AK: Add GenericLexer API to consume an escaped Unicode code point

This parsing is already duplicated between LibJS and LibRegex, and will
shortly be needed in more places in those libraries. Move it to AK to
prevent further duplication.

This API will consume escaped Unicode code points of the form:
    \\u{code point}
    \\unnnn (where each n is a hexadecimal digit)
    \\unnnn\\unnnn (where the two escaped values are a surrogate pair)

AK/GenericLexer.cpp

@@ -5,9 +5,11 @@
  */
 
 #include <AK/Assertions.h>
+#include <AK/CharacterTypes.h>
 #include <AK/GenericLexer.h>
 #include <AK/String.h>
 #include <AK/StringBuilder.h>
+#include <AK/Utf16View.h>
 
 namespace AK {
 // Consume a number of characters
@@ -128,4 +130,74 @@ String GenericLexer::consume_and_unescape_string(char escape_char)
     return builder.to_string();
 }
 
+auto GenericLexer::consume_escaped_code_point(bool combine_surrogate_pairs) -> Result<u32, UnicodeEscapeError>
+{
+    if (!consume_specific("\\u"sv))
+        return UnicodeEscapeError::MalformedUnicodeEscape;
+
+    if (next_is('{'))
+        return decode_code_point();
+    return decode_single_or_paired_surrogate(combine_surrogate_pairs);
+}
+
+auto GenericLexer::decode_code_point() -> Result<u32, UnicodeEscapeError>
+{
+    bool starts_with_open_bracket = consume_specific('{');
+    VERIFY(starts_with_open_bracket);
+
+    u32 code_point = 0;
+
+    while (true) {
+        if (!next_is(is_ascii_hex_digit))
+            return UnicodeEscapeError::MalformedUnicodeEscape;
+
+        auto new_code_point = (code_point << 4u) | parse_ascii_hex_digit(consume());
+        if (new_code_point < code_point)
+            return UnicodeEscapeError::UnicodeEscapeOverflow;
+
+        code_point = new_code_point;
+        if (consume_specific('}'))
+            break;
+    }
+
+    if (is_unicode(code_point))
+        return code_point;
+    return UnicodeEscapeError::UnicodeEscapeOverflow;
+}
+
+auto GenericLexer::decode_single_or_paired_surrogate(bool combine_surrogate_pairs) -> Result<u32, UnicodeEscapeError>
+{
+    constexpr size_t surrogate_length = 4;
+
+    auto decode_one_surrogate = [&]() -> Optional<u16> {
+        u16 surrogate = 0;
+
+        for (size_t i = 0; i < surrogate_length; ++i) {
+            if (!next_is(is_ascii_hex_digit))
+                return {};
+
+            surrogate = (surrogate << 4u) | parse_ascii_hex_digit(consume());
+        }
+
+        return surrogate;
+    };
+
+    auto high_surrogate = decode_one_surrogate();
+    if (!high_surrogate.has_value())
+        return UnicodeEscapeError::MalformedUnicodeEscape;
+    if (!Utf16View::is_high_surrogate(*high_surrogate))
+        return *high_surrogate;
+    if (!combine_surrogate_pairs || !consume_specific("\\u"sv))
+        return *high_surrogate;
+
+    auto low_surrogate = decode_one_surrogate();
+    if (!low_surrogate.has_value())
+        return UnicodeEscapeError::MalformedUnicodeEscape;
+    if (Utf16View::is_low_surrogate(*low_surrogate))
+        return Utf16View::decode_surrogate_pair(*high_surrogate, *low_surrogate);
+
+    retreat(6);
+    return *high_surrogate;
+}
+
 }