LibLine: Implement basic cut-and-paste functionality

Every cut operation (erase last word backward/forward, erase line till
start/end) stores the erased characters in `m_last_erased` u32 vector.

The last erased characters will get inserted into the buffer when
`Ctrl+Y` (`insert_last_erased()` internal function) is pressed.

Userland/Libraries/LibLine/Editor.cpp

@@ -164,6 +164,7 @@ void Editor::set_default_keybinds()
 
     // ^[.: alt-.: insert last arg of previous command (similar to `!$`)
     register_key_input_callback(Key { '.', Key::Alt }, EDITOR_INTERNAL_FUNCTION(insert_last_words));
+    register_key_input_callback(ctrl('Y'), EDITOR_INTERNAL_FUNCTION(insert_last_erased));
     register_key_input_callback(Key { 'b', Key::Alt }, EDITOR_INTERNAL_FUNCTION(cursor_left_word));
     register_key_input_callback(Key { 'f', Key::Alt }, EDITOR_INTERNAL_FUNCTION(cursor_right_word));
     register_key_input_callback(Key { ctrl('B'), Key::Alt }, EDITOR_INTERNAL_FUNCTION(cursor_left_nonspace_word));

Userland/Libraries/LibLine/Editor.h

@@ -124,6 +124,7 @@ struct Configuration {
     M(transpose_characters)                    \
     M(transpose_words)                         \
     M(insert_last_words)                       \
+    M(insert_last_erased)                      \
     M(erase_alnum_word_backwards)              \
     M(erase_alnum_word_forwards)               \
     M(capitalize_word)                         \
@@ -505,6 +506,7 @@ private:
     RefPtr<Core::Notifier> m_notifier;
 
     Vector<u32> m_paste_buffer;
+    Vector<u32> m_last_erased;
 
     bool m_initialized { false };
     bool m_refresh_needed { false };

Userland/Libraries/LibLine/InternalFunctions.cpp

@@ -221,8 +221,15 @@ void Editor::finish_edit()
 
 void Editor::kill_line()
 {
-    for (size_t i = 0; i < m_cursor; ++i)
+    if (m_cursor == 0)
+        return;
+
+    m_last_erased.clear_with_capacity();
+
+    for (size_t i = 0; i < m_cursor; ++i) {
+        m_last_erased.append(m_buffer[0]);
         remove_at_index(0);
+    }
     m_cursor = 0;
     m_inline_search_cursor = m_cursor;
     m_refresh_needed = true;
@@ -230,6 +237,11 @@ void Editor::kill_line()
 
 void Editor::erase_word_backwards()
 {
+    if (m_cursor == 0)
+        return;
+
+    m_last_erased.clear_with_capacity();
+
     // A word here is space-separated. `foo=bar baz` is two words.
     bool has_seen_nonspace = false;
     while (m_cursor > 0) {
@@ -239,20 +251,36 @@ void Editor::erase_word_backwards()
         } else {
             has_seen_nonspace = true;
         }
+
+        m_last_erased.append(m_buffer[m_cursor - 1]);
         erase_character_backwards();
     }
+
+    m_last_erased.reverse();
 }
 
 void Editor::erase_to_end()
 {
-    while (m_cursor < m_buffer.size())
+    if (m_cursor == m_buffer.size())
+        return;
+
+    m_last_erased.clear_with_capacity();
+
+    while (m_cursor < m_buffer.size()) {
+        m_last_erased.append(m_buffer[m_cursor]);
         erase_character_forwards();
+    }
 }
 
 void Editor::erase_to_beginning()
 {
 }
 
+void Editor::insert_last_erased()
+{
+    insert(Utf32View { m_last_erased.data(), m_last_erased.size() });
+}
+
 void Editor::transpose_characters()
 {
     if (m_cursor > 0 && m_buffer.size() >= 2) {
@@ -507,6 +535,11 @@ void Editor::insert_last_words()
 
 void Editor::erase_alnum_word_backwards()
 {
+    if (m_cursor == 0)
+        return;
+
+    m_last_erased.clear_with_capacity();
+
     // A word here is contiguous alnums. `foo=bar baz` is three words.
     bool has_seen_alnum = false;
     while (m_cursor > 0) {
@@ -516,12 +549,21 @@ void Editor::erase_alnum_word_backwards()
         } else {
             has_seen_alnum = true;
         }
+
+        m_last_erased.append(m_buffer[m_cursor - 1]);
         erase_character_backwards();
     }
+
+    m_last_erased.reverse();
 }
 
 void Editor::erase_alnum_word_forwards()
 {
+    if (m_cursor == m_buffer.size())
+        return;
+
+    m_last_erased.clear_with_capacity();
+
     // A word here is contiguous alnums. `foo=bar baz` is three words.
     bool has_seen_alnum = false;
     while (m_cursor < m_buffer.size()) {
@@ -531,6 +573,8 @@ void Editor::erase_alnum_word_forwards()
         } else {
             has_seen_alnum = true;
         }
+
+        m_last_erased.append(m_buffer[m_cursor]);
         erase_character_forwards();
     }
 }