perf(regex_cache): replace half-wipe eviction with LRU

Stored entries now carry a monotonic access counter refreshed on every
hit. When the cache fills, the 10% least-recently-used entries are
dropped instead of an arbitrary half of the table, which previously
caused thrashing under diverse input by evicting hot entries.

lib/aprsme/regex_cache.ex

@@ -1,7 +1,11 @@
 defmodule Aprsme.RegexCache do
   @moduledoc """
-  A simple in-memory cache for compiled regex patterns to avoid recompilation.
-  Uses ETS for thread-safe access.
+  In-memory cache for compiled regex patterns that avoids recompilation.
+
+  Stored values carry a monotonic access counter so eviction can drop the
+  least-recently-used entries instead of an arbitrary half of the table —
+  previously a thrashing pattern under diverse input would churn the cache
+  and keep paying compile cost on every miss.
   """
 
   use GenServer
@@ -10,6 +14,9 @@ defmodule Aprsme.RegexCache do
 
   @table_name :regex_cache
   @max_cache_size 1000
+  # When the cache is full, drop this fraction of least-recently-used entries.
+  # 10% keeps hot entries warm instead of wiping half the cache every churn.
+  @evict_fraction 0.1
 
   def start_link(_opts) do
     GenServer.start_link(__MODULE__, %{}, name: __MODULE__)
@@ -17,9 +24,9 @@ defmodule Aprsme.RegexCache do
 
   @impl true
   def init(_) do
-    # Create ETS table for caching compiled regexes
+    # Table stores {pattern, regex, last_access_counter}
     :ets.new(@table_name, [:set, :protected, :named_table, read_concurrency: true])
-    {:ok, %{}}
+    {:ok, %{counter: 0}}
   end
 
   @doc """
@@ -30,27 +37,31 @@ defmodule Aprsme.RegexCache do
   end
 
   @impl true
-  def handle_call({:get_or_compile, pattern_string}, _from, state) do
-    result =
-      case :ets.lookup(@table_name, pattern_string) do
-        [{^pattern_string, regex}] ->
-          {:ok, regex}
+  def handle_call({:get_or_compile, pattern_string}, _from, %{counter: counter} = state) do
+    next_counter = counter + 1
 
-        [] ->
-          compile_and_cache(pattern_string)
-      end
+    case :ets.lookup(@table_name, pattern_string) do
+      [{^pattern_string, regex, _last}] ->
+        # Refresh the access counter so subsequent evictions see this as recent.
+        :ets.insert(@table_name, {pattern_string, regex, next_counter})
+        {:reply, {:ok, regex}, %{state | counter: next_counter}}
 
-    {:reply, result, state}
+      [] ->
+        case compile_and_cache(pattern_string, next_counter) do
+          {:ok, regex} -> {:reply, {:ok, regex}, %{state | counter: next_counter}}
+          error -> {:reply, error, state}
+        end
+    end
   end
 
-  defp compile_and_cache(pattern_string) do
+  defp compile_and_cache(pattern_string, counter) do
     case Regex.compile(pattern_string) do
       {:ok, regex} ->
         if :ets.info(@table_name, :size) >= @max_cache_size do
-          clear_oldest_entries()
+          evict_lru()
         end
 
-        :ets.insert(@table_name, {pattern_string, regex})
+        :ets.insert(@table_name, {pattern_string, regex, counter})
         {:ok, regex}
 
       error ->
@@ -58,14 +69,15 @@ defmodule Aprsme.RegexCache do
     end
   end
 
-  defp clear_oldest_entries do
-    # Simple strategy: clear half the cache
-    # In production, you might want LRU eviction
+  # Evict the @evict_fraction oldest entries by access counter. This is O(n)
+  # over the table, but only runs when the table is full, not on every insert.
+  defp evict_lru do
     entries = :ets.tab2list(@table_name)
-    to_remove = Enum.take(entries, div(length(entries), 2))
+    target = max(1, trunc(length(entries) * @evict_fraction))
 
-    Enum.each(to_remove, fn {key, _} ->
-      :ets.delete(@table_name, key)
-    end)
+    entries
+    |> Enum.sort_by(fn {_key, _regex, last} -> last end)
+    |> Enum.take(target)
+    |> Enum.each(fn {key, _regex, _last} -> :ets.delete(@table_name, key) end)
   end
 end