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+---
+summary: API for managing MUC hats
+---
+
+# Introduction
+
+This module provides an internal API (i.e. to other modules) to manage
+'hats' for users in MUC rooms.
+
+Hats (first defined in XEP-0317, currently deferred) are additional identifiers
+that can be attached to users in a group chat. For example in an educational
+context, you may have a 'Teacher' hat that allows students to identify their
+teachers.
+
+Hats consist of a machine-readable unique identifier (a URI), and optionally
+a human-readable label.
+
+XEP-0317 suggests a protocol for users to manage their own hats, but though the
+API in this module allows for both user-managed and system-managed hats, there is
+currently no protocol implemented for users to manage their own hats, which is
+rarely desired in real-world implementations.
+
+The rest of this documentation is designed for developers who use this module.
+
+## Data model
+
+### User
+
+```
+{
+  "hats": {
+    "urn:uuid:164c41a2-7461-4cff-bdae-3f93078a6607": {
+      "active": false
+    },
+    "http://example.com/hats/foo": {
+      "active": true,
+      "required": true,
+      "title": "Awesome"
+    }
+}
+```
+
+| Field | Type   | Description                                                  |
+|-------|--------|--------------------------------------------------------------|
+| hats  | object | An object where mapping hat ids (key) to attachments (value) |
+
+Hat IDs must be a URI that uniquely identifies the hat.
+
+### Attachment
+
+```
+{
+  "active": true,
+  "required": true,
+  "title": "My Awesome Hat"
+}
+```
+
+| Field    | Type    | Description                                                 |
+|----------|---------|-------------------------------------------------------------|
+| active   | boolean | If true, indicates the user is currently displaying the hat |
+| required | boolean | If true, indicates the user is not able to remove the hat   |
+| title    | string  | A human-readable display name or label for the hat          |
+
+All fields are optional, omitted boolean values are equivalent to false.
+
+## API
+
+All methods return 'nil, err' on failure as standard throughout the Prosody codebase.
+
+Example of using this module from another module:
+
+```
+local muc_hats = module:depends("muc_hats_api");
+
+muc_hats.add_user_hat("user@localhost", "room@conference.localhost", "urn:uuid:164c41a2-7461-4cff-bdae-3f93078a6607", { active = true });
+```
+
+Note that the module only works when loaded on a MUC host, which generally means any
+module that uses it must also be loaded on the MUC host that it is managing.
+
+### add_user_hat
+
+`add_user_hat(user_jid, room_jid, hat_id, attachment)`
+
+Adds the identified hat to a user's... wardrobe? The user must already
+have an affiliation with the room (i.e. member, admin or owner).
+
+If `attachment` is omitted, it defaults to `{}`.
+
+#### Error cases
+
+item-not-found
+: Supplied room JID was not found on the current host
+
+item-not-found
+: Supplied user JID was not affiliated with the room
+
+### remove_user_hat
+
+`remove_user_hat(user_jid, room_jid, hat_id)`
+
+If the identified hat is currently available to the user, it is removed.
+
+#### Error cases
+
+item-not-found
+: Supplied room JID was not found on the current host
+
+item-not-found
+: Supplied user JID was not affiliated with the room
+
+### set_user_hats
+
+`set_user_hats(user_jid, room_jid, hats)`
+
+Ensures the listed hats are the hats available to a user, automatically
+adding/removing as necessary.
+
+The `hats` parameter should be an object mapping hat ids (keys) to attachment
+objects (values).
+
+#### Error cases
+
+item-not-found
+: Supplied room JID was not found on the current host
+
+item-not-found
+: Supplied user JID was not affiliated with the room