Instead of having required and unique flags for every field, enforce both
requirements on the first field, and neither on the rest. This mirrors the
subject/body format people are used to in note-taking apps. The subject
defines the object being learnt, and the remaining fields represent properties
of that object.
In the past, duplicate checking served two purposes: it quickly notified the
user that they're entering the same fact twice, and it notified the user if
they'd accidentally mistyped a secondary field. The former behaviour is
important for avoiding wasted effort, and so it should be done in real time.
The latter behaviour is not essential however - a typo is not wasted effort,
and it could be fixed in a periodic 'find duplicates' function. Given that
some users ended up with sluggish decks due to the overhead a large number of
facts * a large number of unique fields caused, this seems like a change for
the better.
This also means Anki will let you add notes as long as as the first field has
been filled out. Again, this is not a big deal: Anki is still checking to make
sure one or more cards will be generated, and the user can easily add any
missing fields later.
As a bonus, this change simplifies field configuration somewhat. As the card
layout and field dialogs are a popular point of confusion, the more they can
be simplified, the better.
Decks now have an "update sequence number". All objects also have a USN, which
is set to the deck USN each time they are modified. When syncing, each side
sends any objects with a USN >= clientUSN. When objects are copied via sync,
they have their USNs bumped to the current serverUSN. After a sync, the USN on
both sides is set to serverUSN + 1.
This solves the failing three way test, ensures we receive all changes
regardless of clock drift, and as the revlog also has a USN now, ensures that
old revlog entries are imported properly too.
Objects retain a separate modification time, which is used for conflict
resolution, deck subscriptions/importing, and info for the user.
Note that if the clock is too far off, it will still cause confusion for
users, as the due counts may be different depending on the time. For this
reason it's probably a good idea to keep a limit on how far the clock can
deviate.
We still keep track of the last sync time, but only so we can determine if the
schema has changed since the last sync.
The media code needs to be updated to use USNs too.
