From 4abf18a5f11f4e70b8596fb81a0ed9c56b84472a Mon Sep 17 00:00:00 2001
From: Bruno Windels <274386+bwindels@users.noreply.github.com>
Date: Fri, 25 Nov 2022 11:26:53 +0100
Subject: [PATCH] don't look at tiles before the inserted tile, there is no
 need

also clarify with comments how the current algorithm works
---
 src/domain/session/room/timeline/TilesCollection.js | 12 ++++++++----
 src/domain/session/room/timeline/tiles/DateTile.ts  |  3 +++
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/src/domain/session/room/timeline/TilesCollection.js b/src/domain/session/room/timeline/TilesCollection.js
index 29e0bdfc..e4ed9d24 100644
--- a/src/domain/session/room/timeline/TilesCollection.js
+++ b/src/domain/session/room/timeline/TilesCollection.js
@@ -152,9 +152,11 @@ export class TilesCollection extends BaseObservableList {
 
     _evaluateDateHeaderAtIdx(tileIdx) {
         //console.log("_evaluateDateHeaderAtIdx", tileIdx);
-        // consider the two adjacent tiles where the previous sibling changed:
-        // the new tile and the next tile
-        for (let i = -2; i < 3; i += 1) {
+        // consider two tiles after the inserted tile, because
+        // the first of the two tiles may be a DateTile in which case,
+        // we remove it after looking at the needsDateSeparator prop of the
+        // next next tile
+        for (let i = 0; i < 2; i += 1) {
             const idx = tileIdx + i;
             if (idx < 0) {
                 continue;
@@ -173,10 +175,12 @@ export class TilesCollection extends BaseObservableList {
                     this.emitUpdate(idx - 1, prevTile, "date");
                 } else {
                     //console.log("    add", idx, tile.shape, tile.eventId);
+                    // adding a tile shift all the indices we need to consider
+                    // especially given we consider removals for the tile that
+                    // comes after a datetile
                     tileIdx += 1;
                     this._addTileAt(idx, tile.createDateSeparator());
                 }
-                // TODO must be looking at the wrong index to find the old date separator??
             } else if (hasDateSeparator) {
                 // this is never triggered because needsDateSeparator is not cleared
                 // when loading more items because we don't do anything once the
diff --git a/src/domain/session/room/timeline/tiles/DateTile.ts b/src/domain/session/room/timeline/tiles/DateTile.ts
index 413ec2b2..92bd42e5 100644
--- a/src/domain/session/room/timeline/tiles/DateTile.ts
+++ b/src/domain/session/room/timeline/tiles/DateTile.ts
@@ -119,6 +119,9 @@ export class DateTile extends ViewModel implements ITile<BaseEventEntry> {
 
     // let item know it has a new sibling
     updatePreviousSibling(prev: ITile<BaseEntry> | undefined): void {
+        // forward the sibling update to our next tile, so it is informed
+        // about it's previous sibling beyond the date header (which is it's direct previous sibling)
+        // so it can recalculate whether it still needs a date header
         this._firstTileInDay.updatePreviousSibling(prev);
     }