The error messages produced by various conversions in `automerge-wasm`
were quite uninformative - often consisting of just returning the
offending value with no description of the problem. The logic of these
error messages was often hard to trace due to the use of `JsValue` to
represent both error conditions and valid values - evidenced by most of
the public functions of `automerge-wasm` having return types of
`Result<JsValue, JsValue>`. Change these return types to mention
specific errors, thus enlisting the compilers help in ensuring that
specific error messages are emitted.
Transactions with no ops in them are generally undesirable. They take up
space in the change log but do nothing else. They are not useless
though, it may occasionally be necessary to create an empty change in
order to list all the current heads of the document as dependents of the
empty change.
The current API makes no distinction between empty changes and non-empty
changes. If the user calls `Transaction::commit` a change is created
regardless of whether there are ops to commit. To provide a more useful
API modify `commit` so that if there is a no-op transaction then no
changes are created, but provide explicit methods to create an empty
change via `Transaction::empty_change`, `Automerge::empty_change` and
`Autocommit::empty_change`. Also make these APIs available in Javascript
and C.
The API of Automerge::generate_sync_message requires that the user keep
track of in flight messages themselves if they want to avoid sending
duplicate messages. To avoid this add a flag to `automerge::sync::State`
to track if there are any in flight messages and return `None` from
`generate_sync_message` if there are.
Generating patches to text objects (a la the edit-trace benchmark) was
very slow due to appending to the back of a Vec. Use the SequenceTree
(effectively a B-tree) instead so as to speed up sequence patch
generation.
Sometimes you need a cheap copy of a document at a given set of heads
just so you can see what has changed. Cloning the document to do this is
quite expensive when you don't need a writable copy. Add automerge.view
to allow a cheap read only copy of a document at a given set of heads
and add an additional heads argument to clone for when you do want a
writable copy.
After some discussion with PVH I realise that the repo structure in the
last reorg was very rust-centric. In an attempt to put each language on
a level footing move the rust code and project files into ./rust
