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Definitions
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===========
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(a relatively recent (Oct 2014) new list of definitions from Erik)
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# *Event* -- A JSON object that represents a piece of information to be
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distributed to the the room. The object includes a payload and metadata,
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including a ``type`` used to indicate what the payload is for and how to process
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them. It also includes one or more references to previous events.
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# *Event graph* -- Events and their references to previous events form a
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directed acyclic graph. All events must be a descendant of the first event in a
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room, except for a few special circumstances.
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# *State event* -- A state event is an event that has a non-null string valued
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`state_key` field. It may also include a ``prev_state`` key referencing exactly
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one state event with the same type and state key, in the same event graph.
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# *State tree* -- A state tree is a tree formed by a collection of state events
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that have the same type and state key (all in the same event graph.
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# *State resolution algorithm* -- An algorithm that takes a state tree as input
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and selects a single leaf node.
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# *Current state event* -- The leaf node of a given state tree that has been
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selected by the state resolution algorithm.
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# *Room state* / *state dictionary* / *current state* -- A mapping of the pair
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(event type, state key) to the current state event for that pair.
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# *Room* -- An event graph and its associated state dictionary. An event is in
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the room if it is part of the event graph.
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# *Topological ordering* -- The partial ordering that can be extracted from the
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event graph due to it being a DAG.
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(The state definitions are purposely slightly ill-defined, since if we allow
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deleting events we might end up with multiple state trees for a given event
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type and state key pair.)
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Federation specific
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-------------------
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# *(Persistent data unit) PDU* -- An encoding of an event for distribution of
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the server to server protocol.
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# *(Ephemeral data unit) EDU* -- A piece of information that is sent between
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servers and doesn't encode an event.
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Client specific
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---------------
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# *Child events* -- Events that reference a single event in the same room
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independently of the event graph.
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# *Collapsed events* -- Events that have all child events that reference it
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included in the JSON object.
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