/*
 * DeterministicRandom.cpp
 *
 * This source file is part of the FoundationDB open source project
 *
 * Copyright 2013-2026 Apple Inc. and the FoundationDB project 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
 *
 *     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.
 */

#include "fmt/format.h"
#include "flow/Arena.h"
#include "flow/DeterministicRandom.h"
#include "flow/UnitTest.h"

#include <cstring>

uint64_t DeterministicRandom::gen64() {
	uint64_t curr = next;

	// RE: the previous implementation of this function: order of
	// evaluation of arguments to the ^ operator is not specified, so
	// two rng() calls in the same ^ expression may not produce a
	// consistent 64-bit value across compilations, because we don't
	// know if the first call was used for the higher order bits and
	// the second call for the low order bits, or vice-versa.
	// See https://en.cppreference.com/w/cpp/language/eval_order.html
	next = (uint64_t(rng()) << 32);
	next ^= rng();
	if (TRACE_SAMPLE())
		TraceEvent(SevSample, "Random").log();
	return curr;
}

DeterministicRandom::DeterministicRandom(uint32_t seed, bool useRandLog)
  : rng((unsigned long)seed), next(0), useRandLog(useRandLog) {
	next = (uint64_t(rng()) << 32);
	next ^= rng();
}

double DeterministicRandom::random01() {
	double d = gen64() / double(uint64_t(-1));
	if (randLog && useRandLog)
		fprintf(randLog, "R01  %f\n", d);
	return d;
}

int DeterministicRandom::randomInt(int min, int maxPlusOne) {
	ASSERT_LT(min, maxPlusOne);
	unsigned int range;
	if (maxPlusOne < 0)
		range = std::abs(maxPlusOne - min);
	else {
		range = maxPlusOne;
		range -= min;
	}
	uint64_t v = (gen64() % range);
	int i;
	if (min < 0 && (-static_cast<unsigned int>(min + 1)) >= v)
		i = -static_cast<int>(-static_cast<unsigned int>(min + 1) - v) - 1;
	else
		i = v + min;
	if (randLog && useRandLog)
		fprintf(randLog, "Rint %d\n", i);
	return i;
}

int64_t DeterministicRandom::randomInt64(int64_t min, int64_t maxPlusOne) {
	ASSERT_LT(min, maxPlusOne);
	uint64_t range;
	if (maxPlusOne < 0)
		range = std::abs(maxPlusOne - min);
	else {
		range = maxPlusOne;
		range -= min;
	}
	uint64_t v = (gen64() % range);
	int64_t i;
	if (min < 0 && (-static_cast<uint64_t>(min + 1)) >= v)
		i = -static_cast<int64_t>(-static_cast<uint64_t>(min + 1) - v) - 1;
	else
		i = v + min;
	if (randLog && useRandLog)
		fmt::print(randLog, "Rint64 {}\n", i);
	return i;
}

uint32_t DeterministicRandom::randomUInt32() {
	return gen64();
}

uint64_t DeterministicRandom::randomUInt64() {
	return gen64();
}

uint32_t DeterministicRandom::randomSkewedUInt32(uint32_t min, uint32_t maxPlusOne) {
	ASSERT_LT(min, maxPlusOne);
	std::uniform_real_distribution<double> distribution(std::log(std::max<double>(min, 1.0 / M_E)),
	                                                    std::log(maxPlusOne));
	double exponent = distribution(rng);
	uint32_t value = static_cast<uint32_t>(std::pow(M_E, exponent));
	return std::max(std::min(value, maxPlusOne - 1), min);
}

UID DeterministicRandom::randomUniqueID() {
	uint64_t x, y;
	x = gen64();
	y = gen64();
	if (randLog && useRandLog)
		fmt::print(randLog, "Ruid {0} {1}\n", x, y);
	return UID(x, y);
}

char DeterministicRandom::randomAlphaNumeric() {
	static const char alphanum[] = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
	char c = alphanum[gen64() % 62];
	if (randLog && useRandLog)
		fprintf(randLog, "Rchar %c\n", c);
	return c;
}

std::string DeterministicRandom::randomAlphaNumeric(int length) {
	std::string s;
	s.reserve(length);
	for (int i = 0; i < length; i++)
		s += randomAlphaNumeric();
	return s;
}

void DeterministicRandom::randomBytes(uint8_t* buf, int length) {
	constexpr const int unitLen = sizeof(decltype(gen64()));
	for (int i = 0; i < length; i += unitLen) {
		auto val = gen64();
		memcpy(buf + i, &val, std::min(unitLen, length - i));
	}
	if (randLog && useRandLog) {
		constexpr const int cutOff = 32;
		bool tooLong = length > cutOff;
		fmt::print(randLog,
		           "Rbytes[{}] {}{}\n",
		           length,
		           StringRef(buf, std::min(cutOff, length)).printable(),
		           tooLong ? "..." : "");
	}
}

bool DeterministicRandom::truePercent(const int percent) {
	ASSERT_GT(percent, 0);
	ASSERT_LT(percent, 100);
	return this->randomInt(1, 101) <= percent;
}

uint64_t DeterministicRandom::peek() const {
	return next;
}

void DeterministicRandom::addref() {
	ReferenceCounted<DeterministicRandom>::addref();
}
void DeterministicRandom::delref() {
	ReferenceCounted<DeterministicRandom>::delref();
}

// The nature of IRandom::truePercent API is that the output is random (still deterministic).
// Testing randomness is tricky because there's no fixed value one can assert on.
// For example, if truePercent(1%) is called 10,000 times, it's not always true that exactly
// 100 times the function will return true.
// This test solves such problems in two ways:
//       1. It models the output of truePercent as a binomial distribution, and asserts that output is within a range.
//          The range is decided based on standard deviations, typically 3 standard deviations are used.
//       2. For each part of the test, a fixed seed is picked. This means that if one run passes, other runs should pass
//          as well. We still need #1 above because rng implementation or platforms can change over time.
// A more detailed discussion and math of this can be found here:
//     https://github.com/apple/foundationdb/pull/12440/files#r2430230239
TEST_CASE("/flow/DeterministicRandom/truePercent") {
	constexpr int trials = 10000;

	{
		// Test with a fixed seed for reproducibility
		DeterministicRandom rng(12345);

		// Test 1% probability - should be rare
		int trueCount1 = 0;
		for (int i = 0; i < trials; i++) {
			if (rng.truePercent(1)) {
				trueCount1++;
			}
		}

		// With 1% probability, expect around 100 true out of 10000
		// Allow for some variance (between 70-130 for 99% confidence)
		ASSERT(trueCount1 >= 70 && trueCount1 <= 130);
	}

	{
		// Test with a fixed seed for reproducibility
		DeterministicRandom rng(54321);

		// Test 50% probability - should be roughly half
		int trueCount50 = 0;
		for (int i = 0; i < trials; i++) {
			if (rng.truePercent(50)) {
				trueCount50++;
			}
		}

		// With 50% probability, expect around 5000 true out of 10000
		// Allow for variance (between 4800-5200 for 99% confidence)
		ASSERT(trueCount50 >= 4800 && trueCount50 <= 5200);
	}

	{
		// Test with a fixed seed for reproducibility
		DeterministicRandom rng(99999);

		// Test 99% probability - should be almost always true
		int trueCount99 = 0;
		for (int i = 0; i < trials; i++) {
			if (rng.truePercent(99)) {
				trueCount99++;
			}
		}

		// With 99% probability, expect around 9900 true out of 10000
		// Allow for variance (between 9870-9930 for 99% confidence)
		ASSERT(trueCount99 >= 9870 && trueCount99 <= 9930);
	}

	{
		// Test determinism - same seed should produce same results
		DeterministicRandom rng1(7777);
		DeterministicRandom rng2(7777);
		for (int i = 0; i < 100; i++) {
			ASSERT(rng1.truePercent(75) == rng2.truePercent(75));
		}
	}

	{
		// Test with a fixed seed for reproducibility
		DeterministicRandom rng(8888);

		// Test different percentages produce expected ordering
		int count10 = 0, count30 = 0, count70 = 0, count90 = 0;
		for (int i = 0; i < trials; i++) {
			DeterministicRandom rngTemp(8888 + i); // Different seed for each trial
			if (rngTemp.truePercent(10))
				count10++;
			if (rngTemp.truePercent(30))
				count30++;
			if (rngTemp.truePercent(70))
				count70++;
			if (rngTemp.truePercent(90))
				count90++;
		}

		// Verify ordering: count10 < count30 < count70 < count90
		ASSERT(count10 < count30);
		ASSERT(count30 < count70);
		ASSERT(count70 < count90);
	}

	return Void();
}