[core] Optimize value_accuracy_to_buf to avoid snprintf (#15596)

tests/components/core/test_helpers.cpp

@@ -117,4 +117,100 @@ TEST(FormatHexChar, UppercaseDigits) {
   EXPECT_EQ(format_hex_pretty_char(15), 'F');
 }
 
+// --- small_pow10() ---
+
+TEST(SmallPow10, Zero) { EXPECT_EQ(small_pow10(0), 1u); }
+TEST(SmallPow10, One) { EXPECT_EQ(small_pow10(1), 10u); }
+TEST(SmallPow10, Two) { EXPECT_EQ(small_pow10(2), 100u); }
+TEST(SmallPow10, Three) { EXPECT_EQ(small_pow10(3), 1000u); }
+
+// --- frac_to_str_unchecked() ---
+
+TEST(FracToStr, OneDigit) {
+  char buf[8];
+  char *end = frac_to_str_unchecked(buf, 5, 1);
+  *end = '\0';
+  EXPECT_STREQ(buf, "5");
+  EXPECT_EQ(end - buf, 1);
+}
+
+TEST(FracToStr, TwoDigits) {
+  char buf[8];
+  char *end = frac_to_str_unchecked(buf, 46, 10);
+  *end = '\0';
+  EXPECT_STREQ(buf, "46");
+}
+
+TEST(FracToStr, ThreeDigits) {
+  char buf[8];
+  char *end = frac_to_str_unchecked(buf, 456, 100);
+  *end = '\0';
+  EXPECT_STREQ(buf, "456");
+  EXPECT_EQ(end - buf, 3);
+}
+
+TEST(FracToStr, LeadingZeros) {
+  char buf[8];
+  char *end = frac_to_str_unchecked(buf, 1, 100);
+  *end = '\0';
+  EXPECT_STREQ(buf, "001");
+
+  end = frac_to_str_unchecked(buf, 5, 10);
+  *end = '\0';
+  EXPECT_STREQ(buf, "05");
+}
+
+TEST(FracToStr, AllZeros) {
+  char buf[8];
+  char *end = frac_to_str_unchecked(buf, 0, 100);
+  *end = '\0';
+  EXPECT_STREQ(buf, "000");
+
+  end = frac_to_str_unchecked(buf, 0, 1);
+  *end = '\0';
+  EXPECT_STREQ(buf, "0");
+}
+
+TEST(FracToStr, ZeroDivisor) {
+  char buf[8];
+  buf[0] = 'X';
+  char *end = frac_to_str_unchecked(buf, 0, 0);
+  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::core::testing