From f947e0df48f73596609d240f1875962d96a01a4f Mon Sep 17 00:00:00 2001
From: Charles Haley <cbhaley@i.wont.say.com>
Date: Sun, 16 Oct 2022 13:24:26 +0100
Subject: [PATCH] Two performance improvements related to large libraries. See
 https://www.mobileread.com/forums/showthread.php?p=4265937#post4265937 for an
 explanation.

The "exotic" part is changing _map_filtered to a property with a setter. The setter constructs a dict {book_id:row}. view.id_to_index() uses the dict instead of .index on the list.
---
 src/calibre/db/view.py                      | 28 ++++++++++++++-------
 src/calibre/gui2/library/alternate_views.py |  5 ++--
 2 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/src/calibre/db/view.py b/src/calibre/db/view.py
index c03d82dea8..1d6ad37c92 100644
--- a/src/calibre/db/view.py
+++ b/src/calibre/db/view.py
@@ -149,6 +149,8 @@ class View:
                 fmt = partial(format_is_multiple, sep=sep)
             self._field_getters[idx] = partial(func, label, fmt=fmt) if func == self._get else func
 
+        self._real_map_filtered = tuple()
+        self._real_map_filtered_dict = dict()
         self._map = tuple(sorted(self.cache.all_book_ids()))
         self._map_filtered = tuple(self._map)
         self.full_map_is_sorted = True
@@ -171,6 +173,15 @@ class View:
     def sanitize_sort_field_name(self, field):
         return sanitize_sort_field_name(self.field_metadata, field)
 
+    @property
+    def _map_filtered(self):
+        return self._real_map_filtered
+
+    @_map_filtered.setter
+    def _map_filtered(self, v):
+        self._real_map_filtered = v
+        self._real_map_filtered_dict = {id_:row for row, id_ in enumerate(self._map_filtered)}
+
     @property
     def field_metadata(self):
         return self.cache.field_metadata
@@ -215,12 +226,16 @@ class View:
         return self._map_filtered[idx]
 
     def id_to_index(self, book_id):
-        return self._map_filtered.index(book_id)
+        try:
+            return self._real_map_filtered_dict[book_id]
+        except KeyError:
+            raise ValueError(f'No such book_id {book_id}')
     row = index_to_id
 
     def index(self, book_id, cache=False):
-        x = self._map if cache else self._map_filtered
-        return x.index(book_id)
+        if cache:
+            return self._map.index(book_id)
+        return self.id_to_index(book_id)
 
     def _get(self, field, idx, index_is_id=True, default_value=None, fmt=lambda x:x):
         id_ = idx if index_is_id else self.index_to_id(idx)
@@ -452,12 +467,7 @@ class View:
 
         # The ids list can contain invalid ids (deleted etc). We want to filter
         # those out while keeping the valid ones.
-        def f(id_):
-            try:
-                return self.id_to_index(id_)
-            except ValueError:
-                return None
-        res = [i for i in map(f, ids) if i is not None]
+        res = [self._real_map_filtered_dict[id_] for id_ in ids if id_ in self._real_map_filtered_dict]
         return res if res else None
 
     def remove(self, book_id):
diff --git a/src/calibre/gui2/library/alternate_views.py b/src/calibre/gui2/library/alternate_views.py
index d6bcf162e7..52c280b2f6 100644
--- a/src/calibre/gui2/library/alternate_views.py
+++ b/src/calibre/gui2/library/alternate_views.py
@@ -110,8 +110,9 @@ def event_has_mods(self, event=None):
 
 def mousePressEvent(self, event):
     ep = event.pos()
-    if self.indexAt(ep) in self.selectionModel().selectedIndexes() and \
-            event.button() == Qt.MouseButton.LeftButton and not self.event_has_mods():
+    # for performance, check the selection only once we know we need it
+    if event.button() == Qt.MouseButton.LeftButton and not self.event_has_mods() \
+                and self.indexAt(ep) in self.selectionModel().selectedIndexes():
         self.drag_start_pos = ep
     if hasattr(self, 'handle_mouse_press_event'):
         return self.handle_mouse_press_event(event)