perf: fix PacketBatcher O(n) length/1 to O(1) counter

Ultraworked with Sisyphus

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

lib/aprsme_web/live/map_live/packet_batcher.ex

@@ -10,11 +10,12 @@ defmodule AprsmeWeb.MapLive.PacketBatcher do
   # milliseconds
   @batch_timeout 100
 
-  defstruct [:parent_pid, :buffer, :timer_ref]
+  defstruct [:parent_pid, :buffer, :timer_ref, :buffer_size]
 
   @type t :: %__MODULE__{
           parent_pid: pid(),
           buffer: [map()],
+          buffer_size: non_neg_integer(),
           timer_ref: reference() | nil
         }
 
@@ -50,27 +51,28 @@ defmodule AprsmeWeb.MapLive.PacketBatcher do
      %__MODULE__{
        parent_pid: parent_pid,
        buffer: [],
+       buffer_size: 0,
        timer_ref: nil
      }}
   end
 
   @impl true
   def handle_cast({:add_packet, packet}, state) do
-    # Add packet to buffer
     new_buffer = [packet | state.buffer]
+    new_size = state.buffer_size + 1
 
     # Cancel existing timer if any
     state = cancel_timer(state)
 
     # Check if we should process immediately or wait
-    if length(new_buffer) >= @batch_size do
+    if new_size >= @batch_size do
       # Process immediately
       process_batch(new_buffer, state.parent_pid)
-      {:noreply, %{state | buffer: [], timer_ref: nil}}
+      {:noreply, %{state | buffer: [], buffer_size: 0, timer_ref: nil}}
     else
       # Set timer for batch timeout
       timer_ref = Process.send_after(self(), :batch_timeout, @batch_timeout)
-      {:noreply, %{state | buffer: new_buffer, timer_ref: timer_ref}}
+      {:noreply, %{state | buffer: new_buffer, buffer_size: new_size, timer_ref: timer_ref}}
     end
   end
 
@@ -78,20 +80,20 @@ defmodule AprsmeWeb.MapLive.PacketBatcher do
   def handle_cast(:flush, state) do
     state = cancel_timer(state)
 
-    if state.buffer != [] do
+    if state.buffer_size > 0 do
       process_batch(state.buffer, state.parent_pid)
     end
 
-    {:noreply, %{state | buffer: [], timer_ref: nil}}
+    {:noreply, %{state | buffer: [], buffer_size: 0, timer_ref: nil}}
   end
 
   @impl true
   def handle_info(:batch_timeout, state) do
-    if state.buffer != [] do
+    if state.buffer_size > 0 do
       process_batch(state.buffer, state.parent_pid)
     end
 
-    {:noreply, %{state | buffer: [], timer_ref: nil}}
+    {:noreply, %{state | buffer: [], buffer_size: 0, timer_ref: nil}}
   end
 
   @impl true