Kernel+LibVT: Add function for deleting a range of characters

Previously, this was done by telling the client to put a space at each
character in the range. This was inefficient, because a large number of
function calls took place and incorrect, as the ANSI standard dictates
that character attributes should be cleared as well.

The newly added `clear_in_line` function solves this issue. It performs
just one bounds check when it's called and can be implemented as a
pretty tight loop.

1 files changed, 5 insertions, 8 deletions

diff --git a/Userland/Libraries/LibVT/Line.cpp b/Userland/Libraries/LibVT/Line.cpp
index d807877f17..30b971b9d9 100644
--- a/Userland/Libraries/LibVT/Line.cpp
+++ b/Userland/Libraries/LibVT/Line.cpp
@@ -25,15 +25,12 @@ void Line::set_length(size_t new_length)
     m_cells.resize(new_length);
 }
 
-void Line::clear(const Attribute& attribute)
+void Line::clear_range(size_t first_column, size_t last_column, const Attribute& attribute)
 {
-    if (m_dirty) {
-        for (auto& cell : m_cells) {
-            cell = Cell { .code_point = ' ', .attribute = attribute };
-        }
-        return;
-    }
-    for (auto& cell : m_cells) {
+    VERIFY(first_column <= last_column);
+    VERIFY(last_column < m_cells.size());
+    for (size_t i = first_column; i <= last_column; ++i) {
+        auto& cell = m_cells[i];
         if (!m_dirty)
             m_dirty = cell.code_point != ' ' || cell.attribute != attribute;
         cell = Cell { .code_point = ' ', .attribute = attribute };