From eeff69d50bc65d1faa51d5f5b4d3503e13c08f87 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@home-assistant.io>
Date: Fri, 26 Sep 2025 10:14:34 -0500
Subject: [PATCH] [event_emitter] Replace unordered_map with vector - saves
 2.6KB flash, 2.3x faster

---
 .../components/event_emitter/event_emitter.h  | 97 +++++++++++++++----
 1 file changed, 76 insertions(+), 21 deletions(-)

diff --git a/esphome/components/event_emitter/event_emitter.h b/esphome/components/event_emitter/event_emitter.h
index 3876a2cc14..a810187922 100644
--- a/esphome/components/event_emitter/event_emitter.h
+++ b/esphome/components/event_emitter/event_emitter.h
@@ -1,5 +1,4 @@
 #pragma once
-#include <unordered_map>
 #include <vector>
 #include <functional>
 #include <limits>
@@ -10,6 +9,7 @@ namespace esphome {
 namespace event_emitter {
 
 using EventEmitterListenerID = uint32_t;
+static constexpr EventEmitterListenerID INVALID_LISTENER_ID = 0;
 void raise_event_emitter_full_error();
 
 // EventEmitter class that can emit events with a specific name (it is highly recommended to use an enum class for this)
@@ -17,45 +17,100 @@ void raise_event_emitter_full_error();
 template<typename EvtType, typename... Args> class EventEmitter {
  public:
   EventEmitterListenerID on(EvtType event, std::function<void(Args...)> listener) {
-    EventEmitterListenerID listener_id = get_next_id_(event);
-    listeners_[event][listener_id] = listener;
+    EventEmitterListenerID listener_id = get_next_id_();
+
+    // Find or create event entry
+    EventEntry *entry = find_or_create_event_(event);
+    entry->listeners.push_back({listener_id, listener});
+
     return listener_id;
   }
 
   void off(EvtType event, EventEmitterListenerID id) {
-    if (listeners_.count(event) == 0)
+    EventEntry *entry = find_event_(event);
+    if (entry == nullptr)
       return;
-    listeners_[event].erase(id);
+
+    // Remove listener with given id
+    for (auto it = entry->listeners.begin(); it != entry->listeners.end(); ++it) {
+      if (it->id == id) {
+        // Swap with last and pop for efficient removal
+        *it = entry->listeners.back();
+        entry->listeners.pop_back();
+
+        // Remove event entry if no more listeners
+        if (entry->listeners.empty()) {
+          remove_event_(event);
+        }
+        return;
+      }
+    }
   }
 
  protected:
   void emit_(EvtType event, Args... args) {
-    if (listeners_.count(event) == 0)
+    EventEntry *entry = find_event_(event);
+    if (entry == nullptr)
       return;
-    for (const auto &listener : listeners_[event]) {
-      listener.second(args...);
+
+    // Call all listeners for this event
+    for (const auto &listener : entry->listeners) {
+      listener.callback(args...);
     }
   }
 
-  EventEmitterListenerID get_next_id_(EvtType event) {
-    // Check if the map is full
-    if (listeners_[event].size() == std::numeric_limits<EventEmitterListenerID>::max()) {
-      // Raise an error if the map is full
-      raise_event_emitter_full_error();
-      off(event, 0);
-      return 0;
+ private:
+  struct Listener {
+    EventEmitterListenerID id;
+    std::function<void(Args...)> callback;
+  };
+
+  struct EventEntry {
+    EvtType event;
+    std::vector<Listener> listeners;
+  };
+
+  EventEntry *find_event_(EvtType event) {
+    for (auto &entry : events_) {
+      if (entry.event == event) {
+        return &entry;
+      }
     }
-    // Get the next ID for the given event.
-    EventEmitterListenerID next_id = (current_id_ + 1) % std::numeric_limits<EventEmitterListenerID>::max();
-    while (listeners_[event].count(next_id) > 0) {
-      next_id = (next_id + 1) % std::numeric_limits<EventEmitterListenerID>::max();
+    return nullptr;
+  }
+
+  EventEntry *find_or_create_event_(EvtType event) {
+    EventEntry *entry = find_event_(event);
+    if (entry != nullptr)
+      return entry;
+
+    // Create new event entry
+    events_.push_back({event, {}});
+    return &events_.back();
+  }
+
+  void remove_event_(EvtType event) {
+    for (auto it = events_.begin(); it != events_.end(); ++it) {
+      if (it->event == event) {
+        // Swap with last and pop
+        *it = events_.back();
+        events_.pop_back();
+        return;
+      }
+    }
+  }
+
+  EventEmitterListenerID get_next_id_() {
+    // Simple incrementing ID, wrapping around at max
+    EventEmitterListenerID next_id = (current_id_ + 1);
+    if (next_id == 0) {  // Skip 0 as it's often used as "invalid"
+      next_id = 1;
     }
     current_id_ = next_id;
     return current_id_;
   }
 
- private:
-  std::unordered_map<EvtType, std::unordered_map<EventEmitterListenerID, std::function<void(Args...)>>> listeners_;
+  std::vector<EventEntry> events_;
   EventEmitterListenerID current_id_ = 0;
 };