Add Cursor::go_up() and go_down()

They currently allow going to EOL

src/core/GapBuffer.cc

@@ -184,6 +184,35 @@ bool GapBuffer::Cursor::go_right()
     return false;
 }
 
+bool GapBuffer::Cursor::go_up(size_t target_column)
+{
+    Cursor c2 = *this;
+    c2.go_start_of_line();
+    if (!c2.m_iterator.check_go_back())
+    {
+        return false;
+    }
+    c2.go_start_of_line();
+    *this = c2;
+    m_line--;
+    forward_to_column(target_column);
+    return true;
+}
+
+bool GapBuffer::Cursor::go_down(size_t target_column)
+{
+    Cursor c2 = *this;
+    c2.go_end_of_line();
+    if (!c2.m_iterator.check_go_forward())
+    {
+        return false;
+    }
+    *this = c2;
+    m_line++;
+    forward_to_column(target_column);
+    return true;
+}
+
 void GapBuffer::Cursor::init_column()
 {
     if (*m_iterator == '\t')
@@ -206,3 +235,20 @@ void GapBuffer::Cursor::calculate_column()
     }
     m_column = tmp_cursor.m_column;
 }
+
+void GapBuffer::Cursor::forward_to_column(size_t target_column)
+{
+    int32_t diff = abs((int32_t)(target_column - m_column));
+    if (diff == 0)
+        return;
+    while (go_right())
+    {
+        int32_t new_diff = abs((int32_t)(target_column - m_column));
+        if (new_diff > diff)
+        {
+            go_left();
+            return;
+        }
+        diff = new_diff;
+    }
+}

src/core/GapBuffer.h

@@ -70,6 +70,8 @@ public:
         bool go_end_of_line();
         bool go_left();
         bool go_right();
+        bool go_up(size_t target_column);
+        bool go_down(size_t target_column);
         bool operator<(const Cursor & other)
         {
             return m_iterator < other.m_iterator;
@@ -82,6 +84,7 @@ public:
 
         void init_column();
         void calculate_column();
+        void forward_to_column(size_t target_column);
     };
 
     uint8_t tabstop;