// Copyright 2017 The Abseil Authors.
//
// 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
//
//      https://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.

#include "absl/random/internal/pool_urbg.h"

#include <algorithm>
#include <bitset>
#include <cmath>
#include <cstdint>
#include <iterator>

#include "gtest/gtest.h"
#include "absl/meta/type_traits.h"
#include "absl/types/span.h"

using absl::random_internal::PoolURBG;
using absl::random_internal::RandenPool;

namespace {

// is_randen_pool trait is true when parameterized by an RandenPool
template <typename T>
using is_randen_pool = typename absl::disjunction<  //
    std::is_same<T, RandenPool<uint8_t>>,           //
    std::is_same<T, RandenPool<uint16_t>>,          //
    std::is_same<T, RandenPool<uint32_t>>,          //
    std::is_same<T, RandenPool<uint64_t>>>;         //

// MyFill either calls RandenPool::Fill() or std::generate(..., rng)
template <typename T, typename V>
typename absl::enable_if_t<absl::negation<is_randen_pool<T>>::value, void>  //
MyFill(T& rng, absl::Span<V> data) {  // NOLINT(runtime/references)
  std::generate(std::begin(data), std::end(data), rng);
}

template <typename T, typename V>
typename absl::enable_if_t<is_randen_pool<T>::value, void>  //
MyFill(T& rng, absl::Span<V> data) {  // NOLINT(runtime/references)
  rng.Fill(data);
}

template <typename EngineType>
class PoolURBGTypedTest : public ::testing::Test {};

using EngineTypes = ::testing::Types<  //
    RandenPool<uint8_t>,               //
    RandenPool<uint16_t>,              //
    RandenPool<uint32_t>,              //
    RandenPool<uint64_t>,              //
    PoolURBG<uint8_t, 2>,              //
    PoolURBG<uint16_t, 2>,             //
    PoolURBG<uint32_t, 2>,             //
    PoolURBG<uint64_t, 2>,             //
    PoolURBG<unsigned int, 8>,         // NOLINT(runtime/int)
    PoolURBG<unsigned long, 8>,        // NOLINT(runtime/int)
    PoolURBG<unsigned long int, 4>,    // NOLINT(runtime/int)
    PoolURBG<unsigned long long, 4>>;  // NOLINT(runtime/int)

TYPED_TEST_SUITE(PoolURBGTypedTest, EngineTypes);

// This test is checks that the engines meet the URBG interface requirements
// defined in [rand.req.urbg].
TYPED_TEST(PoolURBGTypedTest, URBGInterface) {
  using E = TypeParam;
  using T = typename E::result_type;

  static_assert(std::is_copy_constructible<E>::value,
                "engine must be copy constructible");

  static_assert(absl::is_copy_assignable<E>::value,
                "engine must be copy assignable");

  E e;
  const E x;

  e();

  static_assert(std::is_same<decltype(e()), T>::value,
                "return type of operator() must be result_type");

  E u0(x);
  u0();

  E u1 = e;
  u1();
}

// This validates that sequences are independent.
TYPED_TEST(PoolURBGTypedTest, VerifySequences) {
  using E = TypeParam;
  using result_type = typename E::result_type;

  E rng;
  (void)rng();  // Discard one value.

  constexpr int kNumOutputs = 64;
  result_type a[kNumOutputs];
  result_type b[kNumOutputs];
  std::fill(std::begin(b), std::end(b), 0);

  // Fill a using Fill or generate, depending on the engine type.
  {
    E x = rng;
    MyFill(x, absl::MakeSpan(a));
  }

  // Fill b using std::generate().
  {
    E x = rng;
    std::generate(std::begin(b), std::end(b), x);
  }

  // Test that generated sequence changed as sequence of bits, i.e. if about
  // half of the bites were flipped between two non-correlated values.
  size_t changed_bits = 0;
  size_t unchanged_bits = 0;
  size_t total_set = 0;
  size_t total_bits = 0;
  size_t equal_count = 0;
  for (size_t i = 0; i < kNumOutputs; ++i) {
    equal_count += (a[i] == b[i]) ? 1 : 0;
    std::bitset<sizeof(result_type) * 8> bitset(a[i] ^ b[i]);
    changed_bits += bitset.count();
    unchanged_bits += bitset.size() - bitset.count();

    std::bitset<sizeof(result_type) * 8> a_set(a[i]);
    std::bitset<sizeof(result_type) * 8> b_set(b[i]);
    total_set += a_set.count() + b_set.count();
    total_bits += 2 * 8 * sizeof(result_type);
  }
  // On average, half the bits are changed between two calls.
  EXPECT_LE(changed_bits, 0.60 * (changed_bits + unchanged_bits));
  EXPECT_GE(changed_bits, 0.40 * (changed_bits + unchanged_bits));

  // verify using a quick normal-approximation to the binomial.
  EXPECT_NEAR(total_set, total_bits * 0.5, 4 * std::sqrt(total_bits))
      << "@" << total_set / static_cast<double>(total_bits);

  // Also, A[i] == B[i] with probability (1/range) * N.
  // Give this a pretty wide latitude, though.
  const double kExpected = kNumOutputs / (1.0 * sizeof(result_type) * 8);
  EXPECT_LE(equal_count, 1.0 + kExpected);
}

}  // namespace

/*
$ nanobenchmarks 1 RandenPool construct
$ nanobenchmarks 1 PoolURBG construct

RandenPool<uint32_t> | 1    | 1000 |    48482.00 ticks | 48.48 ticks | 13.9 ns
RandenPool<uint32_t> | 10   | 2000 |  1028795.00 ticks | 51.44 ticks | 14.7 ns
RandenPool<uint32_t> | 100  | 1000 |  5119968.00 ticks | 51.20 ticks | 14.6 ns
RandenPool<uint32_t> | 1000 |  500 | 25867936.00 ticks | 51.74 ticks | 14.8 ns

RandenPool<uint64_t> | 1    | 1000 |    49921.00 ticks | 49.92 ticks | 14.3 ns
RandenPool<uint64_t> | 10   | 2000 |  1208269.00 ticks | 60.41 ticks | 17.3 ns
RandenPool<uint64_t> | 100  | 1000 |  5844955.00 ticks | 58.45 ticks | 16.7 ns
RandenPool<uint64_t> | 1000 |  500 | 28767404.00 ticks | 57.53 ticks | 16.4 ns

PoolURBG<uint32_t,8> | 1    | 1000 |    86431.00 ticks | 86.43 ticks | 24.7 ns
PoolURBG<uint32_t,8> | 10   | 1000 |   206191.00 ticks | 20.62 ticks |  5.9 ns
PoolURBG<uint32_t,8> | 100  | 1000 |  1516049.00 ticks | 15.16 ticks |  4.3 ns
PoolURBG<uint32_t,8> | 1000 |  500 |  7613936.00 ticks | 15.23 ticks |  4.4 ns

PoolURBG<uint64_t,4> | 1    | 1000 |    96668.00 ticks | 96.67 ticks | 27.6 ns
PoolURBG<uint64_t,4> | 10   | 1000 |   282423.00 ticks | 28.24 ticks |  8.1 ns
PoolURBG<uint64_t,4> | 100  | 1000 |  2609587.00 ticks | 26.10 ticks |  7.5 ns
PoolURBG<uint64_t,4> | 1000 |  500 | 12408757.00 ticks | 24.82 ticks |  7.1 ns

*/