AK: Implement string formatting for FixedStringBuffers

To ensure this happens without duplicating code, we allow forcing a
StringBuilder object to only use the inline buffer, so the code in the
AK/Format.cpp file doesn't need to deal with different underlying
storage types (expandable or inline-fixed) at all.

AK/FixedStringBuffer.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/Array.h>
+#include <AK/StringBuilder.h>
 #include <AK/StringView.h>
 #include <AK/TypedTransfer.h>
 #include <AK/Userspace.h>
@@ -22,6 +23,24 @@ namespace AK {
 template<size_t Size>
 class FixedStringBuffer {
 public:
+    [[nodiscard]] static ErrorOr<FixedStringBuffer<Size>> vformatted(StringView fmtstr, AK::TypeErasedFormatParams& params)
+    requires(Size < StringBuilder::inline_capacity)
+    {
+        StringBuilder builder { StringBuilder::UseInlineCapacityOnly::Yes };
+        TRY(AK::vformat(builder, fmtstr, params));
+        FixedStringBuffer<Size> buffer {};
+        buffer.store_characters(builder.string_view());
+        return buffer;
+    }
+
+    template<typename... Parameters>
+    [[nodiscard]] static ErrorOr<FixedStringBuffer<Size>> formatted(CheckedFormatString<Parameters...>&& fmtstr, Parameters const&... parameters)
+    requires(Size < StringBuilder::inline_capacity)
+    {
+        AK::VariadicFormatParams<AK::AllowDebugOnlyFormatters::No, Parameters...> variadic_format_parameters { parameters... };
+        return vformatted(fmtstr.view(), variadic_format_parameters);
+    }
+
     void store_characters(StringView characters)
     {
         // NOTE: Only store the characters up to the first null terminator

AK/StringBuilder.cpp

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2021, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2023, Liav A. <liavalb@hotmail.co.il>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -23,6 +24,17 @@ namespace AK {
 
 inline ErrorOr<void> StringBuilder::will_append(size_t size)
 {
+    if (m_use_inline_capacity_only == UseInlineCapacityOnly::Yes) {
+        VERIFY(m_buffer.capacity() == StringBuilder::inline_capacity);
+        Checked<size_t> current_pointer = m_buffer.size();
+        current_pointer += size;
+        VERIFY(!current_pointer.has_overflow());
+        if (current_pointer <= StringBuilder::inline_capacity) {
+            return {};
+        }
+        return Error::from_errno(ENOMEM);
+    }
+
     Checked<size_t> needed_capacity = m_buffer.size();
     needed_capacity += size;
     VERIFY(!needed_capacity.has_overflow());
@@ -48,6 +60,27 @@ StringBuilder::StringBuilder(size_t initial_capacity)
     m_buffer.ensure_capacity(initial_capacity);
 }
 
+StringBuilder::StringBuilder(UseInlineCapacityOnly use_inline_capacity_only)
+    : m_use_inline_capacity_only(use_inline_capacity_only)
+{
+}
+
+size_t StringBuilder::length() const
+{
+    return m_buffer.size();
+}
+
+bool StringBuilder::is_empty() const
+{
+    return m_buffer.is_empty();
+}
+
+void StringBuilder::trim(size_t count)
+{
+    auto decrease_count = min(m_buffer.size(), count);
+    m_buffer.resize(m_buffer.size() - decrease_count);
+}
+
 ErrorOr<void> StringBuilder::try_append(StringView string)
 {
     if (string.is_empty())
@@ -129,6 +162,16 @@ ErrorOr<FlyString> StringBuilder::to_fly_string() const
 }
 #endif
 
+u8* StringBuilder::data()
+{
+    return m_buffer.data();
+}
+
+u8 const* StringBuilder::data() const
+{
+    return m_buffer.data();
+}
+
 StringView StringBuilder::string_view() const
 {
     return StringView { data(), m_buffer.size() };

AK/StringBuilder.h

@@ -16,11 +16,19 @@ namespace AK {
 
 class StringBuilder {
 public:
+    static constexpr size_t inline_capacity = 256;
+
     using OutputType = DeprecatedString;
 
     static ErrorOr<StringBuilder> create(size_t initial_capacity = inline_capacity);
 
     explicit StringBuilder(size_t initial_capacity = inline_capacity);
+
+    enum class UseInlineCapacityOnly {
+        Yes,
+        No,
+    };
+    explicit StringBuilder(UseInlineCapacityOnly use_inline_capacity_only);
     ~StringBuilder() = default;
 
     ErrorOr<void> try_append(StringView);
@@ -73,9 +81,9 @@ public:
     [[nodiscard]] StringView string_view() const;
     void clear();
 
-    [[nodiscard]] size_t length() const { return m_buffer.size(); }
+    [[nodiscard]] size_t length() const;
-    [[nodiscard]] bool is_empty() const { return m_buffer.is_empty(); }
+    [[nodiscard]] bool is_empty() const;
-    void trim(size_t count) { m_buffer.resize(m_buffer.size() - count); }
+    void trim(size_t count);
 
     template<class SeparatorType, class CollectionType>
     void join(SeparatorType const& separator, CollectionType const& collection, StringView fmtstr = "{}"sv)
@@ -98,10 +106,10 @@ public:
 
 private:
     ErrorOr<void> will_append(size_t);
-    u8* data() { return m_buffer.data(); }
+    u8* data();
-    u8 const* data() const { return m_buffer.data(); }
+    u8 const* data() const;
 
-    static constexpr size_t inline_capacity = 256;
+    UseInlineCapacityOnly m_use_inline_capacity_only { UseInlineCapacityOnly::No };
     Detail::ByteBuffer<inline_capacity> m_buffer;
 };