AK: Introduce the new String, replacement for DeprecatedString

DeprecatedString (formerly String) has been with us since the start,
and it has served us well. However, it has a number of shortcomings
that I'd like to address.

Some of these issues are hard if not impossible to solve incrementally
inside of DeprecatedString, so instead of doing that, let's build a new
String class and then incrementally move over to it instead.

Problems in DeprecatedString:

- It assumes string allocation never fails. This makes it impossible
  to use in allocation-sensitive contexts, and is the reason we had to
  ban DeprecatedString from the kernel entirely.

- The awkward null state. DeprecatedString can be null. It's different
  from the empty state, although null strings are considered empty.
  All code is immediately nicer when using Optional<DeprecatedString>
  but DeprecatedString came before Optional, which is how we ended up
  like this.

- The encoding of the underlying data is ambiguous. For the most part,
  we use it as if it's always UTF-8, but there have been cases where
  we pass around strings in other encodings (e.g ISO8859-1)

- operator[] and length() are used to iterate over DeprecatedString one
  byte at a time. This is done all over the codebase, and will *not*
  give the right results unless the string is all ASCII.

How we solve these issues in the new String:

- Functions that may allocate now return ErrorOr<String> so that ENOMEM
  errors can be passed to the caller.

- String has no null state. Use Optional<String> when needed.

- String is always UTF-8. This is validated when constructing a String.
  We may need to add a bypass for this in the future, for cases where
  you have a known-good string, but for now: validate all the things!

- There is no operator[] or length(). You can get the underlying data
  with bytes(), but for iterating over code points, you should be using
  an UTF-8 iterator.

Furthermore, it has two nifty new features:

- String implements a small string optimization (SSO) for strings that
  can fit entirely within a pointer. This means up to 3 bytes on 32-bit
  platforms, and 7 bytes on 64-bit platforms. Such small strings will
  not be heap-allocated.

- String can create substrings without making a deep copy of the
  substring. Instead, the superstring gets +1 refcount from the
  substring, and it acts like a view into the superstring. To make
  substrings like this, use the substring_with_shared_superstring() API.

One caveat:

- String does not guarantee that the underlying data is null-terminated
  like DeprecatedString does today. While this was nifty in a handful of
  places where we were calling C functions, it did stand in the way of
  shared-superstring substrings.

10 files changed, 616 insertions, 1 deletions

diff --git a/AK/CMakeLists.txt b/AK/CMakeLists.txt
index 46ec2ee1a4..295e41a328 100644
--- a/AK/CMakeLists.txt
+++ b/AK/CMakeLists.txt
@@ -15,6 +15,7 @@ set(AK_SOURCES
     LexicalPath.cpp
     Random.cpp
     StackInfo.cpp
+    String.cpp
     StringBuilder.cpp
     StringFloatingPointConversions.cpp
     StringImpl.cpp
diff --git a/AK/Forward.h b/AK/Forward.h
index a981707994..31561e6de4 100644
--- a/AK/Forward.h
+++ b/AK/Forward.h
@@ -31,6 +31,7 @@ class StringView;
 class Time;
 class URL;
 class FlyString;
+class String;
 class Utf16View;
 class Utf32View;
 class Utf8CodePointIterator;
@@ -188,6 +189,7 @@ using AK::RefPtr;
 using AK::SinglyLinkedList;
 using AK::Span;
 using AK::StackInfo;
+using AK::String;
 using AK::StringBuilder;
 using AK::StringImpl;
 using AK::StringView;
diff --git a/AK/String.cpp b/AK/String.cpp
new file mode 100644
index 0000000000..ac443a0ce5
--- /dev/null
+++ b/AK/String.cpp
@@ -0,0 +1,337 @@
+/*
+ * Copyright (c) 2018-2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/Checked.h>
+#include <AK/Format.h>
+#include <AK/Memory.h>
+#include <AK/String.h>
+#include <AK/StringBuilder.h>
+#include <AK/Utf8View.h>
+#include <stdlib.h>
+
+namespace AK {
+
+namespace Detail {
+
+class StringData final : public RefCounted<StringData> {
+public:
+    static ErrorOr<NonnullRefPtr<StringData>> create_uninitialized(size_t, u8*& buffer);
+    static ErrorOr<NonnullRefPtr<StringData>> create_substring(StringData const& superstring, size_t start, size_t byte_count);
+    static ErrorOr<NonnullRefPtr<StringData>> from_utf8(char const* utf8_bytes, size_t);
+
+    struct SubstringData {
+        StringData const* superstring { nullptr };
+        u32 start_offset { 0 };
+    };
+
+    void operator delete(void* ptr);
+
+    ~StringData();
+
+    SubstringData const& substring_data() const
+    {
+        return *reinterpret_cast<SubstringData const*>(m_bytes_or_substring_data);
+    }
+
+    // NOTE: There is no guarantee about null-termination.
+    ReadonlyBytes bytes() const
+    {
+        if (m_substring) {
+            auto const& data = substring_data();
+            return data.superstring->bytes().slice(data.start_offset, m_byte_count);
+        }
+        return { &m_bytes_or_substring_data[0], m_byte_count };
+    }
+
+    StringView bytes_as_string_view() const { return { bytes() }; }
+
+    bool operator==(StringData const& other) const
+    {
+        return bytes_as_string_view() == other.bytes_as_string_view();
+    }
+
+    unsigned hash() const
+    {
+        if (!m_has_hash)
+            compute_hash();
+        return m_hash;
+    }
+
+private:
+    explicit StringData(size_t byte_count);
+    StringData(StringData const& superstring, size_t start, size_t byte_count);
+
+    void compute_hash() const;
+
+    u32 m_byte_count { 0 };
+    mutable unsigned m_hash { 0 };
+    mutable bool m_has_hash { false };
+    bool m_substring { false };
+
+    u8 m_bytes_or_substring_data[0];
+};
+
+void StringData::operator delete(void* ptr)
+{
+    free(ptr);
+}
+
+StringData::StringData(size_t byte_count)
+    : m_byte_count(byte_count)
+{
+}
+
+StringData::StringData(StringData const& superstring, size_t start, size_t byte_count)
+    : m_byte_count(byte_count)
+    , m_substring(true)
+{
+    auto& data = const_cast<SubstringData&>(substring_data());
+    data.start_offset = start;
+    data.superstring = &superstring;
+    superstring.ref();
+}
+
+StringData::~StringData()
+{
+    if (m_substring)
+        substring_data().superstring->unref();
+}
+
+constexpr size_t allocation_size_for_string_data(size_t length)
+{
+    return sizeof(StringData) + (sizeof(char) * length) + sizeof(char);
+}
+
+ErrorOr<NonnullRefPtr<StringData>> StringData::create_uninitialized(size_t byte_count, u8*& buffer)
+{
+    VERIFY(byte_count);
+    void* slot = malloc(allocation_size_for_string_data(byte_count));
+    if (!slot) {
+        return Error::from_errno(ENOMEM);
+    }
+    auto new_string_data = adopt_ref(*new (slot) StringData(byte_count));
+    buffer = const_cast<u8*>(new_string_data->bytes().data());
+    return new_string_data;
+}
+
+ErrorOr<NonnullRefPtr<StringData>> StringData::from_utf8(char const* utf8_data, size_t byte_count)
+{
+    // Strings of MAX_SHORT_STRING_BYTE_COUNT bytes or less should be handled by the String short string optimization.
+    VERIFY(byte_count > String::MAX_SHORT_STRING_BYTE_COUNT);
+
+    Utf8View view(StringView(utf8_data, byte_count));
+    if (!view.validate())
+        return Error::from_string_literal("StringData::from_utf8: Input was not valid UTF-8");
+
+    VERIFY(utf8_data);
+    u8* buffer = nullptr;
+    auto new_string_data = TRY(create_uninitialized(byte_count, buffer));
+    memcpy(buffer, utf8_data, byte_count * sizeof(char));
+    return new_string_data;
+}
+
+ErrorOr<NonnullRefPtr<StringData>> StringData::create_substring(StringData const& superstring, size_t start, size_t byte_count)
+{
+    // Strings of MAX_SHORT_STRING_BYTE_COUNT bytes or less should be handled by the String short string optimization.
+    VERIFY(byte_count > String::MAX_SHORT_STRING_BYTE_COUNT);
+
+    void* slot = malloc(sizeof(StringData) + sizeof(StringData::SubstringData));
+    if (!slot) {
+        return Error::from_errno(ENOMEM);
+    }
+    return adopt_ref(*new (slot) StringData(superstring, start, byte_count));
+}
+
+void StringData::compute_hash() const
+{
+    auto bytes = this->bytes();
+    if (bytes.size() == 0)
+        m_hash = 0;
+    else
+        m_hash = string_hash(reinterpret_cast<char const*>(bytes.data()), bytes.size());
+    m_has_hash = true;
+}
+
+}
+
+String::String(NonnullRefPtr<Detail::StringData> data)
+    : m_data(&data.leak_ref())
+{
+}
+
+String::String(ShortString short_string)
+    : m_short_string(short_string)
+{
+}
+
+String::String(String const& other)
+    : m_data(other.m_data)
+{
+    if (!is_short_string())
+        m_data->ref();
+}
+
+String::String(String&& other)
+    : m_data(exchange(other.m_data, nullptr))
+{
+}
+
+String& String::operator=(String&& other)
+{
+    m_data = exchange(other.m_data, nullptr);
+    return *this;
+}
+
+String& String::operator=(String const& other)
+{
+    if (&other != this) {
+        m_data = other.m_data;
+        if (!is_short_string())
+            m_data->ref();
+    }
+    return *this;
+}
+
+String::~String()
+{
+    if (!is_short_string() && m_data)
+        m_data->unref();
+}
+
+String::String()
+{
+    // This is an empty string, it's always short and zero-length.
+    m_short_string.byte_count_and_short_string_flag = SHORT_STRING_FLAG;
+}
+
+ErrorOr<String> String::from_utf8(StringView view)
+{
+    if (view.length() <= MAX_SHORT_STRING_BYTE_COUNT) {
+        ShortString short_string;
+        if (!view.is_empty())
+            memcpy(short_string.storage, view.characters_without_null_termination(), view.length());
+        short_string.byte_count_and_short_string_flag = (view.length() << 1) | SHORT_STRING_FLAG;
+        return String { short_string };
+    }
+    auto data = TRY(Detail::StringData::from_utf8(view.characters_without_null_termination(), view.length()));
+    return String { move(data) };
+}
+
+StringView String::bytes_as_string_view() const
+{
+    return StringView(bytes());
+}
+
+ReadonlyBytes String::bytes() const
+{
+    if (is_short_string())
+        return m_short_string.bytes();
+    return m_data->bytes();
+}
+
+bool String::is_empty() const
+{
+    return bytes().size() == 0;
+}
+
+ErrorOr<String> String::vformatted(StringView fmtstr, TypeErasedFormatParams& params)
+{
+    StringBuilder builder;
+    TRY(vformat(builder, fmtstr, params));
+    return builder.to_string();
+}
+
+bool String::operator==(String const& other) const
+{
+    if (is_short_string())
+        return m_data == other.m_data;
+    return bytes_as_string_view() == other.bytes_as_string_view();
+}
+
+bool String::operator==(StringView other) const
+{
+    return bytes_as_string_view() == other;
+}
+
+ErrorOr<String> String::substring_from_byte_offset(size_t start, size_t byte_count) const
+{
+    if (!byte_count)
+        return String {};
+    return String::from_utf8(bytes_as_string_view().substring_view(start, byte_count));
+}
+
+ErrorOr<String> String::substring_from_byte_offset_with_shared_superstring(size_t start, size_t byte_count) const
+{
+    if (!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)) };
+}
+
+bool String::operator==(char const* c_string) const
+{
+    return bytes_as_string_view() == c_string;
+}
+
+u32 String::hash() const
+{
+    if (is_short_string()) {
+        auto bytes = this->bytes();
+        return string_hash(reinterpret_cast<char const*>(bytes.data()), bytes.size());
+    }
+    return m_data->hash();
+}
+
+Utf8View String::code_points() const
+{
+    return Utf8View(bytes_as_string_view());
+}
+
+ErrorOr<void> Formatter<String>::format(FormatBuilder& builder, String const& utf8_string)
+{
+    return Formatter<StringView>::format(builder, utf8_string.bytes_as_string_view());
+}
+
+ErrorOr<String> String::replace(StringView needle, StringView replacement, ReplaceMode replace_mode) const
+{
+    return StringUtils::replace(*this, needle, replacement, replace_mode);
+}
+
+bool String::is_short_string() const
+{
+    return reinterpret_cast<uintptr_t>(m_data) & SHORT_STRING_FLAG;
+}
+
+ReadonlyBytes String::ShortString::bytes() const
+{
+    return { storage, byte_count() };
+}
+
+size_t String::ShortString::byte_count() const
+{
+    return byte_count_and_short_string_flag >> 1;
+}
+
+unsigned Traits<String>::hash(String const& string)
+{
+    return string.hash();
+}
+
+DeprecatedString String::to_deprecated_string() const
+{
+    return DeprecatedString(bytes_as_string_view());
+}
+
+ErrorOr<String> String::from_deprecated_string(DeprecatedString const& deprecated_string)
+{
+    Utf8View view(deprecated_string);
+    if (!view.validate())
+        return Error::from_string_literal("String::from_deprecated_string: Input was not valid UTF-8");
+    return String::from_utf8(deprecated_string.view());
+}
+
+}
diff --git a/AK/String.h b/AK/String.h
new file mode 100644
index 0000000000..3ce07bdb89
--- /dev/null
+++ b/AK/String.h
@@ -0,0 +1,132 @@
+/*
+ * Copyright (c) 2018-2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Format.h>
+#include <AK/Forward.h>
+#include <AK/RefCounted.h>
+#include <AK/Span.h>
+#include <AK/StringView.h>
+#include <AK/Traits.h>
+#include <AK/Types.h>
+
+namespace AK {
+
+namespace Detail {
+class StringData;
+}
+
+// String is a strongly owned sequence of Unicode code points encoded as UTF-8.
+// The data may or may not be heap-allocated, and may or may not be reference counted.
+// There is no guarantee that the underlying bytes are null-terminated.
+class String {
+public:
+    // NOTE: For short strings, we avoid heap allocations by storing them in the data pointer slot.
+    static constexpr size_t MAX_SHORT_STRING_BYTE_COUNT = sizeof(Detail::StringData*) - 1;
+
+    String(String const&);
+    String(String&&);
+
+    String& operator=(String&&);
+    String& operator=(String const&);
+
+    ~String();
+
+    // Creates an empty (zero-length) String.
+    String();
+
+    // Creates a new String from a sequence of UTF-8 encoded code points.
+    static ErrorOr<String> from_utf8(StringView);
+
+    // Creates a substring with a deep copy of the specified data window.
+    ErrorOr<String> substring_from_byte_offset(size_t start, size_t byte_count) const;
+
+    // Creates a substring that strongly references the origin superstring instead of making a deep copy of the data.
+    ErrorOr<String> substring_from_byte_offset_with_shared_superstring(size_t start, size_t byte_count) const;
+
+    // Returns an iterable view over the Unicode code points.
+    [[nodiscard]] Utf8View code_points() const;
+
+    // Returns the underlying UTF-8 encoded bytes.
+    // NOTE: There is no guarantee about null-termination.
+    [[nodiscard]] ReadonlyBytes bytes() const;
+
+    // Returns true if the String is zero-length.
+    [[nodiscard]] bool is_empty() const;
+
+    // Returns a StringView covering the full length of the string. Note that iterating this will go byte-at-a-time, not code-point-at-a-time.
+    [[nodiscard]] StringView bytes_as_string_view() const;
+
+    ErrorOr<String> replace(StringView needle, StringView replacement, ReplaceMode replace_mode) const;
+
+    [[nodiscard]] bool operator==(String const&) const;
+    [[nodiscard]] bool operator!=(String const& other) const { return !(*this == other); }
+
+    [[nodiscard]] bool operator==(StringView) const;
+    [[nodiscard]] bool operator!=(StringView other) const { return !(*this == other); }
+
+    [[nodiscard]] bool operator==(char const* cstring) const;
+    [[nodiscard]] bool operator!=(char const* cstring) const { return !(*this == cstring); }
+
+    [[nodiscard]] u32 hash() const;
+
+    template<typename T>
+    static ErrorOr<String> number(T value)
+    requires IsArithmetic<T>
+    {
+        return formatted("{}", value);
+    }
+
+    static ErrorOr<String> vformatted(StringView fmtstr, TypeErasedFormatParams&);
+
+    template<typename... Parameters>
+    static ErrorOr<String> formatted(CheckedFormatString<Parameters...>&& fmtstr, Parameters const&... parameters)
+    {
+        VariadicFormatParams variadic_format_parameters { parameters... };
+        return vformatted(fmtstr.view(), variadic_format_parameters);
+    }
+
+    // NOTE: This is primarily interesting to unit tests.
+    [[nodiscard]] bool is_short_string() const;
+
+    // FIXME: Remove these once all code has been ported to String
+    [[nodiscard]] DeprecatedString to_deprecated_string() const;
+    static ErrorOr<String> from_deprecated_string(DeprecatedString const&);
+
+private:
+    // NOTE: If the least significant bit of the pointer is set, this is a short string.
+    static constexpr uintptr_t SHORT_STRING_FLAG = 1;
+
+    struct ShortString {
+        ReadonlyBytes bytes() const;
+        size_t byte_count() const;
+
+        // NOTE: This is the byte count shifted left 1 step and or'ed with a 1 (the SHORT_STRING_FLAG)
+        u8 byte_count_and_short_string_flag { 0 };
+        u8 storage[MAX_SHORT_STRING_BYTE_COUNT] = { 0 };
+    };
+
+    explicit String(NonnullRefPtr<Detail::StringData>);
+    explicit String(ShortString);
+
+    union {
+        ShortString m_short_string;
+        Detail::StringData* m_data { nullptr };
+    };
+};
+
+template<>
+struct Traits<String> : public GenericTraits<String> {
+    static unsigned hash(String const&);
+};
+
+template<>
+struct Formatter<String> : Formatter<StringView> {
+    ErrorOr<void> format(FormatBuilder&, String const&);
+};
+
+}
diff --git a/AK/StringBuilder.cpp b/AK/StringBuilder.cpp
index 9db7c9f23e..932699e208 100644
--- a/AK/StringBuilder.cpp
+++ b/AK/StringBuilder.cpp
@@ -8,6 +8,7 @@
 #include <AK/Checked.h>
 #include <AK/PrintfImplementation.h>
 #include <AK/StdLibExtras.h>
+#include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <AK/StringView.h>
 #include <AK/UnicodeUtils.h>
@@ -115,6 +116,11 @@ DeprecatedString StringBuilder::build() const
 {
     return to_deprecated_string();
 }
+
+ErrorOr<String> StringBuilder::to_string() const
+{
+    return String::from_utf8(string_view());
+}
 #endif
 
 StringView StringBuilder::string_view() const
diff --git a/AK/StringBuilder.h b/AK/StringBuilder.h
index e5c62cb4d0..9bde5b102d 100644
--- a/AK/StringBuilder.h
+++ b/AK/StringBuilder.h
@@ -62,7 +62,9 @@ public:
 #ifndef KERNEL
     [[nodiscard]] DeprecatedString build() const;
     [[nodiscard]] DeprecatedString to_deprecated_string() const;
+    ErrorOr<String> to_string() const;
 #endif
+
     [[nodiscard]] ByteBuffer to_byte_buffer() const;
 
     [[nodiscard]] StringView string_view() const;
diff --git a/AK/StringUtils.cpp b/AK/StringUtils.cpp
index f72d3ec6ea..58157d148e 100644
--- a/AK/StringUtils.cpp
+++ b/AK/StringUtils.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2020, Andreas Kling <awesomekling@gmail.com>
+ * Copyright (c) 2018-2022, Andreas Kling <awesomekling@gmail.com>
  * Copyright (c) 2020, Fei Wu <f.eiwu@yahoo.com>
  *
  * SPDX-License-Identifier: BSD-2-Clause
@@ -9,6 +9,7 @@
 #include <AK/MemMem.h>
 #include <AK/Memory.h>
 #include <AK/Optional.h>
+#include <AK/String.h>
 #include <AK/StringBuilder.h>
 #include <AK/StringUtils.h>
 #include <AK/StringView.h>
@@ -533,6 +534,35 @@ DeprecatedString replace(StringView str, StringView needle, StringView replaceme
     replaced_string.append(str.substring_view(last_position, str.length() - last_position));
     return replaced_string.build();
 }
+
+ErrorOr<String> replace(String const& haystack, StringView needle, StringView replacement, ReplaceMode replace_mode)
+{
+    if (haystack.is_empty())
+        return haystack;
+
+    // FIXME: Propagate Vector allocation failures (or do this without putting positions in a vector)
+    Vector<size_t> positions;
+    if (replace_mode == ReplaceMode::All) {
+        positions = haystack.bytes_as_string_view().find_all(needle);
+        if (!positions.size())
+            return haystack;
+    } else {
+        auto pos = haystack.bytes_as_string_view().find(needle);
+        if (!pos.has_value())
+            return haystack;
+        positions.append(pos.value());
+    }
+
+    StringBuilder replaced_string;
+    size_t last_position = 0;
+    for (auto& position : positions) {
+        replaced_string.append(haystack.bytes_as_string_view().substring_view(last_position, position - last_position));
+        replaced_string.append(replacement);
+        last_position = position + needle.length();
+    }
+    replaced_string.append(haystack.bytes_as_string_view().substring_view(last_position, haystack.bytes_as_string_view().length() - last_position));
+    return replaced_string.to_string();
+}
 #endif
 
 // TODO: Benchmark against KMP (AK/MemMem.h) and switch over if it's faster for short strings too
diff --git a/AK/StringUtils.h b/AK/StringUtils.h
index a531380323..173d8c941b 100644
--- a/AK/StringUtils.h
+++ b/AK/StringUtils.h
@@ -103,6 +103,8 @@ DeprecatedString to_titlecase(StringView);
 DeprecatedString invert_case(StringView);
 
 DeprecatedString replace(StringView, StringView needle, StringView replacement, ReplaceMode);
+ErrorOr<String> replace(String const&, StringView needle, StringView replacement, ReplaceMode);
+
 size_t count(StringView, StringView needle);
 
 }
diff --git a/Tests/AK/CMakeLists.txt b/Tests/AK/CMakeLists.txt
index be5538afa0..c0d5d18763 100644
--- a/Tests/AK/CMakeLists.txt
+++ b/Tests/AK/CMakeLists.txt
@@ -62,6 +62,7 @@ set(AK_TEST_SOURCES
     TestSpan.cpp
     TestStack.cpp
     TestStdLibExtras.cpp
+    TestString.cpp
     TestStringFloatingPointConversions.cpp
     TestStringUtils.cpp
     TestStringView.cpp
diff --git a/Tests/AK/TestString.cpp b/Tests/AK/TestString.cpp
new file mode 100644
index 0000000000..c453e6d74c
--- /dev/null
+++ b/Tests/AK/TestString.cpp
@@ -0,0 +1,102 @@
+/*
+ * Copyright (c) 2022, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibTest/TestCase.h>
+
+#include <AK/String.h>
+#include <AK/StringBuilder.h>
+#include <AK/Try.h>
+#include <AK/Utf8View.h>
+#include <AK/Vector.h>
+
+TEST_CASE(construct_empty)
+{
+    String empty;
+    EXPECT(empty.is_empty());
+    EXPECT_EQ(empty.bytes().size(), 0u);
+
+    auto empty2 = MUST(String::from_utf8(""sv));
+    EXPECT(empty2.is_empty());
+    EXPECT_EQ(empty, empty2);
+    EXPECT_EQ(empty, ""sv);
+}
+
+TEST_CASE(short_strings)
+{
+#ifdef AK_ARCH_64_BIT
+    auto string = MUST(String::from_utf8("abcdefg"sv));
+    EXPECT_EQ(string.is_short_string(), true);
+    EXPECT_EQ(string.bytes().size(), 7u);
+    EXPECT_EQ(string.bytes_as_string_view(), "abcdefg"sv);
+#else
+    auto string = MUST(String::from_utf8("abc"sv));
+    EXPECT_EQ(string.is_short_string(), true);
+    EXPECT_EQ(string.bytes().size(), 3u);
+    EXPECT_EQ(string.bytes_as_string_view(), "abc"sv);
+#endif
+}
+
+TEST_CASE(long_strings)
+{
+    auto string = MUST(String::from_utf8("abcdefgh"sv));
+    EXPECT_EQ(string.is_short_string(), false);
+    EXPECT_EQ(string.bytes().size(), 8u);
+    EXPECT_EQ(string.bytes_as_string_view(), "abcdefgh"sv);
+}
+
+TEST_CASE(substring)
+{
+    auto superstring = MUST(String::from_utf8("Hello I am a long string"sv));
+    auto short_substring = MUST(superstring.substring_from_byte_offset(0, 5));
+    EXPECT_EQ(short_substring, "Hello"sv);
+
+    auto long_substring = MUST(superstring.substring_from_byte_offset(0, 10));
+    EXPECT_EQ(long_substring, "Hello I am"sv);
+}
+
+TEST_CASE(code_points)
+{
+    auto string = MUST(String::from_utf8("🦬🪒"sv));
+
+    Vector<u32> code_points;
+    for (auto code_point : string.code_points())
+        code_points.append(code_point);
+
+    EXPECT_EQ(code_points[0], 0x1f9acu);
+    EXPECT_EQ(code_points[1], 0x1fa92u);
+}
+
+TEST_CASE(string_builder)
+{
+    StringBuilder builder;
+    builder.append_code_point(0x1f9acu);
+    builder.append_code_point(0x1fa92u);
+
+    auto string = MUST(builder.to_string());
+    EXPECT_EQ(string, "🦬🪒"sv);
+    EXPECT_EQ(string.bytes().size(), 8u);
+}
+
+TEST_CASE(ak_format)
+{
+    auto foo = MUST(String::formatted("Hello {}", MUST(String::from_utf8("friends"sv))));
+    EXPECT_EQ(foo, "Hello friends"sv);
+}
+
+TEST_CASE(replace)
+{
+    {
+        auto haystack = MUST(String::from_utf8("Hello enemies"sv));
+        auto result = MUST(haystack.replace("enemies"sv, "friends"sv, ReplaceMode::All));
+        EXPECT_EQ(result, "Hello friends"sv);
+    }
+
+    {
+        auto base_title = MUST(String::from_utf8("anon@courage:~"sv));
+        auto result = MUST(base_title.replace("[*]"sv, "(*)"sv, ReplaceMode::FirstOnly));
+        EXPECT_EQ(result, "anon@courage:~"sv);
+    }
+}