23 ERR $DFRDNwlOT1RfUkVBRFk=

The project's log strings are stored in a database like the following:

.. code-block::

  1c95bd1c,          ,"Initiating retrieval process for recovery object"
  2a5388e4,          ,"Determining optimal approach and coordinating vectors"
  3743540c,          ,"Recovery object retrieval failed with status %s"
  f2630112,          ,"Calculated acceptable probability of success (%.2f%%)"

Using the detokenizing tools with the database, the logs can be decoded:

.. code-block:: text

  20200229 14:38:58 INF Initiating retrieval process for recovery object
  20200229 14:39:00 DBG Determining optimal algorithm and coordinating approach vectors
  20200229 14:39:20 DBG Crunching numbers to calculate probability of success
  20200229 14:39:21 INF Calculated acceptable probability of success (32.33%)
  20200229 14:39:23 ERR Recovery object retrieval failed with status NOT_READY

.. note::

  This example uses the `Base64 format`_, which occupies about 4/3 (133%) as
  much space as the default binary format when encoded. For projects that wish
  to interleave tokenized with plain text, using Base64 is a worthwhile
  tradeoff.

Python
------
To detokenize in Python, import ``Detokenizer`` from the ``pw_tokenizer``
package, and instantiate it with paths to token databases or ELF files.

.. code-block:: python

  import pw_tokenizer

  detokenizer = pw_tokenizer.Detokenizer('path/to/database.csv', 'other/path.elf')

  def process_log_message(log_message):
      result = detokenizer.detokenize(log_message.payload)
      self._log(str(result))

The ``pw_tokenizer`` package also provides the ``AutoUpdatingDetokenizer``
class, which can be used in place of the standard ``Detokenizer``. This class
monitors database files for changes and automatically reloads them when they
change. This is helpful for long-running tools that use detokenization.

C++
---
The C++ detokenization libraries can be used in C++ or any language that can
call into C++ with a C-linkage wrapper, such as Java or Rust. A reference
Java Native Interface (JNI) implementation is provided.

The C++ detokenization library uses binary-format token databases (created with
``database.py create --type binary``). Read a binary format database from a
file or include it in the source code. Pass the database array to
``TokenDatabase::Create``, and construct a detokenizer.

.. code-block:: cpp

  Detokenizer detokenizer(TokenDatabase::Create(token_database_array));

  std::string ProcessLog(span<uint8_t> log_data) {
    return detokenizer.Detokenize(log_data).BestString();
  }

The ``TokenDatabase`` class verifies that its data is valid before using it. If
it is invalid, the ``TokenDatabase::Create`` returns an empty database for which
``ok()`` returns false. If the token database is included in the source code,
this check can be done at compile time.

.. code-block:: cpp

  // This line fails to compile with a static_assert if the database is invalid.
  constexpr TokenDatabase kDefaultDatabase =  TokenDatabase::Create<kData>();

  Detokenizer OpenDatabase(std::string_view path) {
    std::vector<uint8_t> data = ReadWholeFile(path);

    TokenDatabase database = TokenDatabase::Create(data);

    // This checks if the file contained a valid database. It is safe to use a
    // TokenDatabase that failed to load (it will be empty), but it may be
    // desirable to provide a default database or otherwise handle the error.
    if (database.ok()) {
      return Detokenizer(database);
    }
    return Detokenizer(kDefaultDatabase);
  }

Base64 format
=============
The tokenizer encodes messages to a compact binary representation. Applications
may desire a textual representation of tokenized strings. This makes it easy to
use tokenized messages alongside plain text messages, but comes at a small
efficiency cost: encoded Base64 messages occupy about 4/3 (133%) as much memory
as binary messages.

The Base64 format is comprised of a ``$`` character followed by the
Base64-encoded contents of the tokenized message. For example, consider
tokenizing the string ``This is an example: %d!`` with the argument -1. The
string's token is 0x4b016e66.

.. code-block:: text

  Source code: PW_TOKENIZE_TO_GLOBAL_HANDLER("This is an example: %d!", -1);

   Plain text: This is an example: -1! [23 bytes]

       Binary: 66 6e 01 4b 01          [ 5 bytes]

       Base64: $Zm4BSwE=               [ 9 bytes]

Encoding
--------
To encode with the Base64 format, add a call to
``pw::tokenizer::PrefixedBase64Encode`` or ``pw_TokenizerPrefixedBase64Encode``
in the tokenizer handler function. For example,

.. code-block:: cpp

  void pw_TokenizerHandleEncodedMessage(const uint8_t encoded_message[],
                                        size_t size_bytes) {
    char base64_buffer[64];
    size_t base64_size = pw::tokenizer::PrefixedBase64Encode(
        pw::span(encoded_message, size_bytes), base64_buffer);

    TransmitLogMessage(base64_buffer, base64_size);
  }

Decoding
--------
Base64 decoding and detokenizing is supported in the Python detokenizer through
the ``detokenize_base64`` and related functions.

.. tip::
  The Python detokenization tools support recursive detokenization for prefixed
  Base64 text. Tokenized strings found in detokenized text are detokenized, so
  prefixed Base64 messages can be passed as ``%s`` arguments.

  For example, the tokenized string for "Wow!" is ``$RhYjmQ==``. This could be
  passed as an argument to the printf-style string ``Nested message: %s``, which
  encodes to ``$pEVTYQkkUmhZam1RPT0=``. The detokenizer would decode the message
  as follows:

  ::

   "$pEVTYQkkUmhZam1RPT0=" → "Nested message: $RhYjmQ==" → "Nested message: Wow!"

Base64 decoding is supported in C++ or C with the
``pw::tokenizer::PrefixedBase64Decode`` or ``pw_TokenizerPrefixedBase64Decode``
functions.

.. code-block:: cpp

  void pw_TokenizerHandleEncodedMessage(const uint8_t encoded_message[],
                                        size_t size_bytes) {
    char base64_buffer[64];
    size_t base64_size = pw::tokenizer::PrefixedBase64Encode(
        pw::span(encoded_message, size_bytes), base64_buffer);

    TransmitLogMessage(base64_buffer, base64_size);
  }

Deployment war story
====================
The tokenizer module was developed to bring tokenized logging to an
in-development product. The product already had an established text-based
logging system. Deploying tokenization was straightforward and had substantial
benefits.

Results
-------
  * Log contents shrunk by over 50%, even with Base64 encoding.

    * Significant size savings for encoded logs, even using the less-efficient
      Base64 encoding required for compatibility with the existing log system.
    * Freed valuable communication bandwidth.
    * Allowed storing many more logs in crash dumps.

  * Substantial flash savings.

    * Reduced the size firmware images by up to 18%.

  * Simpler logging code.

    * Removed CPU-heavy ``snprintf`` calls.
    * Removed complex code for forwarding log arguments to a low-priority task.

This section describes the tokenizer deployment process and highlights key
insights.

Firmware deployment
-------------------
  * In the project's logging macro, calls to the underlying logging function
    were replaced with a ``PW_TOKENIZE_TO_GLOBAL_HANDLER_WITH_PAYLOAD``
    invocation.
  * The log level was passed as the payload argument to facilitate runtime log
    level control.
  * For this project, it was necessary to encode the log messages as text. In
    ``pw_TokenizerHandleEncodedMessageWithPayload``, the log messages were
    encoded in the $-prefixed `Base64 format`_, then dispatched as normal log
    messages.
  * Asserts were tokenized using ``PW_TOKENIZE_TO_CALLBACK``.

.. attention::
  Do not encode line numbers in tokenized strings. This results in a huge
  number of lines being added to the database, since every time code moves,
  new strings are tokenized. If line numbers are desired in a tokenized
  string, add a ``"%d"`` to the string and pass ``__LINE__`` as an argument.

Database management
-------------------
  * The token database was stored as a CSV file in the project's Git repo.
  * The token database was automatically updated as part of the build, and
    developers were expected to check in the database changes alongside their
    code changes.
  * A presubmit check verified that all strings added by a change were added to
    the token database.
  * The token database included logs and asserts for all firmware images in the
    project.
  * No strings were purged from the token database.

.. tip::
  Merge conflicts may be a frequent occurrence with an in-source database. If
  the database is in-source, make sure there is a simple script to resolve any
  merge conflicts. The script could either keep both sets of lines or discard
  local changes and regenerate the database.

Decoding tooling deployment
---------------------------
  * The Python detokenizer in ``pw_tokenizer`` was deployed to two places:

      * Product-specific Python command line tools, using
        ``pw_tokenizer.Detokenizer``.
      * Standalone script for decoding prefixed Base64 tokens in files or
        live output (e.g. from ``adb``), using ``detokenize.py``'s command line
        interface.

  * The C++ detokenizer library was deployed to two Android apps with a Java
    Native Interface (JNI) layer.

      * The binary token database was included as a raw resource in the APK.
      * In one app, the built-in token database could be overridden by copying a
        file to the phone.

.. tip::
  Make the tokenized logging tools simple to use for your project.

  * Provide simple wrapper shell scripts that fill in arguments for the
    project. For example, point ``detokenize.py`` to the project's token
    databases.
  * Use ``pw_tokenizer.AutoReloadingDetokenizer`` to decode in
    continuously-running tools, so that users don't have to restart the tool
    when the token database updates.
  * Integrate detokenization everywhere it is needed. Integrating the tools
    takes just a few lines of code, and token databases can be embedded in
    APKs or binaries.

Limitations and future work
===========================

GCC bug: tokenization in template functions
-------------------------------------------
GCC incorrectly ignores the section attribute for template
`functions <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70435>`_ and
`variables <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88061>`_. Due to this
bug, tokenized strings in template functions may be emitted into ``.rodata``
instead of the special tokenized string section. This causes two problems:

  1. Tokenized strings will not be discovered by the token database tools.
  2. Tokenized strings may not be removed from the final binary.

clang does **not** have this issue! Use clang to avoid this.

It is possible to work around this bug in GCC. One approach would be to tag
format strings so that the database tools can find them in ``.rodata``. Then, to
remove the strings, compile two binaries: one metadata binary with all tokenized
strings and a second, final binary that removes the strings. The strings could
be removed by providing the appropriate linker flags or by removing the ``used``
attribute from the tokenized string character array declaration.

64-bit tokenization
-------------------
The Python and C++ detokenizing libraries currently assume that strings were
tokenized on a system with 32-bit ``long``, ``size_t``, ``intptr_t``, and
``ptrdiff_t``. Decoding may not work correctly for these types if a 64-bit
device performed the tokenization.

Supporting detokenization of strings tokenized on 64-bit targets would be
simple. This could be done by adding an option to switch the 32-bit types to
64-bit. The tokenizer stores the sizes of these types in the
``.pw_tokenizer_info`` ELF section, so the sizes of these types can be verified
by checking the ELF file, if necessary.

Tokenization in headers
-----------------------
Tokenizing code in header files (inline functions or templates) may trigger
warnings such as ``-Wlto-type-mismatch`` under certain conditions. That
is because tokenization requires declaring a character array for each tokenized
string. If the tokenized string includes macros that change value, the size of
this character array changes, which means the same static variable is defined
with different sizes. It should be safe to suppress these warnings, but, when
possible, code that tokenizes strings with macros that can change value should
be moved to source files rather than headers.

Tokenized strings as ``%s`` arguments
-------------------------------------
Encoding ``%s`` string arguments is inefficient, since ``%s`` strings are
encoded 1:1, with no tokenization. It would be better to send a tokenized string
literal as an integer instead of a string argument, but this is not yet
supported.

A string token could be sent by marking an integer % argument in a way
recognized by the detokenization tools. The detokenizer would expand the
argument to the string represented by the integer.

.. code-block:: cpp

  #define PW_TOKEN_ARG PRIx32 "<PW_TOKEN]"

  constexpr uint32_t answer_token = PW_TOKENIZE_STRING("Uh, who is there");

  PW_TOKENIZE_TO_GLOBAL_HANDLER("Knock knock: %" PW_TOKEN_ARG "?", answer_token);

Strings with arguments could be encoded to a buffer, but since printf strings
are null-terminated, a binary encoding would not work. These strings can be
prefixed Base64-encoded and sent as ``%s`` instead. See `Base64 format`_.

Another possibility: encode strings with arguments to a ``uint64_t`` and send
them as an integer. This would be efficient and simple, but only support a small
number of arguments.

Compatibility
=============
  * C11
  * C++11
  * Python 3

Dependencies
============
  * ``pw_varint`` module
  * ``pw_preprocessor`` module
  * ``pw_span`` module