Terminal: Defer scrollback text calculation

In high load situations, the conversion of cells to std::u32string's can
become a bottleneck. To address this, the calculation is deferred until
it is needed.

Change-Id: I0cc3c941fa680d8b9ebc3a5cba13cdc5c2db50f9
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Cristian Adam <cristian.adam@qt.io>

src/plugins/terminal/scrollback.cpp

@@ -7,14 +7,23 @@
 
 #include <cassert>
 #include <cstring>
+#include <future>
 
 namespace Terminal::Internal {
 
 Scrollback::Line::Line(int cols, const VTermScreenCell *cells, VTermState *vts)
     : m_cols(cols)
     , m_cells(std::make_unique<VTermScreenCell[]>(cols))
-
 {
+    m_textFuture = std::async(std::launch::async, [this, cols] {
+        std::u32string text;
+        text.reserve(cols);
+        for (int i = 0; i < cols; ++i) {
+            text += cellToString(m_cells[i]);
+        }
+        return text;
+    });
+
     memcpy(m_cells.get(), cells, cols * sizeof(cells[0]));
     for (int i = 0; i < cols; ++i) {
         vterm_state_convert_color_to_rgb(vts, &m_cells[i].fg);
@@ -49,6 +58,13 @@ const QTextLayout &Scrollback::Line::layout(int version, const QFont &font, qrea
     return *m_layout;
 }
 
+const std::u32string &Scrollback::Line::text() const
+{
+    if (!m_text)
+        m_text = m_textFuture.get();
+    return *m_text;
+}
+
 Scrollback::Scrollback(size_t capacity)
     : m_capacity(capacity)
 {}
@@ -57,13 +73,8 @@ void Scrollback::emplace(int cols, const VTermScreenCell *cells, VTermState *vts
 {
     m_deque.emplace_front(cols, cells, vts);
     while (m_deque.size() > m_capacity) {
-        m_currentText = m_currentText.substr(m_deque.back().cols());
         m_deque.pop_back();
     }
-
-    for (int i = 0; i < cols; i++) {
-        m_currentText += cellToString(cells[i]);
-    }
 }
 
 void Scrollback::popto(int cols, VTermScreenCell *cells)
@@ -82,7 +93,6 @@ void Scrollback::popto(int cols, VTermScreenCell *cells)
     }
 
     m_deque.pop_front();
-    m_currentText.resize(m_currentText.size() - cols);
 }
 
 size_t Scrollback::scroll(int delta)
@@ -96,12 +106,15 @@ void Scrollback::clear()
 {
     m_offset = 0;
     m_deque.clear();
-    m_currentText.clear();
 }
 
 std::u32string Scrollback::currentText()
 {
-    return m_currentText;
+    std::u32string currentText;
+    for (auto it = m_deque.rbegin(); it != m_deque.rend(); ++it) {
+        currentText += it->text();
+    }
+    return currentText;
 }
 
 } // namespace Terminal::Internal

src/plugins/terminal/scrollback.h

@@ -7,6 +7,7 @@
 #include <vterm.h>
 
 #include <deque>
+#include <future>
 #include <memory>
 
 #include <QFont>
@@ -29,12 +30,15 @@ public:
         const VTermScreenCell *cells() const { return &m_cells[0]; };
 
         const QTextLayout &layout(int version, const QFont &font, qreal lineSpacing) const;
+        const std::u32string &text() const;
 
     private:
         int m_cols;
         std::unique_ptr<VTermScreenCell[]> m_cells;
         mutable std::unique_ptr<QTextLayout> m_layout;
         mutable int m_layoutVersion{-1};
+        mutable std::optional<std::u32string> m_text;
+        mutable std::future<std::u32string> m_textFuture;
     };
 
 public:
@@ -63,8 +67,6 @@ private:
     size_t m_capacity;
     size_t m_offset{0};
     std::deque<Line> m_deque;
-
-    std::u32string m_currentText;
 };
 
 } // namespace Terminal::Internal