[core] Refactor str_snake_case and str_sanitize to use constexpr helpers (#12454)

esphome/core/helpers.cpp

@@ -200,22 +200,27 @@ template<int (*fn)(int)> std::string str_ctype_transform(const std::string &str)
 }
 std::string str_lower_case(const std::string &str) { return str_ctype_transform<std::tolower>(str); }
 std::string str_upper_case(const std::string &str) { return str_ctype_transform<std::toupper>(str); }
+// Convert char to snake_case: lowercase and spaces to underscores
+static constexpr char to_snake_case_char(char c) {
+  return (c == ' ') ? '_' : (c >= 'A' && c <= 'Z') ? c + ('a' - 'A') : c;
+}
+// Sanitize char: keep alphanumerics, dashes, underscores; replace others with underscore
+static constexpr char to_sanitized_char(char c) {
+  return (c == '-' || c == '_' || (c >= '0' && c <= '9') || (c >= 'a' && c <= 'z') || (c >= 'A' && c <= 'Z')) ? c : '_';
+}
 std::string str_snake_case(const std::string &str) {
-  std::string result;
-  result.resize(str.length());
-  std::transform(str.begin(), str.end(), result.begin(), ::tolower);
-  std::replace(result.begin(), result.end(), ' ', '_');
+  std::string result = str;
+  for (char &c : result) {
+    c = to_snake_case_char(c);
+  }
   return result;
 }
 std::string str_sanitize(const std::string &str) {
-  std::string out = str;
-  std::replace_if(
-      out.begin(), out.end(),
-      [](const char &c) {
-        return c != '-' && c != '_' && (c < '0' || c > '9') && (c < 'a' || c > 'z') && (c < 'A' || c > 'Z');
-      },
-      '_');
-  return out;
+  std::string result = str;
+  for (char &c : result) {
+    c = to_sanitized_char(c);
+  }
+  return result;
 }
 std::string str_snprintf(const char *fmt, size_t len, ...) {
   std::string str;