server/posts: add getting posts around id

Leave undocumented since it add almost no value for the client
applications.

server/szurubooru/api/__init__.py

@@ -20,7 +20,8 @@ from szurubooru.api.post_api import (
     PostDetailApi,
     PostFeatureApi,
     PostScoreApi,
-    PostFavoriteApi)
+    PostFavoriteApi,
+    PostsAroundApi)
 from szurubooru.api.snapshot_api import SnapshotListApi
 from szurubooru.api.info_api import InfoApi
 from szurubooru.api.context import Context, Request

server/szurubooru/api/post_api.py

@@ -152,3 +152,15 @@ class PostFavoriteApi(BaseApi):
         favorites.unset_favorite(post, ctx.user)
         ctx.session.commit()
         return _serialize_post(ctx, post)
+
+class PostsAroundApi(BaseApi):
+    def __init__(self):
+        super().__init__()
+        self._search_executor = search.Executor(
+            search.configs.PostSearchConfig())
+
+    def get(self, ctx, post_id):
+        auth.verify_privilege(ctx.user, 'posts:list')
+        self._search_executor.config.user = ctx.user
+        return self._search_executor.get_around_and_serialize(
+            ctx, post_id, lambda post: _serialize_post(ctx, post))

server/szurubooru/app.py

@@ -109,6 +109,7 @@ def create_app():
     app.add_route('/post/{post_id}', api.PostDetailApi())
     app.add_route('/post/{post_id}/score', api.PostScoreApi())
     app.add_route('/post/{post_id}/favorite', api.PostFavoriteApi())
+    app.add_route('/post/{post_id}/around', api.PostsAroundApi())
 
     app.add_route('/comments/', api.CommentListApi())
     app.add_route('/comment/{comment_id}', api.CommentDetailApi())

server/szurubooru/search/configs/base_search_config.py

@@ -13,6 +13,10 @@ class BaseSearchConfig(object):
     def create_count_query(self):
         return self.create_filter_query()
 
+    @property
+    def id_column(self):
+        return None
+
     @property
     def anonymous_filter(self):
         return None

server/szurubooru/search/configs/post_search_config.py

@@ -105,6 +105,10 @@ class PostSearchConfig(BaseSearchConfig):
     def finalize_query(self, query):
         return query.order_by(db.Post.creation_time.desc())
 
+    @property
+    def id_column(self):
+        return db.Post.post_id
+
     @property
     def anonymous_filter(self):
         return search_util.create_subquery_filter(

server/szurubooru/search/executor.py

@@ -27,6 +27,34 @@ class Executor(object):
         self.config = search_config
         self.parser = parser.Parser()
 
+    def get_around(self, query_text, entity_id):
+        search_query = self.parser.parse(query_text)
+        self.config.on_search_query_parsed(search_query)
+        filter_query = self.config \
+            .create_filter_query() \
+            .options(sqlalchemy.orm.lazyload('*'))
+        filter_query = self._prepare_db_query(filter_query, search_query, False)
+        prev_filter_query = filter_query \
+            .filter(self.config.id_column < entity_id) \
+            .order_by(None) \
+            .order_by(sqlalchemy.func.abs(self.config.id_column - entity_id).asc()) \
+            .limit(1)
+        next_filter_query = filter_query \
+            .filter(self.config.id_column > entity_id) \
+            .order_by(None) \
+            .order_by(sqlalchemy.func.abs(self.config.id_column - entity_id).asc()) \
+            .limit(1)
+        return [
+            next_filter_query.one_or_none(),
+            prev_filter_query.one_or_none()]
+
+    def get_around_and_serialize(self, ctx, entity_id, serializer):
+        entities = self.get_around(ctx.get_param_as_string('query'), entity_id)
+        return {
+            'next': serializer(entities[0]),
+            'prev': serializer(entities[1]),
+        }
+
     def execute(self, query_text, page, page_size):
         '''
         Parse input and return tuple containing total record count and filtered