Less intrusive auto discard.

Author: ColinH

include/tao/pegtl/internal/input_with_fakes.hpp

@@ -41,12 +41,6 @@ namespace TAO_PEGTL_NAMESPACE::internal
       void discard() const noexcept
       {}
 
-      void inc_rewind_guards() const noexcept
-      {}
-
-      void dec_rewind_guards() const noexcept
-      {}
-
       // The stream_foo() member functions are only for actual stream inputs.
    };
 

include/tao/pegtl/internal/rewind_guard.hpp

@@ -12,6 +12,12 @@
 
 namespace TAO_PEGTL_NAMESPACE::internal
 {
+   template< typename Input, typename = void >
+   inline constexpr bool has_auto_discard = false;
+
+   template< typename Input >
+   inline constexpr bool has_auto_discard< Input, decltype( (void)std::declval< Input >().inc_rewind_guards() ) > = true;
+
    template< typename ParseInput >
    class rewind_guard
    {
@@ -20,7 +26,9 @@ namespace TAO_PEGTL_NAMESPACE::internal
          : m_input( &in ),
            m_saved( in.rewind_position() )
       {
-         in.inc_rewind_guards();
+         if constexpr( has_auto_discard< ParseInput > ) {
+            in.inc_rewind_guards();
+         }
       }
 
       rewind_guard( rewind_guard&& rg ) noexcept
@@ -36,7 +44,9 @@ namespace TAO_PEGTL_NAMESPACE::internal
       {
          if( active() ) {
             m_input->rewind_to_position( m_saved );
-            m_input->dec_rewind_guards();
+            if constexpr( has_auto_discard< ParseInput > ) {
+               m_input->dec_rewind_guards();
+            }
          }
       }
 
@@ -51,7 +61,9 @@ namespace TAO_PEGTL_NAMESPACE::internal
       [[nodiscard]] bool operator()( const bool result ) noexcept
       {
          if( result ) {
-            m_input->dec_rewind_guards();
+            if constexpr( has_auto_discard< ParseInput > ) {
+               m_input->dec_rewind_guards();
+            }
             m_input = nullptr;
             return true;
          }

include/tao/pegtl/stream/stream_input_base.hpp

@@ -139,12 +139,6 @@ namespace TAO_PEGTL_NAMESPACE::internal
          return std::size_t( this->buffer_start() + this->buffer_capacity() - m_end );
       }
 
-      void inc_rewind_guards() const noexcept
-      {}
-
-      void dec_rewind_guards() const noexcept
-      {}
-
    protected:
       const data_t* m_current;
       const data_t* m_end;

include/tao/pegtl/stream/stream_input_with_auto_discard.hpp

@@ -37,6 +37,11 @@ namespace TAO_PEGTL_NAMESPACE::internal
          auto_discard();
       }
 
+      [[nodiscard]] std::size_t get_rewind_guards() const noexcept
+      {
+         return m_rewind_guards;
+      }
+
    private:
       std::size_t m_rewind_guards = 0;
 

src/test/stream_endless_input.cpp

@@ -38,17 +38,25 @@ namespace TAO_PEGTL_NAMESPACE
    {
       array_endless_auto_input< void > in( 'a' );
       TAO_PEGTL_TEST_ASSERT( in.buffer_capacity() < 10000 );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
 
       for( std::size_t i = 0; i < 42; ++i ) {
          TAO_PEGTL_TEST_ASSERT( parse< rep< 3000, one< 'a' > > >( in ) );
+         TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       }
       for( std::size_t i = 0; i < 42; ++i ) {
          TAO_PEGTL_TEST_ASSERT( parse< rep< 3000, one< 'a' > >, nothing, normal, apply_mode::action, rewind_mode::required >( in ) );
+         TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       }
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       TAO_PEGTL_TEST_THROWS( parse< rep< 12000, one< 'a' > >, nothing, normal, apply_mode::action, rewind_mode::required >( in ) );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       TAO_PEGTL_TEST_ASSERT( parse< rep< 12000, one< 'a' > > >( in ) );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       TAO_PEGTL_TEST_THROWS( parse< at< rep< 12000, one< 'a' > > > >( in ) );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       TAO_PEGTL_TEST_THROWS( parse< rep< 12000, one< 'a' > >, nop_action >( in ) );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
    }
 
    void test_multiple()
@@ -69,17 +77,25 @@ namespace TAO_PEGTL_NAMESPACE
       array_endless_auto_input< void > in( "abc" );
       TAO_PEGTL_TEST_ASSERT( parse< not_at< eof > >( in ) );
       TAO_PEGTL_TEST_ASSERT( in.buffer_capacity() < 10000 );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
 
       for( std::size_t i = 0; i < 42; ++i ) {
          TAO_PEGTL_TEST_ASSERT( parse< rep< 1000, string< 'a', 'b', 'c' > > >( in ) );
+         TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       }
       for( std::size_t i = 0; i < 42; ++i ) {
          TAO_PEGTL_TEST_ASSERT( parse< rep< 1000, string< 'a', 'b', 'c' > >, nothing, normal, apply_mode::action, rewind_mode::required >( in ) );
+         TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       }
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       TAO_PEGTL_TEST_THROWS( parse< rep< 4000, string< 'a', 'b', 'c' > >, nothing, normal, apply_mode::action, rewind_mode::required >( in ) );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       TAO_PEGTL_TEST_ASSERT( parse< rep< 4000, string< 'a', 'b', 'c' > > >( in ) );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       TAO_PEGTL_TEST_THROWS( parse< at< rep< 4000, string< 'a', 'b', 'c' > > > >( in ) );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
       TAO_PEGTL_TEST_THROWS( parse< rep< 4000, string< 'a', 'b', 'c' > >, nop_action >( in ) );
+      TAO_PEGTL_TEST_ASSERT( in.get_rewind_guards() == 0 );
    }
 
    void unit_test()