[core] Simplify ESPTime::strftime() and save 20 bytes flash

esphome/core/time.cpp

@@ -46,23 +46,17 @@ struct tm ESPTime::to_c_tm() {
   return c_tm;
 }
 
-std::string ESPTime::strftime(const std::string &format) {
+std::string ESPTime::strftime(const char *format, size_t format_len) {
-  std::string timestr;
-  timestr.resize(format.size() * 4);
   struct tm c_tm = this->to_c_tm();
-  size_t len = ::strftime(&timestr[0], timestr.size(), format.c_str(), &c_tm);
+  char buf[128];
-  while (len == 0) {
+  size_t len = ::strftime(buf, sizeof(buf), format, &c_tm);
-    if (timestr.size() >= 128) {
+  if (len > 0) {
-      // strftime has failed for reasons unrelated to the size of the buffer
+    return std::string(buf, len);
-      // so return a formatting error
+  }
   return "ERROR";
 }
-    timestr.resize(timestr.size() * 2);
+
-    len = ::strftime(&timestr[0], timestr.size(), format.c_str(), &c_tm);
+std::string ESPTime::strftime(const std::string &format) { return this->strftime(format.c_str(), format.size()); }
-  }
-  timestr.resize(len);
-  return timestr;
-}
 
 bool ESPTime::strptime(const std::string &time_to_parse, ESPTime &esp_time) {
   uint16_t year;

esphome/core/time.h

@@ -55,6 +55,23 @@ struct ESPTime {
    */
   std::string strftime(const std::string &format);
 
+  /** Convert this ESPTime struct to a string as specified by the format argument.
+   * @see https://www.gnu.org/software/libc/manual/html_node/Formatting-Calendar-Time.html#index-strftime
+   *
+   * This overload is optimized for string literals and avoids std::string parameter overhead.
+   *
+   * @param format The format string (null-terminated C string)
+   * @param format_len Optional length of the format string. If 0 (default), strlen() will be called.
+   *
+   * @warning This method uses dynamically allocated strings which can cause heap fragmentation with some
+   * microcontrollers.
+   *
+   * @warning This method can return "ERROR" when the underlying strftime() call fails, e.g. when the
+   * format string contains unsupported specifiers or when the format string doesn't produce any
+   * output.
+   */
+  std::string strftime(const char *format, size_t format_len = 0);
+
   /// Check if this ESPTime is valid (all fields in range and year is greater than 2018)
   bool is_valid() const { return this->year >= 2019 && this->fields_in_range(); }