in-place CString replace when possible

libs/utils/src/CString.cpp

@@ -77,9 +77,24 @@ CString& CString::replace(size_type const pos, size_type len, char const* str, s
 
     len = std::min(len, size() - pos);
 
-    // The new size of the string, after the replacement.
     const size_type newSize = size() - len + l;
 
+    // if the new string is not longer, we can do it in-place, which is much faster.
+    if (newSize <= size()) {
+        if (mCStr) {
+            // This is equivalent to l <= len because capacity() == size()
+            // move the tail of the string. +1 to move the null-terminator
+            std::copy(mCStr + pos + len, mCStr + size() + 1, mCStr + pos + l);
+            // copy the new content
+            std::copy_n(str, l, mCStr + pos);
+            // update the size
+            (mData - 1)->length = newSize;
+        }
+        // if mCStr is null, newSize<=size() implies l<=len and since size is 0, len is 0, so l is 0.
+        // so we're replacing nothing with nothing and there is nothing to do.
+        return *this;
+    }
+
     // Allocate enough memory to hold the new string.
     Data* const p = static_cast<Data*>(std::malloc(sizeof(Data) + newSize + 1));
     assert(p);