[string] new StringFind()/StringEndsWith() for a sub-string (#6733)

This commit adds a new flavor of function `StringFind()` which finds
the first occurrence of a given sub-string within a string. It also
adds `StringEndsWith()` function that works with a sub-string. The
unit test `test_string` is also updated to cover the new behavior of
the newly added function.

src/core/common/string.cpp

@@ -66,6 +66,26 @@ const char *StringFind(const char *aString, char aChar)
     return ret;
 }
 
+const char *StringFind(const char *aString, const char *aSubString)
+{
+    const char *ret    = nullptr;
+    size_t      len    = strlen(aString);
+    size_t      subLen = strlen(aSubString);
+
+    VerifyOrExit(subLen <= len);
+
+    for (size_t index = 0; index <= static_cast<size_t>(len - subLen); index++)
+    {
+        if (memcmp(&aString[index], aSubString, subLen) == 0)
+        {
+            ExitNow(ret = &aString[index]);
+        }
+    }
+
+exit:
+    return ret;
+}
+
 bool StringEndsWith(const char *aString, char aChar)
 {
     size_t len = strlen(aString);
@@ -73,6 +93,14 @@ bool StringEndsWith(const char *aString, char aChar)
     return len > 0 && aString[len - 1] == aChar;
 }
 
+bool StringEndsWith(const char *aString, const char *aSubString)
+{
+    size_t len    = strlen(aString);
+    size_t subLen = strlen(aSubString);
+
+    return (subLen > 0) && (len >= subLen) && (memcmp(aSubString, &aString[len - subLen], subLen) == 0);
+}
+
 StringWriter::StringWriter(char *aBuffer, uint16_t aSize)
     : mBuffer(aBuffer)
     , mLength(0)

src/core/common/string.hpp

@@ -78,6 +78,17 @@ uint16_t StringLength(const char *aString, uint16_t aMaxLength);
  */
 const char *StringFind(const char *aString, char aChar);
 
+/**
+ * This function finds the first occurrence of a given sub-string in a null-terminated string.
+ *
+ * @param[in] aString     A pointer to the string.
+ * @param[in] aSubString  A sub-string to search for.
+ *
+ * @returns The pointer to first occurrence of the @p aSubString in @p aString, or nullptr if cannot be found.
+ *
+ */
+const char *StringFind(const char *aString, const char *aSubString);
+
 /**
  * This function checks whether a null-terminated string ends with a given character.
  *
@@ -90,6 +101,18 @@ const char *StringFind(const char *aString, char aChar);
  */
 bool StringEndsWith(const char *aString, char aChar);
 
+/**
+ * This function checks whether a null-terminated string ends with a given sub-string.
+ *
+ * @param[in] aString      A pointer to the string.
+ * @param[in] aSubString   A sun-string to check against.
+ *
+ * @retval TRUE   If @p aString ends with sub-string @p aSubString.
+ * @retval FALSE  If @p aString does not end with sub-string @p aSubString.
+ *
+ */
+bool StringEndsWith(const char *aString, const char *aSubString);
+
 /**
  * This class implements writing to a string buffer.
  *

tests/unit/test_string.cpp

@@ -145,7 +145,8 @@ void TestUtf8(void)
 void TestStringFind(void)
 {
     char emptyString[1] = {'\0'};
-    char testString[]   = "foo.bar\\.";
+    char testString[]   = "foo.bar.bar\\.";
+    char testString2[]  = "abcabcabcdabc";
 
     printf("\nTest 6: StringFind() function\n");
 
@@ -153,13 +154,53 @@ void TestStringFind(void)
     VerifyOrQuit(StringFind(testString, 'o') == &testString[1], "StringFind() failed");
     VerifyOrQuit(StringFind(testString, '.') == &testString[3], "StringFind() failed");
     VerifyOrQuit(StringFind(testString, 'r') == &testString[6], "StringFind() failed");
-    VerifyOrQuit(StringFind(testString, '\\') == &testString[7], "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, '\\') == &testString[11], "StringFind() failed");
     VerifyOrQuit(StringFind(testString, 'x') == nullptr, "StringFind() failed");
     VerifyOrQuit(StringFind(testString, ',') == nullptr, "StringFind() failed");
 
     VerifyOrQuit(StringFind(emptyString, 'f') == nullptr, "StringFind() failed");
     VerifyOrQuit(StringFind(emptyString, '.') == nullptr, "StringFind() failed");
 
+    VerifyOrQuit(StringFind(testString, "foo") == &testString[0], "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, "oo") == &testString[1], "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, "bar") == &testString[4], "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, "bar\\") == &testString[8], "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, "\\.") == &testString[11], "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, testString) == testString, "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, "fooo") == nullptr, "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, "far") == nullptr, "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, "bar\\..") == nullptr, "StringFind() failed");
+    VerifyOrQuit(StringFind(testString, "") == &testString[0], "StringFind() failed");
+
+    VerifyOrQuit(StringFind(emptyString, "foo") == nullptr, "StringFind() failed");
+    VerifyOrQuit(StringFind(emptyString, "bar") == nullptr, "StringFind() failed");
+    VerifyOrQuit(StringFind(emptyString, "") == &emptyString[0], "StringFind() failed");
+
+    // Verify when sub-string has repeated patterns
+    VerifyOrQuit(StringFind(testString2, "abcabc") == &testString2[0], "StringFind() failed");
+    VerifyOrQuit(StringFind(testString2, "abcabcd") == &testString2[3], "StringFind() failed");
+
+    printf(" -- PASS\n");
+}
+
+void TestStringEndsWith(void)
+{
+    printf("\nTest 7: StringEndsWith() function\n");
+
+    VerifyOrQuit(StringEndsWith("foobar", 'r'), "StringEndsWith() failed");
+    VerifyOrQuit(!StringEndsWith("foobar", 'a'), "StringEndsWith() failed");
+    VerifyOrQuit(!StringEndsWith("foobar", '\0'), "StringEndsWith() failed");
+    VerifyOrQuit(StringEndsWith("a", 'a'), "StringEndsWith() failed");
+    VerifyOrQuit(!StringEndsWith("a", 'b'), "StringEndsWith() failed");
+
+    VerifyOrQuit(StringEndsWith("foobar", "bar"), "StringEndsWith() failed");
+    VerifyOrQuit(!StringEndsWith("foobar", "ba"), "StringEndsWith() failed");
+    VerifyOrQuit(StringEndsWith("foobar", "foobar"), "StringEndsWith() failed");
+    VerifyOrQuit(!StringEndsWith("foobar", "foobarr"), "StringEndsWith() failed");
+
+    VerifyOrQuit(!StringEndsWith("", 'a'), "StringEndsWith() failed");
+    VerifyOrQuit(!StringEndsWith("", "foo"), "StringEndsWith() failed");
+
     printf(" -- PASS\n");
 }
 
@@ -171,6 +212,7 @@ int main(void)
     ot::TestStringLength();
     ot::TestUtf8();
     ot::TestStringFind();
+    ot::TestStringEndsWith();
     printf("\nAll tests passed.\n");
     return 0;
 }