// 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_FEEFRAC_H

#define BITCOIN_UTIL_FEEFRAC_H

#include <stdint.h>

#include <compare>

#include <vector>

#include <span.h>

#include <util/check.h>

/** Data structure storing a fee and size, ordered by increasing fee/size.

 *

 * The size of a FeeFrac cannot be zero unless the fee is also zero.

 *

 * FeeFracs have a total ordering, first by increasing feerate (ratio of fee over size), and then

 * by decreasing size. The empty FeeFrac (fee and size both 0) sorts last. So for example, the

 * following FeeFracs are in sorted order:

 *

 * - fee=0 size=1 (feerate 0)

 * - fee=1 size=2 (feerate 0.5)

 * - fee=2 size=3 (feerate 0.667...)

 * - fee=2 size=2 (feerate 1)

 * - fee=1 size=1 (feerate 1)

 * - fee=3 size=2 (feerate 1.5)

 * - fee=2 size=1 (feerate 2)

 * - fee=0 size=0 (undefined feerate)

 *

 * A FeeFrac is considered "better" if it sorts after another, by this ordering. All standard

 * comparison operators (<=>, ==, !=, >, <, >=, <=) respect this ordering.

 *

 * The FeeRateCompare, and >> and << operators only compare feerate and treat equal feerate but

 * different size as equivalent. The empty FeeFrac is neither lower or higher in feerate than any

 * other.

 */

struct FeeFrac

{

    /** Fallback version for Mul (see below).

     *

     * Separate to permit testing on platforms where it isn't actually needed.

     */

    static inline std::pair<int64_t, uint32_t> MulFallback(int64_t a, int32_t b) noexcept

    {

        // Otherwise, emulate 96-bit multiplication using two 64-bit multiplies.

        int64_t low = int64_t{static_cast<uint32_t>(a)} * b;

        int64_t high = (a >> 32) * b;

        return {high + (low >> 32), static_cast<uint32_t>(low)};

    }

    // Compute a * b, returning an unspecified but totally ordered type.

#ifdef __SIZEOF_INT128__

    static inline __int128 Mul(int64_t a, int32_t b) noexcept

    {

        // If __int128 is available, use 128-bit wide multiply.

        return __int128{a} * b;

    }

#else

    static constexpr auto Mul = MulFallback;

#endif

    int64_t fee;

    int32_t size;

    /** Construct an IsEmpty() FeeFrac. */

    constexpr inline FeeFrac() noexcept : fee{0}, size{0} {}

    /** Construct a FeeFrac with specified fee and size. */

    constexpr inline FeeFrac(int64_t f, int32_t s) noexcept : fee{f}, size{s} {}

    constexpr inline FeeFrac(const FeeFrac&) noexcept = default;

    constexpr inline FeeFrac& operator=(const FeeFrac&) noexcept = default;

    /** Check if this is empty (size and fee are 0). */

    bool inline IsEmpty() const noexcept {

        return size == 0;

    }

    /** Add fee and size of another FeeFrac to this one. */

    void inline operator+=(const FeeFrac& other) noexcept

    {

        fee += other.fee;

        size += other.size;

    }

    /** Subtract fee and size of another FeeFrac from this one. */

    void inline operator-=(const FeeFrac& other) noexcept

    {

        fee -= other.fee;

        size -= other.size;

    }

    /** Sum fee and size. */

    friend inline FeeFrac operator+(const FeeFrac& a, const FeeFrac& b) noexcept

    {

        return {a.fee + b.fee, a.size + b.size};

    }

    /** Subtract both fee and size. */

    friend inline FeeFrac operator-(const FeeFrac& a, const FeeFrac& b) noexcept

    {

        return {a.fee - b.fee, a.size - b.size};

    }

    /** Check if two FeeFrac objects are equal (both same fee and same size). */

    friend inline bool operator==(const FeeFrac& a, const FeeFrac& b) noexcept

    {

        return a.fee == b.fee && a.size == b.size;

    }

    /** Compare two FeeFracs just by feerate. */

    friend inline std::weak_ordering FeeRateCompare(const FeeFrac& a, const FeeFrac& b) noexcept

    {

        auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);

        return cross_a <=> cross_b;

    }

    /** Check if a FeeFrac object has strictly lower feerate than another. */

    friend inline bool operator<<(const FeeFrac& a, const FeeFrac& b) noexcept

    {

        auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);

        return cross_a < cross_b;

    }

    /** Check if a FeeFrac object has strictly higher feerate than another. */

    friend inline bool operator>>(const FeeFrac& a, const FeeFrac& b) noexcept

    {

        auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);

        return cross_a > cross_b;

    }

    /** Compare two FeeFracs. <, >, <=, and >= are auto-generated from this. */

    friend inline std::strong_ordering operator<=>(const FeeFrac& a, const FeeFrac& b) noexcept

    {

        auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);

        if (cross_a == cross_b) return b.size <=> a.size;

        return cross_a <=> cross_b;

    }

    /** Swap two FeeFracs. */

    friend inline void swap(FeeFrac& a, FeeFrac& b) noexcept

    {

        std::swap(a.fee, b.fee);

        std::swap(a.size, b.size);

    }

};

/** Compare the feerate diagrams implied by the provided sorted chunks data.

 *

 * The implied diagram for each starts at (0, 0), then contains for each chunk the cumulative fee

 * and size up to that chunk, and then extends infinitely to the right with a horizontal line.

 *

 * The caller must guarantee that the sum of the FeeFracs in either of the chunks' data set do not

 * overflow (so sum fees < 2^63, and sum sizes < 2^31).

 */

std::partial_ordering CompareChunks(Span<const FeeFrac> chunks0, Span<const FeeFrac> chunks1);

#endif // BITCOIN_UTIL_FEEFRAC_H