LibWeb: Frame/Position: Implement cursor increment/decrement methods

This introduces methods to increment and decrement the cursor position.
This is non-trivial as the cursor position is specified in bytes rather
than codepoints. Thus, it sometimes needs to be incremented or
decremented by more than one, depending on the codepoint to "jump over".

Because the cursor blink cycle needs to be reset after moving the
cursor, methods calling the ones in DOM::Position are implemented in
Frame. Furthermore, this allows the cursor_position() getter to stay
const. :^)

Additionally, it adds a offset_is_at_end_of_node() method which checks
if the current offset points to the end of the node.

Userland/Libraries/LibWeb/DOM/Position.cpp

@@ -1,11 +1,14 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, Max Wipfli <mail@maxwipfli.ch>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/Utf8View.h>
 #include <LibWeb/DOM/Node.h>
 #include <LibWeb/DOM/Position.h>
+#include <LibWeb/DOM/Text.h>
 
 namespace Web::DOM {
 
@@ -26,4 +29,56 @@ String Position::to_string() const
     return String::formatted("DOM::Position({} ({})), {})", node()->node_name(), node(), offset());
 }
 
+bool Position::increment_offset()
+{
+    if (!is<DOM::Text>(*m_node))
+        return false;
+
+    auto& node = downcast<DOM::Text>(*m_node);
+    auto text = Utf8View(node.data());
+
+    for (auto iterator = text.begin(); !iterator.done(); ++iterator) {
+        if (text.byte_offset_of(iterator) >= m_offset) {
+            // NOTE: If the current offset is inside a multi-byte codepoint, it will be moved to the start of the next codepoint.
+            m_offset = text.byte_offset_of(++iterator);
+            return true;
+        }
+    }
+    // NOTE: Already at end of current node.
+    return false;
+}
+
+bool Position::decrement_offset()
+{
+    if (m_offset == 0 || !is<DOM::Text>(*m_node))
+        return false;
+
+    auto& node = downcast<DOM::Text>(*m_node);
+    auto text = Utf8View(node.data());
+
+    size_t last_smaller_offset = 0;
+
+    for (auto iterator = text.begin(); !iterator.done(); ++iterator) {
+        auto byte_offset = text.byte_offset_of(iterator);
+        if (byte_offset >= m_offset) {
+            break;
+        }
+        last_smaller_offset = text.byte_offset_of(iterator);
+    }
+
+    // NOTE: If the current offset is inside a multi-byte codepoint, it will be moved to the start of that codepoint.
+    m_offset = last_smaller_offset;
+    return true;
+}
+
+bool Position::offset_is_at_end_of_node() const
+{
+    if (!is<DOM::Text>(*m_node))
+        return false;
+
+    auto& node = downcast<DOM::Text>(*m_node);
+    auto text = node.data();
+    return m_offset == text.length();
+}
+
 }

Userland/Libraries/LibWeb/DOM/Position.h

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, Max Wipfli <mail@maxwipfli.ch>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
@@ -25,7 +26,10 @@ public:
     const Node* node() const { return m_node; }
 
     unsigned offset() const { return m_offset; }
+    bool offset_is_at_end_of_node() const;
     void set_offset(unsigned value) { m_offset = value; }
+    bool increment_offset();
+    bool decrement_offset();
 
     bool operator==(const Position& other) const
     {

Userland/Libraries/LibWeb/Page/Frame.cpp

@@ -297,4 +297,20 @@ bool Frame::is_frame_nesting_allowed(URL const& url) const
     return m_frame_nesting_levels.get(url).value_or(0) < 3;
 }
 
+bool Frame::increment_cursor_position_offset()
+{
+    if (!m_cursor_position.increment_offset())
+        return false;
+    reset_cursor_blink_cycle();
+    return true;
+}
+
+bool Frame::decrement_cursor_position_offset()
+{
+    if (!m_cursor_position.decrement_offset())
+        return false;
+    reset_cursor_blink_cycle();
+    return true;
+}
+
 }

Userland/Libraries/LibWeb/Page/Frame.h

@@ -74,6 +74,8 @@ public:
 
     const DOM::Position& cursor_position() const { return m_cursor_position; }
     void set_cursor_position(DOM::Position);
+    bool increment_cursor_position_offset();
+    bool decrement_cursor_position_offset();
 
     bool cursor_blink_state() const { return m_cursor_blink_state; }