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/*
* Copyright 2021 The Android Open Source Project
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <limits>
#include <type_traits>
#include <ftl/details/cast.h>
namespace android::ftl {
enum class CastSafety { kSafe, kUnderflow, kOverflow };
// Returns whether static_cast<R>(v) is safe, or would result in underflow or overflow.
//
// static_assert(ftl::cast_safety<uint8_t>(-1) == ftl::CastSafety::kUnderflow);
// static_assert(ftl::cast_safety<int8_t>(128u) == ftl::CastSafety::kOverflow);
//
// static_assert(ftl::cast_safety<uint32_t>(-.1f) == ftl::CastSafety::kUnderflow);
// static_assert(ftl::cast_safety<int32_t>(static_cast<float>(INT32_MAX)) ==
// ftl::CastSafety::kOverflow);
//
// static_assert(ftl::cast_safety<float>(-DBL_MAX) == ftl::CastSafety::kUnderflow);
//
template <typename R, typename T>
constexpr CastSafety cast_safety(T v) {
static_assert(std::is_arithmetic_v<T>);
static_assert(std::is_arithmetic_v<R>);
constexpr bool kFromSigned = std::is_signed_v<T>;
constexpr bool kToSigned = std::is_signed_v<R>;
using details::max_exponent;
// If the R range contains the T range, then casting is always safe.
if constexpr ((kFromSigned == kToSigned && max_exponent<R> >= max_exponent<T>) ||
(!kFromSigned && kToSigned && max_exponent<R> > max_exponent<T>)) {
return CastSafety::kSafe;
}
using C = std::common_type_t<R, T>;
if constexpr (kFromSigned) {
using L = details::safe_limits<R, T>;
if constexpr (kToSigned) {
// Signed to signed.
if (v < L::lowest()) return CastSafety::kUnderflow;
return v <= L::max() ? CastSafety::kSafe : CastSafety::kOverflow;
} else {
// Signed to unsigned.
if (v < 0) return CastSafety::kUnderflow;
return static_cast<C>(v) <= static_cast<C>(L::max()) ? CastSafety::kSafe
: CastSafety::kOverflow;
}
} else {
using L = std::numeric_limits<R>;
if constexpr (kToSigned) {
// Unsigned to signed.
return static_cast<C>(v) <= static_cast<C>(L::max()) ? CastSafety::kSafe
: CastSafety::kOverflow;
} else {
// Unsigned to unsigned.
return v <= L::max() ? CastSafety::kSafe : CastSafety::kOverflow;
}
}
}
} // namespace android::ftl
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