Extract buf_append_sep_str helper, add tests

esphome/core/helpers.cpp

@@ -569,21 +569,9 @@ size_t value_accuracy_with_uom_to_buf(std::span<char, VALUE_ACCURACY_MAX_LEN> bu
   if (unit_of_measurement.empty()) {
     return len;
   }
-  // Append " <uom>" directly
-  char *p = buf.data() + len;
-  size_t remaining = buf.size() - len;
-  size_t uom_len = unit_of_measurement.size();
-  // Need space for: ' ' + uom + '\0'
-  if (remaining < 2) {
-    return len;
-  }
-  *p++ = ' ';
-  remaining--;
-  size_t copy_len = std::min(uom_len, remaining - 1);
-  memcpy(p, unit_of_measurement.c_str(), copy_len);
-  p += copy_len;
-  *p = '\0';
-  return static_cast<size_t>(p - buf.data());
+  char *end = buf_append_sep_str(buf.data() + len, buf.size() - len, ' ', unit_of_measurement.c_str(),
+                                 unit_of_measurement.size());
+  return static_cast<size_t>(end - buf.data());
 }
 
 int8_t step_to_accuracy_decimals(float step) {

esphome/core/helpers.h

@@ -1295,6 +1295,21 @@ inline char *int8_to_str(char *buf, int8_t val) {
   return buf;
 }
 
+/// Append a separator char and a string to a buffer, respecting remaining space.
+/// Returns pointer past last char written (null terminator is written).
+inline char *buf_append_sep_str(char *buf, size_t remaining, char separator, const char *str, size_t str_len) {
+  if (remaining < 2) {
+    return buf;
+  }
+  *buf++ = separator;
+  remaining--;
+  size_t copy_len = std::min(str_len, remaining - 1);
+  memcpy(buf, str, copy_len);
+  buf += copy_len;
+  *buf = '\0';
+  return buf;
+}
+
 /// Return 10^n for small non-negative n (0-3) as uint32_t, avoiding float.
 inline uint32_t small_pow10(int8_t n) {
   if (n == 1) {

tests/components/core/test_helpers.cpp

@@ -116,4 +116,39 @@ TEST(FracToStr, ZeroDivisor) {
   EXPECT_EQ(end, buf);  // writes nothing
 }
 
+// --- buf_append_sep_str() ---
+
+TEST(BufAppendSepStr, Basic) {
+  char buf[32] = "23.46";
+  char *start = buf + 5;
+  char *end = buf_append_sep_str(start, sizeof(buf) - 5, ' ', "°C", 3);
+  EXPECT_STREQ(buf, "23.46 °C");
+  EXPECT_EQ(end - buf, 9);  // "°C" is 3 bytes (UTF-8)
+}
+
+TEST(BufAppendSepStr, EmptyString) {
+  char buf[32] = "100";
+  char *start = buf + 3;
+  char *end = buf_append_sep_str(start, sizeof(buf) - 3, ' ', "", 0);
+  EXPECT_STREQ(buf, "100 ");
+  EXPECT_EQ(end - start, 1);  // just the separator
+}
+
+TEST(BufAppendSepStr, NoRoom) {
+  char buf[8] = "1234567";
+  char *start = buf + 7;
+  char *end = buf_append_sep_str(start, 1, ' ', "unit", 4);
+  EXPECT_EQ(end, start);  // nothing written
+}
+
+TEST(BufAppendSepStr, Truncation) {
+  char buf[8] = "val";
+  char *start = buf + 3;
+  // remaining = 5, separator takes 1, so 3 chars of string fit + null
+  char *end = buf_append_sep_str(start, 5, ' ', "longunit", 8);
+  *end = '\0';
+  EXPECT_STREQ(buf, "val lon");
+  EXPECT_EQ(end - buf, 7);
+}
+
 }  // namespace esphome::testing