Introduce isPrivate for permissions service

h/static/scripts/directives/annotation.coffee

@@ -82,7 +82,7 @@ AnnotationController = [
     # @returns {boolean} True if the annotation is private to the current user.
     ###
     this.isPrivate = ->
-      model.user and angular.equals(model.permissions?.read or [], [model.user])
+      permissions.isPrivate model.permissions, model.user
 
     ###*
     # @ngdoc method

h/static/scripts/permissions-service.coffee

@@ -53,6 +53,18 @@ class Permissions
   isPublic: (permissions) ->
     GROUP_WORLD in (permissions?.read or [])
 
+  ###*
+  # @ngdoc method
+  # @name permissions#isPrivate
+  #
+  # @param {Object} permissions
+  # @param {String} user
+  #
+  # @returns {boolean} True if the annotation is private to the user.
+  ###
+  isPrivate: (permissions, user) ->
+    user and angular.equals(permissions?.read or [], [user])
+
   ###*
   # @ngdoc method
   # @name permissions#permits

tests/js/permissions-service-test.coffee

@@ -58,6 +58,22 @@ describe 'h', ->
         permission.read = ['one', 'two', 'three']
         assert.isFalse(permissions.isPublic(permission))
 
+    describe 'isPrivate', ->
+      it 'returns true if the given user is in the permissions', ->
+        user = 'acct:angry@birds.com'
+        permission = {read: [user]}
+        assert.isTrue(permissions.isPrivate(permission, user))
+
+      it 'returns false if another user is in the permissions', ->
+        users = ['acct:angry@birds.com', 'acct:angry@joe.com']
+        permission = {read: users}
+        assert.isFalse(permissions.isPrivate(permission, 'acct:angry@birds.com'))
+
+      it 'returns false if different user in the permissions', ->
+        user = 'acct:angry@joe.com'
+        permission = {read: ['acct:angry@birds.com']}
+        assert.isFalse(permissions.isPrivate(permission, user))
+
     describe 'permits', ->
       it 'returns true when annotation has no permissions', ->
         annotation = {}