include/ftl/cast.h

/*
 * 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