[core] Add uint32_to_str helper and use in preferences (#15597)

tests/components/core/test_uint32_to_str.cpp

@@ -0,0 +1,77 @@
+#include <gtest/gtest.h>
+
+#include "esphome/core/helpers.h"
+
+namespace esphome::core::testing {
+
+// --- uint32_to_str_unchecked() (internal, raw pointer) ---
+
+TEST(Uint32ToStr, InternalZero) {
+  char buf[UINT32_MAX_STR_SIZE];
+  char *end = uint32_to_str_unchecked(buf, 0);
+  *end = '\0';
+  EXPECT_STREQ(buf, "0");
+  EXPECT_EQ(end - buf, 1);
+}
+
+TEST(Uint32ToStr, InternalSingleDigit) {
+  char buf[UINT32_MAX_STR_SIZE];
+  char *end = uint32_to_str_unchecked(buf, 7);
+  *end = '\0';
+  EXPECT_STREQ(buf, "7");
+}
+
+TEST(Uint32ToStr, InternalMultiDigit) {
+  char buf[UINT32_MAX_STR_SIZE];
+  char *end = uint32_to_str_unchecked(buf, 12345);
+  *end = '\0';
+  EXPECT_STREQ(buf, "12345");
+  EXPECT_EQ(end - buf, 5);
+}
+
+TEST(Uint32ToStr, InternalMaxValue) {
+  char buf[UINT32_MAX_STR_SIZE];
+  char *end = uint32_to_str_unchecked(buf, 4294967295u);
+  *end = '\0';
+  EXPECT_STREQ(buf, "4294967295");
+  EXPECT_EQ(end - buf, 10);
+}
+
+TEST(Uint32ToStr, InternalPowersOfTen) {
+  char buf[UINT32_MAX_STR_SIZE];
+  char *end;
+
+  end = uint32_to_str_unchecked(buf, 10);
+  *end = '\0';
+  EXPECT_STREQ(buf, "10");
+
+  end = uint32_to_str_unchecked(buf, 100);
+  *end = '\0';
+  EXPECT_STREQ(buf, "100");
+
+  end = uint32_to_str_unchecked(buf, 1000000);
+  *end = '\0';
+  EXPECT_STREQ(buf, "1000000");
+}
+
+// --- uint32_to_str() (public, span API) ---
+
+TEST(Uint32ToStr, SpanZero) {
+  char buf[UINT32_MAX_STR_SIZE];
+  EXPECT_EQ(uint32_to_str(buf, 0), 1u);
+  EXPECT_STREQ(buf, "0");
+}
+
+TEST(Uint32ToStr, SpanMultiDigit) {
+  char buf[UINT32_MAX_STR_SIZE];
+  EXPECT_EQ(uint32_to_str(buf, 12345), 5u);
+  EXPECT_STREQ(buf, "12345");
+}
+
+TEST(Uint32ToStr, SpanMaxValue) {
+  char buf[UINT32_MAX_STR_SIZE];
+  EXPECT_EQ(uint32_to_str(buf, 4294967295u), 10u);
+  EXPECT_STREQ(buf, "4294967295");
+}
+
+}  // namespace esphome::core::testing