AK/GenericLexer: constexpr where possible

Problem:
- Much of the `GenericLexer` can be `constexpr`, but is not.

Solution:
- Make it `constexpr` and de-duplicate code.
- Extend some of `StringView` with `constexpr` to support.
- Add tests to ensure `constexpr` behavior.

Note:
- Construction of `StringView` from pointer and length is not
  `constexpr`-compatible at the moment because the VERIFY cannot be,
  yet.

AK/GenericLexer.cpp

@@ -10,92 +10,6 @@
 #include <AK/StringBuilder.h>
 
 namespace AK {
-
-GenericLexer::GenericLexer(const StringView& input)
-    : m_input(input)
-{
-}
-
-GenericLexer::~GenericLexer()
-{
-}
-
-// Tells whether the parser's index has reached input's end
-bool GenericLexer::is_eof() const
-{
-    return m_index >= m_input.length();
-}
-
-// Returns the current character at the parser index, plus `offset` if specified
-char GenericLexer::peek(size_t offset) const
-{
-    return (m_index + offset < m_input.length()) ? m_input[m_index + offset] : '\0';
-}
-
-// Tests the next character in the input
-bool GenericLexer::next_is(char expected) const
-{
-    return peek() == expected;
-}
-
-// Tests if the `expected` string comes next in the input
-bool GenericLexer::next_is(StringView expected) const
-{
-    for (size_t i = 0; i < expected.length(); ++i)
-        if (peek(i) != expected[i])
-            return false;
-    return true;
-}
-
-// Tests if the `expected` string comes next in the input
-bool GenericLexer::next_is(const char* expected) const
-{
-    for (size_t i = 0; expected[i] != '\0'; ++i)
-        if (peek(i) != expected[i])
-            return false;
-    return true;
-}
-
-// Go back to the previous character
-void GenericLexer::retreat()
-{
-    VERIFY(m_index > 0);
-    m_index--;
-}
-
-// Consume a character and advance the parser index
-char GenericLexer::consume()
-{
-    VERIFY(!is_eof());
-    return m_input[m_index++];
-}
-
-// Consume the given character if it is next in the input
-bool GenericLexer::consume_specific(char specific)
-{
-    if (peek() != specific)
-        return false;
-
-    ignore();
-    return true;
-}
-
-// Consume the given string if it is next in the input
-bool GenericLexer::consume_specific(StringView str)
-{
-    if (!next_is(str))
-        return false;
-
-    ignore(str.length());
-    return true;
-}
-
-// Consume the given string if it is next in the input
-bool GenericLexer::consume_specific(const char* str)
-{
-    return consume_specific(StringView(str));
-}
-
 // Consume a number of characters
 StringView GenericLexer::consume(size_t count)
 {
@@ -214,46 +128,4 @@ String GenericLexer::consume_and_unescape_string(char escape_char)
     return builder.to_string();
 }
 
-char GenericLexer::consume_escaped_character(char escape_char, const StringView& escape_map)
-{
-    if (!consume_specific(escape_char))
-        return consume();
-
-    auto c = consume();
-
-    for (size_t i = 0; i < escape_map.length(); i += 2) {
-        if (c == escape_map[i])
-            return escape_map[i + 1];
-    }
-
-    return c;
-}
-
-// Ignore a number of characters (1 by default)
-void GenericLexer::ignore(size_t count)
-{
-    count = min(count, m_input.length() - m_index);
-    m_index += count;
-}
-
-// Ignore characters until `stop` is peek'd
-// The `stop` character is ignored as it is user-defined
-void GenericLexer::ignore_until(char stop)
-{
-    while (!is_eof() && peek() != stop)
-        m_index++;
-
-    ignore();
-}
-
-// Ignore characters until the string `stop` is found
-// The `stop` string is ignored, as it is user-defined
-void GenericLexer::ignore_until(const char* stop)
-{
-    while (!is_eof() && !next_is(stop))
-        m_index++;
-
-    ignore(__builtin_strlen(stop));
-}
-
 }

AK/GenericLexer.h

@@ -12,29 +12,95 @@ namespace AK {
 
 class GenericLexer {
 public:
-    explicit GenericLexer(const StringView& input);
-    virtual ~GenericLexer();
+    constexpr explicit GenericLexer(const StringView& input)
+        : m_input(input)
+    {
+    }
 
-    size_t tell() const { return m_index; }
-    size_t tell_remaining() const { return m_input.length() - m_index; }
+    constexpr size_t tell() const { return m_index; }
+    constexpr size_t tell_remaining() const { return m_input.length() - m_index; }
 
     StringView remaining() const { return m_input.substring_view(m_index); }
 
-    bool is_eof() const;
+    constexpr bool is_eof() const { return m_index >= m_input.length(); }
 
-    char peek(size_t offset = 0) const;
+    constexpr char peek(size_t offset = 0) const
+    {
+        return (m_index + offset < m_input.length()) ? m_input[m_index + offset] : '\0';
+    }
 
-    bool next_is(char) const;
-    bool next_is(StringView) const;
-    bool next_is(const char*) const;
+    constexpr bool next_is(char expected) const
+    {
+        return peek() == expected;
+    }
 
-    void retreat();
+    constexpr bool next_is(StringView expected) const
+    {
+        for (size_t i = 0; i < expected.length(); ++i)
+            if (peek(i) != expected[i])
+                return false;
+        return true;
+    }
+
+    constexpr bool next_is(const char* expected) const
+    {
+        for (size_t i = 0; expected[i] != '\0'; ++i)
+            if (peek(i) != expected[i])
+                return false;
+        return true;
+    }
+
+    constexpr void retreat()
+    {
+        VERIFY(m_index > 0);
+        --m_index;
+    }
+
+    constexpr char consume()
+    {
+        VERIFY(!is_eof());
+        return m_input[m_index++];
+    }
+
+    template<typename T>
+    constexpr bool consume_specific(const T& next)
+    {
+        if (!next_is(next))
+            return false;
+
+        if constexpr (requires { next.length(); }) {
+            ignore(next.length());
+        } else {
+            ignore(sizeof(next));
+        }
+        return true;
+    }
+
+    bool consume_specific(const String& next)
+    {
+        return consume_specific(StringView { next });
+    }
+
+    constexpr bool consume_specific(const char* next)
+    {
+        return consume_specific(StringView { next });
+    }
+
+    constexpr char consume_escaped_character(char escape_char = '\\', const StringView& escape_map = "n\nr\rt\tb\bf\f")
+    {
+        if (!consume_specific(escape_char))
+            return consume();
+
+        auto c = consume();
+
+        for (size_t i = 0; i < escape_map.length(); i += 2) {
+            if (c == escape_map[i])
+                return escape_map[i + 1];
+        }
+
+        return c;
+    }
 
-    char consume();
-    bool consume_specific(char);
-    bool consume_specific(StringView);
-    bool consume_specific(const char*);
-    char consume_escaped_character(char escape_char = '\\', const StringView& escape_map = "n\nr\rt\tb\bf\f");
     StringView consume(size_t count);
     StringView consume_all();
     StringView consume_line();
@@ -43,9 +109,27 @@ public:
     StringView consume_quoted_string(char escape_char = 0);
     String consume_and_unescape_string(char escape_char = '\\');
 
-    void ignore(size_t count = 1);
-    void ignore_until(char);
-    void ignore_until(const char*);
+    constexpr void ignore(size_t count = 1)
+    {
+        count = min(count, m_input.length() - m_index);
+        m_index += count;
+    }
+
+    constexpr void ignore_until(char stop)
+    {
+        while (!is_eof() && peek() != stop) {
+            ++m_index;
+        }
+        ignore();
+    }
+
+    constexpr void ignore_until(const char* stop)
+    {
+        while (!is_eof() && !next_is(stop)) {
+            ++m_index;
+        }
+        ignore(__builtin_strlen(stop));
+    }
 
     /*
      * Conditions are used to match arbitrary characters. You can use lambdas,
@@ -57,19 +141,19 @@ public:
      */
 
     // Test the next character against a Condition
-    template<typename C>
-    bool next_is(C condition) const
+    template<typename TPredicate>
+    constexpr bool next_is(TPredicate pred) const
     {
-        return condition(peek());
+        return pred(peek());
     }
 
-    // Consume and return characters while `condition` returns true
-    template<typename C>
-    StringView consume_while(C condition)
+    // Consume and return characters while `pred` returns true
+    template<typename TPredicate>
+    StringView consume_while(TPredicate pred)
     {
         size_t start = m_index;
-        while (!is_eof() && condition(peek()))
-            m_index++;
+        while (!is_eof() && pred(peek()))
+            ++m_index;
         size_t length = m_index - start;
 
         if (length == 0)
@@ -77,13 +161,13 @@ public:
         return m_input.substring_view(start, length);
     }
 
-    // Consume and return characters until `condition` return true
-    template<typename C>
-    StringView consume_until(C condition)
+    // Consume and return characters until `pred` return true
+    template<typename TPredicate>
+    StringView consume_until(TPredicate pred)
     {
         size_t start = m_index;
-        while (!is_eof() && !condition(peek()))
-            m_index++;
+        while (!is_eof() && !pred(peek()))
+            ++m_index;
         size_t length = m_index - start;
 
         if (length == 0)
@@ -91,21 +175,21 @@ public:
         return m_input.substring_view(start, length);
     }
 
-    // Ignore characters while `condition` returns true
-    template<typename C>
-    void ignore_while(C condition)
+    // Ignore characters while `pred` returns true
+    template<typename TPredicate>
+    constexpr void ignore_while(TPredicate pred)
     {
-        while (!is_eof() && condition(peek()))
-            m_index++;
+        while (!is_eof() && pred(peek()))
+            ++m_index;
     }
 
-    // Ignore characters until `condition` return true
+    // Ignore characters until `pred` return true
     // We don't skip the stop character as it may not be a unique value
-    template<typename C>
-    void ignore_until(C condition)
+    template<typename TPredicate>
+    constexpr void ignore_until(TPredicate pred)
     {
-        while (!is_eof() && !condition(peek()))
-            m_index++;
+        while (!is_eof() && !pred(peek()))
+            ++m_index;
     }
 
 protected:

AK/StringView.cpp

@@ -172,17 +172,6 @@ bool StringView::equals_ignoring_case(const StringView& other) const
     return StringUtils::equals_ignoring_case(*this, other);
 }
 
-StringView StringView::substring_view(size_t start, size_t length) const
-{
-    VERIFY(start + length <= m_length);
-    return { m_characters + start, length };
-}
-StringView StringView::substring_view(size_t start) const
-{
-    VERIFY(start <= m_length);
-    return { m_characters + start, length() - start };
-}
-
 StringView StringView::substring_view_starting_from_substring(const StringView& substring) const
 {
     const char* remaining_characters = substring.characters_without_null_termination();

AK/StringView.h

@@ -46,15 +46,15 @@ public:
     StringView(const String&);
     StringView(const FlyString&);
 
-    [[nodiscard]] bool is_null() const { return !m_characters; }
-    [[nodiscard]] bool is_empty() const { return m_length == 0; }
+    [[nodiscard]] constexpr bool is_null() const { return !m_characters; }
+    [[nodiscard]] constexpr bool is_empty() const { return m_length == 0; }
 
     [[nodiscard]] const char* characters_without_null_termination() const { return m_characters; }
-    [[nodiscard]] size_t length() const { return m_length; }
+    [[nodiscard]] constexpr size_t length() const { return m_length; }
 
     [[nodiscard]] ReadonlyBytes bytes() const { return { m_characters, m_length }; }
 
-    const char& operator[](size_t index) const { return m_characters[index]; }
+    constexpr const char& operator[](size_t index) const { return m_characters[index]; }
 
     using ConstIterator = SimpleIterator<const StringView, const char>;
 
@@ -84,8 +84,17 @@ public:
     Optional<size_t> find(const StringView&) const;
     Optional<size_t> find(char c) const;
 
-    [[nodiscard]] StringView substring_view(size_t start, size_t length) const;
-    [[nodiscard]] StringView substring_view(size_t start) const;
+    [[nodiscard]] constexpr StringView substring_view(size_t start, size_t length) const
+    {
+        VERIFY(start + length <= m_length);
+        return { m_characters + start, length };
+    }
+
+    [[nodiscard]] constexpr StringView substring_view(size_t start) const
+    {
+        return substring_view(start, length() - start);
+    }
+
     [[nodiscard]] Vector<StringView> split_view(char, bool keep_empty = false) const;
     [[nodiscard]] Vector<StringView> split_view(const StringView&, bool keep_empty = false) const;
 
@@ -166,7 +175,7 @@ public:
 
     bool operator==(const String&) const;
 
-    bool operator==(const StringView& other) const
+    constexpr bool operator==(const StringView& other) const
     {
         if (is_null())
             return other.is_null();
@@ -177,7 +186,7 @@ public:
         return !__builtin_memcmp(m_characters, other.m_characters, m_length);
     }
 
-    bool operator!=(const StringView& other) const
+    constexpr bool operator!=(const StringView& other) const
     {
         return !(*this == other);
     }

AK/Tests/CMakeLists.txt

@@ -21,6 +21,7 @@ set(AK_TEST_SOURCES
     TestEnumBits.cpp
     TestFind.cpp
     TestFormat.cpp
+    TestGenericLexer.cpp
     TestHashFunctions.cpp
     TestHashMap.cpp
     TestHashTable.cpp

AK/Tests/TestGenericLexer.cpp

@@ -0,0 +1,160 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/TestSuite.h>
+
+#include <AK/GenericLexer.h>
+#include <AK/StringView.h>
+
+TEST_CASE(should_constexpr_construct_from_empty_string_view)
+{
+    constexpr GenericLexer sut(StringView {});
+    static_assert(sut.is_eof());
+}
+
+TEST_CASE(should_construct_from_string_view)
+{
+    constexpr GenericLexer sut(StringView { "abcdef" });
+    static_assert(!sut.is_eof());
+}
+
+TEST_CASE(should_constexpr_tell)
+{
+    constexpr GenericLexer sut(StringView { "abcdef" });
+    static_assert(sut.tell() == 0);
+}
+
+TEST_CASE(should_constexpr_tell_remaining)
+{
+    constexpr GenericLexer sut(StringView { "abcdef" });
+    static_assert(sut.tell_remaining() == 6);
+}
+
+TEST_CASE(should_constexpr_peek)
+{
+    constexpr GenericLexer sut(StringView { "abcdef" });
+    static_assert(sut.peek() == 'a');
+    static_assert(sut.peek(2) == 'c');
+    static_assert(sut.peek(100) == '\0');
+}
+
+TEST_CASE(should_constexpr_next_is)
+{
+    constexpr GenericLexer sut(StringView { "abcdef" });
+    static_assert(sut.next_is('a'));
+    static_assert(sut.next_is("abc"));
+    static_assert(sut.next_is(StringView { "abc" }));
+}
+
+TEST_CASE(should_constexpr_retreat)
+{
+    constexpr auto sut = [] {
+        GenericLexer sut(StringView { "abcdef" });
+        sut.consume();
+        sut.retreat();
+        return sut;
+    }();
+    static_assert(sut.peek() == 'a');
+}
+
+TEST_CASE(should_constexpr_consume_1)
+{
+    constexpr auto sut = [] {
+        GenericLexer sut(StringView { "abcdef" });
+        sut.consume();
+        return sut;
+    }();
+    static_assert(sut.peek() == 'b');
+}
+
+TEST_CASE(should_constexpr_consume_specific_char)
+{
+    constexpr auto sut = [] {
+        GenericLexer sut(StringView { "abcdef" });
+        sut.consume_specific('a');
+        return sut;
+    }();
+    static_assert(sut.peek() == 'b');
+}
+
+TEST_CASE(should_constexpr_consume_specific_string_view)
+{
+    constexpr auto sut = [] {
+        GenericLexer sut(StringView { "abcdef" });
+        sut.consume_specific(StringView { "ab" });
+        return sut;
+    }();
+    static_assert(sut.peek() == 'c');
+}
+
+TEST_CASE(should_constexpr_consume_specific_cstring)
+{
+    constexpr auto sut = [] {
+        GenericLexer sut(StringView { "abcdef" });
+        sut.consume_specific("abcd");
+        return sut;
+    }();
+    static_assert(sut.peek() == 'e');
+}
+
+TEST_CASE(should_constexpr_ignore_until)
+{
+    constexpr auto sut = [] {
+        GenericLexer sut(StringView { "abcdef" });
+        sut.ignore_until('d');
+        return sut;
+    }();
+    static_assert(sut.peek() == 'e');
+}
+
+TEST_CASE(should_constexpr_ignore_until_cstring)
+{
+    constexpr auto sut = [] {
+        GenericLexer sut(StringView { "abcdef" });
+        sut.ignore_until("cde");
+        return sut;
+    }();
+    static_assert(sut.peek() == 'f');
+}
+
+TEST_CASE(should_constexpr_next_is_pred)
+{
+    constexpr auto pred = [](auto c) {
+        return c == 'a';
+    };
+    constexpr GenericLexer sut(StringView { "abcdef" });
+    static_assert(sut.next_is(pred));
+}
+
+TEST_CASE(should_constexpr_ignore_while_pred)
+{
+    constexpr auto sut = [] {
+        constexpr auto pred = [](auto c) {
+            return c == 'a';
+        };
+
+        GenericLexer sut(StringView { "abcdef" });
+        sut.ignore_while(pred);
+        return sut;
+    }();
+    static_assert(sut.peek() == 'b');
+}
+
+TEST_CASE(should_constexpr_ignore_until_pred)
+{
+    constexpr auto sut = [] {
+        constexpr auto pred = [](auto c) {
+            return c == 'c';
+        };
+
+        GenericLexer sut(StringView { "abcdef" });
+        sut.ignore_until(pred);
+        return sut;
+    }();
+    static_assert(sut.peek() == 'c');
+}
+
+TEST_MAIN(GenericLexer)