From d516627957778427af22ce57c2d5c1294e9948d5 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Sun, 9 Nov 2025 16:37:14 -0600
Subject: [PATCH] [uart] Store static data in flash and use function pointers
 for lambdas (#11784)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>
---
 esphome/components/uart/__init__.py  |  7 ++++--
 esphome/components/uart/automation.h | 35 +++++++++++++++-------------
 2 files changed, 24 insertions(+), 18 deletions(-)

diff --git a/esphome/components/uart/__init__.py b/esphome/components/uart/__init__.py
index eb911ed0073..cbc11d0db04 100644
--- a/esphome/components/uart/__init__.py
+++ b/esphome/components/uart/__init__.py
@@ -31,7 +31,7 @@ from esphome.const import (
     PLATFORM_HOST,
     PlatformFramework,
 )
-from esphome.core import CORE
+from esphome.core import CORE, ID
 import esphome.final_validate as fv
 from esphome.yaml_util import make_data_base
 
@@ -446,7 +446,10 @@ async def uart_write_to_code(config, action_id, template_arg, args):
         templ = await cg.templatable(data, args, cg.std_vector.template(cg.uint8))
         cg.add(var.set_data_template(templ))
     else:
-        cg.add(var.set_data_static(cg.ArrayInitializer(*data)))
+        # Generate static array in flash to avoid RAM copy
+        arr_id = ID(f"{action_id}_data", is_declaration=True, type=cg.uint8)
+        arr = cg.static_const_array(arr_id, cg.ArrayInitializer(*data))
+        cg.add(var.set_data_static(arr, len(data)))
     return var
 
 
diff --git a/esphome/components/uart/automation.h b/esphome/components/uart/automation.h
index ad2c4d2bf1d..c2eb308eb8d 100644
--- a/esphome/components/uart/automation.h
+++ b/esphome/components/uart/automation.h
@@ -10,32 +10,35 @@ namespace uart {
 
 template<typename... Ts> class UARTWriteAction : public Action<Ts...>, public Parented<UARTComponent> {
  public:
-  void set_data_template(std::function<std::vector<uint8_t>(Ts...)> func) {
-    this->data_func_ = func;
-    this->static_ = false;
+  void set_data_template(std::vector<uint8_t> (*func)(Ts...)) {
+    // Stateless lambdas (generated by ESPHome) implicitly convert to function pointers
+    this->code_.func = func;
+    this->len_ = -1;  // Sentinel value indicates template mode
   }
-  void set_data_static(std::vector<uint8_t> &&data) {
-    this->data_static_ = std::move(data);
-    this->static_ = true;
-  }
-  void set_data_static(std::initializer_list<uint8_t> data) {
-    this->data_static_ = std::vector<uint8_t>(data);
-    this->static_ = true;
+
+  // Store pointer to static data in flash (no RAM copy)
+  void set_data_static(const uint8_t *data, size_t len) {
+    this->code_.data = data;
+    this->len_ = len;  // Length >= 0 indicates static mode
   }
 
   void play(const Ts &...x) override {
-    if (this->static_) {
-      this->parent_->write_array(this->data_static_);
+    if (this->len_ >= 0) {
+      // Static mode: use pointer and length
+      this->parent_->write_array(this->code_.data, static_cast<size_t>(this->len_));
     } else {
-      auto val = this->data_func_(x...);
+      // Template mode: call function
+      auto val = this->code_.func(x...);
       this->parent_->write_array(val);
     }
   }
 
  protected:
-  bool static_{false};
-  std::function<std::vector<uint8_t>(Ts...)> data_func_{};
-  std::vector<uint8_t> data_static_{};
+  ssize_t len_{-1};  // -1 = template mode, >=0 = static mode with length
+  union Code {
+    std::vector<uint8_t> (*func)(Ts...);  // Function pointer (stateless lambdas)
+    const uint8_t *data;                  // Pointer to static data in flash
+  } code_;
 };
 
 }  // namespace uart