MOSS Decoder

Python package implemented in Rust for high-performance decoding of readout data from the MOSS chip (Stitched Monolithic Pixel Sensor prototype).

• MOSS Decoder
  • Installation
    • Example
  • Features
    • 5 types of idempotent functions are provided
  • MOSS event data packet protocol FSM
  • MOSS event data packet decoder FSM
  • Event packet hit decoder FSM
  • Running tests
    • Testing local changes
  • Motivation & Purpose
  • @CERN Gitlab installation for CentOS and similar distributions from local build
    • Troubleshooting

Installation

$ pip install moss-decoder

Import in python and use for decoding raw data.

Example

import moss_decoder

moss_packet = moss_decoder.decode_from_file("path/to/raw_data.raw")
print(moss_packet[0])
# Unit ID: 6 Hits: 44
#  [MossHit { region: 0, row: 3, column: 11 }, MossHit { region: 0, row: 18, column: 243 }, ...
print(moss_packet[0].hits[0])
# reg: 0 row: 3 col: 11

Features

See python types for the type information the package exposes to Python.

Two classes are provided: MossPacket & MossHit.

5 types of idempotent functions are provided

decode_event(arg: bytes)  -> tuple[MossPacket, int]: ...
# allows decoding a single event from an iterable of bytes

Returns: the decoded MossPacket and the index the unit frame trailer was found. Throws if no valid MossPacket is found.

decode_all_events(arg: bytes) -> tuple[list[MossPacket], int]: ...
# returns as many `MossPacket`s as can be decoded from the bytes iterable.
# This is much more effecient than calling `decode_event` multiple times.

Returns: A list of MossPackets and the index of the last observed unit frame trailer. Throws if no valid MossPackets are found or a protocol error is encountered.

decode_from_file(arg: str | Path) -> list[MossPacket]: ...
# takes a `Path` and returns as many `MossPacket` as can be decoded from file.
# This is the most effecient way of decoding data from a file.

Returns: A list of MossPackets. Throws if the file is not found, no valid MossPackets are found, or a protocol error is encountered.

decode_n_events(
    bytes: bytes,
    take: int,
    skip: Optional[int] = None,
    prepend_buffer: Optional[bytes] = None,
) -> tuple[list[MossPacket], int]: ...
# Decode N events from bytes
# Optionally provide either (not both):
#    - Skip M events before decoding N events
#    - Prepend a buffer before decoding N events.

Returns: A list of MossPackets. Throws if the file is not found, no valid MossPackets are found, or a protocol error is encountered.

A BytesWarning exception is thrown if the end of the bytes is reached while decoding a packet (no trailer is found)

def skip_n_take_all(
    bytes: bytes, skip: int = None
) -> tuple[list[MossPacket], Optional[bytes]]: ...
# Decode all events, optionally skip N events first.
# return all decoded events and the remaining bytes after decoding the last MossPacket

Returns: All decoded events and any remaining bytes after the last trailer seen.

Using decode_n_events and skip_n_take_all it is possible to continuously decode multiple files that potentially ends or starts with partial events.

MOSS event data packet protocol FSM

The a MOSS half-unit event data packet follows the states seen in the FSM below. The region header state is simplified here.

stateDiagram-v2
  frame_header : Unit Frame Header
  frame_trailer : Unit Frame Trailer
  region_header : Region Header
  data_0 : Data 0
  data_1 : Data 1
  data_2 : Data 2
  idle : Idle

    [*] --> frame_header

    frame_header --> region_header
    frame_header --> frame_trailer

    region_header --> region_header
    region_header --> frame_trailer
    region_header --> DATA

    state DATA {
      direction LR
      [*] --> data_0
      data_0 --> data_1
      data_1 --> data_2
      data_2 --> data_0
      data_2 --> [*]
    }

    DATA --> idle
    DATA --> region_header
    DATA --> frame_trailer


    idle --> DATA
    idle --> idle
    idle --> frame_trailer

    frame_trailer --> [*]

MOSS event data packet decoder FSM

The raw data is decoded using the following FSM. The delimiter is expected to be 0xFA. The default Idle value 0xFF is also assumed.

stateDiagram-v2
direction LR
  delimiter : Event Delimiter
  frame_header : Unit Frame Header
  frame_trailer : Unit Frame Trailer


  [*] --> delimiter
  delimiter --> EVENT
  delimiter --> delimiter

  state EVENT {

    [*] --> frame_header

    frame_header --> HITS
    frame_header --> frame_trailer

    state HITS {
      [*] --> [*] : decode hits
    }

    HITS --> frame_trailer

    frame_trailer --> [*]

  }

The EVENT state is reached by finding decoding a Unit Frame Header and then the decoder enters the HITS substate which is depicted in the FSM in the next section.

Event packet hit decoder FSM

stateDiagram-v2

  region_header0 : Region Header 0
  region_header1 : Region Header 1
  region_header2 : Region Header 2
  region_header3 : Region Header 3
  data_0 : Data 0
  data_1 : Data 1
  data_2 : Data 2
  idle : Idle

  [*] --> region_header0
  region_header0 --> region_header1
  region_header0 --> region_header2
  region_header0 --> region_header3
  region_header0 --> DATA
  region_header0 --> [*]
  DATA --> region_header1
  region_header1 --> region_header2
  region_header1 --> region_header3
  region_header1 --> DATA
  region_header1 --> [*]
  DATA --> region_header2
  region_header2 --> region_header3
  region_header2 --> DATA
  region_header2 --> [*]
  DATA --> region_header3
  region_header3 --> DATA
  DATA --> [*]
  region_header3 --> [*]


    state DATA {
      [*] --> data_0
      data_0 --> data_1
      data_1 --> data_2
      data_2 --> data_0
      data_2 --> idle
      idle --> idle
      idle --> [*]
      idle --> data_0
      data_2 --> [*]

    }

Decoding hits using the FSM above leads to higher performance and assures correct decoding by validating the state transitions.

Running tests

Rust unit and integration tests can be executed with cargo test --no-default-features.

The --no-default-features flag has to be supplied to be able to run tests that links to Python types e.g. throwing Python exceptions, this is a temporary workaround see more.

Python integration tests can be run by running ìntegration.py with Python.

Testing local changes

Testing against local changes in the Rust code requires first compiling and installing the wheel package, the tool maturin is used for this, you can look at the shell script performance_dev_py.sh for inspiration. If you have access to bash you can simply run the shell script performance_dev_py.sh which will compile and install it for you, but it will also run a little benchmark with hyperfine, if you are not interested in the benchmark, just don't run the hyperfine command in the end of the measure_performance_dev function.

Motivation & Purpose

Decoding in native Python is slow and the MOSS verification team at CERN got to a point where we needed more performance.

Earliest version of a Rust package gave massive improvements as shown in the benchmark below.

Decoding 10 MB MOSS readout data with 100k event data packets and ~2.7 million hits. Performed on CentOS Stream 9 with Python 3.11

@CERN Gitlab installation for CentOS and similar distributions from local build

If you update the package source code and want to build and install without publishing and fetching from PyPI, you can follow these steps.

The .CERN-gitlab-ci.yml file contains a build-centos manual job which will build the MOSS decoder package from source and saves the package as an artifact.

1. Start the job, download the artifacts.
2. Unzip the artifacts and you will find a wheels package in /target/wheels/ with the .whl extension
3. Run python -m pip install <wheels package>.whl
4. Confirm the installation with python -m pip freeze | grep moss, it should display something containing moss_decoder @ file:<your-path-to-wheels-package>

Troubleshooting

if you get ERROR: Could not find a version that satisfies the requirement ... make sure to add .whl when performing step 3 above.

if you don't see the expected message at step 4, try running the installation command in step 3 with any or all of these options --upgrade --no-cache-dir --force-reinstall.