Add 'third_party/abseil_cpp/' from commit '768eb2ca28'

git-subtree-dir: third_party/abseil_cpp
git-subtree-mainline: ffb2ae54be
git-subtree-split: 768eb2ca28

third_party/abseil_cpp/absl/random/examples_test.cc

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+// 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 <cinttypes>
+#include <random>
+#include <sstream>
+#include <vector>
+
+#include "gtest/gtest.h"
+#include "absl/random/random.h"
+
+template <typename T>
+void Use(T) {}
+
+TEST(Examples, Basic) {
+  absl::BitGen gen;
+  std::vector<int> objs = {10, 20, 30, 40, 50};
+
+  // Choose an element from a set.
+  auto elem = objs[absl::Uniform(gen, 0u, objs.size())];
+  Use(elem);
+
+  // Generate a uniform value between 1 and 6.
+  auto dice_roll = absl::Uniform<int>(absl::IntervalClosedClosed, gen, 1, 6);
+  Use(dice_roll);
+
+  // Generate a random byte.
+  auto byte = absl::Uniform<uint8_t>(gen);
+  Use(byte);
+
+  // Generate a fractional value from [0f, 1f).
+  auto fraction = absl::Uniform<float>(gen, 0, 1);
+  Use(fraction);
+
+  // Toss a fair coin; 50/50 probability.
+  bool coin_toss = absl::Bernoulli(gen, 0.5);
+  Use(coin_toss);
+
+  // Select a file size between 1k and 10MB, biased towards smaller file sizes.
+  auto file_size = absl::LogUniform<size_t>(gen, 1000, 10 * 1000 * 1000);
+  Use(file_size);
+
+  // Randomize (shuffle) a collection.
+  std::shuffle(std::begin(objs), std::end(objs), gen);
+}
+
+TEST(Examples, CreateingCorrelatedVariateSequences) {
+  // Unexpected PRNG correlation is often a source of bugs,
+  // so when using absl::BitGen it must be an intentional choice.
+  // NOTE: All of these only exhibit process-level stability.
+
+  // Create a correlated sequence from system entropy.
+  {
+    auto my_seed = absl::MakeSeedSeq();
+
+    absl::BitGen gen_1(my_seed);
+    absl::BitGen gen_2(my_seed);  // Produces same variates as gen_1.
+
+    EXPECT_EQ(absl::Bernoulli(gen_1, 0.5), absl::Bernoulli(gen_2, 0.5));
+    EXPECT_EQ(absl::Uniform<uint32_t>(gen_1), absl::Uniform<uint32_t>(gen_2));
+  }
+
+  // Create a correlated sequence from an existing URBG.
+  {
+    absl::BitGen gen;
+
+    auto my_seed = absl::CreateSeedSeqFrom(&gen);
+    absl::BitGen gen_1(my_seed);
+    absl::BitGen gen_2(my_seed);
+
+    EXPECT_EQ(absl::Bernoulli(gen_1, 0.5), absl::Bernoulli(gen_2, 0.5));
+    EXPECT_EQ(absl::Uniform<uint32_t>(gen_1), absl::Uniform<uint32_t>(gen_2));
+  }
+
+  // An alternate construction which uses user-supplied data
+  // instead of a random seed.
+  {
+    const char kData[] = "A simple seed string";
+    std::seed_seq my_seed(std::begin(kData), std::end(kData));
+
+    absl::BitGen gen_1(my_seed);
+    absl::BitGen gen_2(my_seed);
+
+    EXPECT_EQ(absl::Bernoulli(gen_1, 0.5), absl::Bernoulli(gen_2, 0.5));
+    EXPECT_EQ(absl::Uniform<uint32_t>(gen_1), absl::Uniform<uint32_t>(gen_2));
+  }
+}
+