ianthomas23 · Oct 20, 2025 · Sep 19, 2025 · Sep 22, 2025 · Oct 10, 2025 · Oct 10, 2025
diff --git a/CMakeLists.txt b/CMakeLists.txt
@@ -58,11 +58,16 @@ set(GIT2CPP_SRC
     ${GIT2CPP_SOURCE_DIR}/subcommand/reset_subcommand.hpp
     ${GIT2CPP_SOURCE_DIR}/subcommand/status_subcommand.cpp
     ${GIT2CPP_SOURCE_DIR}/subcommand/status_subcommand.hpp
+    ${GIT2CPP_SOURCE_DIR}/utils/ansi_code.cpp
+    ${GIT2CPP_SOURCE_DIR}/utils/ansi_code.hpp
     ${GIT2CPP_SOURCE_DIR}/utils/common.cpp
     ${GIT2CPP_SOURCE_DIR}/utils/common.hpp
     ${GIT2CPP_SOURCE_DIR}/utils/git_exception.cpp
     ${GIT2CPP_SOURCE_DIR}/utils/git_exception.hpp
+    ${GIT2CPP_SOURCE_DIR}/utils/output.cpp
     ${GIT2CPP_SOURCE_DIR}/utils/output.hpp
+    ${GIT2CPP_SOURCE_DIR}/utils/terminal_pager.cpp
+    ${GIT2CPP_SOURCE_DIR}/utils/terminal_pager.hpp
     ${GIT2CPP_SOURCE_DIR}/wrapper/annotated_commit_wrapper.cpp
     ${GIT2CPP_SOURCE_DIR}/wrapper/annotated_commit_wrapper.hpp
     ${GIT2CPP_SOURCE_DIR}/wrapper/branch_wrapper.cpp

diff --git a/src/subcommand/log_subcommand.cpp b/src/subcommand/log_subcommand.cpp
@@ -7,6 +7,7 @@
 #include <termcolor/termcolor.hpp>
 
 #include "log_subcommand.hpp"
+#include "../utils/terminal_pager.hpp"
 #include "../wrapper/repository_wrapper.hpp"
 #include "../wrapper/commit_wrapper.hpp"
 
@@ -90,6 +91,8 @@ void log_subcommand::run()
     git_revwalk_new(&walker, repo);
     git_revwalk_push_head(walker);
 
+    terminal_pager pager;
+
     std::size_t i=0;
     git_oid commit_oid;
     while (!git_revwalk_next(&commit_oid, walker) && i<m_max_count_flag)
@@ -100,4 +103,6 @@ void log_subcommand::run()
     }
 
     git_revwalk_free(walker);
+
+    pager.show();
 }
diff --git a/src/utils/ansi_code.cpp b/src/utils/ansi_code.cpp
@@ -0,0 +1,24 @@
+#include "ansi_code.hpp"
+
+namespace ansi_code
+{
+    std::string cursor_to_row(size_t row)
+    {
+        return "\e[" + std::to_string(row) + "H";
+    }
+
+    bool is_down_arrow(std::string str)
+    {
+        return str == "\e[B" || str == "\e[1B]";
+    }
+
+    bool is_escape_char(char ch)
+    {
+        return ch == '\e';
+    }
+
+    bool is_up_arrow(std::string str)
+    {
+        return str == "\e[A" || str == "\e[1A]";
+    }
+}
diff --git a/src/utils/ansi_code.hpp b/src/utils/ansi_code.hpp
@@ -0,0 +1,29 @@
+#pragma once
+
+#include <string>
+
+/**
+ * ANSI escape codes.
+ * Use `termcolor` for colours.
+ */
+namespace ansi_code
+{
+    // Constants.
+    const std::string bel = "\a";  // ASCII 7, used for audio/visual feedback.
+    const std::string cursor_to_top = "\e[H";
+    const std::string erase_screen = "\e[2J";
+
+    const std::string enable_alternative_buffer = "\e[?1049h";
+    const std::string disable_alternative_buffer = "\e[?1049l";
+
+    const std::string hide_cursor = "\e[?25l";
+    const std::string show_cursor = "\e[?25h";
+
+    // Functions.
+    std::string cursor_to_row(size_t row);
+
+    bool is_escape_char(char ch);
+
+    bool is_down_arrow(std::string str);
+    bool is_up_arrow(std::string str);
+}
diff --git a/src/utils/output.cpp b/src/utils/output.cpp
@@ -0,0 +1,23 @@
+#include "output.hpp"
+
+// OS-specific libraries.
+#include <sys/ioctl.h>
+
+alternative_buffer::alternative_buffer()
+{
+    tcgetattr(fileno(stdin), &m_previous_termios);
+    auto new_termios = m_previous_termios;
+    // Disable canonical mode (buffered I/O) and echo from stdin to stdout.
+    new_termios.c_lflag &= (~ICANON & ~ECHO);
+    tcsetattr(fileno(stdin), TCSANOW, &new_termios);
+
+    std::cout << ansi_code::enable_alternative_buffer;
+}
+
+alternative_buffer::~alternative_buffer()
+{
+    std::cout << ansi_code::disable_alternative_buffer;
+
+    // Restore previous termios settings.
+    tcsetattr(fileno(stdin), TCSANOW, &m_previous_termios);
+}
diff --git a/src/utils/output.hpp b/src/utils/output.hpp
@@ -1,20 +1,37 @@
 #pragma once
 
 #include <iostream>
+#include "ansi_code.hpp"
 #include "common.hpp"
 
+// OS-specific libraries.
+#include <termios.h>
+
 // Scope object to hide the cursor. This avoids
 // cursor twinkling when rewritting the same line
 // too frequently.
 struct cursor_hider : noncopyable_nonmovable
 {
     cursor_hider()
     {
-        std::cout << "\e[?25l";
+        std::cout << ansi_code::hide_cursor;
     }
 
     ~cursor_hider()
     {
-        std::cout << "\e[?25h";
+        std::cout << ansi_code::show_cursor;
     }
 };
+
+// Scope object to use alternative output buffer for
+// fullscreen interactive terminal input/output.
+class alternative_buffer : noncopyable_nonmovable
+{
+public:
+    alternative_buffer();
+
+    ~alternative_buffer();
+
+private:
+    struct termios m_previous_termios;
+};
diff --git a/src/utils/terminal_pager.cpp b/src/utils/terminal_pager.cpp
@@ -0,0 +1,221 @@
+#include <cctype>
+#include <cstdint>
+#include <cstdio>
+#include <iostream>
+#include <ranges>
+
+// OS-specific libraries.
+#include <sys/ioctl.h>
+
+#include <termcolor/termcolor.hpp>
+
+#include "ansi_code.hpp"
+#include "output.hpp"
+#include "terminal_pager.hpp"
+
+terminal_pager::terminal_pager()
+    : m_rows(0), m_columns(0), m_start_row_index(0)
+{
+    maybe_grab_cout();
+}
+
+terminal_pager::~terminal_pager()
+{
+    release_cout();
+}
+
+std::string terminal_pager::get_input() const
+{
+    // Blocks until input received.
+    std::string str;
+    char ch;
+    std::cin.get(ch);
+    str += ch;
+
+    if (ansi_code::is_escape_char(ch))  // Start of ANSI escape sequence.
+    {
+        do
+        {
+            std::cin.get(ch);
+            str += ch;
+        } while (!std::isalpha(ch));  // ANSI escape sequence ends with a letter.
+    }
+
+    return str;
+}
+
+void terminal_pager::maybe_grab_cout()
+{
+    // Unfortunately need to access _internal namespace of termcolor to check if a tty.
+    if (termcolor::_internal::is_atty(std::cout))
+    {
+        // Should we do anything with cerr?
+        m_cout_rdbuf = std::cout.rdbuf(&m_stringbuf);
+    }
+    else
+    {
+        m_cout_rdbuf = std::cout.rdbuf();
+    }
+}
+
+bool terminal_pager::process_input(std::string input)
+{
+    if (input.size() == 0)
+    {
+        return true;
+    }
+
+    switch (input[0])
+    {
+        case 'q':
+        case 'Q':
+            return true;  // Exit pager.
+        case 'u':
+        case 'U':
+            scroll(true, true);  // Up a page.
+            return false;
+        case 'd':
+        case 'D':
+        case ' ':
+            scroll(false, true);  // Down a page.
+            return false;
+        case '\n':
+            scroll(false, false);  // Down a line.
+            return false;
+        case '\e':  // ANSI escape sequence.
+            // Cannot switch on a std::string.
+            if (ansi_code::is_up_arrow(input))
+            {
+                scroll(true, false);  // Up a line.
+                return false;
+            }
+            else if (ansi_code::is_down_arrow(input))
+            {
+                scroll(false, false);  // Down a line.
+                return false;
+            }
+    }
+
+    std::cout << ansi_code::bel;
+    return false;
+}
+
+void terminal_pager::release_cout()
+{
+    std::cout.rdbuf(m_cout_rdbuf);
+}
+
+void terminal_pager::render_terminal() const
+{
+    auto end_row_index = m_start_row_index + m_rows - 1;
+
+    std::cout << ansi_code::erase_screen;
+    std::cout << ansi_code::cursor_to_top;
+
+    for (size_t i = m_start_row_index; i < end_row_index; i++)
+    {
+        if (i >= m_lines.size())
+        {
+            break;
+        }
+        std::cout << m_lines[i] << std::endl;
+    }
+
+    std::cout << ansi_code::cursor_to_row(m_rows);  // Move cursor to bottom row of terminal.
+    std::cout << ":";
+}
+
+void terminal_pager::scroll(bool up, bool page)
+{
+    update_terminal_size();
+    const auto old_start_row_index = m_start_row_index;
+    size_t offset = page ? m_rows - 1 : 1;
+
+    if (up)
+    {
+        // Care needed to avoid underflow of unsigned size_t.
+        if (m_start_row_index >= offset)
+        {
+            m_start_row_index -= offset;
+        }
+        else
+        {
+            m_start_row_index = 0;
+        }
+    }
+    else
+    {
+        m_start_row_index += offset;
+        auto end_row_index = m_start_row_index + m_rows - 1;
+        if (end_row_index > m_lines.size())
+        {
+            m_start_row_index = m_lines.size() - (m_rows - 1);
+        }
+    }
+
+    if (m_start_row_index == old_start_row_index)
+    {
+        std::cout << ansi_code::bel;
+    }
+    else
+    {
+        render_terminal();
+    }
+}
+
+void terminal_pager::show()
+{
+    release_cout();
+
+    split_input_at_newlines(m_stringbuf.view());
+
+    update_terminal_size();
+    if (m_rows == 0 || m_lines.size() <= m_rows - 1)
+    {
+        // Don't need to use pager, can display directly.
+        for (auto line : m_lines)
+        {
+            std::cout << line << std::endl;
+        }
+        m_lines.clear();
+        return;
+    }
+
+    alternative_buffer alt_buffer;
+
+    m_start_row_index = 0;
+    render_terminal();
+
+    bool stop = false;
+    do
+    {
+        stop = process_input(get_input());
+    } while (!stop);
+
+    m_lines.clear();
+    m_start_row_index = 0;
+}
+
+void terminal_pager::split_input_at_newlines(std::string_view str)
+{
+    auto split = str | std::ranges::views::split('\n')
+                     | std::ranges::views::transform([](auto&& range) {
+                           return std::string(range.begin(), std::ranges::distance(range));
+                       });
+    m_lines = std::vector<std::string>{split.begin(), split.end()};
+}
+
+void terminal_pager::update_terminal_size()
+{
+    struct winsize size;
+    int err = ioctl(fileno(stdout), TIOCGWINSZ, &size);
+    if (err == 0)
+    {
+        m_rows = size.ws_row;
+        m_columns = size.ws_col;
+    }
+    else
+    {
+        m_rows = m_columns = 0;
+    }
+}