include/private/SkTFitsIn.h - skia - Git at Google

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */

 #ifndef SkTFitsIn_DEFINED
 #define SkTFitsIn_DEFINED

 #include "../private/SkTLogic.h"
 #include <limits>
 #include <type_traits>

 /**
  * In C++ an unsigned to signed cast where the source value cannot be represented in the destination
  * type results in an implementation defined destination value. Unlike C, C++ does not allow a trap.
  * This makes "(S)(D)s == s" a possibly useful test. However, there are two cases where this is
  * incorrect:
  *
  * when testing if a value of a smaller signed type can be represented in a larger unsigned type
  * (int8_t)(uint16_t)-1 == -1 => (int8_t)0xFFFF == -1 => [implementation defined] == -1
  *
  * when testing if a value of a larger unsigned type can be represented in a smaller signed type
  * (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true.
  *
  * Consider the cases:
  * u = unsigned, s = signed, X = more digits, x = less digits
  * ux -> uX: (ux)(uX)ux == ux, trivially true
  * uX -> ux: (uX)(ux)uX == uX, both casts well defined, test works
  * sx -> sX: (sx)(sX)sx == sx, trivially true
  * sX -> sx: (sX)(sx)sX == sX, first cast implementation value, second cast defined, test works
  * sx -> uX: (sx)(uX)sx == sx, this is bad, the second cast results in implementation defined value
  * sX -> ux: (sX)(ux)sX == sX, the second cast is required to prevent promotion of rhs to unsigned
  * ux -> sX: (ux)(sX)ux == ux, trivially true
  * uX -> sx: (uX)(sx)uX == uX, this is bad,
  *                             first cast results in implementation defined value,
  *                             second cast is defined. However, this creates false positives
  *                             uint16_t x = 0xFFFF
  *                                (uint16_t)(int8_t)x == x
  *                             => (uint16_t)-1        == x
  *                             => 0xFFFF              == x
  *                             => true
  *
  * So for the eight cases three are trivially true, three more are valid casts, and two are special.
  * The two 'full' checks which otherwise require two comparisons are valid cast checks.
  * The two remaining checks uX -> sx [uX < max(sx)] and sx -> uX [sx > 0] can be done with one op.
  */

 namespace sktfitsin {
 namespace Private {

 /** SkTMux::type = (a && b) ? Both : (a) ? A : (b) ? B : Neither; */
 template <bool a, bool b, typename Both, typename A, typename B, typename Neither>
 struct SkTMux {
     using type = skstd::conditional_t<a, skstd::conditional_t<b, Both, A>,
                                          skstd::conditional_t<b, B, Neither>>;
 };

 /** SkTHasMoreDigits = (digits(A) >= digits(B)) ? true_type : false_type. */
 template <typename A, typename B> struct SkTHasMoreDigits
     : skstd::bool_constant<std::numeric_limits<A>::digits >= std::numeric_limits<B>::digits>
 { };

 /** Returns true.
  *  Used when it is statically known that source values are in the range of the Destination.
  */
 template <typename S> struct SkTInRange_True {
     static constexpr bool fits(S) {
         return true;
     }
 };

 /** Tests that (S)(D)s == s.
  *  This is not valid for uX -> sx and sx -> uX conversions.
  */
 template <typename D, typename S> struct SkTInRange_Cast {
     static constexpr bool fits(S s) {
         using S_is_bigger = SkTHasMoreDigits<S, D>;
         using D_is_bigger = SkTHasMoreDigits<D, S>;

         using S_is_signed = skstd::bool_constant<std::numeric_limits<S>::is_signed>;
         using D_is_signed = skstd::bool_constant<std::numeric_limits<D>::is_signed>;

         using precondition = skstd::bool_constant<
             !((!S_is_signed::value &&  D_is_signed::value && S_is_bigger::value) ||
               ( S_is_signed::value && !D_is_signed::value && D_is_bigger::value)   )>;
         static_assert(precondition::value, "not valid for uX -> sx and sx -> uX conversions");

         return static_cast<S>(static_cast<D>(s)) == s;
     }
 };

 /** Tests if the source value <= Max(D).
  *  Assumes that Max(S) >= Max(D).
  */
 template <typename D, typename S> struct SkTInRange_LE_MaxD {
     static constexpr bool fits(S s) {
         using precondition = SkTHasMoreDigits<S, D>;
         static_assert(precondition::value, "maxS < maxD");

         return s <= static_cast<S>((std::numeric_limits<D>::max)());

     }
 };

 /** Tests if the source value >= 0. */
 template <typename D, typename S> struct SkTInRange_GE_Zero {
     static constexpr bool fits(S s) {
         return static_cast<S>(0) <= s;
     }
 };

 /** SkTFitsIn_Unsigned2Unsiged::type is an SkTInRange with an fits(S s) method
  *  the implementation of which is tailored for the source and destination types.
  *  Assumes that S and D are unsigned integer types.
  */
 template <typename D, typename S> struct SkTFitsIn_Unsigned2Unsiged {
     using CastCheck = SkTInRange_Cast<D, S>;
     using NoCheck = SkTInRange_True<S>;

     // If std::numeric_limits<D>::digits >= std::numeric_limits<S>::digits, nothing to check.
     using sourceFitsInDesitination = SkTHasMoreDigits<D, S>;
     using type = skstd::conditional_t<sourceFitsInDesitination::value, NoCheck, CastCheck>;
 };

 /** SkTFitsIn_Signed2Signed::type is an SkTInRange with an fits(S s) method
  *  the implementation of which is tailored for the source and destination types.
  *  Assumes that S and D are signed integer types.
  */
 template <typename D, typename S> struct SkTFitsIn_Signed2Signed {
     using CastCheck = SkTInRange_Cast<D, S>;
     using NoCheck = SkTInRange_True<S>;

     // If std::numeric_limits<D>::digits >= std::numeric_limits<S>::digits, nothing to check.
     using sourceFitsInDesitination = SkTHasMoreDigits<D, S>;
     using type = skstd::conditional_t<sourceFitsInDesitination::value, NoCheck, CastCheck>;
 };

 /** SkTFitsIn_Signed2Unsigned::type is an SkTInRange with an fits(S s) method
  *  the implementation of which is tailored for the source and destination types.
  *  Assumes that S is a signed integer type and D is an unsigned integer type.
  */
 template <typename D, typename S> struct SkTFitsIn_Signed2Unsigned {
     using CastCheck = SkTInRange_Cast<D, S>;
     using LowSideOnlyCheck = SkTInRange_GE_Zero<D, S>;

     // If std::numeric_limits<D>::max() >= std::numeric_limits<S>::max(),
     // no need to check the high side. (Until C++11, assume more digits means greater max.)
     // This also protects the precondition of SkTInRange_Cast.
     using sourceCannotExceedDest = SkTHasMoreDigits<D, S>;
     using type = skstd::conditional_t<sourceCannotExceedDest::value, LowSideOnlyCheck, CastCheck>;
 };

 /** SkTFitsIn_Unsigned2Signed::type is an SkTInRange with an fits(S s) method
  *  the implementation of which is tailored for the source and destination types.
  *  Assumes that S is an usigned integer type and D is a signed integer type.
  */
 template <typename D, typename S> struct SkTFitsIn_Unsigned2Signed {
     using HighSideCheck = SkTInRange_LE_MaxD<D, S>;
     using NoCheck = SkTInRange_True<S>;

     // If std::numeric_limits<D>::max() >= std::numeric_limits<S>::max(), nothing to check.
     // (Until C++11, assume more digits means greater max.)
     using sourceCannotExceedDest = SkTHasMoreDigits<D, S>;
     using type = skstd::conditional_t<sourceCannotExceedDest::value, NoCheck, HighSideCheck>;
 };

 /** SkTFitsIn::type is an SkTInRange with an fits(S s) method
  *  the implementation of which is tailored for the source and destination types.
  *  Assumes that S and D are integer types.
  */
 template <typename D, typename S> struct SkTFitsIn {
     // One of the following will be the 'selector' type.
     using S2S = SkTFitsIn_Signed2Signed<D, S>;
     using S2U = SkTFitsIn_Signed2Unsigned<D, S>;
     using U2S = SkTFitsIn_Unsigned2Signed<D, S>;
     using U2U = SkTFitsIn_Unsigned2Unsiged<D, S>;

     using S_is_signed = skstd::bool_constant<std::numeric_limits<S>::is_signed>;
     using D_is_signed = skstd::bool_constant<std::numeric_limits<D>::is_signed>;

     using selector = typename SkTMux<S_is_signed::value, D_is_signed::value,
                                      S2S, S2U, U2S, U2U>::type;
     // This type is an SkTInRange.
     using type = typename selector::type;
 };

 template <typename T, bool = std::is_enum<T>::value> struct underlying_type {
     using type = skstd::underlying_type_t<T>;
 };
 template <typename T> struct underlying_type<T, false> {
     using type = T;
 };

 } // namespace Private
 } // namespace sktfitsin

 /** Returns true if the integer source value 's' will fit in the integer destination type 'D'. */
 template <typename D, typename S> constexpr inline bool SkTFitsIn(S s) {
     static_assert(std::is_integral<S>::value || std::is_enum<S>::value, "S must be integral.");
     static_assert(std::is_integral<D>::value || std::is_enum<D>::value, "D must be integral.");

     using RealS = typename sktfitsin::Private::underlying_type<S>::type;
     using RealD = typename sktfitsin::Private::underlying_type<D>::type;

     return sktfitsin::Private::SkTFitsIn<RealD, RealS>::type::fits(s);
 }

 #endif
	/*
	* Copyright 2013 Google Inc.
	*
	* Use of this source code is governed by a BSD-style license that can be
	* found in the LICENSE file.
	*/

	#ifndef SkTFitsIn_DEFINED
	#define SkTFitsIn_DEFINED

	#include "../private/SkTLogic.h"
	#include <limits>
	#include <type_traits>

	/**
	* In C++ an unsigned to signed cast where the source value cannot be represented in the destination
	* type results in an implementation defined destination value. Unlike C, C++ does not allow a trap.
	* This makes "(S)(D)s == s" a possibly useful test. However, there are two cases where this is
	* incorrect:
	*
	* when testing if a value of a smaller signed type can be represented in a larger unsigned type
	* (int8_t)(uint16_t)-1 == -1 => (int8_t)0xFFFF == -1 => [implementation defined] == -1
	*
	* when testing if a value of a larger unsigned type can be represented in a smaller signed type
	* (uint16_t)(int8_t)0xFFFF == 0xFFFF => (uint16_t)-1 == 0xFFFF => 0xFFFF == 0xFFFF => true.
	*
	* Consider the cases:
	* u = unsigned, s = signed, X = more digits, x = less digits
	* ux -> uX: (ux)(uX)ux == ux, trivially true
	* uX -> ux: (uX)(ux)uX == uX, both casts well defined, test works
	* sx -> sX: (sx)(sX)sx == sx, trivially true
	* sX -> sx: (sX)(sx)sX == sX, first cast implementation value, second cast defined, test works
	* sx -> uX: (sx)(uX)sx == sx, this is bad, the second cast results in implementation defined value
	* sX -> ux: (sX)(ux)sX == sX, the second cast is required to prevent promotion of rhs to unsigned
	* ux -> sX: (ux)(sX)ux == ux, trivially true
	* uX -> sx: (uX)(sx)uX == uX, this is bad,
	* first cast results in implementation defined value,
	* second cast is defined. However, this creates false positives
	* uint16_t x = 0xFFFF
	* (uint16_t)(int8_t)x == x
	* => (uint16_t)-1 == x
	* => 0xFFFF == x
	* => true
	*
	* So for the eight cases three are trivially true, three more are valid casts, and two are special.
	* The two 'full' checks which otherwise require two comparisons are valid cast checks.
	* The two remaining checks uX -> sx [uX < max(sx)] and sx -> uX [sx > 0] can be done with one op.
	*/

	namespace sktfitsin {
	namespace Private {

	/** SkTMux::type = (a && b) ? Both : (a) ? A : (b) ? B : Neither; */
	template <bool a, bool b, typename Both, typename A, typename B, typename Neither>
	struct SkTMux {
	using type = skstd::conditional_t<a, skstd::conditional_t<b, Both, A>,
	skstd::conditional_t<b, B, Neither>>;
	};

	/** SkTHasMoreDigits = (digits(A) >= digits(B)) ? true_type : false_type. */
	template <typename A, typename B> struct SkTHasMoreDigits
	: skstd::bool_constant<std::numeric_limits<A>::digits >= std::numeric_limits<B>::digits>
	{ };

	/** Returns true.
	* Used when it is statically known that source values are in the range of the Destination.
	*/
	template <typename S> struct SkTInRange_True {
	static constexpr bool fits(S) {
	return true;
	}
	};

	/** Tests that (S)(D)s == s.
	* This is not valid for uX -> sx and sx -> uX conversions.
	*/
	template <typename D, typename S> struct SkTInRange_Cast {
	static constexpr bool fits(S s) {
	using S_is_bigger = SkTHasMoreDigits<S, D>;
	using D_is_bigger = SkTHasMoreDigits<D, S>;

	using S_is_signed = skstd::bool_constant<std::numeric_limits<S>::is_signed>;
	using D_is_signed = skstd::bool_constant<std::numeric_limits<D>::is_signed>;

	using precondition = skstd::bool_constant<
	!((!S_is_signed::value && D_is_signed::value && S_is_bigger::value) \|\|
	( S_is_signed::value && !D_is_signed::value && D_is_bigger::value) )>;
	static_assert(precondition::value, "not valid for uX -> sx and sx -> uX conversions");

	return static_cast<S>(static_cast<D>(s)) == s;
	}
	};

	/** Tests if the source value <= Max(D).
	* Assumes that Max(S) >= Max(D).
	*/
	template <typename D, typename S> struct SkTInRange_LE_MaxD {
	static constexpr bool fits(S s) {
	using precondition = SkTHasMoreDigits<S, D>;
	static_assert(precondition::value, "maxS < maxD");

	return s <= static_cast<S>((std::numeric_limits<D>::max)());

	}
	};

	/** Tests if the source value >= 0. */
	template <typename D, typename S> struct SkTInRange_GE_Zero {
	static constexpr bool fits(S s) {
	return static_cast<S>(0) <= s;
	}
	};

	/** SkTFitsIn_Unsigned2Unsiged::type is an SkTInRange with an fits(S s) method
	* the implementation of which is tailored for the source and destination types.
	* Assumes that S and D are unsigned integer types.
	*/
	template <typename D, typename S> struct SkTFitsIn_Unsigned2Unsiged {
	using CastCheck = SkTInRange_Cast<D, S>;
	using NoCheck = SkTInRange_True<S>;

	// If std::numeric_limits<D>::digits >= std::numeric_limits<S>::digits, nothing to check.
	using sourceFitsInDesitination = SkTHasMoreDigits<D, S>;
	using type = skstd::conditional_t<sourceFitsInDesitination::value, NoCheck, CastCheck>;
	};

	/** SkTFitsIn_Signed2Signed::type is an SkTInRange with an fits(S s) method
	* the implementation of which is tailored for the source and destination types.
	* Assumes that S and D are signed integer types.
	*/
	template <typename D, typename S> struct SkTFitsIn_Signed2Signed {
	using CastCheck = SkTInRange_Cast<D, S>;
	using NoCheck = SkTInRange_True<S>;

	// If std::numeric_limits<D>::digits >= std::numeric_limits<S>::digits, nothing to check.
	using sourceFitsInDesitination = SkTHasMoreDigits<D, S>;
	using type = skstd::conditional_t<sourceFitsInDesitination::value, NoCheck, CastCheck>;
	};

	/** SkTFitsIn_Signed2Unsigned::type is an SkTInRange with an fits(S s) method
	* the implementation of which is tailored for the source and destination types.
	* Assumes that S is a signed integer type and D is an unsigned integer type.
	*/
	template <typename D, typename S> struct SkTFitsIn_Signed2Unsigned {
	using CastCheck = SkTInRange_Cast<D, S>;
	using LowSideOnlyCheck = SkTInRange_GE_Zero<D, S>;

	// If std::numeric_limits<D>::max() >= std::numeric_limits<S>::max(),
	// no need to check the high side. (Until C++11, assume more digits means greater max.)
	// This also protects the precondition of SkTInRange_Cast.
	using sourceCannotExceedDest = SkTHasMoreDigits<D, S>;
	using type = skstd::conditional_t<sourceCannotExceedDest::value, LowSideOnlyCheck, CastCheck>;
	};

	/** SkTFitsIn_Unsigned2Signed::type is an SkTInRange with an fits(S s) method
	* the implementation of which is tailored for the source and destination types.
	* Assumes that S is an usigned integer type and D is a signed integer type.
	*/
	template <typename D, typename S> struct SkTFitsIn_Unsigned2Signed {
	using HighSideCheck = SkTInRange_LE_MaxD<D, S>;
	using NoCheck = SkTInRange_True<S>;

	// If std::numeric_limits<D>::max() >= std::numeric_limits<S>::max(), nothing to check.
	// (Until C++11, assume more digits means greater max.)
	using sourceCannotExceedDest = SkTHasMoreDigits<D, S>;
	using type = skstd::conditional_t<sourceCannotExceedDest::value, NoCheck, HighSideCheck>;
	};

	/** SkTFitsIn::type is an SkTInRange with an fits(S s) method
	* the implementation of which is tailored for the source and destination types.
	* Assumes that S and D are integer types.
	*/
	template <typename D, typename S> struct SkTFitsIn {
	// One of the following will be the 'selector' type.
	using S2S = SkTFitsIn_Signed2Signed<D, S>;
	using S2U = SkTFitsIn_Signed2Unsigned<D, S>;
	using U2S = SkTFitsIn_Unsigned2Signed<D, S>;
	using U2U = SkTFitsIn_Unsigned2Unsiged<D, S>;

	using S_is_signed = skstd::bool_constant<std::numeric_limits<S>::is_signed>;
	using D_is_signed = skstd::bool_constant<std::numeric_limits<D>::is_signed>;

	using selector = typename SkTMux<S_is_signed::value, D_is_signed::value,
	S2S, S2U, U2S, U2U>::type;
	// This type is an SkTInRange.
	using type = typename selector::type;
	};

	template <typename T, bool = std::is_enum<T>::value> struct underlying_type {
	using type = skstd::underlying_type_t<T>;
	};
	template <typename T> struct underlying_type<T, false> {
	using type = T;
	};

	} // namespace Private
	} // namespace sktfitsin

	/** Returns true if the integer source value 's' will fit in the integer destination type 'D'. */
	template <typename D, typename S> constexpr inline bool SkTFitsIn(S s) {
	static_assert(std::is_integral<S>::value \|\| std::is_enum<S>::value, "S must be integral.");
	static_assert(std::is_integral<D>::value \|\| std::is_enum<D>::value, "D must be integral.");

	using RealS = typename sktfitsin::Private::underlying_type<S>::type;
	using RealD = typename sktfitsin::Private::underlying_type<D>::type;

	return sktfitsin::Private::SkTFitsIn<RealD, RealS>::type::fits(s);
	}

	#endif