GTextEditor: Add a "span" mechanism for having custom-style text ranges

It's now possible to give GTextEditor a vector of Span objects.
Spans currently tell the editor which color to use for each character
in the span. This can be used to implement syntax highlighting :^)

Libraries/LibGUI/GTextEditor.cpp

@@ -364,7 +364,27 @@ void GTextEditor::paint_event(GPaintEvent& event)
 #ifdef DEBUG_GTEXTEDITOR
             painter.draw_rect(visual_line_rect, Color::Cyan);
 #endif
-            painter.draw_text(visual_line_rect, visual_line_text, m_text_alignment, Color::Black);
+            if (m_spans.is_empty()) {
+                // Fast-path for plain text
+                painter.draw_text(visual_line_rect, visual_line_text, m_text_alignment, Color::Black);
+            } else {
+                int advance = font().glyph_width(' ') + font().glyph_spacing();
+                Rect character_rect = { visual_line_rect.location(), { font().glyph_width(' '), line_height() } };
+                for (int i = 0; i < visual_line_text.length(); ++i) {
+                    Color color;
+                    int physical_line = line_index;
+                    int physical_column = start_of_visual_line + i;
+                    // FIXME: This is *horribly* inefficient.
+                    for (auto& span : m_spans) {
+                        if (!span.contains(GTextPosition(physical_line, physical_column)))
+                            continue;
+                        color = span.color;
+                        break;
+                    }
+                    painter.draw_text(character_rect, visual_line_text.substring_view(i, 1), m_text_alignment, color);
+                    character_rect.move_by(advance, 0);
+                }
+            }
             bool physical_line_has_selection = has_selection && line_index >= selection.start().line() && line_index <= selection.end().line();
             if (physical_line_has_selection) {
 

Libraries/LibGUI/GTextEditor.h

@@ -167,6 +167,26 @@ public:
     void set_cursor(int line, int column);
     void set_cursor(const GTextPosition&);
 
+    struct Span {
+        bool contains(const GTextPosition& position) const
+        {
+            if (!(position.line() > start.line() || (position.line() == start.line() && position.column() >= start.column())))
+                return false;
+            if (!(position.line() < end.line() || (position.line() == end.line() && position.column() <= end.column())))
+                return false;
+            return true;
+        }
+
+        GTextPosition start;
+        GTextPosition end;
+        Color color;
+    };
+
+    void set_spans(const Vector<Span>& spans)
+    {
+        m_spans = spans;
+    }
+
 protected:
     GTextEditor(Type, GWidget* parent);
 
@@ -187,7 +207,6 @@ protected:
     virtual void resize_event(GResizeEvent&) override;
 
 private:
-
     void create_actions();
     void paint_ruler(Painter&);
     void update_content_size();
@@ -274,6 +293,8 @@ private:
     RefPtr<GAction> m_delete_action;
     CElapsedTimer m_triple_click_timer;
     NonnullRefPtrVector<GAction> m_custom_context_menu_actions;
+
+    Vector<Span> m_spans;
 };
 
 inline const LogStream& operator<<(const LogStream& stream, const GTextPosition& value)