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.. _module-pw_checksum:
The ``pw_checksum`` module provides functions for calculating checksums.
.. cpp:namespace:: pw::checksum
.. cpp:var:: constexpr uint16_t kCcittCrc16DefaultInitialValue = 0xFFFF
The default initial value for the CRC16.
.. cpp:function:: uint16_t CcittCrc16(span<const std::byte> data, uint16_t initial_value = kCcittCrc16DefaultInitialValue)
Calculates the CRC16 of the provided data using polynomial 0x1021, with a
default initial value of :cpp:expr:`0xFFFF`.
To incrementally calculate a CRC16, use the previous value as the initial
.. code-block:: cpp
uint16_t crc = CcittCrc16(my_data);
crc = CcittCrc16(more_data, crc);
.. cpp:var:: constexpr uint32_t kCrc32InitialValue = 0xFFFFFFFF
The initial value for the CRC32.
.. cpp:function:: uint32_t Crc32(span<const std::byte> data)
Calculates the initial / one-time CRC32 of the provided data using polynomial
0x4C11DB7, with an initial value of :cpp:expr:`0xFFFFFFFF`.
.. code-block:: cpp
uint32_t crc = Crc32(my_data);
.. cpp:function:: uint32_t Crc32(span<const std::byte> data, uint32_t previous_result)
Incrementally append calculation of a CRC32, need to pass in the previous
.. code-block:: cpp
uint32_t crc = Crc32(my_data);
crc = Crc32(more_data, crc);
.. _CRC32 Implementations:
Pigweed provides 3 different CRC32 implementations with different size and
runtime tradeoffs. The below table summarizes the variants. For more detailed
size information see the :ref:`pw_checksum-size-report` below. Instructions counts were
calculated by hand by analyzing the
`assembly <>`_.
.. list-table::
:header-rows: 1
* - Variant
- Relative size (see Size Report below)
- Speed
- Lookup table size (entries)
- Instructions/byte (M33/-Os)
* - 8 bits per iteration (default)
- large
- fastest
- 256
- 8
* - 4 bits per iteration
- small
- fast
- 16
- 13
* - 1 bit per iteration
- smallest
- slow
- 0
- 43
The default implementation provided by the APIs above can be selected through
:ref:`Module Configuration Options`. Additionally ``pw_checksum`` provides
variants of the C++ API to explicitly use each of the implementations. These
classes provide the same API as ``Crc32``:
* ``Crc32EightBit``
* ``Crc32FourBit``
* ``Crc32OneBit``
.. _pw_checksum-size-report:
Size report
The CRC module currently optimizes for speed instead of binary size, by using
pre-computed 256-entry tables to reduce the CPU cycles per byte CRC
.. include:: size_report
* C
* C++17
* ``pw_span``
.. _Module Configuration Options:
Module Configuration Options
The following configurations can be adjusted via compile-time configuration of
this module, see the
:ref:`module documentation <module-structure-compile-time-configuration>` for
more details.
Selects which of the :ref:`CRC32 Implementations` the default CRC32 APIs
use. Set to one of the following values:
To enable ``pw_checksum`` for Zephyr add ``CONFIG_PIGWEED_CHECKSUM=y`` to the
project's configuration.