AK: Forward substring creation with shared superstring to StringBase

AK/String.cpp

@@ -252,11 +252,7 @@ ErrorOr<String> String::substring_from_byte_offset(size_t start) const
 
 ErrorOr<String> String::substring_from_byte_offset_with_shared_superstring(size_t start, size_t byte_count) const
 {
-    if (!byte_count)
+    return String { TRY(StringBase::substring_from_byte_offset_with_shared_superstring(start, byte_count)) };
-        return String {};
-    if (byte_count <= MAX_SHORT_STRING_BYTE_COUNT)
-        return String::from_utf8(bytes_as_string_view().substring_view(start, byte_count));
-    return String { TRY(Detail::StringData::create_substring(*m_data, start, byte_count)) };
 }
 
 ErrorOr<String> String::substring_from_byte_offset_with_shared_superstring(size_t start) const

AK/String.h

@@ -196,6 +196,11 @@ public:
 
 private:
     using ShortString = Detail::ShortString;
+
+    explicit constexpr String(StringBase&& base)
+        : StringBase(move(base))
+    {
+    }
 };
 
 template<>

AK/StringBase.cpp

@@ -70,6 +70,13 @@ u32 StringBase::hash() const
     return m_data->hash();
 }
 
+size_t StringBase::byte_count() const
+{
+    if (is_short_string())
+        return m_short_string.byte_count_and_short_string_flag >> 1;
+    return m_data->byte_count();
+}
+
 bool StringBase::operator==(StringBase const& other) const
 {
     if (is_short_string())
@@ -88,6 +95,20 @@ ErrorOr<Bytes> StringBase::replace_with_uninitialized_buffer(size_t byte_count)
     return Bytes { buffer, byte_count };
 }
 
+ErrorOr<StringBase> StringBase::substring_from_byte_offset_with_shared_superstring(size_t start, size_t length) const
+{
+    VERIFY(start + length <= byte_count());
+
+    if (length == 0)
+        return StringBase {};
+    if (length <= MAX_SHORT_STRING_BYTE_COUNT) {
+        StringBase result;
+        bytes().slice(start, length).copy_to(result.replace_with_uninitialized_short_string(length));
+        return result;
+    }
+    return StringBase { TRY(Detail::StringData::create_substring(*m_data, start, length)) };
+}
+
 void StringBase::destroy_string()
 {
     if (!is_short_string())

AK/StringBase.h

@@ -64,6 +64,7 @@ public:
     // NOTE: There is no guarantee about null-termination.
     [[nodiscard]] ReadonlyBytes bytes() const;
     [[nodiscard]] u32 hash() const;
+    [[nodiscard]] size_t byte_count() const;
 
     [[nodiscard]] bool operator==(StringBase const&) const;
 
@@ -100,6 +101,10 @@ protected:
             callback(buffer);
     }
 
+    // This is not a trivial operation with storage, so it does not belong here. Unfortunately, it
+    // is impossible to implement it without access to StringData.
+    ErrorOr<StringBase> substring_from_byte_offset_with_shared_superstring(size_t start, size_t byte_count) const;
+
     union {
         ShortString m_short_string;
         Detail::StringData const* m_data { nullptr };

AK/StringInternals.h

@@ -59,6 +59,8 @@ public:
     bool is_fly_string() const { return m_is_fly_string; }
     void set_fly_string(bool is_fly_string) const { m_is_fly_string = is_fly_string; }
 
+    size_t byte_count() const { return m_byte_count; }
+
 private:
     explicit StringData(size_t byte_count);
     StringData(StringData const& superstring, size_t start, size_t byte_count);