Allow to undo and redo selection changes

From the issue:

> It often happens to me that I carefully craft a selection with multiple
> cursors, ready to make changes elegantly, only to completely mess it
> up by pressing a wrong key (by merging the cursors for example). Being
> able to undo the last selection change (even if only until the previous
> buffer change) would make this much less painful.

Fix this by recording selection changes and allowing simple linear
undo/redo of selection changes.

The preliminary key bindings are <c-h> and <c-k>.
Here are some other vacant normal mode keys I considered

	X Y
	<backspace> <minus>
	# ^ =
	<plus> '

unfortunately none of them is super convenient to type.  Maybe we
can kick out some other normal mode command?

---

This feature has some overlap with the jump list (<c-o>/<c-i>) and
with undo (u) but each of the three features have their moment.

Currently there's no special integration with either peer feature;
the three histories are completely independent.  In future we might
want to synchronize them so we can implement Sublime Text's "Soft
undo" feature.

Note that it is possible to restore selections that predate a buffer
modification. Depending on the buffer modification, the selections
might look different of course. (When trying to apply an old buffer's
selection to the new buffer, Kakoune computes a diff of the buffers
and updates the selection accordingly. This works quite well for
many practical examples.)

This makes us record the full history of all selections for each
client. This seems wasteful, we could set a limit. I don't expect
excessive memory usage in practice (we also keep the full history of
buffer changes) but I could be wrong.

Closes mawww#898

doc/pages/keys.asciidoc

@@ -314,6 +314,12 @@ Yanking (copying) and pasting use the *"* register by default (See <<registers#,
 *<a-U>*::
     move forward in history
 
+*<c-h>*::
+    undo last selection change
+
+*<c-k>*::
+    redo last selection change
+
 *&*::
     align selections, align the cursor of each selection by inserting spaces
     before the first character of each selection

src/client.cc

@@ -170,7 +170,7 @@ DisplayLine Client::generate_mode_line() const
     return modeline;
 }
 
-void Client::change_buffer(Buffer& buffer)
+void Client::change_buffer(Buffer& buffer, Optional<FunctionRef<void()>> set_selections)
 {
     if (m_buffer_reload_dialog_opened)
         close_buffer_reload_dialog();
@@ -181,12 +181,20 @@ void Client::change_buffer(Buffer& buffer)
     m_window->options().unregister_watcher(*this);
     m_window->set_client(nullptr);
     client_manager.add_free_window(std::move(m_window),
-                                   std::move(context().selections()));
+                                   context().selections());
 
     m_window = std::move(ws.window);
     m_window->set_client(this);
     m_window->options().register_watcher(*this);
-    context().selections_write_only() = std::move(ws.selections);
+
+    if (set_selections)
+        (*set_selections)();
+    else
+    {
+        ScopedSelectionEdition selection_edition{context()};
+        context().selections_write_only() = std::move(ws.selections);
+    }
+
     context().set_window(*m_window);
 
     m_window->set_dimensions(m_ui->dimensions());

src/client.hh

@@ -65,7 +65,7 @@ public:
     InputHandler& input_handler() { return m_input_handler; }
     const InputHandler& input_handler() const { return m_input_handler; }
 
-    void change_buffer(Buffer& buffer);
+    void change_buffer(Buffer& buffer, Optional<FunctionRef<void()>> set_selection);
 
     StringView get_env_var(StringView name) const;
 

src/context.cc

@@ -16,11 +16,11 @@ Context::Context(InputHandler& input_handler, SelectionList selections,
                  Flags flags, String name)
     : m_flags(flags),
       m_input_handler{&input_handler},
-      m_selections{std::move(selections)},
+      m_selection_history{*this, std::move(selections)},
       m_name(std::move(name))
 {}
 
-Context::Context(EmptyContextFlag) {}
+Context::Context(EmptyContextFlag) : m_selection_history{*this} {}
 
 Buffer& Context::buffer() const
 {
@@ -164,7 +164,143 @@ void JumpList::forget_buffer(Buffer& buffer)
     }
 }
 
-void Context::change_buffer(Buffer& buffer)
+Context::SelectionHistory::SelectionHistory(Context& context) : m_context(context) {}
+
+Context::SelectionHistory::SelectionHistory(Context& context, SelectionList selections)
+    : m_context(context),
+      m_history{HistoryNode{std::move(selections), HistoryId::Invalid}},
+      m_history_id(HistoryId::First) {}
+
+void Context::SelectionHistory::initialize(SelectionList selections)
+{
+    kak_assert(empty());
+    m_history = {HistoryNode{std::move(selections), HistoryId::Invalid}};
+    m_history_id = HistoryId::First;
+}
+
+SelectionList& Context::SelectionHistory::selections(bool update)
+{
+    if (empty())
+        throw runtime_error("no selections in context");
+    auto& sels = m_staging ? m_staging->selections : current_history_node().selections;
+    if (update)
+        sels.update();
+    return sels;
+}
+
+void Context::SelectionHistory::begin_edition()
+{
+    if (not in_edition())
+        m_staging = HistoryNode{selections(), m_history_id};
+    m_in_edition.set();
+}
+
+void Context::SelectionHistory::end_edition()
+{
+    kak_assert(in_edition());
+    m_in_edition.unset();
+    if (in_edition())
+        return;
+
+    if (m_history_id != HistoryId::Invalid and current_history_node().selections == m_staging->selections)
+        m_history[(size_t)m_history_id].selections.force_timestamp(m_staging->selections.timestamp());
+    else
+    {
+        m_history_id = next_history_id();
+        m_history.push_back(std::move(*m_staging));
+    }
+    m_staging.reset();
+}
+
+void Context::SelectionHistory::undo()
+{
+    if (in_edition())
+        throw runtime_error("selection undo is only supported at top-level");
+    kak_assert(not empty());
+    begin_edition();
+    auto end = on_scope_end([&] {
+        kak_assert(current_history_node().selections == m_staging->selections);
+        end_edition();
+    });
+    HistoryId parent = current_history_node().parent;
+    if (parent == HistoryId::Invalid)
+        throw runtime_error("no selection change to undo");
+    auto select_parent = [&, parent] {
+        HistoryId before_undo = m_history_id;
+        m_history_id = parent;
+        current_history_node().redo_child = before_undo;
+        m_staging = current_history_node();
+    };
+    if (&history_node(parent).selections.buffer() == &m_context.buffer())
+        select_parent();
+    else
+        m_context.change_buffer(history_node(parent).selections.buffer(), { std::move(select_parent) });
+                                                                           // });
+}
+
+void Context::SelectionHistory::redo()
+{
+    if (in_edition())
+        throw runtime_error("selection redo is only supported at top-level");
+    kak_assert(not empty());
+    begin_edition();
+    auto end = on_scope_end([&] {
+        kak_assert(current_history_node().selections == m_staging->selections);
+        end_edition();
+    });
+    HistoryId child = current_history_node().redo_child;
+    if (child == HistoryId::Invalid)
+        throw runtime_error("no selection change to redo");
+    auto select_child = [&, child] {
+        m_history_id = child;
+        m_staging = current_history_node();
+    };
+    if (&history_node(child).selections.buffer() == &m_context.buffer())
+        select_child();
+    else
+        m_context.change_buffer(history_node(child).selections.buffer(), { std::move(select_child) });
+}
+
+void Context::SelectionHistory::forget_buffer(Buffer& buffer)
+{
+    Vector<HistoryId, MemoryDomain::Selections> new_ids;
+    size_t bias = 0;
+    for (size_t i = 0; i < m_history.size(); ++i)
+    {
+        auto& node = history_node((HistoryId)i);
+        HistoryId id;
+        if (&node.selections.buffer() == &buffer)
+        {
+            id = HistoryId::Invalid;
+            ++bias;
+        }
+        else
+            id = (HistoryId)(i - bias);
+        new_ids.push_back(id);
+    }
+    auto new_id = [&new_ids](HistoryId old_id) -> HistoryId {
+        return old_id == HistoryId::Invalid ? HistoryId::Invalid : new_ids[(size_t)old_id];
+    };
+
+    m_history.erase(remove_if(m_history, [&buffer](const auto& node) {
+        return &node.selections.buffer() == &buffer;
+    }), m_history.end());
+
+    for (auto& node : m_history)
+    {
+        node.parent = new_id(node.parent);
+        node.redo_child = new_id(node.redo_child);
+    }
+    m_history_id = new_id(m_history_id);
+    if (m_staging)
+    {
+        m_staging->parent = new_id(m_staging->parent);
+        kak_assert(m_staging->redo_child == HistoryId::Invalid);
+    }
+    kak_assert(m_history_id != HistoryId::Invalid or m_staging);
+}
+
+void Context::change_buffer(Buffer& buffer, Optional<FunctionRef<void()>> set_selections)
 {
     if (has_buffer() and &buffer == &this->buffer())
         return;
@@ -176,12 +312,18 @@ void Context::change_buffer(Buffer& buffer)
     {
         client().info_hide();
         client().menu_hide();
-        client().change_buffer(buffer);
+        client().change_buffer(buffer, std::move(set_selections));
     }
     else
     {
         m_window.reset();
-        m_selections = SelectionList{buffer, Selection{}};
+        if (m_selection_history.empty())
+            m_selection_history.initialize(SelectionList{buffer, Selection{}});
+        else
+        {
+            ScopedSelectionEdition selection_edition{*this};
+            selections_write_only() = SelectionList{buffer, Selection{}};
+        }
     }
 
     if (has_input_handler())
@@ -200,6 +342,8 @@ void Context::forget_buffer(Buffer& buffer)
 
     auto last_buffer = this->last_buffer();
     change_buffer(last_buffer ? *last_buffer : BufferManager::instance().get_first_buffer());
+
+    m_selection_history.forget_buffer(buffer);
 }
 
 Buffer* Context::last_buffer() const
@@ -225,11 +369,17 @@ Buffer* Context::last_buffer() const
 
 SelectionList& Context::selections(bool update)
 {
-    if (not m_selections)
-        throw runtime_error("no selections in context");
-    if (update)
-        (*m_selections).update();
-    return *m_selections;
+    return m_selection_history.selections(update);
+}
+
+void Context::undo_selection_change()
+{
+    m_selection_history.undo();
+}
+
+void Context::redo_selection_change()
+{
+    m_selection_history.redo();
 }
 
 SelectionList& Context::selections_write_only()

src/context.hh

@@ -72,7 +72,7 @@ public:
     Context& operator=(const Context&) = delete;
 
     Buffer& buffer() const;
-    bool has_buffer() const { return (bool)m_selections; }
+    bool has_buffer() const { return not m_selection_history.empty(); }
 
     Window& window() const;
     bool has_window() const { return (bool)m_window; }
@@ -90,7 +90,11 @@ public:
     // Return potentially out of date selections
     SelectionList& selections_write_only();
 
-    void change_buffer(Buffer& buffer);
+    void end_selection_edition() { m_selection_history.end_edition(); }
+    void undo_selection_change();
+    void redo_selection_change();
+
+    void change_buffer(Buffer& buffer, Optional<FunctionRef<void()>> set_selection = {});
     void forget_buffer(Buffer& buffer);
 
     void set_client(Client& client);
@@ -113,6 +117,7 @@ public:
 
     bool is_editing() const { return m_edition_level!= 0; }
     void disable_undo_handling() { m_edition_level = -1; }
+    bool is_editing_selection() const { return m_selection_history.in_edition(); }
 
     NestedBool& hooks_disabled() { return m_hooks_disabled; }
     const NestedBool& hooks_disabled() const { return m_hooks_disabled; }
@@ -145,14 +150,53 @@ private:
     size_t m_edition_timestamp = 0;
 
     friend struct ScopedEdition;
+    friend struct ScopedSelectionEdition;
 
     Flags m_flags = Flags::None;
 
     SafePtr<InputHandler> m_input_handler;
     SafePtr<Window>       m_window;
     SafePtr<Client>       m_client;
 
-    Optional<SelectionList> m_selections;
+    class SelectionHistory {
+    public:
+        SelectionHistory(Context& context);
+        SelectionHistory(Context& context, SelectionList selections);
+        void initialize(SelectionList selections);
+        bool empty() const { return m_history.empty() and not m_staging; }
+        SelectionList& selections(bool update = true);
+
+        void begin_edition();
+        void end_edition();
+        bool in_edition() const { return m_in_edition; }
+
+        void undo();
+        void redo();
+        void forget_buffer(Buffer& buffer);
+    private:
+        enum class HistoryId : size_t { First = 0, Invalid = (size_t)-1 };
+
+        struct HistoryNode
+        {
+            HistoryNode(SelectionList selections, HistoryId parent) : selections(selections), parent(parent) {}
+
+            SelectionList selections;
+            HistoryId parent;
+            HistoryId redo_child = HistoryId::Invalid;
+        };
+
+              HistoryId next_history_id() const noexcept    { return (HistoryId)m_history.size(); }
+              HistoryNode& history_node(HistoryId id)       { return m_history[(size_t)id]; }
+        const HistoryNode& history_node(HistoryId id) const { return m_history[(size_t)id]; }
+              HistoryNode& current_history_node()           { kak_assert((size_t)m_history_id < m_history.size()); return m_history[(size_t)m_history_id]; }
+
+        Context&              m_context;
+        Vector<HistoryNode>   m_history;
+        HistoryId             m_history_id = HistoryId::Invalid;
+        Optional<HistoryNode> m_staging;
+        NestedBool            m_in_edition;
+    };
+    SelectionHistory m_selection_history;
 
     String m_name;
 
@@ -184,11 +228,12 @@ struct ScopedSelectionEdition
 {
     ScopedSelectionEdition(Context& context)
         : m_context{context},
-          m_buffer{context.has_buffer() ? &context.buffer() : nullptr} {}
+          m_buffer{context.has_buffer() ? &context.buffer() : nullptr}
+    { if (m_buffer) m_context.m_selection_history.begin_edition(); }
     ScopedSelectionEdition(ScopedSelectionEdition&& other) : m_context{other.m_context}, m_buffer{other.m_buffer}
     { other.m_buffer = nullptr; }
 
-    ~ScopedSelectionEdition() {}
+    ~ScopedSelectionEdition() { if (m_buffer) m_context.m_selection_history.end_edition(); }
 private:
     Context& m_context;
     SafePtr<Buffer> m_buffer;

src/input_handler.cc

@@ -100,6 +100,7 @@ struct MouseHandler
             switch (key.mouse_button())
             {
             case Key::MouseButton::Right: {
+                kak_assert(not context.is_editing_selection());
                 m_dragging.reset();
                 cursor = context.window().buffer_coord(key.coord());
                 ScopedSelectionEdition selection_edition{context};
@@ -113,6 +114,7 @@ struct MouseHandler
             }
 
             case Key::MouseButton::Left: {
+                kak_assert(not context.is_editing_selection());
                 m_dragging.reset(new ScopedSelectionEdition{context});
                 m_anchor = context.window().buffer_coord(key.coord());
                 if (not (key.modifiers & Key::Modifiers::Control))