LibURL/Pattern: Implement the constructor string parser

This is missing one small bit of functionality where the not-yet
impplemented component compilation is required.

Libraries/LibURL/Pattern/ConstructorStringParser.h

@@ -0,0 +1,106 @@
+/*
+ * Copyright (c) 2025, Shannon Booth <shannon@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibURL/Pattern/Init.h>
+#include <LibURL/Pattern/PatternError.h>
+#include <LibURL/Pattern/Tokenizer.h>
+
+namespace URL::Pattern {
+
+// https://urlpattern.spec.whatwg.org/#constructor-string-parser
+class ConstructorStringParser {
+public:
+    static PatternErrorOr<Init> parse(Utf8View const& input);
+
+private:
+    ConstructorStringParser(Utf8View const& input, Vector<Token> token_list);
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-state
+    enum class State {
+        Initial,
+        Protocol,
+        Authority,
+        Username,
+        Password,
+        Hostname,
+        Port,
+        Pathname,
+        Search,
+        Hash,
+        Done,
+    };
+    StringView state_to_string() const;
+
+    void rewind();
+    void rewind_and_set_state(State);
+
+    bool next_is_authority_slashes() const;
+
+    bool is_an_identity_terminator() const;
+    bool is_a_port_prefix() const;
+    bool is_a_pathname_start() const;
+    bool is_a_password_prefix() const;
+    bool is_a_search_prefix() const;
+    bool is_a_hash_prefix() const;
+    bool is_a_protocol_suffix() const;
+    bool is_an_ipv6_open() const;
+    bool is_an_ipv6_close() const;
+    bool is_a_group_open() const;
+    bool is_a_group_close() const;
+
+    Token const& get_a_safe_token(u32 index) const;
+    bool is_a_non_special_pattern_char(u32 index, char value) const;
+    void change_state(State, u32 skip);
+    String make_a_component_string() const;
+    PatternErrorOr<void> compute_protocol_matches_a_special_scheme_flag();
+
+    Optional<String> const& result_for_active_state() const;
+    void set_result_for_active_state(Optional<String> value);
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-input
+    // A constructor string parser has an associated input, a string, which must be set upon creation.
+    Utf8View m_input;
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-token-list
+    // A constructor string parser has an associated token list, a token list, which must be set upon creation.
+    Vector<Token> m_token_list;
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-result
+    // A constructor string parser has an associated result, a URLPatternInit, initially set to a new URLPatternInit.
+    Init m_result;
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-component-start
+    // A constructor string parser has an associated component start, a number, initially set to 0.
+    u32 m_component_start { 0 };
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-token-index
+    // A constructor string parser has an associated token index, a number, initially set to 0.
+    u32 m_token_index { 0 };
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-token-increment
+    // A constructor string parser has an associated token increment, a number, initially set to 1.
+    u32 m_token_increment { 1 };
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-group-depth
+    // A constructor string parser has an associated group depth, a number, initially set to 0.
+    u32 m_group_depth { 0 };
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-hostname-ipv6-bracket-depth
+    // A constructor string parser has an associated hostname IPv6 bracket depth, a number, initially set to 0.
+    u32 m_hostname_ipv6_bracket_depth { 0 };
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-protocol-matches-a-special-scheme-flag
+    // A constructor string parser has an associated protocol matches a special scheme flag, a boolean, initially set to false.
+    bool m_protocol_matches_a_special_scheme { false };
+
+    // https://urlpattern.spec.whatwg.org/#constructor-string-parser-state
+    // A constructor string parser has an associated state, a string, initially set to "init".
+    State m_state { State::Initial };
+};
+
+}