/* Copyright 2015 The TensorFlow Authors. All Rights Reserved.

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.
==============================================================================*/

#ifndef TENSORFLOW_CORE_UTIL_OVERFLOW_H_
#define TENSORFLOW_CORE_UTIL_OVERFLOW_H_

#include "tensorflow/core/platform/logging.h"
#include "tensorflow/core/platform/macros.h"
#include "tensorflow/core/platform/types.h"

namespace tensorflow {

// Multiply two nonnegative int64's, returning negative for overflow
// If any of the arguments is negative, return negative too.
inline int64_t MultiplyWithoutOverflow(int64_t x, int64_t y) {
  if (TF_PREDICT_FALSE(x < 0)) return -1;
  if (TF_PREDICT_FALSE(y < 0)) return -1;
  if (TF_PREDICT_FALSE(x == 0)) return 0;

  // Multiply in uint64 rather than int64 since signed overflow is undefined.
  // Negative values will wrap around to large unsigned values in the casts
  // (see section 4.7 [conv.integral] of the C++14 standard).
  const uint64_t ux = x;
  const uint64_t uy = y;
  const uint64_t uxy = ux * uy;

  // Check if we overflow uint64, using a cheap check if both inputs are small
  if (TF_PREDICT_FALSE((ux | uy) >> 32 != 0)) {
    // Otherwise, detect overflow using a division
    if (uxy / ux != uy) return -1;
  }

  // Cast back to signed. A negative value signals an overflow.
  return static_cast<int64_t>(uxy);
}

// Add two nonnegative int64's, returning negative for overflow
// If any of the arguments is negative, return negative too.
inline int64_t AddWithoutOverflow(int64_t x, int64_t y) {
  if (TF_PREDICT_FALSE((x < 0)) || (y < 0)) return -1;

  // Add in uint64 rather than int64 since signed overflow is undefined.
  // Negative values will wrap around to large unsigned values in the casts
  // (see section 4.7 [conv.integral] of the C++14 standard).
  const uint64_t ux = x;
  const uint64_t uy = y;
  const uint64_t uxy = ux + uy;

  // Cast back to signed. A negative value signals an overflow.
  return static_cast<int64_t>(uxy);
}

}  // namespace tensorflow

#endif  // TENSORFLOW_CORE_UTIL_OVERFLOW_H_
