x86_64/vga: fix scrolling implementation

3 files changed, 53 insertions, 18 deletions

diff --git a/arch/x86_64/include/x86_64/boot/boot.hpp b/arch/x86_64/include/x86_64/boot/boot.hpp
index 2c44659..0db2396 100644
--- a/arch/x86_64/include/x86_64/boot/boot.hpp
+++ b/arch/x86_64/include/x86_64/boot/boot.hpp
@@ -60,7 +60,7 @@ namespace teachos::boot
     multiboot2::information_view const * mbi;
 
     //! The index of the next character to be written in the VGA text buffer after handoff.
-    std::size_t vga_buffer_index;
+    std::ptrdiff_t vga_buffer_index;
   };
 
 }  // namespace teachos::boot
diff --git a/arch/x86_64/include/x86_64/vga/text.hpp b/arch/x86_64/include/x86_64/vga/text.hpp
index bb593e7..9f80f94 100644
--- a/arch/x86_64/include/x86_64/vga/text.hpp
+++ b/arch/x86_64/include/x86_64/vga/text.hpp
@@ -3,9 +3,11 @@
 
 #include "kapi/cio.hpp"
 
+#include <cstddef>
 #include <cstdint>
 #include <span>
 #include <string_view>
+#include <utility>
 
 namespace teachos::vga::x86_64::text
 {
@@ -123,14 +125,17 @@ namespace teachos::vga::x86_64::text
     {
       write(text, common_attributes::green_on_black);
     }
+
     auto writeln(std::string_view text) -> void override
     {
       writeln(text, common_attributes::green_on_black);
     }
+
     auto write_error(std::string_view text) -> void override
     {
       write(text, common_attributes::red_on_black);
     }
+
     auto writeln_error(std::string_view text) -> void override
     {
       writeln(text, common_attributes::red_on_black);
@@ -139,11 +144,19 @@ namespace teachos::vga::x86_64::text
   private:
     using glyph = std::pair<char, std::byte>;
 
+    [[nodiscard]] auto column() const noexcept -> std::ptrdiff_t;
+    [[nodiscard]] auto line() const noexcept -> std::ptrdiff_t;
+
     /**
      * @brief Move the cursor to a new line, scrolling the buffer if necessary.
      */
     auto newline() -> void;
 
+    //! Scroll the screen by a number of lines.
+    //!
+    //! @param nof_lines The number of lines to scroll up.
+    auto scroll(std::ptrdiff_t nof_lines = 1) -> void;
+
     /**
      * @brief Write a string of code points to the VGA text buffer.
      *
@@ -155,6 +168,10 @@ namespace teachos::vga::x86_64::text
      */
     auto write(std::string_view code_points, attribute attribute) -> void;
 
+    //! Write a single code point to the VGA text buffer.
+    //!
+    //! @param code_point A single (8-bit) code point
+    //! @param attribute The #attribute to apply to the code point.
     auto write(char code_point, attribute attribute) -> void;
 
     /**
@@ -170,7 +187,7 @@ namespace teachos::vga::x86_64::text
 
     std::span<glyph> static const buffer;
 
-    std::size_t m_position{};
+    std::ptrdiff_t m_position{};
   };
 
 }  // namespace teachos::vga::x86_64::text
diff --git a/arch/x86_64/src/vga/text.cpp b/arch/x86_64/src/vga/text.cpp
index c9eee71..d4548a2 100644
--- a/arch/x86_64/src/vga/text.cpp
+++ b/arch/x86_64/src/vga/text.cpp
@@ -19,8 +19,8 @@ namespace teachos::vga::x86_64::text
   namespace
   {
     constexpr auto BUFFER_BASE_ADDRESS = std::uintptr_t{0xb8000};
-    constexpr auto DEFAULT_TEXT_BUFFER_WIDTH = 80U;
-    constexpr auto DEFAULT_TEXT_BUFFER_HEIGHT = 25U;
+    constexpr auto DEFAULT_TEXT_BUFFER_WIDTH = 80z;
+    constexpr auto DEFAULT_TEXT_BUFFER_HEIGHT = 25z;
     constexpr auto CURSOR_ENABLED_BIT = 5U;
   }  // namespace
 
@@ -47,22 +47,33 @@ namespace teachos::vga::x86_64::text
     crtc::data::write(crtc::data::read() | cursor_disable_byte);
   }
 
+  [[nodiscard]] auto device::column() const noexcept -> std::ptrdiff_t
+  {
+    return m_position % DEFAULT_TEXT_BUFFER_WIDTH;
+  }
+
+  [[nodiscard]] auto device::line() const noexcept -> std::ptrdiff_t
+  {
+    return m_position / DEFAULT_TEXT_BUFFER_WIDTH;
+  }
+
   auto device::newline() -> void
   {
-    auto current_line = m_position / DEFAULT_TEXT_BUFFER_WIDTH;
-    auto next_line = current_line + 1;
+    auto free_glyphs_in_line = DEFAULT_TEXT_BUFFER_WIDTH - column();
+    m_position += free_glyphs_in_line;
+  }
 
-    if (std::cmp_greater_equal(next_line, DEFAULT_TEXT_BUFFER_HEIGHT))
-    {
-      auto begin = buffer.begin() + DEFAULT_TEXT_BUFFER_WIDTH;
-      auto end = buffer.begin() + DEFAULT_TEXT_BUFFER_WIDTH * DEFAULT_TEXT_BUFFER_HEIGHT;
-      std::ranges::move(begin, end, buffer.begin());
-      m_position = current_line * DEFAULT_TEXT_BUFFER_WIDTH;
-    }
-    else
-    {
-      m_position = next_line * DEFAULT_TEXT_BUFFER_WIDTH;
-    }
+  auto device::scroll(std::ptrdiff_t nof_lines) -> void
+  {
+    auto scroll_count = std::min(nof_lines, DEFAULT_TEXT_BUFFER_HEIGHT);
+
+    auto scroll_start = buffer.begin() + (scroll_count * DEFAULT_TEXT_BUFFER_WIDTH);
+    std::ranges::move(scroll_start, buffer.end(), buffer.begin());
+
+    auto clear_start = buffer.begin() + (DEFAULT_TEXT_BUFFER_HEIGHT - scroll_count) * DEFAULT_TEXT_BUFFER_WIDTH;
+    std::ranges::fill(clear_start, buffer.end(), glyph{});
+
+    m_position = (line() - scroll_count) * DEFAULT_TEXT_BUFFER_WIDTH;
   }
 
   auto device::write(std::string_view code_points, attribute attribute) -> void
@@ -72,13 +83,20 @@ namespace teachos::vga::x86_64::text
 
   auto device::write(char code_point, attribute attribute) -> void
   {
+    if (m_position + 1 > DEFAULT_TEXT_BUFFER_HEIGHT * DEFAULT_TEXT_BUFFER_WIDTH)
+    {
+      scroll();
+    }
     buffer[m_position++] = std::pair{code_point, std::bit_cast<std::byte>(attribute)};
   };
 
   auto device::writeln(std::string_view code_points, attribute attribute) -> void
   {
     std::ranges::for_each(code_points, [&](auto code_point) -> void { write(code_point, attribute); });
-    newline();
+    if (column())
+    {
+      newline();
+    }
   }
 
 }  // namespace teachos::vga::x86_64::text