Add method to forcefully set a lexer's position

Author: trflynn89

fly/types/string/lexer.hpp

@@ -60,6 +60,17 @@ class BasicLexer
      */
     constexpr std::size_t position() const;
 
+    /**
+     * Set the lexer's current position into the C-string literal.
+     *
+     * The provided position is not checked for validity. If the position is past-the-end of the
+     * C-string literal, further operations will fail (i.e. |peek| and |consume| will return
+     * std::nullopt).
+     *
+     * @param position The lexer's new position.
+     */
+    constexpr void set_position(std::size_t position);
+
     /**
      * If a character is available at the current position (or some offset from the current
      * position) in the C-string literal, return that character.
@@ -166,6 +177,13 @@ constexpr std::size_t BasicLexer<CharType>::position() const
     return m_index;
 }
 
+//==================================================================================================
+template <typename CharType>
+constexpr void BasicLexer<CharType>::set_position(std::size_t position)
+{
+    m_index = position;
+}
+
 //==================================================================================================
 template <typename CharType>
 constexpr std::optional<CharType> BasicLexer<CharType>::peek(std::size_t offset)

test/types/string/lexer.cpp

@@ -62,6 +62,30 @@ CATCH_TEMPLATE_TEST_CASE("BasicLexer", "[string]", char, wchar_t, char8_t, char1
         CATCH_CHECK_FALSE(lexer.consume());
     }
 
+    CATCH_SECTION("Setting the lexer's position mutates internal pointer")
+    {
+        Lexer lexer(FLY_ARR(char_type, "ab"));
+        CATCH_CHECK(lexer.position() == 0);
+
+        lexer.set_position(1);
+        CATCH_CHECK(lexer.position() == 1);
+
+        auto p1 = lexer.peek();
+        CATCH_REQUIRE(p1.has_value());
+        CATCH_CHECK(p1.value() == FLY_CHR(char_type, 'b'));
+
+        lexer.set_position(0);
+        CATCH_CHECK(lexer.position() == 0);
+
+        auto p2 = lexer.peek();
+        CATCH_REQUIRE(p2.has_value());
+        CATCH_CHECK(p2.value() == FLY_CHR(char_type, 'a'));
+
+        lexer.set_position(2);
+        CATCH_CHECK(lexer.position() == 2);
+        CATCH_CHECK_FALSE(lexer.peek());
+    }
+
     CATCH_SECTION("Peeking does not advance internal pointer")
     {
         Lexer lexer(FLY_ARR(char_type, "ab"));