[core] Add portable strcasestr implementation named str_contains_ignore_case (#18497)

Co-authored-by: J. Nick Koston <nick@koston.org>

esphome/components/audio/audio.cpp

@@ -86,7 +86,7 @@ AudioFileType detect_audio_file_type(const char *content_type, const char *url)
     // Match "audio/ogg" with a codecs parameter containing "opus"
     // Valid forms: audio/ogg;codecs=opus, audio/ogg; codecs="opus", etc.
     // Plain "audio/ogg" without opus is not matched (almost always Ogg Vorbis)
-    if (strncasecmp(content_type, "audio/ogg", 9) == 0 && strcasestr(content_type + 9, "opus") != nullptr) {
+    if (strncasecmp(content_type, "audio/ogg", 9) == 0 && str_contains_ignore_case(content_type + 9, "opus")) {
       return AudioFileType::OPUS;
     }
 #endif

esphome/core/helpers.cpp

@@ -220,6 +220,19 @@ bool str_endswith_ignore_case(const char *str, size_t str_len, const char *suffi
   return strncasecmp(str + str_len - suffix_len, suffix, suffix_len) == 0;
 }
 
+bool str_contains_ignore_case_fallback(const char *haystack, const char *needle) {
+  const size_t needle_len = strlen(needle);
+  if (needle_len == 0) {
+    return true;
+  }
+  for (const char *p = haystack; *p != '\0'; p++) {
+    if (strncasecmp(p, needle, needle_len) == 0) {
+      return true;
+    }
+  }
+  return false;
+}
+
 // str_truncate, str_until, str_lower_case, str_upper_case, str_snake_case moved to alloc_helpers.cpp
 char *str_sanitize_to(char *buffer, size_t buffer_size, const char *str) {
   if (buffer_size == 0) {

esphome/core/helpers.h

@@ -981,6 +981,25 @@ inline bool str_endswith_ignore_case(const std::string &str, const char *suffix)
   return str_endswith_ignore_case(str.c_str(), str.size(), suffix, strlen(suffix));
 }
 
+/// Fallback implementation for case insensitive substring comparison.
+bool str_contains_ignore_case_fallback(const char *haystack, const char *needle);
+
+/// Case-insensitive check if needle string is contained in haystack (no heap allocation).
+inline bool str_contains_ignore_case(const char *haystack, const char *needle) {
+  if (!needle || !haystack) {
+    return false;
+  }
+
+// strcasestr is a GNU extension: newlib only declares it when _GNU_SOURCE is set.
+// ESP32/ESP8266/host builds get it from their framework or from g++ on Linux;
+// LibreTiny, RP2 and Zephyr do not, so they use the hand-rolled fallback.
+#if defined(USE_LIBRETINY) || defined(USE_RP2) || defined(USE_ZEPHYR)
+  return str_contains_ignore_case_fallback(haystack, needle);
+#else   // defined(USE_LIBRETINY) || defined(USE_RP2) || defined(USE_ZEPHYR)
+  return strcasestr(haystack, needle) != nullptr;
+#endif  // defined(USE_LIBRETINY) || defined(USE_RP2) || defined(USE_ZEPHYR)
+}
+
 // str_truncate moved to alloc_helpers.h - remove this include before 2026.11.0
 
 // str_until, str_lower_case, str_upper_case moved to alloc_helpers.h - remove this comment before 2026.11.0

tests/components/core/helpers_test.cpp

@@ -83,4 +83,50 @@ TEST(StaticVectorTest, ConvertingConstructorSameSize) {
   EXPECT_EQ(dst[2], 3);
 }
 
+TEST(StringContainsIgnoreCaseTest, NullPointerAlwaysFalse) {
+  const char *haystack = nullptr;
+  const char *needle = nullptr;
+
+  EXPECT_FALSE(str_contains_ignore_case(haystack, needle));
+  EXPECT_FALSE(str_contains_ignore_case("Hello World", needle));
+  EXPECT_FALSE(str_contains_ignore_case(haystack, "anything"));
+}
+
+TEST(StringContainsIgnoreCaseTest, EmptySearchMatches) {
+  const char *haystack = "Hello World";
+
+  EXPECT_TRUE(str_contains_ignore_case_fallback(haystack, ""));
+}
+
+TEST(StringContainsIgnoreCaseTest, MiscCaseMatches) {
+  const char *haystack = "Hello World";
+
+  EXPECT_TRUE(str_contains_ignore_case_fallback(haystack, "Hello"));
+  EXPECT_TRUE(str_contains_ignore_case_fallback(haystack, "hello"));
+  EXPECT_TRUE(str_contains_ignore_case_fallback(haystack, "HELLO"));
+  EXPECT_TRUE(str_contains_ignore_case_fallback(haystack, "hELLO"));
+}
+
+TEST(StringContainsIgnoreCaseTest, MiscNotMatching) {
+  const char *haystack = "Hello World";
+
+  // Expected to match
+  EXPECT_TRUE(str_contains_ignore_case_fallback(haystack, "Hell"));
+
+  // Expected not to match
+  EXPECT_FALSE(str_contains_ignore_case_fallback(haystack, "Heaven"));
+  EXPECT_FALSE(str_contains_ignore_case_fallback(haystack, "Hello!"));
+  EXPECT_FALSE(str_contains_ignore_case_fallback(haystack, "world!"));
+}
+
+TEST(StringContainsIgnoreCaseTest, FallbackMatchesLibc) {
+  const char *haystack = "Hello World";
+  for (const char *needle : {"", "Hello", "hELLO", "Hell", "world", "Heaven", "Hello!", "d"}) {
+    EXPECT_EQ(str_contains_ignore_case_fallback(haystack, needle), str_contains_ignore_case(haystack, needle))
+        << "needle: " << needle;
+  }
+  EXPECT_EQ(str_contains_ignore_case_fallback("", ""), str_contains_ignore_case("", ""));
+  EXPECT_EQ(str_contains_ignore_case_fallback("ab", "abc"), str_contains_ignore_case("ab", "abc"));
+}
+
 }  // namespace esphome