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.

1 files changed, 55 insertions, 0 deletions

diff --git a/Userland/Libraries/LibWeb/DOM/Position.cpp b/Userland/Libraries/LibWeb/DOM/Position.cpp
index 4bd0135a23..155e94ecbe 100644
--- a/Userland/Libraries/LibWeb/DOM/Position.cpp
+++ b/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();
+}
+
 }