[uptime] Format text sensor output on stack to avoid heap allocations (#13150)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

esphome/components/uptime/text_sensor/uptime_text_sensor.cpp

@@ -9,6 +9,19 @@ namespace uptime {
 
 static const char *const TAG = "uptime.sensor";
 
+// Clamp position to valid buffer range when snprintf indicates truncation
+static size_t clamp_buffer_pos(size_t pos, size_t buf_size) { return pos < buf_size ? pos : buf_size - 1; }
+
+static void append_unit(char *buf, size_t buf_size, size_t &pos, const char *separator, unsigned value,
+                        const char *label) {
+  if (pos > 0) {
+    pos += snprintf(buf + pos, buf_size - pos, "%s", separator);
+    pos = clamp_buffer_pos(pos, buf_size);
+  }
+  pos += snprintf(buf + pos, buf_size - pos, "%u%s", value, label);
+  pos = clamp_buffer_pos(pos, buf_size);
+}
+
 void UptimeTextSensor::setup() {
   this->last_ms_ = millis();
   if (this->last_ms_ < 60 * 1000)
@@ -16,11 +29,6 @@ void UptimeTextSensor::setup() {
   this->update();
 }
 
-void UptimeTextSensor::insert_buffer_(std::string &buffer, const char *key, unsigned value) const {
-  buffer.insert(0, this->separator_);
-  buffer.insert(0, str_sprintf("%u%s", value, key));
-}
-
 void UptimeTextSensor::update() {
   auto now = millis();
   // get whole seconds since last update. Note that even if the millis count has overflowed between updates,
@@ -29,36 +37,58 @@ void UptimeTextSensor::update() {
   this->last_ms_ = now - delta % 1000;  // save remainder for next update
   delta /= 1000;
   this->uptime_ += delta;
-  auto uptime = this->uptime_;
+  uint32_t uptime = this->uptime_;
   unsigned interval = this->get_update_interval() / 1000;
-  std::string buffer{};
-  // display from the largest unit that corresponds to the update interval, drop larger units that are zero.
-  while (true) {  // enable use of break for early exit
-    unsigned remainder = uptime % 60;
-    uptime /= 60;
-    if (interval < 30) {
-      this->insert_buffer_(buffer, this->seconds_text_, remainder);
-      if (!this->expand_ && uptime == 0)
-        break;
+
+  // Calculate all time units
+  unsigned seconds = uptime % 60;
+  uptime /= 60;
+  unsigned minutes = uptime % 60;
+  uptime /= 60;
+  unsigned hours = uptime % 24;
+  uptime /= 24;
+  unsigned days = uptime;
+
+  // Determine which units to display based on interval thresholds
+  bool seconds_enabled = interval < 30;
+  bool minutes_enabled = interval < 1800;
+  bool hours_enabled = interval < 12 * 3600;
+
+  // Show from highest non-zero unit (or all in expand mode) down to smallest enabled
+  bool show_days = this->expand_ || days > 0;
+  bool show_hours = hours_enabled && (show_days || hours > 0);
+  bool show_minutes = minutes_enabled && (show_hours || minutes > 0);
+  bool show_seconds = seconds_enabled && (show_minutes || seconds > 0);
+
+  // If nothing shown, show smallest enabled unit
+  if (!show_days && !show_hours && !show_minutes && !show_seconds) {
+    if (seconds_enabled) {
+      show_seconds = true;
+    } else if (minutes_enabled) {
+      show_minutes = true;
+    } else if (hours_enabled) {
+      show_hours = true;
+    } else {
+      show_days = true;
     }
-    remainder = uptime % 60;
-    uptime /= 60;
-    if (interval < 1800) {
-      this->insert_buffer_(buffer, this->minutes_text_, remainder);
-      if (!this->expand_ && uptime == 0)
-        break;
-    }
-    remainder = uptime % 24;
-    uptime /= 24;
-    if (interval < 12 * 3600) {
-      this->insert_buffer_(buffer, this->hours_text_, remainder);
-      if (!this->expand_ && uptime == 0)
-        break;
-    }
-    this->insert_buffer_(buffer, this->days_text_, (unsigned) uptime);
-    break;
   }
-  this->publish_state(buffer);
+
+  // Build output string on stack
+  // Home Assistant max state length is 255 chars + null terminator
+  char buf[256];
+  size_t pos = 0;
+  buf[0] = '\0';  // Initialize for empty case
+
+  if (show_days)
+    append_unit(buf, sizeof(buf), pos, this->separator_, days, this->days_text_);
+  if (show_hours)
+    append_unit(buf, sizeof(buf), pos, this->separator_, hours, this->hours_text_);
+  if (show_minutes)
+    append_unit(buf, sizeof(buf), pos, this->separator_, minutes, this->minutes_text_);
+  if (show_seconds)
+    append_unit(buf, sizeof(buf), pos, this->separator_, seconds, this->seconds_text_);
+
+  this->publish_state(buf);
 }
 
 float UptimeTextSensor::get_setup_priority() const { return setup_priority::HARDWARE; }

esphome/components/uptime/text_sensor/uptime_text_sensor.h

@@ -29,7 +29,6 @@ class UptimeTextSensor : public text_sensor::TextSensor, public PollingComponent
   void set_seconds(const char *seconds_text) { this->seconds_text_ = seconds_text; }
 
  protected:
-  void insert_buffer_(std::string &buffer, const char *key, unsigned value) const;
   const char *days_text_;
   const char *hours_text_;
   const char *minutes_text_;