From 9c85a7eff34d0268321609dd755528c3a5aeaed7 Mon Sep 17 00:00:00 2001
From: "J. Nick Koston" <nick@koston.org>
Date: Sun, 14 Sep 2025 16:50:38 -0500
Subject: [PATCH] [core] Optimize MAC address formatting to eliminate sprintf
 dependency (#10713)

---
 .../ethernet/ethernet_component.cpp           |  4 ++-
 esphome/core/helpers.cpp                      | 15 +++++-----
 esphome/core/helpers.h                        | 29 +++++++++++++++++++
 3 files changed, 40 insertions(+), 8 deletions(-)

diff --git a/esphome/components/ethernet/ethernet_component.cpp b/esphome/components/ethernet/ethernet_component.cpp
index cd73333222..a48fd27383 100644
--- a/esphome/components/ethernet/ethernet_component.cpp
+++ b/esphome/components/ethernet/ethernet_component.cpp
@@ -682,7 +682,9 @@ void EthernetComponent::get_eth_mac_address_raw(uint8_t *mac) {
 std::string EthernetComponent::get_eth_mac_address_pretty() {
   uint8_t mac[6];
   get_eth_mac_address_raw(mac);
-  return str_snprintf("%02X:%02X:%02X:%02X:%02X:%02X", 17, mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
+  char buf[18];
+  format_mac_addr_upper(mac, buf);
+  return std::string(buf);
 }
 
 eth_duplex_t EthernetComponent::get_duplex_mode() {
diff --git a/esphome/core/helpers.cpp b/esphome/core/helpers.cpp
index 43d6f1153c..f1560711ef 100644
--- a/esphome/core/helpers.cpp
+++ b/esphome/core/helpers.cpp
@@ -255,23 +255,22 @@ size_t parse_hex(const char *str, size_t length, uint8_t *data, size_t count) {
 }
 
 std::string format_mac_address_pretty(const uint8_t *mac) {
-  return str_snprintf("%02X:%02X:%02X:%02X:%02X:%02X", 17, mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
+  char buf[18];
+  format_mac_addr_upper(mac, buf);
+  return std::string(buf);
 }
 
-static char format_hex_char(uint8_t v) { return v >= 10 ? 'a' + (v - 10) : '0' + v; }
 std::string format_hex(const uint8_t *data, size_t length) {
   std::string ret;
   ret.resize(length * 2);
   for (size_t i = 0; i < length; i++) {
-    ret[2 * i] = format_hex_char((data[i] & 0xF0) >> 4);
+    ret[2 * i] = format_hex_char(data[i] >> 4);
     ret[2 * i + 1] = format_hex_char(data[i] & 0x0F);
   }
   return ret;
 }
 std::string format_hex(const std::vector<uint8_t> &data) { return format_hex(data.data(), data.size()); }
 
-static char format_hex_pretty_char(uint8_t v) { return v >= 10 ? 'A' + (v - 10) : '0' + v; }
-
 // Shared implementation for uint8_t and string hex formatting
 static std::string format_hex_pretty_uint8(const uint8_t *data, size_t length, char separator, bool show_length) {
   if (data == nullptr || length == 0)
@@ -280,7 +279,7 @@ static std::string format_hex_pretty_uint8(const uint8_t *data, size_t length, c
   uint8_t multiple = separator ? 3 : 2;  // 3 if separator is not \0, 2 otherwise
   ret.resize(multiple * length - (separator ? 1 : 0));
   for (size_t i = 0; i < length; i++) {
-    ret[multiple * i] = format_hex_pretty_char((data[i] & 0xF0) >> 4);
+    ret[multiple * i] = format_hex_pretty_char(data[i] >> 4);
     ret[multiple * i + 1] = format_hex_pretty_char(data[i] & 0x0F);
     if (separator && i != length - 1)
       ret[multiple * i + 2] = separator;
@@ -591,7 +590,9 @@ bool HighFrequencyLoopRequester::is_high_frequency() { return num_requests > 0;
 std::string get_mac_address() {
   uint8_t mac[6];
   get_mac_address_raw(mac);
-  return str_snprintf("%02x%02x%02x%02x%02x%02x", 12, mac[0], mac[1], mac[2], mac[3], mac[4], mac[5]);
+  char buf[13];
+  format_mac_addr_lower_no_sep(mac, buf);
+  return std::string(buf);
 }
 
 std::string get_mac_address_pretty() {
diff --git a/esphome/core/helpers.h b/esphome/core/helpers.h
index a6741925d0..21aa159b25 100644
--- a/esphome/core/helpers.h
+++ b/esphome/core/helpers.h
@@ -380,6 +380,35 @@ template<typename T, enable_if_t<std::is_unsigned<T>::value, int> = 0> optional<
   return parse_hex<T>(str.c_str(), str.length());
 }
 
+/// Convert a nibble (0-15) to lowercase hex char
+inline char format_hex_char(uint8_t v) { return v >= 10 ? 'a' + (v - 10) : '0' + v; }
+
+/// Convert a nibble (0-15) to uppercase hex char (used for pretty printing)
+/// This always uses uppercase (A-F) for pretty/human-readable output
+inline char format_hex_pretty_char(uint8_t v) { return v >= 10 ? 'A' + (v - 10) : '0' + v; }
+
+/// Format MAC address as XX:XX:XX:XX:XX:XX (uppercase)
+inline void format_mac_addr_upper(const uint8_t *mac, char *output) {
+  for (size_t i = 0; i < 6; i++) {
+    uint8_t byte = mac[i];
+    output[i * 3] = format_hex_pretty_char(byte >> 4);
+    output[i * 3 + 1] = format_hex_pretty_char(byte & 0x0F);
+    if (i < 5)
+      output[i * 3 + 2] = ':';
+  }
+  output[17] = '\0';
+}
+
+/// Format MAC address as xxxxxxxxxxxxxx (lowercase, no separators)
+inline void format_mac_addr_lower_no_sep(const uint8_t *mac, char *output) {
+  for (size_t i = 0; i < 6; i++) {
+    uint8_t byte = mac[i];
+    output[i * 2] = format_hex_char(byte >> 4);
+    output[i * 2 + 1] = format_hex_char(byte & 0x0F);
+  }
+  output[12] = '\0';
+}
+
 /// Format the six-byte array \p mac into a MAC address.
 std::string format_mac_address_pretty(const uint8_t mac[6]);
 /// Format the byte array \p data of length \p len in lowercased hex.