Respond to CR.

Signal/src/ViewControllers/ConversationView/ConversationViewController.m

@@ -708,16 +708,24 @@ typedef enum : NSUInteger {
 - (NSIndexPath *_Nullable)indexPathOfMessageOnOpen
 {
     OWSAssert(self.focusMessageIdOnOpen);
+    OWSAssert(self.dynamicInteractions.focusMessagePosition);
 
-    NSInteger row = 0;
-    for (ConversationViewItem *viewItem in self.viewItems) {
-        if ([viewItem.interaction.uniqueId isEqualToString:self.focusMessageIdOnOpen]) {
-            return [NSIndexPath indexPathForRow:row inSection:0];
-        }
-        row++;
+    if (!self.dynamicInteractions.focusMessagePosition) {
+        // This might happen if the focus message has disappeared
+        // before this view could appear.
+        OWSFail(@"%@ focus message has unknown position.", self.logTag);
+        return nil;
     }
+    NSUInteger focusMessagePosition = self.dynamicInteractions.focusMessagePosition.unsignedIntegerValue;
+    if (focusMessagePosition >= self.viewItems.count) {
+        // This might happen if the focus message is outside the maximum
+        // valid load window size for this view.
+        OWSFail(@"%@ focus message has invalid position.", self.logTag);
         return nil;
     }
+    NSInteger row = (NSInteger)((self.viewItems.count - 1) - focusMessagePosition);
+    return [NSIndexPath indexPathForRow:row inSection:0];
+}
 
 - (void)scrollToDefaultPosition
 {

SignalMessaging/utils/ThreadUtil.h

@@ -25,6 +25,10 @@ NS_ASSUME_NONNULL_BEGIN
 // to include the unread indicator.
 @property (nonatomic, nullable, readonly) NSNumber *unreadIndicatorPosition;
 
+// Represents the "reverse index" of the focus message, if any.
+// The "reverse index" is the distance of this interaction from
+// the last interaction in the thread.  Therefore the last interaction
+// will have a "reverse index" of zero.
 @property (nonatomic, nullable, readonly) NSNumber *focusMessagePosition;
 
 // If there are unseen messages in the thread, this is the timestamp

SignalMessaging/utils/ThreadUtil.m

@@ -639,23 +639,29 @@ NS_ASSUME_NONNULL_BEGIN
     OWSAssert(transaction);
     OWSAssert(focusMessageId);
 
-    // Enumerate in reverse to count the number of messages after the "focus message".
-    __block NSUInteger count = 0;
-    __block BOOL didMatch = NO;
-    [[transaction ext:TSMessageDatabaseViewExtensionName]
-        enumerateKeysInGroup:thread.uniqueId
-                 withOptions:NSEnumerationReverse
-                  usingBlock:^(NSString *collection, NSString *key, NSUInteger index, BOOL *stop) {
-                      if ([key isEqualToString:focusMessageId]) {
-                          didMatch = YES;
-                          *stop = YES;
-                          return;
+    YapDatabaseViewTransaction *databaseView = [transaction ext:TSMessageDatabaseViewExtensionName];
+
+    NSString *_Nullable group = nil;
+    NSUInteger index;
+    BOOL success =
+        [databaseView getGroup:&group index:&index forKey:focusMessageId inCollection:TSInteraction.collection];
+    if (!success) {
+        // This might happen if the focus message has disappeared
+        // before this view could appear.
+        OWSFail(@"%@ failed to find focus message index.", self.logTag);
+        return nil;
     }
-
-                      count++;
-                  }];
-
-    return didMatch ? @(count) : nil;
+    if (![group isEqualToString:thread.uniqueId]) {
+        OWSFail(@"%@ focus message has invalid group.", self.logTag);
+        return nil;
+    }
+    NSUInteger count = [databaseView numberOfItemsInGroup:thread.uniqueId];
+    if (index >= count) {
+        OWSFail(@"%@ focus message has invalid index.", self.logTag);
+        return nil;
+    }
+    NSUInteger position = (count - index) - 1;
+    return @(position);
 }
 
 + (BOOL)shouldShowGroupProfileBannerInThread:(TSThread *)thread blockingManager:(OWSBlockingManager *)blockingManager