Get working string formatting for unsigned numbers.

8 files changed, 1080 insertions, 231 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 12c0727..6a3076b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -102,6 +102,7 @@ set(FUD_HEADERS
     "include/fud_c_file.hpp"
     "include/fud_c_string.hpp"
     "include/fud_directory.hpp"
+    "include/fud_fixed_vector.hpp"
     "include/fud_fud_type_traits.hpp"
     "include/fud_memory.hpp"
     "include/fud_option.hpp"
diff --git a/include/fud_fixed_vector.hpp b/include/fud_fixed_vector.hpp
new file mode 100644
index 0000000..d0c0f0e
--- /dev/null
+++ b/include/fud_fixed_vector.hpp
@@ -0,0 +1,276 @@
+/*
+ * libfud
+ * Copyright 2024 Dominick Allen
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef FUD_FIXED_VECTOR_HPP
+#define FUD_FIXED_VECTOR_HPP
+
+#include "fud_assert.hpp"
+#include "fud_config.hpp"
+#include "fud_option.hpp"
+#include "fud_span.hpp"
+
+#include <cstddef>
+
+namespace fud {
+
+template <typename T, size_t Size>
+class FixedVector {
+    static_assert(Size > 0);
+    using ValueType = T;
+
+  private:
+    T m_data[Size]; // NOLINT(cppcoreguidelines-avoid-c-arrays)
+    size_t m_length;
+
+  public:
+    [[nodiscard]] constexpr size_t size() const
+    {
+        return Size;
+    }
+
+    constexpr T* data() noexcept
+    {
+        return m_data;
+    }
+
+    constexpr const T* data() const noexcept
+    {
+        return m_data;
+    }
+
+    constexpr T* begin() noexcept
+    {
+        return m_data;
+    }
+
+    constexpr const T* begin() const noexcept
+    {
+        return m_data;
+    }
+
+    constexpr T* end() noexcept
+    {
+        return m_data + Size;
+    }
+
+    constexpr const T* end() const noexcept
+    {
+        return m_data + Size;
+    }
+
+    constexpr T& operator[](size_t index)
+    {
+        if constexpr (fudBoundsChecking) {
+            fudAssert(m_data != nullptr);
+            fudAssert(index < m_length);
+        }
+        return m_data[index];
+    }
+
+    constexpr const T& operator[](size_t index) const
+    {
+        if constexpr (fudBoundsChecking) {
+            fudAssert(m_data != nullptr);
+            fudAssert(index < m_length);
+        }
+        return m_data[index];
+    }
+
+    constexpr bool operator==(const FixedVector<T, Size>&) const noexcept = default;
+
+    constexpr auto operator<=>(const FixedVector<T, Size>& other) const noexcept = default;
+
+    Span<T, Size> span()
+    {
+        return Span<T, Size>{data(), Size};
+    }
+
+    FudStatus resize(size_t count)
+    {
+        if (count == m_length) {
+            return FudStatus::Success;
+        }
+
+        if (count < m_length) {
+            m_length = count;
+            return FudStatus::Success;
+        }
+
+        for (size_t index = m_length; index < count; ++index) {
+            m_data = std::move(T());
+        }
+
+        m_length = count;
+        return FudStatus::Success;
+    }
+
+    FudStatus clear()
+    {
+        m_length = 0;
+        return FudStatus::Success;
+    }
+
+    constexpr Option<T&> front()
+    {
+        if (m_length > 0) {
+            return m_data[0];
+        }
+        return NullOpt;
+    }
+
+    constexpr Option<const T&> front() const
+    {
+        if (m_length > 0) {
+            return m_data[0];
+        }
+        return NullOpt;
+    }
+
+    constexpr Option<T&> back()
+    {
+        if (m_length > 0) {
+            return m_data[m_length - 1];
+        }
+        return NullOpt;
+    }
+
+    constexpr Option<const T&> back() const
+    {
+        if (m_length > 0) {
+            return m_data[m_length - 1];
+        }
+        return NullOpt;
+    }
+
+    FudStatus pushBack(const T& value)
+    {
+        if (m_length == Size) {
+            return FudStatus::Full;
+        }
+
+        m_data[m_length] = value;
+        m_length++;
+        return FudStatus::Success;
+    }
+
+    FudStatus pushBack(T&& value)
+    {
+        if (m_length == Size) {
+            return FudStatus::Full;
+        }
+
+        m_data[m_length] = std::move(value);
+        m_length++;
+        return FudStatus::Success;
+    }
+
+    Result<T, FudStatus> popBack()
+    {
+        if (m_data == nullptr) {
+            return FudStatus::ObjectInvalid;
+        }
+        if (m_length == 0) {
+            return FudStatus::Empty;
+        }
+        auto result{std::move(m_data[m_length - 1])};
+        m_length--;
+        return result;
+    }
+
+    FudStatus eraseBack()
+    {
+        if (m_data == nullptr) {
+            return FudStatus::ObjectInvalid;
+        }
+        if (m_length == 0) {
+            return FudStatus::Empty;
+        }
+        m_length--;
+        return FudStatus::Success;
+    }
+
+    FudStatus insert(size_t index, const T& value)
+    {
+        if (index > m_length) {
+            return FudStatus::IndexInvalid;
+        }
+        if (index == m_length) {
+            return pushBack(value);
+        }
+        if (m_length == Size) {
+            return FudStatus::Full;
+        }
+
+        m_data[m_length] = std::move(m_data[m_length - 1]);
+
+        for (size_t backIndex = m_length - 1; backIndex > index; --backIndex) {
+            m_data[backIndex] = std::move(m_data[backIndex - 1]);
+        }
+        m_data[index] = value;
+        m_length++;
+        return FudStatus::Success;
+    }
+
+    FudStatus insert(size_t index, T&& value)
+    {
+        if (index > m_length) {
+            return FudStatus::IndexInvalid;
+        }
+        if (index == m_length) {
+            return pushBack(std::move(value));
+        }
+        if (m_length == Size) {
+            return FudStatus::Full;
+        }
+
+        m_data[m_length] = std::move(m_data[m_length - 1]);
+
+        for (size_t backIndex = m_length - 1; backIndex > index; --backIndex) {
+            m_data[backIndex] = std::move(m_data[backIndex - 1]);
+        }
+        m_data[index] = std::move(value);
+        m_length++;
+        return FudStatus::Success;
+    }
+
+    FudStatus erase(size_t index)
+    {
+        if (index >= m_length) {
+            return FudStatus::IndexInvalid;
+        }
+
+        for (size_t fwdIndex = index; fwdIndex + 1 < m_length; fwdIndex++) {
+            m_data[fwdIndex] = std::move(m_data[fwdIndex + 1]);
+        }
+
+        m_length--;
+        return FudStatus::Success;
+    }
+
+  private:
+    FudStatus cleanup() noexcept
+    {
+        auto status = clear();
+
+        m_length = 0;
+        return status;
+    }
+};
+
+} // namespace fud
+
+#endif
diff --git a/include/fud_format.hpp b/include/fud_format.hpp
index 1d09422..c42cd03 100644
--- a/include/fud_format.hpp
+++ b/include/fud_format.hpp
@@ -19,6 +19,7 @@
 #define FUD_FORMAT_HPP
 
 // #include "fud_assert.hpp"
+#include "fud_array.hpp"
 #include "fud_option.hpp"
 #include "fud_result.hpp"
 #include "fud_status.hpp"
@@ -27,12 +28,17 @@
 #include "fud_string_view.hpp"
 #include "fud_utf8.hpp"
 
+#include <concepts>
 #include <cstdint>
 #include <limits>
 #include <variant>
 
 namespace fud {
 
+// TODO: constants file?
+constexpr size_t bitsPerOctal = 3;
+constexpr size_t maxIntCharCount = bitsPerOctal * sizeof(uint64_t) + 4;
+
 struct FormatString {
     template <size_t N>
     consteval FormatString(const char (&input)[N]) : m_size{N - 1}, m_data{input}
@@ -199,8 +205,9 @@ struct FormatSpec {
 
     bool hasLocale : 1 = false;
 
-    [[nodiscard]] constexpr bool takesPosition() const {
-        return position == positionUnspecified;
+    [[nodiscard]] constexpr bool takesPosition() const
+    {
+        return position != positionUnspecified;
     }
 
     static Result<FormatSpec, FudStatus> parse(StringView formatView, size_t& specLength);
@@ -216,7 +223,6 @@ struct FormatHandle {
 } // namespace impl
 
 using FormatArgument = std::variant<
-    std::monostate,
     bool,
     utf8,
     int32_t,
@@ -240,6 +246,11 @@ struct FormatArguments {
         return Size;
     }
 
+    constexpr const FormatArgument& operator[](size_t index) const
+    {
+        return arguments[index];
+    }
+
     template <typename... Args>
     static auto makeFormatArguments(Args&&... args) -> FormatArguments
     {
@@ -266,7 +277,7 @@ struct FormatResult {
 };
 
 /* TODO : require concept of a sink that takes pushBack() -> FudStatus */
-/* TODO : sink concept also requires append() -> FudStatus */
+/* TODO : sink concept also requires drain() -> FudStatus */
 template <typename Sink>
 FormatResult format(Sink& sink, FormatCharMode formatMode, FormatString fmt);
 
@@ -274,58 +285,48 @@ template <typename Sink, typename... Args>
 FormatResult format(Sink& sink, FormatCharMode formatMode, FormatString fmt, Args&&... args);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, bool arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, bool arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, utf8 arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, utf8 arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, int32_t arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, int32_t arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, uint32_t arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, uint32_t arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, int64_t arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, int64_t arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, uint64_t arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, uint64_t arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, float arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, float arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, double arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, double arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, long double arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, long double arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, const utf8* arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, const utf8* arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, StringView arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, StringView arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, const void* arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, const void* arg);
 
 template <typename Sink>
-FormatResult format(Sink& sink, FormatCharMode formatMode, FormatSpec& formatSpec, impl::FormatHandle arg);
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, impl::FormatHandle arg);
 
 namespace impl {
 
-Result<size_t, FudStatus> validateSpecHelper(FormatString fmt);
-
-struct FormatInfo {
-    uint16_t numArgs{0};
-    uint16_t maxArg{0};
-    bool positional{false};
-};
-
-Result<FormatInfo, FudStatus> validateSpecHelperV2(FormatString fmt);
-
 template <typename Sink, size_t Size>
-FormatResult vFormat(Sink& sink, FormatCharMode formatMode, FormatString fmt, FormatArguments<Size>& args);
+FormatResult vFormat(Sink& sink, FormatCharMode formatMode, FormatString fmt, const FormatArguments<Size>& args);
 
 } // namespace impl
 
@@ -339,16 +340,6 @@ FormatResult format(Sink& sink, FormatCharMode formatMode, FormatString fmt, Arg
 
     constexpr size_t numArgs = sizeof...(args);
     auto formatArguments{FormatArguments<numArgs>::makeFormatArguments(std::forward<Args>(args)...)};
-    static_cast<void>(formatArguments);
-
-    auto numSpecsResult = impl::validateSpecHelper(fmt);
-    if (numSpecsResult.isError()) {
-        return {0, numSpecsResult.takeError()};
-    }
-
-    if (numSpecsResult.getOkay() < numArgs) {
-        return {0, FudStatus::FormatInvalid};
-    }
 
     return impl::vFormat(sink, formatMode, fmt, formatArguments);
 }
@@ -367,150 +358,95 @@ FormatResult format(Sink& sink, FormatCharMode formatMode, FormatString fmt)
 
 namespace impl {
 
-bool nestingHelper(bool& sawBracket, int& nesting, int increment)
-{
-    auto inSpec = nesting > 0;
-    if (not inSpec && not sawBracket) {
-        sawBracket = true;
-        return false;
-    }
-    if (not inSpec && sawBracket) {
-        sawBracket = false;
-        return false;
-    }
-
-    nesting += increment;
-    return true;
-}
+size_t findSpec(StringView scanView);
 
-Result<size_t, FudStatus> validateSpecHelper(FormatString fmt)
-{
-    auto scanView{fmt.view()};
-    const auto length = scanView.length();
-    const auto* data = scanView.data();
-    int nesting = 0;
-    size_t index = 0;
-    size_t numGroups = 0;
+template <size_t Size>
+Result<uint32_t, FudStatus> getSpecField(FormatSpec& formatSpec, uint32_t index, const FormatArguments<Size>& args);
 
-    bool sawOpenBracket = false;
-    bool sawCloseBracket = false;
-    while (index < length) {
-        auto letter = data[index];
+template <size_t Size>
+Result<FormatSpec, FudStatus> vFormatPrepareSpec(
+    StringView& scanView,
+    const FormatArguments<Size>& args,
+    const uint32_t& argIndex,
+    bool& firstSpec,
+    bool& takesPosition);
 
-        if (letter == FormatSpec::openBracket) {
-            auto incremented = nestingHelper(sawOpenBracket, nesting, 1);
-            if (incremented) {
-                numGroups++;
-            }
-        } else if (letter == FormatSpec::closeBracket) {
-            nestingHelper(sawCloseBracket, nesting, -1);
-            if (sawOpenBracket) {
-                sawOpenBracket = false;
-                sawCloseBracket = false;
-                numGroups++;
-            }
-        } else {
-            if (sawOpenBracket) {
-                nesting++;
-                numGroups++;
-            }
-            sawOpenBracket = false;
-            sawCloseBracket = false;
-        }
-        if (nesting < 0 || nesting > 2) {
-            return FudStatus::FormatInvalid;
-        }
-        ++index;
-    }
+template <size_t Size>
+Result<FormatSpec, FudStatus> vFormatPrepareFirstSpec(
+    StringView& scanView,
+    const FormatArguments<Size>& args,
+    const uint32_t& argIndex,
+    bool& takesPosition);
 
-    if (nesting != 0 || sawOpenBracket || sawCloseBracket) {
-        return FudStatus::FormatInvalid;
-    }
+template <size_t Size>
+FudStatus vFormatPreparePositionalSpec(FormatSpec& formatSpec, const FormatArguments<Size>& args);
 
-    return numGroups;
-}
+template <size_t Size>
+FudStatus vFormatPrepareNonPositionalSpec(
+    FormatSpec& formatSpec,
+    const FormatArguments<Size>& args,
+    const uint32_t& argIndex);
 
-// NOLINTBEGIN(readability-function-cognitive-complexity)
-Result<FormatInfo, FudStatus> validateSpecHelperV2(FormatString fmt)
+template <typename Sink, size_t Size>
+FormatResult vFormat(Sink& sink, FormatCharMode formatMode, FormatString fmt, const FormatArguments<Size>& args)
 {
-    auto scanView{fmt.view()};
-    const auto length = scanView.length();
-    const auto* data = scanView.data();
-    int nesting = 0;
-    size_t index = 0;
-    FormatInfo formatInfo{};
-
-    bool sawOpenBracket = false;
-    bool sawCloseBracket = false;
-    bool enteredGroup = false;
-
-    bool firstGroup = true;
+    FormatResult result{0, FudStatus::Success};
+    auto scanView = fmt.view();
+    bool firstSpec = true;
+    bool takesPosition = false;
+    uint32_t argIndex = 0;
+    while (scanView.length() > 0) {
+        if (argIndex > Size) {
+            result.status = FudStatus::FormatInvalid;
+            return result;
+        }
 
-    while (index < length) {
-        auto inSpec = nesting > 0;
-        auto letter = data[index];
+        auto specIndex = findSpec(scanView);
+        fudAssert(specIndex <= scanView.length());
+        StringView run{specIndex, scanView.data()};
+        auto drainResult = sink.drain(run);
+        result.bytesWritten += drainResult.bytesWritten;
+        if (drainResult.status != FudStatus::Success) {
+            result.status = drainResult.status;
+            return result;
+        }
+        scanView.advanceUnsafe(specIndex);
+        if (specIndex == scanView.length()) {
+            return result;
+        }
 
-        if (letter == FormatSpec::openBracket) {
-            if (not inSpec) {
-                sawOpenBracket = not sawOpenBracket;
-            } else {
-                nesting++;
-                formatInfo.numArgs++;
-                enteredGroup = true;
-            }
-        } else if (letter == FormatSpec::closeBracket) {
-            if (not inSpec) {
-                sawCloseBracket = not sawCloseBracket;
-            } else {
-                nesting--;
-            }
-            if (sawOpenBracket) {
-                sawOpenBracket = false;
-                sawCloseBracket = false;
-                enteredGroup = false;
-                formatInfo.numArgs++;
-            }
-        } else if (enteredGroup) {
-            nesting++;
-            formatInfo.numArgs++;
-            enteredGroup = false;
-            sawOpenBracket = false;
-            sawCloseBracket = false;
-
-            if (classify::isDigit(letter)) {
-                if (length <= index) {
-                    return FudStatus::FormatInvalid;
-                }
-                StringView specView{length - index, data + index};
-                auto positionResult = fromString<decltype(formatInfo.maxArg)>(specView, Radix::Decimal);
-                if (positionResult.isError())
-                {
-                    return positionResult.takeError();
-                }
-                auto positionValue = positionResult.takeOkay();
-                if (not firstGroup && not formatInfo.positional) {
-                    return FudStatus::FormatInvalid;
-                }
-                formatInfo.maxArg = std::max(formatInfo.maxArg, positionValue.value);
-                formatInfo.numArgs++;
-                firstGroup = false;
+        auto specResult{vFormatPrepareSpec(scanView, args, argIndex, firstSpec, takesPosition)};
+        if (specResult.isError()) {
+            result.status = specResult.takeError();
+            return result;
+        }
+        auto formatSpec{specResult.takeOkay()};
 
+        uint32_t formatArgIndex = argIndex;
+        if (formatSpec.takesPosition()) {
+            if (formatSpec.position >= Size) {
+                result.status = specResult.takeError();
+                return result;
             }
+            formatArgIndex = formatSpec.position;
         }
-
-        if (nesting < 0 || nesting > 2) {
-            return FudStatus::FormatInvalid;
+        auto argResult{std::visit(
+            [&](const auto& arg) -> FormatResult { return format(sink, formatMode, formatSpec, arg); },
+            args[formatArgIndex])};
+        if (argResult.status != FudStatus::Success) {
+            result.status = argResult.status;
+            return result;
+        }
+        result.bytesWritten += argResult.bytesWritten;
+        if (!formatSpec.takesPosition()) {
+            argIndex++;
+            argIndex += static_cast<uint8_t>(formatSpec.takesWidth);
+            argIndex += static_cast<uint8_t>(formatSpec.takesPrecision);
         }
-        ++index;
-    }
-
-    if (nesting != 0 || sawOpenBracket || sawCloseBracket) {
-        return FudStatus::FormatInvalid;
     }
 
-    return formatInfo;
+    return result;
 }
-// NOLINTEND(readability-function-cognitive-complexity)
 
 size_t findSpec(StringView scanView)
 {
@@ -533,80 +469,605 @@ size_t findSpec(StringView scanView)
 }
 
 template <size_t Size>
-Result<FormatSpec, FudStatus> vFormatPrepareArgs(
+Result<uint32_t, FudStatus> getSpecField(FormatSpec& formatSpec, uint32_t index, const FormatArguments<Size>& args)
+{
+    static_assert(not std::is_signed_v<utf8>);
+
+    if (not formatSpec.takesPosition() || not formatSpec.takesWidth) {
+        return FudStatus::ArgumentInvalid;
+    }
+
+    if (index >= Size) {
+        return FudStatus::FormatInvalid;
+    }
+
+    if (std::holds_alternative<utf8>(args[index])) {
+        return std::get<utf8>(args[index]);
+    }
+    if (std::holds_alternative<int32_t>(args[index])) {
+        auto value = std::get<int32_t>(args[index]);
+        if (value < 0) {
+            return FudStatus::FormatInvalid;
+        }
+        return static_cast<uint32_t>(value);
+    }
+    if (std::holds_alternative<int64_t>(args[index])) {
+        auto value = std::get<int64_t>(args[index]);
+        if (value < 0 || value > std::numeric_limits<uint32_t>::max()) {
+            return FudStatus::FormatInvalid;
+        }
+        return static_cast<uint32_t>(value);
+    }
+    if (std::holds_alternative<uint32_t>(args[index])) {
+        return std::get<uint32_t>(args[index]);
+    }
+    if (std::holds_alternative<uint32_t>(args[index])) {
+        auto value = std::get<uint64_t>(args[index]);
+        if (value > std::numeric_limits<uint32_t>::max()) {
+            return FudStatus::FormatInvalid;
+        }
+        return static_cast<uint32_t>(value);
+    }
+
+    return FudStatus::FormatInvalid;
+}
+
+template <size_t Size>
+Result<FormatSpec, FudStatus> vFormatPrepareSpec(
     StringView& scanView,
-    FormatArguments<Size>& args,
-    size_t& argIndex,
+    const FormatArguments<Size>& args,
+    const uint32_t& argIndex,
     bool& firstSpec,
     bool& takesPosition)
 {
+    if (firstSpec) {
+        firstSpec = false;
+        return vFormatPrepareFirstSpec(scanView, args, argIndex, takesPosition);
+    }
+
     size_t specLength{0};
     auto formatSpecResult{FormatSpec::parse(scanView, specLength)};
     if (formatSpecResult.isError()) {
         return formatSpecResult;
     }
     fudAssert(specLength <= scanView.length());
-    auto formatSpec{formatSpecResult.takeOkay()};
-    bool specTakesPosition = formatSpec.position != FormatSpec::positionUnspecified;
-    if (firstSpec) {
-        takesPosition = specTakesPosition;
-        firstSpec = false;
-    } else if (takesPosition != specTakesPosition) {
-        return FudStatus::FormatInvalid;
-    } else if (takesPosition && formatSpec.position >= Size) {
+    auto formatSpec{formatSpecResult.getOkay()};
+
+    if (takesPosition != formatSpec.takesPosition() || (takesPosition && formatSpec.position >= Size)) {
         return FudStatus::FormatInvalid;
     }
-    if (formatSpec.takesWidth) {
-        if (takesPosition) {
-            auto widthPosition = formatSpec.width;
-            if (widthPosition >= args.size()) {
-                return FudStatus::FormatInvalid;
-            }
+
+    if (takesPosition) {
+        auto status = vFormatPreparePositionalSpec(formatSpec, args);
+        if (status != FudStatus::Success) {
+            return status;
+        }
+    } else {
+        auto status = vFormatPrepareNonPositionalSpec(formatSpec, args, argIndex);
+        if (status != FudStatus::Success) {
+            return status;
         }
     }
-        // auto formatResult{format(sink,
-        scanView.advanceUnsafe(specLength);
+
+    scanView.advanceUnsafe(specLength);
 
     return formatSpec;
 }
 
-template <typename Sink, size_t Size>
-FormatResult vFormat(Sink& sink, FormatCharMode formatMode, FormatString fmt, FormatArguments<Size>& args)
+template <size_t Size>
+Result<FormatSpec, FudStatus> vFormatPrepareFirstSpec(
+    StringView& scanView,
+    const FormatArguments<Size>& args,
+    const uint32_t& argIndex,
+    bool& takesPosition)
 {
-    FormatResult result{0, FudStatus::Failure};
-    auto scanView = fmt.view();
-    bool firstSpec = false;
-    bool takesPosition = false;
-    size_t argIndex = 0;
-    while (scanView.length() > 0) {
-        auto specIndex = findSpec(scanView);
-        fudAssert(specIndex <= scanView.length());
-        StringView run{specIndex, scanView.data()};
-        auto status = sink.append(run);
+    size_t specLength{0};
+    auto formatSpecResult{FormatSpec::parse(scanView, specLength)};
+    if (formatSpecResult.isError()) {
+        return formatSpecResult;
+    }
+    fudAssert(specLength <= scanView.length());
+    auto formatSpec{formatSpecResult.takeOkay()};
+    takesPosition = formatSpec.takesPosition();
+
+    if (takesPosition) {
+        auto status = vFormatPreparePositionalSpec(formatSpec, args);
         if (status != FudStatus::Success) {
-            result.status = status;
-            return result;
+            return status;
         }
-        result.bytesWritten += specIndex;
-        scanView.advanceUnsafe(specIndex);
-        if (specIndex == scanView.length()) {
-            return result;
+    } else {
+        auto status = vFormatPrepareNonPositionalSpec(formatSpec, args, argIndex);
+        if (status != FudStatus::Success) {
+            return status;
         }
+    }
+
+    scanView.advanceUnsafe(specLength);
+
+    return formatSpec;
+}
 
-        auto specResult{vFormatPrepareArgs(scanView, args, argIndex, firstSpec, takesPosition)};
+template <size_t Size>
+FudStatus vFormatPreparePositionalSpec(FormatSpec& formatSpec, const FormatArguments<Size>& args)
+{
+    if (not formatSpec.takesPosition()) {
+        return FudStatus::OperationInvalid;
+    }
+
+    if (formatSpec.takesWidth) {
+        auto widthResult = getSpecField(formatSpec, formatSpec.width, args);
+        if (widthResult.isError()) {
+            return widthResult.takeError();
+        }
+        formatSpec.width = widthResult.takeOkay();
+    }
+
+    if (formatSpec.takesPrecision) {
+        auto precisionResult = getSpecField(formatSpec, formatSpec.precision, args);
+        if (precisionResult.isError()) {
+            return precisionResult.takeError();
+        }
+        formatSpec.precision = precisionResult.takeOkay();
+    }
+
+    return FudStatus::Success;
+}
+
+template <size_t Size>
+FudStatus vFormatPrepareNonPositionalSpec(
+    FormatSpec& formatSpec,
+    const FormatArguments<Size>& args,
+    const uint32_t& argIndex)
+{
+    if (formatSpec.takesPosition()) {
+        return FudStatus::OperationInvalid;
+    }
+
+    uint32_t fieldIndex = argIndex + 1;
+
+    if (formatSpec.takesWidth) {
+        if (fieldIndex < argIndex) {
+            return FudStatus::FormatInvalid;
+        }
+        auto widthResult = getSpecField(formatSpec, fieldIndex, args);
+        if (widthResult.isError()) {
+            return widthResult.takeError();
+        }
+        formatSpec.width = widthResult.takeOkay();
+        fieldIndex++;
+    }
+
+    if (formatSpec.takesPrecision) {
+        if (fieldIndex < argIndex) {
+            return FudStatus::FormatInvalid;
+        }
+        auto precisionResult = getSpecField(formatSpec, fieldIndex, args);
+        if (precisionResult.isError()) {
+            return precisionResult.takeError();
+        }
+        formatSpec.precision = precisionResult.takeOkay();
+    }
+
+    return FudStatus::Success;
+}
+
+template <typename T>
+    requires std::is_integral_v<T>
+[[nodiscard]] StringView validateFormatType(
+    FormatResult& result,
+    FormatType formatType,
+    Radix& radix,
+    bool& uppercase,
+    T arg)
+{
+    StringView prefix{""};
+    switch (formatType) {
+    case FormatType::Unspecified:
+        break;
+    case FormatType::BinaryLower:
+        prefix = StringView{"0b"};
+        radix = Radix::Binary;
+        break;
+    case FormatType::BinaryUpper:
+        prefix = StringView{"0B"};
+        radix = Radix::Binary;
+        uppercase = true;
+        break;
+    case FormatType::Character:
+        if constexpr (std::is_signed_v<T>) {
+            if (arg < 0) {
+                result.status = FudStatus::FormatInvalid;
+            }
+        }
+        if (arg > std::numeric_limits<utf8>::max()) {
+            result.status = FudStatus::FormatInvalid;
+            break;
+        }
+        break;
+    case FormatType::Decimal:
+        break;
+    case FormatType::Octal:
+        prefix = StringView{"o"};
+        radix = Radix::Octal;
+        break;
+    case FormatType::HexLower:
+        prefix = StringView{"0x"};
+        radix = Radix::Hexadecimal;
+        break;
+    case FormatType::HexUpper:
+        prefix = StringView{"0X"};
+        radix = Radix::Hexadecimal;
+        uppercase = true;
+        break;
+    case FormatType::FloatHexLower:
+    case FormatType::FloatHexUpper:
+    case FormatType::String:
+    case FormatType::Escaped:
+    case FormatType::ScientificLower:
+    case FormatType::ScientificUpper:
+    case FormatType::FixedLower:
+    case FormatType::FixedUpper:
+    case FormatType::GeneralLower:
+    case FormatType::GeneralUpper:
+        result.status = FudStatus::FormatInvalid;
+    default:
+        result.status = FudStatus::Failure;
+    }
+    return prefix;
+}
+
+inline FudStatus fillUnsignedBuffer(
+    Array<utf8, maxIntCharCount>& buffer,
+    uint64_t value,
+    uint8_t& bufferLength,
+    Radix radix,
+    bool uppercase)
+{
+    static_assert(maxIntCharCount < std::numeric_limits<uint8_t>::max());
+    bufferLength = 0;
+
+    constexpr Array<uint8_t, static_cast<size_t>(Radix::Hexadecimal)> lower{
+        {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'}};
+    constexpr Array<uint8_t, static_cast<size_t>(Radix::Hexadecimal)> upper{
+        {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'A', 'B', 'C', 'D', 'E', 'F'}};
+    const auto& lookup = uppercase ? upper : lower;
+
+    if (value == 0) {
+        buffer[0] = '0';
+        bufferLength = 1;
+        return FudStatus::Success;
+    }
+
+    static_assert(std::is_same_v<std::underlying_type_t<Radix>, uint8_t>);
+    auto radixValue = static_cast<uint8_t>(radix);
+    while (value > 0 && bufferLength < maxIntCharCount) {
+        auto digit = static_cast<uint8_t>(value % radixValue);
+        buffer[bufferLength] = lookup[digit];
+        value /= radixValue;
+        bufferLength++;
+    }
+
+    if (value > 0 || bufferLength > maxIntCharCount) {
+        return FudStatus::Failure;
+    }
+
+    // TODO: implement fud_algorithm reverse
+    for (size_t idx = 0; idx < bufferLength / 2; ++idx) {
+        auto rhsIndex = bufferLength - idx - 1;
+        std::swap(buffer[idx], buffer[rhsIndex]);
+    }
+
+    return FudStatus::Success;
+}
+
+template <typename Sink>
+[[nodiscard]] FormatResult fillPad(Sink& sink, utf8 fill, size_t count)
+{
+    FormatResult result{0, FudStatus::Success};
+    constexpr size_t BigBlockSize = 256;
+    constexpr size_t MediumBlockSize = 64;
+    constexpr size_t SmallBlockSize = 16;
+    constexpr size_t SmallestBlockSize = 4;
+
+    auto backingBuffer = Array<utf8, BigBlockSize>::constFill(fill);
+
+    constexpr auto bufferSizes{Array<size_t, 4>{{BigBlockSize, MediumBlockSize, SmallBlockSize, SmallestBlockSize}}};
+
+    for (auto bufferSize : bufferSizes) {
+        while (count > bufferSize) {
+            auto drainResult = sink.drain(StringView{bufferSize, backingBuffer.data()});
+            result.bytesWritten += drainResult.bytesWritten;
+            result.status = drainResult.status;
+            if (result.status != FudStatus::Success) {
+                return result;
+            }
+            count -= result.bytesWritten;
+        }
+    }
+
+    if (count > 0) {
+        fudAssert(count < backingBuffer.size());
+        auto drainResult = sink.drain(StringView{count, backingBuffer.data()});
+        result.bytesWritten += drainResult.bytesWritten;
+        result.status = drainResult.status;
+    }
+
+    return result;
+}
+
+template <typename Sink, typename T>
+    requires std::is_integral_v<T> and std::is_scalar_v<T> /*  and std::is_unsigned_v<T> */
+[[nodiscard]] FormatResult formatUnsigned(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, T arg)
+{
+    FormatResult result{0, FudStatus::Success};
+    if (formatSpec.takesPrecision || formatSpec.precision != FormatSpec::precisionUnspecified) {
+        result.status = FudStatus::FormatInvalid;
+        return result;
+    }
+    auto fill = formatSpec.fill.fill;
+    auto align = formatSpec.fill.align.value;
+    bool padZero = false;
+    if (align == FormatAlign::Value::Default) {
+        align = FormatAlign::Value::Right;
+        fill = formatSpec.leadingZero ? '0' : fill;
+        padZero = formatSpec.leadingZero;
+    }
+    auto width = formatSpec.width;
+    if (width == FormatSpec::widthUnspecified) {
+        width = 0;
+    }
+
+    auto radix = Radix::Decimal;
+    bool uppercase = false;
+
+    auto prefix = validateFormatType(result, formatSpec.formatType, radix, uppercase, arg);
+    if (result.status != FudStatus::Success) {
+        return result;
+    }
+
+    if (formatSpec.formatType == FormatType::Character) {
+        return format(sink, formatMode, formatSpec, static_cast<utf8>(arg));
+    }
+
+    if (not formatSpec.alternateForm) {
+        prefix = StringView{""};
+    }
+
+    auto calculatedWidth = prefix.length();
+
+    Array<utf8, maxIntCharCount> buffer{};
+    uint8_t bufferLength = 0;
+
+    // TODO: constexpr if for signed values
+    result.status = fillUnsignedBuffer(buffer, static_cast<uint64_t>(arg), bufferLength, radix, uppercase);
+
+    if (result.status != FudStatus::Success) {
+        return result;
+    }
+
+    calculatedWidth += bufferLength;
+
+    StringView sign{""};
+    switch (formatSpec.formatSign) {
+    case FormatSign::Plus:
+        sign = StringView{"+"};
+        break;
+    case FormatSign::Space:
+        sign = StringView{" "};
+        break;
+    case FormatSign::Minus:
+    case FormatSign::Default:
+    default:
+        if constexpr (std::is_signed_v<T>) {
+            if (arg < 0) {
+                sign = StringView{"-"};
+            }
+        }
+        break;
+    }
+    calculatedWidth += sign.length();
+
+    if (align == FormatAlign::Value::Right and width > calculatedWidth) {
+        auto leftPadSize = width - calculatedWidth;
+        auto padResult = fillPad(sink, fill, leftPadSize);
+        result.bytesWritten += padResult.bytesWritten;
+        result.status = padResult.status;
+    } else if (align == FormatAlign::Value::Center and width > calculatedWidth) {
+        auto leftPadSize = (width - calculatedWidth) / 2;
+        auto padResult = fillPad(sink, fill, leftPadSize);
+        result.bytesWritten += padResult.bytesWritten;
+        result.status = padResult.status;
+    }
+
+    if (result.status != FudStatus::Success) {
+        return result;
+    }
+
+    if (sign.length() > 0) {
+        auto drainResult = sink.drain(sign);
+        result.bytesWritten += drainResult.bytesWritten;
+        result.status = drainResult.status;
+    }
+
+    if (result.status != FudStatus::Success) {
+        return result;
+    }
+
+    if (prefix.length() > 0) {
+        auto drainResult = sink.drain(sign);
+        result.bytesWritten += drainResult.bytesWritten;
+        result.status = drainResult.status;
+    }
+
+    if (result.status != FudStatus::Success) {
+        return result;
+    }
+
+    if (padZero) {
+        auto leftPadSize = width - calculatedWidth;
+        auto padResult = fillPad(sink, '0', leftPadSize);
+        result.bytesWritten += padResult.bytesWritten;
+        result.status = padResult.status;
+    }
+
+    if (result.status != FudStatus::Success) {
+        return result;
+    }
+
+    auto drainNumberResult = sink.drain(StringView{bufferLength, buffer.data()});
+    result.bytesWritten += drainNumberResult.bytesWritten;
+    result.status = drainNumberResult.status;
+
+    if (result.status != FudStatus::Success) {
+        return result;
+    }
+
+    if (align == FormatAlign::Value::Left and width > calculatedWidth) {
+        auto rightPadSize = width - calculatedWidth;
+        auto padResult = fillPad(sink, fill, rightPadSize);
+        result.bytesWritten += padResult.bytesWritten;
+        result.status = padResult.status;
+    } else if (align == FormatAlign::Value::Center and width > calculatedWidth) {
+        auto leftPadSize = (width - calculatedWidth) / 2;
+        auto rightPadSize = width - leftPadSize;
+        auto padResult = fillPad(sink, fill, rightPadSize);
+        result.bytesWritten += padResult.bytesWritten;
+        result.status = padResult.status;
     }
 
-    static_cast<void>(formatMode);
-    static_cast<void>(args);
     return result;
 }
 
 } // namespace impl
 
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, bool arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, utf8 arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, int32_t arg)
+{
+    return impl::formatUnsigned(sink, formatMode, formatSpec, arg);
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, uint32_t arg)
+{
+    return impl::formatUnsigned(sink, formatMode, formatSpec, arg);
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, int64_t arg)
+{
+    return impl::formatUnsigned(sink, formatMode, formatSpec, arg);
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, uint64_t arg)
+{
+    return impl::formatUnsigned(sink, formatMode, formatSpec, arg);
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, float arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, double arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, long double arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, const utf8* arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, StringView arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, const void* arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
+template <typename Sink>
+FormatResult format(Sink& sink, FormatCharMode formatMode, const FormatSpec& formatSpec, impl::FormatHandle arg)
+{
+    FormatResult result{0, FudStatus::NotImplemented};
+    static_cast<void>(sink);
+    static_cast<void>(formatMode);
+    static_cast<void>(formatSpec);
+    static_cast<void>(arg);
+    return result;
+}
+
 namespace fudTest {
 
 inline void test()
 {
+    StringView prefix{0, ""};
     String sink{};
 
     static_cast<void>(format(sink, FormatCharMode::Unchecked, "HELL YEAH"));
diff --git a/include/fud_string.hpp b/include/fud_string.hpp
index 60a328f..0213524 100644
--- a/include/fud_string.hpp
+++ b/include/fud_string.hpp
@@ -38,6 +38,11 @@ constexpr size_t SSO_BUF_SIZE = SSO_BUF_LENGTH + 1;
 
 using StringResult = Result<String, FudStatus>;
 
+struct DrainResult {
+    size_t bytesWritten;
+    FudStatus status;
+};
+
 class String {
   public:
     static StringResult makeFromCString(const char* cString);
@@ -212,12 +217,20 @@ class String {
 
     FudStatus append(StringView source);
 
+    DrainResult drain(const char* source);
+
+    DrainResult drain(const String& source);
+
+    DrainResult drain(StringView source);
+
     [[nodiscard]] StringResult catenate(const String& rhs) const;
 
     [[nodiscard]] StringResult catenate(const char* rhs) const;
 
     [[nodiscard]] bool compare(const String& rhs) const;
 
+    FudStatus clear();
+
     const utf8* begin() const;
 
     const utf8* end() const;
diff --git a/include/fud_string_view.hpp b/include/fud_string_view.hpp
index 2ced639..718b3d7 100644
--- a/include/fud_string_view.hpp
+++ b/include/fud_string_view.hpp
@@ -45,6 +45,12 @@ struct StringView {
     {
     }
 
+    template <size_t N>
+    StringView(const char (&input)[N]) : m_length{N - 1}, m_data{reinterpret_cast<const utf8*>(input)}
+    {
+        static_assert(N > 0);
+    }
+
     StringView(size_t strLen, const char* strData) :
         m_length(strLen), // line break
         m_data{reinterpret_cast<const utf8*>(strData)} // NOLINT(cppcoreguidelines-pro-type-reinterpret-cast)
diff --git a/source/fud_allocator.cpp b/source/fud_allocator.cpp
index 1e4117a..8daf969 100644
--- a/source/fud_allocator.cpp
+++ b/source/fud_allocator.cpp
@@ -35,7 +35,6 @@ FudStatus FudAllocator::deallocate(void* pointer, size_t bytes)
     if (pointer == nullptr || bytes == 0) {
         return FudStatus::ArgumentInvalid;
     }
-    static_cast<void>(bytes);
     fudFree(pointer);
     return FudStatus::Success;
 }
diff --git a/source/fud_string.cpp b/source/fud_string.cpp
index 6e16741..048cc94 100644
--- a/source/fud_string.cpp
+++ b/source/fud_string.cpp
@@ -364,14 +364,15 @@ FudStatus String::append(StringView source)
         return FudStatus::StringInvalid;
     }
 
-    if (source.data() == m_data) {
-        return FudStatus::Aliased;
+    if (source.data() == nullptr) {
+        return FudStatus::NullPointer;
     }
 
     const size_t newLength = length() + source.length();
     if (newLength < length()) {
         return FudStatus::OperationInvalid;
     }
+
     const size_t newSize = newLength + 1; // cppcheck-suppress knownConditionTrueFalse
     if (newSize < newLength) {            // cppcheck-suppress knownConditionTrueFalse
         return FudStatus::OperationInvalid;
@@ -386,7 +387,7 @@ FudStatus String::append(StringView source)
     }
 
     auto* destPtr = data() + length();
-    auto status = copyMem(destPtr, m_capacity, source.data(), source.length());
+    auto status = copyMem(destPtr, remainingLength(), source.data(), source.length());
     fudAssert(status == FudStatus::Success);
 
     m_length += source.length();
@@ -395,6 +396,87 @@ FudStatus String::append(StringView source)
     return status;
 }
 
+DrainResult String::drain(const char* source)
+{
+    auto lenResult = cStringLength(source);
+    if (lenResult < 0 || lenResult >= SSIZE_MAX) {
+        return {0, FudStatus::ArgumentInvalid};
+    }
+
+    return drain(StringView{static_cast<size_t>(lenResult), source});
+}
+
+DrainResult String::drain(const String& source)
+{
+    return drain(source.asView());
+}
+
+DrainResult String::drain(StringView source)
+{
+    DrainResult result{0, FudStatus::Success};
+    if (!valid()) {
+        result.status = FudStatus::StringInvalid;
+        return result;
+    }
+
+    if (source.data() == nullptr) {
+        result.status = FudStatus::NullPointer;
+        return result;
+    }
+
+    if (source.length() == 0) {
+        return result;
+    }
+
+    if (remainingLength() > 0) {
+        StringView firstPart{source};
+        if (source.length() > remainingLength()) {
+            firstPart.m_length = remainingLength();
+        }
+        auto* destPtr = data() + length();
+        auto status = copyMem(destPtr, remainingLength(), firstPart.m_data, firstPart.m_length);
+        fudAssert(status == FudStatus::Success);
+        m_length += firstPart.m_length;
+        result.bytesWritten += firstPart.m_length;
+        status = nullTerminate();
+        source.advanceUnsafe(firstPart.m_length);
+        fudAssert(status == FudStatus::Success);
+    }
+
+    if (source.length() == 0) {
+        return result;
+    }
+
+    const size_t newLength = length() + source.length();
+    if (newLength < length()) {
+        result.status = FudStatus::OperationInvalid;
+        return result;
+    }
+    const size_t newSize = newLength + 1; // cppcheck-suppress knownConditionTrueFalse
+    if (newSize < newLength) {            // cppcheck-suppress knownConditionTrueFalse
+        result.status = FudStatus::OperationInvalid;
+        return result;
+    }
+    if (newSize >= m_capacity) {
+        auto newCapacity = newSize < SIZE_MAX / 2 ? newSize * 2 : SIZE_MAX;
+        const auto resizeStatus = resize(newCapacity);
+        if (resizeStatus != FudStatus::Success) {
+            result.status = resizeStatus;
+            return result;
+        }
+        fudAssert(m_capacity == newCapacity);
+    }
+
+    auto* destPtr = data() + length();
+    auto status = copyMem(destPtr, remainingLength(), source.data(), source.length());
+    fudAssert(status == FudStatus::Success);
+
+    m_length += source.length();
+    status = nullTerminate();
+
+    return result;
+}
+
 StringResult String::catenate(const char* rhs) const
 {
     if (!valid()) {
@@ -507,6 +589,17 @@ bool String::compare(const String& rhs) const
     return diffResult.getOkay() == 0;
 }
 
+FudStatus String::clear() {
+    if (!valid()) {
+        return FudStatus::StringInvalid;
+    }
+
+    data()[0] = '\0';
+    m_length = 0;
+
+    return FudStatus::Success;
+}
+
 const utf8* String::begin() const
 {
     return data();
diff --git a/test/test_format.cpp b/test/test_format.cpp
index f0ea93f..9c95330 100644
--- a/test/test_format.cpp
+++ b/test/test_format.cpp
@@ -19,6 +19,7 @@
 #include "fud_string_view.hpp"
 
 #include "gtest/gtest.h"
+#include <format>
 
 namespace fud {
 
@@ -410,31 +411,30 @@ TEST(FormatTest, FormatTest)
     EXPECT_EQ(formatSpecResult.takeError(), FudStatus::FormatInvalid);
 }
 
-TEST(FormatTest, ValidateSpecHelperTest)
-{
-    constexpr size_t invalid = 0xFFFF;
-    EXPECT_EQ(impl::validateSpecHelper("").getOkayOr(invalid), 0);
-    EXPECT_EQ(impl::validateSpecHelper("{}").getOkayOr(invalid), 1);
-    EXPECT_EQ(impl::validateSpecHelper("{{{}").getOkayOr(invalid), 1);
-    EXPECT_EQ(impl::validateSpecHelper("{}}}").getOkayOr(invalid), 1);
-    EXPECT_EQ(impl::validateSpecHelper("{{{}}}").getOkayOr(invalid), 1);
-    EXPECT_EQ(impl::validateSpecHelper("}").getErrorOr(FudStatus::Failure), FudStatus::FormatInvalid);
-    EXPECT_EQ(impl::validateSpecHelper("{}}").getErrorOr(FudStatus::Failure), FudStatus::FormatInvalid);
-    EXPECT_EQ(impl::validateSpecHelper("{").getErrorOr(FudStatus::Failure), FudStatus::FormatInvalid);
-    EXPECT_EQ(impl::validateSpecHelper("{{}").getErrorOr(FudStatus::Failure), FudStatus::FormatInvalid);
-    EXPECT_EQ(impl::validateSpecHelper("{{}").getOkayOr(invalid), invalid);
-    EXPECT_EQ(impl::validateSpecHelper("{{{}}} {} {{ }} {{{}}}").getOkayOr(invalid), 3);
-    EXPECT_EQ(impl::validateSpecHelper("{{{}}} {} {{ }} {{ }}}").getOkayOr(invalid), invalid);
-    EXPECT_EQ(
-        impl::validateSpecHelper("{{{}}} {} {{ }} {{{ }}").getErrorOr(FudStatus::Failure),
-        FudStatus::FormatInvalid);
-}
-
 TEST(FormatTest, OneArgFormatTest)
 {
     String sink{};
-    auto formatResult = format(sink, FormatCharMode::Unchecked, "Test {:X}", 42U);
+    auto formatResult = format(sink, FormatCharMode::Unchecked, "Test {:}", 42U);
+    EXPECT_TRUE(formatResult.isOkay());
+
+    ASSERT_EQ(sink.clear(), FudStatus::Success);
+    ASSERT_EQ(sink.drain(StringView{4, "TEST"}).bytesWritten, 4);
+    ASSERT_TRUE(sink.valid());
+    ASSERT_EQ(sink.clear(), FudStatus::Success);
+
+    auto expected = std::format("{:6}", 42U);
+    formatResult = format(sink, FormatCharMode::Unchecked, "{:6}", 42U);
     EXPECT_TRUE(formatResult.isOkay());
+    EXPECT_STREQ(sink.c_str(), expected.c_str());
+
+    /*
+    EXPECT_EQ(std::format("{:6}", 'x')   , "x     ");
+    EXPECT_EQ(std::format("{:*<6}", 'x') , "x*****");
+    EXPECT_EQ(std::format("{:*>6}", 'x') , "*****x");
+    EXPECT_EQ(std::format("{:*^6}", 'x') , "**x***");
+    EXPECT_EQ(std::format("{:6d}", c)    , "   120");
+    EXPECT_EQ(std::format("{:6}", true)  , "true  ");
+     */
 }
 
 } // namespace fud