// Licensed to the Apache Software Foundation (ASF) under one

// or more contributor license agreements.  See the NOTICE file

// distributed with this work for additional information

// regarding copyright ownership.  The ASF licenses this file

// to you 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 <stdint.h>

namespace doris {

// A very simple random number generator.  Not especially good at

// generating truly random bits, but good enough for our needs in this

// package.

class Random {

private:

    uint32_t seed_;

public:

    explicit Random(uint32_t s) : seed_(s & 0x7fffffffu) {

        // Avoid bad seeds.

        if (seed_ == 0 || seed_ == 2147483647L) {

            seed_ = 1;

        }

    }

    uint32_t Next() {

        static const uint32_t M = 2147483647L; // 2^31-1

        static const uint64_t A = 16807;       // bits 14, 8, 7, 5, 2, 1, 0

        // We are computing

        //       seed_ = (seed_ * A) % M,    where M = 2^31-1

        //

        // seed_ must not be zero or M, or else all subsequent computed values

        // will be zero or M respectively.  For all other values, seed_ will end

        // up cycling through every number in [1,M-1]

        uint64_t product = seed_ * A;

        // Compute (product % M) using the fact that ((x << 31) % M) == x.

        seed_ = static_cast<uint32_t>((product >> 31) + (product & M));

        // The first reduction may overflow by 1 bit, so we may need to

        // repeat.  mod == M is not possible; using > allows the faster

        // sign-bit-based test.

        if (seed_ > M) {

            seed_ -= M;

        }

        return seed_;

    }

    // Returns a uniformly distributed value in the range [0..n-1]

    // REQUIRES: n > 0

    uint32_t Uniform(int n) { return Next() % n; }

    // Randomly returns true ~"1/n" of the time, and false otherwise.

    // REQUIRES: n > 0

    bool OneIn(int n) { return (Next() % n) == 0; }

    // Skewed: pick "base" uniformly from range [0,max_log] and then

    // return "base" random bits.  The effect is to pick a number in the

    // range [0,2^max_log-1] with exponential bias towards smaller numbers.

    uint32_t Skewed(int max_log) { return Uniform(1 << Uniform(max_log + 1)); }

};

} // namespace doris