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proposals/4356-recent-emoji.md

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+# MSC4356: Recently used emoji
+
+Like other chat platforms, Matrix supports emoji as a way to visually express ideas or emotions. In
+practice, most people use a limited set of emoji only. Since emoji are commonly used as a quick way
+to react to something, it is desirable for clients to offer users shortcuts to their favorite emoji.
+Some emoji picker libraries support this feature by locally tracking emoji usage. This doesn't work
+well in a multi-device environment, however, because such history cannot easily be shared between
+clients.
+
+This proposal introduces a way for clients to maintain a shared storage of recently used emoji to
+enable emoji suggestions across clients.
+
+## Proposal
+
+A new global account data event `m.recent_emoji` is introduced. In `content`, it contains a single
+property `recent_emoji` that is an array where each element is an object with the following properties:
+
+- `emoji` (string, required): The Unicode emoji as string.
+- `total` (number, required): The number of times the emoji has been used (\<= 2^53-1).
+
+The outer `recent_emoji` array is ordered descendingly by last usage time.
+
+``` json5
+{
+  "type": "m.recent_emoji",
+  "content": {
+    "recent_emoji": [
+      // Most recently used, 7 times overall
+      {
+        "emoji": "😅",
+        "total": 7
+      },
+      // Second most recently used, 84 times overall
+      {
+        "emoji": "👍",
+        "total": 84
+      },
+      ...
+  }
+}
+```
+
+When an emoji is used, the sending client moves (or adds) it to the beginning of the `recent_emoji`
+array and increments (or initializes) its counter. What exactly constitutes emoji usage is left as
+an implementation detail for clients. Obvious choices are sending emoji in messages or annotations.
+
+When an image is sent as an inline image or in a reaction (using [MSC4027]), the `mxc://` URI of the
+image MAY be used as the "emoji" in this event. Clients which do not support such use of images MUST
+tolerate the existence of `mxc://` entries, e.g. by ignoring the entries when deciding what to
+display to the user, while still preserving them when modifying the list.
+
+As new emoji are being used, clients SHOULD limit the length of the `recent_emoji` array by dropping
+elements from the end. A RECOMMENDED maximum length is 100 emoji. Apart from this, no other
+mechanism for resetting counters is mandated as the upper boundary of 2^53-1 seems sufficiently
+large for all practical purposes.
+
+Clients MAY freely customise the logic for generating recommendations from the stored emoji. As an
+example, they could select the 24 first (= most recently used) emoji and stably sort them by their
+counters (so that more recently used emoji are ordered first on ties).
+
+## Potential issues
+
+Clients could choose wildly different ways to generate recommendations from the shared storage
+leading to significantly different UX across clients. A "correct" algorithm is not currently known,
+however, and will require future UI exploration in client implementations.
+
+## Alternatives
+
+Further metadata such as the concrete access time or the room could be tracked together with emoji.
+It is unclear, however, if this would lead to materially better suggestions. A last-used timestamp
+could also be used to cull emoji that haven't been used in a very long time. Given that implementations
+are already encouraged to limit the maximum number of tracked emoji, this doesn't appear necessary,
+however.
+
+Rather than an array of objects, a nested array could be used. This approach has been used in early
+implementations of this proposal under the `io.element.recent_emoji` account data event. While this
+results in a more compact storage layout, it doesn't lend itself well to future extension. Additionally,
+given request compression and the common expectable length of the array, performance benefits should
+be negligible.
+
+## Security considerations
+
+This proposal doesn't mandate encrypting the `m.recent_emoji` account data event. Since emoji are
+most commonly used in annotations which are not encrypted, servers could already track and abuse
+this information today, however.
+
+Malicious or buggy clients could cause undefined behavior on other clients by setting emoji counters
+to negative values or the maximum allowed value. To prevent this, clients SHOULD drop any emojis
+with invalid entries from the list. When observing a count of 2^53-1 for an emoji, clients SHOULD
+normalise the entire set of emoji by dividing all counts by two and rounding up. To prevent race
+conditions, both of these changes SHOULD only be applied when the client is updating the account
+data event due to local emoji usage.
+
+## Unstable prefix
+
+While this MSC is not considered stable, `m.recent_emoji` should be referred to as
+`io.github.johennes.msc4356.recent_emoji`.
+
+## Dependencies
+
+None.
+
+  [MSC4027]: https://github.com/matrix-org/matrix-spec-proposals/pull/4027