LibVT: Let Terminal keep history in a circular buffer

This makes Terminal::scroll_up() O(1) instead of O(n) in the
size of the history. (It's still O(n) in the size of visible
lines.)

Reduces time to run `disasm /bin/id` with the default terminal
window size from 530ms to 409ms (min-of-5) on my system.

2 files changed, 16 insertions, 4 deletions

diff --git a/Libraries/LibVT/Terminal.cpp b/Libraries/LibVT/Terminal.cpp
index bc49f82562..fd651408d2 100644
--- a/Libraries/LibVT/Terminal.cpp
+++ b/Libraries/LibVT/Terminal.cpp
@@ -52,6 +52,8 @@ void Terminal::clear()
 void Terminal::clear_including_history()
 {
     m_history.clear();
+    m_history_start = 0;
+
     clear();
 
     m_client.terminal_history_changed();
@@ -738,9 +740,7 @@ void Terminal::scroll_up()
     invalidate_cursor();
     if (m_scroll_region_top == 0) {
         auto line = move(m_lines.ptr_at(m_scroll_region_top));
-        m_history.append(move(line));
-        while (m_history.size() > max_history_size())
-            m_history.take_first();
+        add_line_to_history(move(line));
         m_client.terminal_history_changed();
     }
     m_lines.remove(m_scroll_region_top);
diff --git a/Libraries/LibVT/Terminal.h b/Libraries/LibVT/Terminal.h
index 49a73fd32e..fb1cea884c 100644
--- a/Libraries/LibVT/Terminal.h
+++ b/Libraries/LibVT/Terminal.h
@@ -76,7 +76,7 @@ public:
     Line& line(size_t index)
     {
         if (index < m_history.size())
-            return m_history[index];
+            return m_history[(m_history_start + index) % m_history.size()];
         return m_lines[index - m_history.size()];
     }
     const Line& line(size_t index) const
@@ -154,7 +154,19 @@ private:
 
     TerminalClient& m_client;
 
+    size_t m_history_start = 0;
     NonnullOwnPtrVector<Line> m_history;
+    void add_line_to_history(NonnullOwnPtr<Line>&& line)
+    {
+        if (m_history.size() < max_history_size()) {
+            ASSERT(m_history_start == 0);
+            m_history.append(move(line));
+            return;
+        }
+        m_history.ptr_at(m_history_start) = move(line);
+        m_history_start = (m_history_start + 1) % m_history.size();
+    }
+
     NonnullOwnPtrVector<Line> m_lines;
 
     size_t m_scroll_region_top { 0 };