1  // Copyright (c) The Bitcoin Core developers
  2  // Distributed under the MIT software license, see the accompanying
  3  // file COPYING or http://www.opensource.org/licenses/mit-license.php.
  4  
  5  #ifndef BITCOIN_UTIL_FEEFRAC_H
  6  #define BITCOIN_UTIL_FEEFRAC_H
  7  
  8  #include <stdint.h>
  9  #include <compare>
 10  #include <vector>
 11  #include <span.h>
 12  #include <util/check.h>
 13  
 14  /** Data structure storing a fee and size, ordered by increasing fee/size.
 15   *
 16   * The size of a FeeFrac cannot be zero unless the fee is also zero.
 17   *
 18   * FeeFracs have a total ordering, first by increasing feerate (ratio of fee over size), and then
 19   * by decreasing size. The empty FeeFrac (fee and size both 0) sorts last. So for example, the
 20   * following FeeFracs are in sorted order:
 21   *
 22   * - fee=0 size=1 (feerate 0)
 23   * - fee=1 size=2 (feerate 0.5)
 24   * - fee=2 size=3 (feerate 0.667...)
 25   * - fee=2 size=2 (feerate 1)
 26   * - fee=1 size=1 (feerate 1)
 27   * - fee=3 size=2 (feerate 1.5)
 28   * - fee=2 size=1 (feerate 2)
 29   * - fee=0 size=0 (undefined feerate)
 30   *
 31   * A FeeFrac is considered "better" if it sorts after another, by this ordering. All standard
 32   * comparison operators (<=>, ==, !=, >, <, >=, <=) respect this ordering.
 33   *
 34   * The FeeRateCompare, and >> and << operators only compare feerate and treat equal feerate but
 35   * different size as equivalent. The empty FeeFrac is neither lower or higher in feerate than any
 36   * other.
 37   */
 38  struct FeeFrac
 39  {
 40      /** Helper function for 32*64 signed multiplication, returning an unspecified but totally
 41       *  ordered type. This is a fallback version, separate so it can be tested on platforms where
 42       *  it isn't actually needed. */
 43      static inline std::pair<int64_t, uint32_t> MulFallback(int64_t a, int32_t b) noexcept
 44      {
 45          int64_t low = int64_t{static_cast<uint32_t>(a)} * b;
 46          int64_t high = (a >> 32) * b;
 47          return {high + (low >> 32), static_cast<uint32_t>(low)};
 48      }
 49  
 50      /** Helper function for 96/32 signed division, rounding towards negative infinity (if
 51       *  round_down) or positive infinity (if !round_down). This is a fallback version, separate so
 52       *  that it can be tested on platforms where it isn't actually needed.
 53       *
 54       * The exact behavior with negative n does not really matter, but this implementation chooses
 55       * to be consistent for testability reasons.
 56       *
 57       * The result must fit in an int64_t, and d must be strictly positive. */
 58      static inline int64_t DivFallback(std::pair<int64_t, uint32_t> n, int32_t d, bool round_down) noexcept
 59      {
 60          Assume(d > 0);
 61          // Compute quot_high = n.first / d, so the result becomes
 62          // (n.second + (n.first - quot_high * d) * 2**32) / d + (quot_high * 2**32), or
 63          // (n.second + (n.first % d) * 2**32) / d + (quot_high * 2**32).
 64          int64_t quot_high = n.first / d;
 65          // Evaluate the parenthesized expression above, so the result becomes
 66          // n_low / d + (quot_high * 2**32)
 67          int64_t n_low = ((n.first % d) << 32) + n.second;
 68          // Evaluate the division so the result becomes quot_low + quot_high * 2**32. It is possible
 69          // that the / operator here rounds in the wrong direction (if n_low is not a multiple of
 70          // size, and is (if round_down) negative, or (if !round_down) positive). If so, make a
 71          // correction.
 72          int64_t quot_low = n_low / d;
 73          int32_t mod_low = n_low % d;
 74          quot_low += (mod_low > 0) - (mod_low && round_down);
 75          // Combine and return the result
 76          return (quot_high << 32) + quot_low;
 77      }
 78  
 79  #ifdef __SIZEOF_INT128__
 80      /** Helper function for 32*64 signed multiplication, returning an unspecified but totally
 81       *  ordered type. This is a version relying on __int128. */
 82      static inline __int128 Mul(int64_t a, int32_t b) noexcept
 83      {
 84          return __int128{a} * b;
 85      }
 86  
 87      /** Helper function for 96/32 signed division, rounding towards negative infinity (if
 88       *  round_down), or towards positive infinity (if !round_down). This is a
 89       *  version relying on __int128.
 90       *
 91       * The result must fit in an int64_t, and d must be strictly positive. */
 92      static inline int64_t Div(__int128 n, int32_t d, bool round_down) noexcept
 93      {
 94          Assume(d > 0);
 95          // Compute the division.
 96          int64_t quot = n / d;
 97          int32_t mod = n % d;
 98          // Correct result if the / operator above rounded in the wrong direction.
 99          return quot + ((mod > 0) - (mod && round_down));
100      }
101  #else
102      static constexpr auto Mul = MulFallback;
103      static constexpr auto Div = DivFallback;
104  #endif
105  
106      int64_t fee;
107      int32_t size;
108  
109      /** Construct an IsEmpty() FeeFrac. */
110      constexpr inline FeeFrac() noexcept : fee{0}, size{0} {}
111  
112      /** Construct a FeeFrac with specified fee and size. */
113      constexpr inline FeeFrac(int64_t f, int32_t s) noexcept : fee{f}, size{s} {}
114  
115      constexpr inline FeeFrac(const FeeFrac&) noexcept = default;
116      constexpr inline FeeFrac& operator=(const FeeFrac&) noexcept = default;
117  
118      /** Check if this is empty (size and fee are 0). */
119      bool inline IsEmpty() const noexcept {
120          return size == 0;
121      }
122  
123      /** Add fee and size of another FeeFrac to this one. */
124      void inline operator+=(const FeeFrac& other) noexcept
125      {
126          fee += other.fee;
127          size += other.size;
128      }
129  
130      /** Subtract fee and size of another FeeFrac from this one. */
131      void inline operator-=(const FeeFrac& other) noexcept
132      {
133          fee -= other.fee;
134          size -= other.size;
135      }
136  
137      /** Sum fee and size. */
138      friend inline FeeFrac operator+(const FeeFrac& a, const FeeFrac& b) noexcept
139      {
140          return {a.fee + b.fee, a.size + b.size};
141      }
142  
143      /** Subtract both fee and size. */
144      friend inline FeeFrac operator-(const FeeFrac& a, const FeeFrac& b) noexcept
145      {
146          return {a.fee - b.fee, a.size - b.size};
147      }
148  
149      /** Check if two FeeFrac objects are equal (both same fee and same size). */
150      friend inline bool operator==(const FeeFrac& a, const FeeFrac& b) noexcept
151      {
152          return a.fee == b.fee && a.size == b.size;
153      }
154  
155      /** Compare two FeeFracs just by feerate. */
156      friend inline std::weak_ordering FeeRateCompare(const FeeFrac& a, const FeeFrac& b) noexcept
157      {
158          auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);
159          return cross_a <=> cross_b;
160      }
161  
162      /** Check if a FeeFrac object has strictly lower feerate than another. */
163      friend inline bool operator<<(const FeeFrac& a, const FeeFrac& b) noexcept
164      {
165          auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);
166          return cross_a < cross_b;
167      }
168  
169      /** Check if a FeeFrac object has strictly higher feerate than another. */
170      friend inline bool operator>>(const FeeFrac& a, const FeeFrac& b) noexcept
171      {
172          auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);
173          return cross_a > cross_b;
174      }
175  
176      /** Compare two FeeFracs. <, >, <=, and >= are auto-generated from this. */
177      friend inline std::strong_ordering operator<=>(const FeeFrac& a, const FeeFrac& b) noexcept
178      {
179          auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);
180          if (cross_a == cross_b) return b.size <=> a.size;
181          return cross_a <=> cross_b;
182      }
183  
184      /** Swap two FeeFracs. */
185      friend inline void swap(FeeFrac& a, FeeFrac& b) noexcept
186      {
187          std::swap(a.fee, b.fee);
188          std::swap(a.size, b.size);
189      }
190  
191      /** Compute the fee for a given size `at_size` using this object's feerate.
192       *
193       * This effectively corresponds to evaluating (this->fee * at_size) / this->size, with the
194       * result rounded towards negative infinity (if RoundDown) or towards positive infinity
195       * (if !RoundDown).
196       *
197       * Requires this->size > 0, at_size >= 0, and that the correct result fits in a int64_t. This
198       * is guaranteed to be the case when 0 <= at_size <= this->size.
199       */
200      template<bool RoundDown>
201      int64_t EvaluateFee(int32_t at_size) const noexcept
202      {
203          Assume(size > 0);
204          Assume(at_size >= 0);
205          if (fee >= 0 && fee < 0x200000000) [[likely]] {
206              // Common case where (this->fee * at_size) is guaranteed to fit in a uint64_t.
207              if constexpr (RoundDown) {
208                  return (uint64_t(fee) * at_size) / uint32_t(size);
209              } else {
210                  return (uint64_t(fee) * at_size + size - 1U) / uint32_t(size);
211              }
212          } else {
213              // Otherwise, use Mul and Div.
214              return Div(Mul(fee, at_size), size, RoundDown);
215          }
216      }
217  
218  public:
219      /** Compute the fee for a given size `at_size` using this object's feerate, rounding down. */
220      int64_t EvaluateFeeDown(int32_t at_size) const noexcept { return EvaluateFee<true>(at_size); }
221      /** Compute the fee for a given size `at_size` using this object's feerate, rounding up. */
222      int64_t EvaluateFeeUp(int32_t at_size) const noexcept { return EvaluateFee<false>(at_size); }
223  };
224  
225  /** Compare the feerate diagrams implied by the provided sorted chunks data.
226   *
227   * The implied diagram for each starts at (0, 0), then contains for each chunk the cumulative fee
228   * and size up to that chunk, and then extends infinitely to the right with a horizontal line.
229   *
230   * The caller must guarantee that the sum of the FeeFracs in either of the chunks' data set do not
231   * overflow (so sum fees < 2^63, and sum sizes < 2^31).
232   */
233  std::partial_ordering CompareChunks(std::span<const FeeFrac> chunks0, std::span<const FeeFrac> chunks1);
234  
235  /** Tagged wrapper around FeeFrac to avoid unit confusion. */
236  template<typename Tag>
237  struct FeePerUnit : public FeeFrac
238  {
239      // Inherit FeeFrac constructors.
240      using FeeFrac::FeeFrac;
241  
242      /** Convert a FeeFrac to a FeePerUnit. */
243      static FeePerUnit FromFeeFrac(const FeeFrac& feefrac) noexcept
244      {
245          return {feefrac.fee, feefrac.size};
246      }
247  };
248  
249  // FeePerUnit instance for satoshi / vbyte.
250  struct VSizeTag {};
251  using FeePerVSize = FeePerUnit<VSizeTag>;
252  
253  // FeePerUnit instance for satoshi / WU.
254  struct WeightTag {};
255  using FeePerWeight = FeePerUnit<WeightTag>;
256  
257  #endif // BITCOIN_UTIL_FEEFRAC_H