Alepha/IOStreams/Filter.h

static_assert( __cplusplus > 2020'99 );

#pragma once

#include <Alepha/Alepha.h>

#include <memory>

#include <boost/noncopyable.hpp>

#include <Alepha/error.h>

#include <Alepha/IOStreams/StackableStreambuf.h>

namespace Alepha::Hydrogen::IOStreams  ::detail::  Filter_m
{
	namespace C
	{
		const bool debug= false;
	}

	inline namespace exports
	{
		class Emitter;

		Emitter nilEmitter();

		class StreamFilter;

		class FilteringStreambuf;

		struct Filter_impl
		{
			std::unique_ptr< StreamFilter > filter;

			explicit
			Filter_impl( std::unique_ptr< StreamFilter > filter )
				: filter( std::move( filter ) )
			{}
		};
		using Filter= PushStack< Filter_impl >;
	}

	// XXX: This should form the new heart of how stackable
	// streambufs work.  One writes StreamFilter objects.
	// When adding a filter to a streambuf in a stack, we build
	// a stack of filters.  The stackable streambuf becomes
	// a stackable filter streambuf.


	class exports::Emitter
	{
		public:
			struct Impl
			{
				virtual ~Impl()= default;

				virtual bool empty() const= 0;

				virtual char next()= 0;
			};

		private:
			std::unique_ptr< Impl > pimpl;

			Impl &impl() { return *pimpl; }
			const Impl &impl() const { return *pimpl; }

		public:
			explicit
			Emitter( std::unique_ptr< Impl > pimpl )
				: pimpl( std::move( pimpl ) ) {}

			bool empty() const { return impl().empty(); }

			char next() { return impl().next(); }

			void
			drain( std::ostream &out )
			{
				while( not empty() )
				{
					out << next();
				}
			}
	};

	inline Emitter
	exports::nilEmitter()
	{
		struct NilImpl : Emitter::Impl
		{
			bool empty() const final { return true; }
			char next() final { throw std::out_of_range( "" ); }
		};

		return Emitter{ std::make_unique< NilImpl >() };
	}

	class exports::StreamFilter
		: boost::noncopyable
	{
		public:
			virtual ~StreamFilter()= default;

			virtual Emitter nextChar( char ch )= 0;

			virtual Emitter
			nextChar( std::string_view chars )
			{
				std::size_t amount= 0;
				for( const char ch: chars )
				{
					++amount;
					auto emitter= nextChar( ch );
					if( not emitter.empty() )
					{
						return emitter;
					}
				}

				return nilEmitter();
			}
	};

	class exports::FilteringStreambuf
		: public virtual IOStreams::StackableStreambuf
	{
		private:
			static const std::size_t limit= 256;

			std::unique_ptr< StreamFilter > filter;

			Emitter nextInput= nilEmitter();
			std::string inBuffer;
			std::string outBuffer= std::string( int( limit ), '\0' );

		public:
			explicit
			FilteringStreambuf( std::ios &stream, std::unique_ptr< StreamFilter > filter )
				: StackableStreambuf( stream ), filter( std::move( filter ) )
			{}

		private:
			void drain() final {}
			void writeChar( char ) final { throw "Unimpl"; }

			int
			underflow() override
			{
				if( C::debug ) error() << "Underflow called..." << std::endl;
				inBuffer.clear();
				while( inBuffer.size() < limit )
				{
					if( nextInput.empty() )
					{
						const auto next= underlying_sbumpc();
						if( next == EOF ) break;
						nextInput= filter->nextChar( next );
						continue;
					}
					inBuffer.push_back( nextInput.next() );
				}
				setg( inBuffer.data(), inBuffer.data(), inBuffer.data() + inBuffer.size() );
				if( inBuffer.empty() ) return EOF;

				if( C::debug ) error() << "Underflow returning: " << inBuffer.front();
				return inBuffer.front();
			}

			int
			overflow( const int nextChar ) override
			{
				sync();

				if( nextChar != EOF )
				{
					Emitter emitter= filter->nextChar( nextChar );
					emitter.drain( out() );
				}

				return 1;
			}

			int
			sync() override
			{
				for( const auto next: std::string_view{ pbase(), pptr() } )
				{
					Emitter emitter= filter->nextChar( next );
					emitter.drain( out() );
				}
				out().rdbuf()->pubsync();

				std::fill_n( begin( outBuffer ), limit, 0 );
				setg( outBuffer.data(), outBuffer.data(), outBuffer.data() + outBuffer.size() );

				return 1;
			}
	};

	inline void
	build_streambuf( std::ios &ios, Filter &&params )
	{
		new FilteringStreambuf( ios, std::move( params.filter ) );
	}
}

namespace Alepha::Hydrogen::IOStreams::inline exports::inline Filter_m
{
	using namespace detail::Filter_m::exports;
}