AK: Add a Utf16View method to retrieve an iterator at a code unit offset

Author: https://github.com/trflynn89
Commit: bbda6d13f7
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5746
Reviewed-by: https://github.com/gmta ✅

AK/Utf16View.h

@@ -425,6 +425,15 @@ public:
         return it.m_iterator.utf16 - m_string.utf16;
     }
 
+    [[nodiscard]] constexpr Utf16CodePointIterator iterator_at_code_unit_offset(size_t code_unit_offset) const
+    {
+        VERIFY(code_unit_offset <= length_in_code_units());
+
+        if (has_ascii_storage())
+            return { m_string.ascii + code_unit_offset, length_in_code_units() - code_unit_offset };
+        return { m_string.utf16 + code_unit_offset, length_in_code_units() - code_unit_offset };
+    }
+
     Utf16String replace(Utf16View const& needle, Utf16View const& replacement, ReplaceMode) const;
     Utf16String escape_html_entities() const;
 

Tests/AK/TestUtf16View.cpp

@@ -571,6 +571,18 @@ TEST_CASE(iterator_offset)
     EXPECT_EQ(view.iterator_offset(view.end()), view.length_in_code_units());
 }
 
+TEST_CASE(iterator_at_code_unit_offset)
+{
+    Utf16View view { u"😂 foo 😀 bar"sv };
+
+    for (size_t i = 0; i < view.length_in_code_units(); ++i) {
+        auto it = view.iterator_at_code_unit_offset(i);
+        EXPECT_EQ(*it, view.code_point_at(i));
+    }
+
+    EXPECT_EQ(view.iterator_at_code_unit_offset(view.length_in_code_units()), view.end());
+}
+
 TEST_CASE(replace)
 {
     auto result = u""sv.replace({}, {}, ReplaceMode::FirstOnly);