Modify the message decoder to avoid std::string

esphome/components/hdmi_cec/cec_decoder.cpp

@@ -153,7 +153,7 @@ template<uint32_t OPERANDS> bool Decoder::do_operand_() {
   }
 }
 
-#define APPEND_LINE(...) (length_ = buf_append_printf(line_.data(), line_.size(), length_, __VA_ARGS__))
+#define APPEND_LINE(...) (length_ = buf_append_printf(line_, size_, length_, __VA_ARGS__))
 
 /**
  * List of specialised operand decode functions, one function per operand type.
@@ -441,16 +441,19 @@ const char *Decoder::find_opcode_name_(uint32_t opcode) const {
 bool Decoder::append_operand_(const char *word, uint8_t offset_incr /* default 1 */) {
   APPEND_LINE("[%s]", word);
   offset_ += offset_incr;
-  return (length_ < line_.size()) && (offset_ < frame_.size());
+  return (length_ < size_) && (offset_ < frame_.size());
 }
 
 /**
  * Entry function 'decode' to call for full decode of a CEC frame
  */
-const char *Decoder::decode() {
-  // prepare text buffer as empty
-  length_ = 0;         // currently accumulated length of text of operands
-  line_[length_] = 0;  // initialise operand text to empty string
+int Decoder::decode() {
+  // prepare text buffer state variables for an empty line
+  length_ = 0;            // currently accumulated length of text of operands
+  line_[length_] = '\0';  // initialise output text to empty string
+
+  if (!line_ || !size_)
+    return 0;
 
   // print src and dest fields into 'line_'
   address_decode_();
@@ -473,7 +476,7 @@ const char *Decoder::decode() {
     OperandDecode_f f = frametype->decode_f_;
     (this->*f)();  // call one of the above 'do_operand_<xx>' methods
   }
-  return line_.data();
+  return length_;
 }
 
 }  // namespace hdmi_cec

esphome/components/hdmi_cec/cec_decoder.h

@@ -53,8 +53,15 @@ template<typename KeyT, typename ValueT, unsigned int SIZE> class LookupTable {
  */
 class Decoder {
  public:
-  Decoder(const Frame &frame) : frame_(frame), length_(0), offset_(2) {}
-  const char *decode();
+  Decoder(const Frame &frame, char *line, unsigned int size)
+      : frame_(frame), line_(line), size_(size), length_(0), offset_(2) {}
+
+  /**
+   * Create a one-line textual representation of the Frame content for logging and debugging
+   * Store the text in the earlier provided 'line' buffer
+   * @return the length of the written text, excluding the terminating null character
+   */
+  int decode();
 
  protected:
   const char *find_opcode_name_(uint32_t opcode) const;
@@ -80,9 +87,10 @@ class Decoder {
   using VendorIdTable = LookupTable<uint32_t, const char *, 28>;
 
   const Frame &frame_;
-  std::array<char, 256> line_;  // to hold the text of the decoded frame
-  unsigned int length_;         // currently accumulated length of output text in 'line'
-  unsigned int offset_;  // current offset in frame to process next operand byte(s) (frame[0] and [1] are skipped)
+  char *const line_;         // buffer to hold the text of the decoded frame
+  const unsigned int size_;  // size of the 'line_' buffer
+  unsigned int length_;      // currently accumulated length of output text in 'line'
+  unsigned int offset_;      // current offset in frame to process next operand byte(s) (frame[0] and [1] are skipped)
 
   /**
    * The HDMI CEC standard specifies a set of distinct operand (parameter) types,

esphome/components/hdmi_cec/hdmi_cec.cpp

@@ -46,25 +46,22 @@ void Frame::set_header(uint8_t initiator_addr, uint8_t target_addr) {
     size_ = 1;
 }
 
-std::string Frame::to_string() const {
-  static constexpr unsigned int HEX_LINE_SIZE = Frame::MAX_LENGTH * 3 + 1;
-  char hex_line[HEX_LINE_SIZE];
+int Frame::to_text(char *line, unsigned int size) const {
   unsigned int offset = 0;
   for (int i = 0; i < size_; i++) {
-    offset = buf_append_printf(hex_line, HEX_LINE_SIZE, offset, "%02X:", data_[i]);
+    offset = buf_append_printf(line, size, offset, "%02X:", data_[i]);
   }
   // clear last printed ':'
   if (offset > 0)
-    hex_line[offset - 1] = '\0';
-  std::string result{hex_line};
+    line[--offset] = '\0';
 
 #ifdef HDMI_CEC_USE_DECODER
-  Decoder decoder(*this);
-  result += " => ";
-  result += decoder.decode();
+  offset = buf_append_printf(line, size, offset, " => ");
+  Decoder decoder(*this, line + offset, size - offset);
+  offset += decoder.decode();
 #endif
 
-  return result;
+  return offset;
 }
 
 void HDMICEC::setup() {
@@ -110,8 +107,11 @@ void HDMICEC::handle_received_message_(const Frame *frame) {
     return;
   }
 
-  auto frame_str = frame->to_string();
-  ESP_LOGD(TAG, "Received: %s", frame_str.c_str());
+#if ESPHOME_LOG_LEVEL >= ESPHOME_LOG_LEVEL_DEBUG
+  std::array<char, 256> line;
+  frame->to_text(line.data(), (unsigned int) (line.size()));
+  ESP_LOGD(TAG, "Received: %s", line.data());
+#endif
 
   std::vector<uint8_t> data(frame->size() - 1);
   for (int i = 1; i < frame->size(); i++)
@@ -362,9 +362,15 @@ void CECTransmit::transmit_message() {
   // Launch the transmit of the frame that is on the front of the queue
   transmit_state_ = TransmitState::BUSY;
   transmit_attempts_++;
+
+#if ESPHOME_LOG_LEVEL >= ESPHOME_LOG_LEVEL_DEBUG
   if (transmit_attempts_ <= 1) {
-    ESP_LOGD(TAG, "Sending: %s", (frame->to_string()).c_str());
+    std::array<char, 256> line;
+    frame->to_text(line.data(), (unsigned int) (line.size()));
+    ESP_LOGD(TAG, "Sending: %s", line.data());
   }
+#endif
+
   // Reset info from receiver on progress of this transmit
   // (The receiver will see this transmit and keep track of acknowledge bits.)
   n_bytes_received_ = 0;
@@ -573,7 +579,7 @@ void IRAM_ATTR CECTransmit::got_byte_eom_ack(bool eom, bool ack) {
   if (eom) {
     // some bus transfer ended. When the bus is free for a little while, we are allowed to transmit.
     required_idle_period_ = SIGNAL_FREE_TIME_AFTER_RECEIVE;
-    ATOMIC_SET(receiver_is_busy_, false);
+    ATOMIC_SET(receiver_is_busy_, false);  // unblock the transmitter
     ATOMIC_SET(eom_received_, true);  // atomic write to synchronize/commit above increments to the transmitter thread
   }
 }

esphome/components/hdmi_cec/hdmi_cec.h

@@ -57,7 +57,15 @@ class Frame {
       data_[size_++] = data;
     }
   }
-  std::string to_string() const;  // NOLINT
+
+  /**
+   * Create a one-line textual representation of the Frame content for logging and debugging
+   * @param line: a buffer in which the text is to be written
+   * @param size: The size of this buffer not to exceed with text
+   * @return the length of the written text, excluding the terminating null character
+   */
+  int to_text(char *line, unsigned int size) const;
+
   constexpr static unsigned int MAX_LENGTH = 16;
   constexpr static unsigned int MAX_PAYLOAD_LENGTH = MAX_LENGTH - 1;  // subtract 1 byte for the src&dst field