src: Allow ga to skip removed buffers

This commit prevents `ga` from returning a “no last buffer” error
when the previously displayed buffer was removed.

Since the jumps list keeps track of the order in which buffers were
displayed already, handling arbitrary `delete-buffer`s as well,
cycle through it to implement `ga` instead of storing a pointer.

Note that this commit doesn't take into account buffer flags that
might exclude some buffers from being cycled over by commands.

Fixes #1840

src/context.cc

@@ -172,12 +172,6 @@ void Context::change_buffer(Buffer& buffer)
     if (has_buffer() and m_edition_level > 0)
        this->buffer().commit_undo_group();
 
-    if (has_buffer())
-    {
-        auto* current = &this->buffer();
-        m_last_buffer = contains(BufferManager::instance(), current) ? current : nullptr;
-    }
-
     if (has_client())
     {
         client().info_hide();
@@ -197,8 +191,6 @@ void Context::change_buffer(Buffer& buffer)
 void Context::forget_buffer(Buffer& buffer)
 {
     m_jump_list.forget_buffer(buffer);
-    if (m_last_buffer.get() == &buffer)
-        m_last_buffer = nullptr;
 
     if (&this->buffer() != &buffer)
         return;
@@ -206,7 +198,29 @@ void Context::forget_buffer(Buffer& buffer)
     if (is_editing() && has_input_handler())
         input_handler().reset_normal_mode();
 
-    change_buffer(m_last_buffer ? *m_last_buffer : BufferManager::instance().get_first_buffer());
+    auto last_buffer = this->last_buffer();
+    change_buffer(last_buffer ? *last_buffer : BufferManager::instance().get_first_buffer());
+}
+
+Buffer* Context::last_buffer() const
+{
+    const auto jump_list = m_jump_list.get_as_list();
+    if (jump_list.empty())
+        return nullptr;
+
+    auto predicate = [this](const auto& sels) {
+        return &sels.buffer() != &this->buffer();
+    };
+
+    auto next_buffer = find_if(jump_list.subrange(m_jump_list.current_index()-1),
+                               predicate);
+    if (next_buffer != jump_list.end())
+        return &next_buffer->buffer();
+
+    auto previous_buffer = find_if(jump_list.subrange(0, m_jump_list.current_index()) | reverse(),
+                                   predicate);
+
+    return previous_buffer != jump_list.rend() ? &previous_buffer->buffer() : nullptr;
 }
 
 SelectionList& Context::selections()

src/context.hh

@@ -35,6 +35,8 @@ struct JumpList
 
     size_t current_index() const { return m_current; }
 
+    ConstArrayView<SelectionList> get_as_list() const { return m_jumps; }
+
 private:
     using Contents = Vector<SelectionList, MemoryDomain::Selections>;
     Contents m_jumps;
@@ -135,7 +137,7 @@ public:
 
     void repeat_last_select() { if (m_last_select) m_last_select(*this); }
 
-    Buffer* last_buffer() const { return m_last_buffer.get(); }
+    Buffer* last_buffer() const;
 private:
     void begin_edition();
     void end_edition();
@@ -149,7 +151,6 @@ private:
     SafePtr<InputHandler> m_input_handler;
     SafePtr<Window>       m_window;
     SafePtr<Client>       m_client;
-    SafePtr<Buffer>       m_last_buffer;
 
     Optional<SelectionList> m_selections;