Architecture
============

.. mermaid::
   :title: Frame architecture

   classDiagram
    Frame *-- FrameState : state
    FrameState *-- HeaderType : header_type
    Answer --|> HeaderType
    Error --|> HeaderType
    Message --|> HeaderType
    Null --|> HeaderType
    Reply --|> HeaderType
    Frame ..> PossibleHeaderType : used during initialization
    FrameState ..> PossibleHeaderType : used during initialization
    HeaderType ..> PossibleHeaderType : used to make the corresponding object
    Frame ..> ContinuationIndicator : used during initialization
    FrameState ..> ContinuationIndicator : used during initialization
    HeaderType ..> ContinuationIndicator : used in make
    Answer ..> ContinuationIndicator : continuation_indicator
    Error ..> ContinuationIndicator : continuation_indicator
    Message ..> ContinuationIndicator : continuation_indicator
    Reply ..> ContinuationIndicator : continuation_indicator
    class ContinuationIndicator {
        <<Enumeration>>
        COMPLETE
        INTERMEDIATE
    }
    class PossibleHeaderType {
        <<Enumeration>>
        ANS
        ERR 
        MSG
        NUL
        RPY
    }
    class Frame {
        +bytes payload
        +FrameState state
        +bytes build()
        +Frame(payload: bytes, channel_number: int, message_number: int, sequence_number: int, answer_number: int | None, continuation_indicator: ContinuationIndicator, possible_header_type: PossibleHeaderType)
    }
    class FrameState {
        +Frame frame
        +HeaderType header_type
        +int channel_number
        +int message_number
        +int sequence_number
        +bytes build()
        +FrameState(frame: Frame, channel_number: int, message_number: int, sequence_number: int, answer_number: int | None, continuation_indicator: ContinuationIndicator, possible_header_type: PossibleHeaderType)
    }
    class HeaderType {
        +FrameState state
        +bytes build()
        +HeaderType make(state: FrameState, continuation_indicator: ContinuationIndicator, possible_header_type: PossibleHeaderType, answer_number: int | None)$
        +HeaderType(state:FrameState)
    }
    class Answer {
        +int answer_number
        +ContinuationIndicator continuation_indicator
        +Answer(state: FrameState, answer_number: int, continuation_indicator: ContinuationIndicator)
    }
    class Error {
        +ContinuationIndicator continuation_indicator
        +Error(state: FrameState, continuation_indicator: ContinuationIndicator)
    }
    class Message {
        +ContinuationIndicator continuation_indicator
        +Message(state: FrameState, continuation_indicator: ContinuationIndicator)
    }
    class Null {}
    class Reply {
        +ContinuationIndicator continuation_indicator
        +Reply(state: FrameState, continuation_indicator: ContinuationIndicator)
    }


State
-----

This library use class as state. A ``Frame`` class can have different header.
Instead of creating multiple class like ``MessageFrame``, ``AnswerFrame``, etc.
I chose to store the header as a state of ``Frame`` (``FrameState`` being the
state holder and manager). As each state can have different behaviors, any
behaviors dependents of the header must be defined in the specific
``HeaderType``.