From cfe59cb4b42c7ec14b35f88d76889e514a70a0cf Mon Sep 17 00:00:00 2001
From: Eoghan Murray <eoghan@getthere.ie>
Date: Wed, 1 Apr 2026 12:00:00 +0800
Subject: [PATCH] Sort events upon creation, and keep ordered (#411)

* Sort events at start, as otherwise we risk misidentifying the last event

* Keep inserted events in the correct order, ensuring we don't misidentify the last event

 - e.g. network conditions mean that 'live' events come in non-sequentially
 - or so that adding custom events to an existing event works

* Ensure we maintain original ordering while inserting a new event which has an identical timestamp to an existing event. This came up with a series of mutations which had the same timestamp but needed to be applied in the correct order

* Fast track the common case of a new event being added which occurs after all prior events
---
 src/replay/index.ts   |  4 ++++
 src/replay/machine.ts | 24 +++++++++++++++++++++++-
 src/replay/timer.ts   |  1 -
 3 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/src/replay/index.ts b/src/replay/index.ts
index 8cdada96..7af8bf63 100644
--- a/src/replay/index.ts
+++ b/src/replay/index.ts
@@ -129,6 +129,8 @@ export class Replayer {
     events: Array<eventWithTime | string>,
     config?: Partial<playerConfig>,
   ) {
+    events.sort((a1, a2) => a1.timestamp - a2.timestamp);
+
     if (!config?.liveMode && events.length < 2) {
       throw new Error('Replayer need at least 2 events.');
     }
@@ -520,6 +522,8 @@ export class Replayer {
         castFn();
       }
       this.service.send({ type: 'CAST_EVENT', payload: { event } });
+
+      // events are kept sorted by timestamp, check if this is the last event
       if (
         event ===
         this.service.state.context.events[
diff --git a/src/replay/machine.ts b/src/replay/machine.ts
index 86459e6a..d2e3313f 100644
--- a/src/replay/machine.ts
+++ b/src/replay/machine.ts
@@ -237,7 +237,29 @@ export function createPlayerService(
           if (machineEvent.type === 'ADD_EVENT') {
             const { event } = machineEvent.payload;
             addDelay(event, baselineTime);
-            events.push(event);
+
+            let end = events.length - 1;
+            if (events[end].timestamp <= event.timestamp) {
+              // fast track
+              events.push(event);
+            } else {
+              let insertion_index = -1;
+              let start = 0;
+              while (start <= end) {
+                let mid = Math.floor((start + end) / 2);
+                if (events[mid].timestamp <= event.timestamp) {
+                  start = mid + 1;
+                } else {
+                  end = mid - 1;
+                }
+              }
+              if (insertion_index === -1) {
+                insertion_index = start;
+              }
+              events.splice(insertion_index, 0, event);
+            }
+
+
             const isSync = event.timestamp < baselineTime;
             const castFn = getCastFn(event, isSync);
             if (isSync) {
diff --git a/src/replay/timer.ts b/src/replay/timer.ts
index e5eefa04..2d2d7c23 100644
--- a/src/replay/timer.ts
+++ b/src/replay/timer.ts
@@ -34,7 +34,6 @@ export class Timer {
   }
 
   public start() {
-    this.actions.sort((a1, a2) => a1.delay - a2.delay);
     this.timeOffset = 0;
     let lastTimestamp = performance.now();
     const { actions } = this;