From 8042003647b50e9fddfe500677a132130449d69e Mon Sep 17 00:00:00 2001
From: Felix Morgner <felix.morgner@ost.ch>
Date: Thu, 18 Dec 2025 14:00:03 +0100
Subject: kapi/cio: implement formatted printing

---
 kernel/src/kapi/cio.cpp | 155 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 155 insertions(+)

(limited to 'kernel')

diff --git a/kernel/src/kapi/cio.cpp b/kernel/src/kapi/cio.cpp
index 66493b6..500d61e 100644
--- a/kernel/src/kapi/cio.cpp
+++ b/kernel/src/kapi/cio.cpp
@@ -1,5 +1,10 @@
 #include "kapi/cio.hpp"
 
+#include <kstd/format>
+
+#include <array>
+#include <cstddef>
+#include <iterator>
 #include <optional>
 #include <string_view>
 #include <utility>
@@ -20,6 +25,63 @@ namespace teachos::cio
     };
 
     constinit null_device null_device::instance;
+
+    struct write_buffer
+    {
+      constexpr auto static size = 128uz;
+
+      write_buffer(write_buffer const &) = delete;
+      write_buffer(write_buffer &&) = delete;
+      auto operator=(write_buffer const &) -> write_buffer & = delete;
+      auto operator=(write_buffer &&) -> write_buffer & = delete;
+
+      explicit write_buffer(output_device * device, bool write_to_error)
+          : m_device{device}
+          , m_write_to_error{write_to_error}
+      {}
+
+      ~write_buffer() noexcept
+      {
+        flush();
+      }
+
+      auto flush() noexcept -> void
+      {
+        if (m_position > 0)
+        {
+          std::string_view chunk{m_buffer.data(), m_position};
+          if (m_write_to_error)
+          {
+            m_device->write_error(chunk);
+          }
+          else
+          {
+            m_device->write(chunk);
+          }
+          m_position = 0;
+        }
+      }
+
+      auto static callback(void * object, std::string_view text) -> void
+      {
+        auto * self = static_cast<write_buffer *>(object);
+        for (char const character : text)
+        {
+          if (self->m_position >= size)
+          {
+            self->flush();
+          }
+          self->m_buffer.at(self->m_position++) = character;
+        }
+      }
+
+    private:
+      output_device * m_device;
+      bool m_write_to_error;
+      std::array<char, size> m_buffer{};
+      std::size_t m_position{};
+    };
+
   }  // namespace
 
   // NOLINTNEXTLINE(cppcoreguidelines-avoid-non-const-global-variables)
@@ -54,4 +116,97 @@ namespace teachos::cio
     active_device->writeln_error(text);
   }
 
+  namespace
+  {
+    auto static vprint_impl(std::string_view fmt, kstd::format_args args, bool write_to_error) -> void
+    {
+      if (!active_device)
+        return;
+
+      auto writer = write_buffer{active_device, write_to_error};
+      auto context = kstd::format_context{.writer = write_buffer::callback, .user_data = &writer};
+
+      auto current = fmt.begin();
+      auto end = fmt.end();
+      auto next_automatic_index = 0uz;
+
+      while (current != end)
+      {
+        if (*current != '{')
+        {
+          auto start = current;
+          while (current != end && *current != '{')
+          {
+            std::advance(current, 1);
+          }
+          context.push(std::string_view(start, current - start));
+          continue;
+        }
+
+        if (std::next(current) != end && *(std::next(current)) == '{')
+        {
+          context.push('{');
+          std::advance(current, 2);
+          continue;
+        }
+
+        std::advance(current, 1);
+
+        auto index = 0uz;
+        if (current != end && *current >= '0' && *current <= '9')
+        {
+          while (current != end && *current >= '0' && *current <= '9')
+          {
+            // NOLINTNEXTLINE(cppcoreguidelines-avoid-magic-numbers)
+            index = index * 10 + static_cast<std::size_t>(*current - '0');
+            std::advance(current, 1);
+          }
+        }
+        else
+        {
+          index = next_automatic_index++;
+        }
+
+        auto remaining_fmt = std::string_view{current, static_cast<std::size_t>(std::distance(current, end))};
+
+        auto const arg = args.get(index);
+        if (arg.format)
+        {
+          auto const after_specs = arg.format(arg.value, remaining_fmt, context);
+          auto const consumed = remaining_fmt.size() - after_specs.size();
+          std::advance(current, consumed);
+        }
+        else
+        {
+          context.push("{?}");
+          while (current != end && *current != '}')
+            std::advance(current, 1);
+        }
+
+        if (current != end && *current == '}')
+        {
+          std::advance(current, 1);
+        }
+        else
+        {
+          context.push("{fmt-err}");
+          while (current != end && *current != '}')
+            std::advance(current, 1);
+          if (current != end)
+            std::advance(current, 1);
+        }
+      }
+    }
+  }  // namespace
+
+  auto vprint(std::string_view fmt, kstd::format_args args) -> void
+  {
+    vprint_impl(fmt, args, false);
+  }
+
+  auto vprint_error(std::string_view fmt, kstd::format_args args) -> void
+  {
+    vprint_impl(fmt, args, true);
+  }
+
 }  // namespace teachos::cio
\ No newline at end of file
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