// Copyright (c) The Bitcoin Core developers

// Distributed under the MIT software license, see the accompanying

// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#ifndef BITCOIN_UTIL_TOKENBUCKET_H

#define BITCOIN_UTIL_TOKENBUCKET_H

#include <util/time.h>

namespace util {

/** A token bucket rate limiter.

 *

 * Tokens are added at a steady rate (m_rate per second) up to a capacity

 * cap (m_cap). Tokens are removed by calling decrement(), which returns

 * false if the bucket is emptied.

 *

 * Typical usage:

 *   bucket.increment(now);   // refill based on elapsed time

 *   if (bucket.value() >= 1) bucket.decrement(1);  // consume a token

 */

template <typename Clock>

class TokenBucket

{

public:

    using clock = Clock;

    using time_point = typename Clock::time_point;

    using duration = typename Clock::duration;

    const double m_rate{1};     //!< Tokens added per second

    const double m_cap{0};      //!< Maximum token balance

    /** @param rate   Tokens added per second.

     *  @param value  Initial token balance (clamped to cap).

     *  @param cap    Maximum token balance. */

    TokenBucket(double rate, double value, double cap) : m_rate{rate}, m_cap{cap}, m_value{std::min(value, cap)} {}

    /** Refill tokens based on elapsed time since last call. No refill

     *  occurs on the first call (establishes the time baseline). */

    void increment(const time_point& now)

    {

        if (now > m_last_updated) {

            if (m_value < m_cap && m_last_updated > MIN_TIME) {

                double inc = m_rate * std::chrono::duration_cast<SecondsDouble>(now - m_last_updated).count();

                m_value = std::min(m_cap, m_value + inc);

            }

        }

        m_last_updated = now;

    }

    /** Consume n tokens. Returns false if the balance dropped to/below the given floor. */

    bool decrement(double n = 1.0, double floor = 0.0)

    {

        m_value -= n;

        return (m_value > floor);

    }

    /** Current token balance. */

    double value() const { return m_value; }

private:

    static constexpr time_point MIN_TIME{time_point::min()};

    time_point m_last_updated{MIN_TIME};

    double m_value{0};

};

} // namespace util

#endif // BITCOIN_UTIL_TOKENBUCKET_H