type_token.hpp
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type_token.hpp
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	/******************************************************************************
	*
	* File: type_token.hpp
	*
	* Contents: Facilities for encoding TypeLists as unsigned integer values.
	*
	* Copyright 2017
	*
	* Author: David Juhasz (david.juhasz@tuwien.ac.at)
	*
	******************************************************************************/

	#ifndef ROSA_SUPPORT_TYPE_TOKEN_HPP
	#define ROSA_SUPPORT_TYPE_TOKEN_HPP

	#include "rosa/support/type_numbers.hpp"

	namespace rosa {

	// NOTE on compatibility between different versions of the type token
	// implementation:
	// Different software versions produce compatible type tokens as long as
	// backward compatibility of BuiltinTypes is maintained (denoted by
	// TypeNumberVersion, see a note on that) and the type token implementation
	// uses the same type as token_t (boiling down to the same token::TokenBits
	// value) and the same token::RepresentationBits value. Thus, interacting
	// software need to cross-validate the aforementioned values to check
	// compatibility. Interoperation between compatible sofware is limited to
	// backward compatiblity, that is builtin types defined in both software
	// versions are handled correctly but a newer system may produce a type token
	// which is invalid in an old one. Therefore, tokens obtained from a compatible
	// remote system need to be validated.

	// Integer type to store type tokens.
	// NOTE: The trade-off between the binary overhead of type encoding and the
	// maximal size of encodable lists can be tuned by using unsigned integer types
	// of different widths as token_t.
	using token_t = uint64_t;

	// Sanity check in case someone would change token_t.
	STATIC_ASSERT(std::is_unsigned<token_t>::value,
	"token_t is not an unsigned integer");

	// A type to cast tokens into in order to output them to streams as
	// numbers and not ASCII-codes.
	// NOTE: Use it for safety, necessary for printing uint8_t values.
	using printable_token_t = PRINTABLE(token_t);

	// Helper preprocessor macro to cast tokens into printable_token_t.
	#define PRINTABLE_TOKEN(T) static_cast<printable_token_t>(T)

	// Nested namespace for protecting constants related to type tokens.
	namespace token {

	// The number of bits in one token.
	constexpr size_t TokenBits = sizeof(token_t) * 8;

	// The number of bits a builtin type can be uniquely encoded into, that is any
	// valid TypeNumber can fit into.
	// NOTE: There is one extra bit position added for encoding so that providing a
	// better chance to maintain backward comaptibility when BuiltinTypes is
	// extended.
	constexpr size_t RepresentationBits = log2(NumberOfBuiltinTypes) + 1;

	// Maximal size of uniquely tokenizable TypeList.
	constexpr size_t MaxTokenizableListSize = TokenBits / RepresentationBits;

	} // End namespace token

	// Generates a token, unsigned integer representation, for the TypeList.
	// NOTE: The TypeList cannot have more than MaxTokenizableListSize elements and
	// must be a subset of BuiltinTypes with respect to squashed integers.
	// NOTE: A generated token uniquely represents a list of types, except for
	// AtomConstant types. Observe that any AtomConstant is encoded as the type
	// AtomValue. The type information on all separate AtomConstant types are lost
	// and replaced by the AtomValue type whose actual value needs to be considered
	// in order to obtain the original AtomConstant type and so the full type
	// information on the encoded TypeList.
	template <typename List> struct TypeListTokenImpl;

	template <> struct TypeListTokenImpl<EmptyTypeList> {
	static constexpr token_t Token = 0;
	};

	template <typename T, typename... Ts>
	struct TypeListTokenImpl<TypeList<T, Ts...>> {
	static constexpr type_nr_t TN = TypeNumber<T>::Value;
	// Check if the generated type number is valid.
	STATIC_ASSERT(validTypeNumber(TN), "non-builtin type");
	static constexpr token_t Token =
	(TypeListTokenImpl<TypeList<Ts...>>::Token << token::RepresentationBits) \|
	TN;
	};

	template <typename List> struct TypeListToken;

	template <typename... Ts> struct TypeListToken<TypeList<Ts...>> {
	// NOTE: TypeNumber is computed against squased_int_t for integral types, so
	// let's do the same here.
	using List = typename SquashedTypeList<TypeList<Ts...>>::Type;
	// Check the length of the list here.
	// NOTE: Type validation is done one-by-one in TypeListTokenImpl.
	STATIC_ASSERT((TypeListSize<List>::Value <= token::MaxTokenizableListSize),
	"too long list of types");
	static constexpr token_t Token = TypeListTokenImpl<List>::Token;
	};

	// Convenience template turning a list of types into a TypeList to generate
	// TypeListToken for it.
	template <typename... Ts> using TypeToken = TypeListToken<TypeList<Ts...>>;

	// Anonymous namespace with helper facilities, consider it private.
	namespace {

	// Extracts the type number of the first encoded type of Token.
	// NOTE: The returned type number is not validated.
	inline type_nr_t typeNumberOfHeadOfToken(const token_t Token) {
	return static_cast<type_nr_t>(Token & ((1 << token::RepresentationBits) - 1));
	}

	} // End namespace


	// Tells if the given Token is valid, can be decoded by the current software.
	// NOTE: Validation gives a correct result only when Token was generated by a
	// compatible software (see note above).
	bool validToken(const token_t Token);

	// Tells if the token does encode an empty list.
	bool emptyToken(const token_t Token);

	// Tells the full memory size of the list encoded in the token.
	// PRE: validToken(Token)
	size_t sizeOfToken(const token_t Token);

	// Tells the memory size of the first type encoded in the token.
	// PRE: !empty(Token) && validToken(Token)
	size_t sizeOfHeadOfToken(const token_t Token);

	// Tells the name of the first type encoded in the token.
	// PRE: !empty(Token) && validToken(Token)
	const char *nameOfHeadOfToken(const token_t Token);

	// Drops the head element of the encoded list.
	void dropHeadOfToken(token_t &Token);

	// Tells if the head of the encoded list is of type T.
	// PRE: !empty(Token) && validToken(Token)
	template <typename T>
	bool isHeadOfTokenTheSameType(const token_t Token) {
	ASSERT(!emptyToken(Token) && validToken(Token));
	return TypeNumber<T>::Value == typeNumberOfHeadOfToken(Token);
	}

	} // End namespace rosa

	#endif // ROSA_SUPPORT_TYPE_TOKEN_HPP

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