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Rework the mainline ordering more dramatically.

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This has undergone a fairly large rework, so needs careful inspection.
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David Robertson 2022-06-06 13:48:13 +01:00
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content/rooms/fragments/v2-state-res.md
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@ -88,28 +88,35 @@ Starting with *i* = 0, repeatedly fetch *P*<sub>*i*+1</sub>, the
Increment *i* and repeat until *P<sub>i</sub>* has no `m.room.power_levels` Increment *i* and repeat until *P<sub>i</sub>* has no `m.room.power_levels`
event in its `auth_events`. event in its `auth_events`.
The *mainline of P*<sub>0</sub> is the list of events The *mainline of P*<sub>0</sub> is the list of events
[*P*<sub>n</sub> , ... , *P*<sub>1</sub>, *P*<sub>0</sub>]. [*P*<sub>n</sub> , ... , *P*<sub>1</sub>, *P*<sub>0</sub>],
ordered from oldest to newest.
Given another event *e* = *e<sub>0</sub>* we can compute a similar list of Let *e* = *e<sub>0</sub>* be another event (possibly another
`m.room.power_levels` events `m.room.power_levels` event). We can compute a similar list of events
[*e*<sub>0</sub>, *e*<sub>1</sub>, ...], [*e*<sub>1</sub>, ..., *e<sub>m</sub>*],
where *e<sub>*j* + 1</sub>* is the `m.room.power_levels` event in the where *e<sub>*j* + 1</sub>* is the `m.room.power_levels` event in the
`auth_events` of *e<sub>j</sub>*. The *closest mainline event to e* `auth_events` of *e<sub>j</sub>*, and where *e<sub>m</sub>* has no
is the first event *e<sub>j</sub>* which belongs to the mainline of *P*. `m.room.power_levels` event in its `auth_events`. (Note that this second list
If no event *e<sub>j</sub>* belongs to the mainline of *P*, then the closest may be empty, because *e* may not cite an `m.room.power_levels` event in its
mainline event to *e* can be considered to be a dummy event that is `auth_events` at all.)
before any other event in the mainline of *P* for the purposes of
condition 1 below. Now compare these two lists as follows.
* Find the smallest index *j* ≥1 for which *e<sub>j</sub>* belongs to the
mainline of *P*.
* If *j* exists, then *e<sub>j</sub>* = *P<sub>i</sub>* for some unique index
*i* ≥ 0. Otherwise set *i* = ∞, where ∞ is a sentinel value greater
than any integer.
* In both cases, the *position of e in the mainline of P* is *i*.
The *mainline ordering based on* *P* of a set of events is the ordering, The *mainline ordering based on* *P* of a set of events is the ordering,
from smallest to largest, using the following comparison relation on from smallest to largest, using the following comparison relation on
events: for events *x* and *y*, *x* &lt; *y* if events: for events *x* and *y*, *x* &lt; *y* if
1. the closest mainline event to *x* appears *before* the closest 1. the position of *x* in *P*'s mainline is **greater** than
mainline event to *y*; or the position of *y* in *P*'s mainline.
2. the closest mainline events are the same, but *x*'s 2. the positions of the events are the same, but *x*'s
`origin_server_ts` is *less* than *y*'s `origin_server_ts`; or `origin_server_ts` is *less* than *y*'s `origin_server_ts`; or
3. the closest mainline events are the same and the events have the 3. the positions of the events are the same and the events have the
same `origin_server_ts`, but *x*'s `event_id` is *less* than *y*'s same `origin_server_ts`, but *x*'s `event_id` is *less* than *y*'s
`event_id`. `event_id`.