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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 <span.h>
9 : : #include <util/check.h>
10 : :
11 : : #include <compare>
12 : : #include <cstdint>
13 : : #include <vector>
14 : :
15 : : /** Data structure storing a fee and size, ordered by increasing fee/size.
16 : : *
17 : : * The size of a FeeFrac cannot be zero unless the fee is also zero.
18 : : *
19 : : * FeeFracs have a total ordering, first by increasing feerate (ratio of fee over size), and then
20 : : * by decreasing size. The empty FeeFrac (fee and size both 0) sorts last. So for example, the
21 : : * following FeeFracs are in sorted order:
22 : : *
23 : : * - fee=0 size=1 (feerate 0)
24 : : * - fee=1 size=2 (feerate 0.5)
25 : : * - fee=2 size=3 (feerate 0.667...)
26 : : * - fee=2 size=2 (feerate 1)
27 : : * - fee=1 size=1 (feerate 1)
28 : : * - fee=3 size=2 (feerate 1.5)
29 : : * - fee=2 size=1 (feerate 2)
30 : : * - fee=0 size=0 (undefined feerate)
31 : : *
32 : : * A FeeFrac is considered "better" if it sorts after another, by this ordering. All standard
33 : : * comparison operators (<=>, ==, !=, >, <, >=, <=) respect this ordering.
34 : : *
35 : : * The FeeRateCompare, and >> and << operators only compare feerate and treat equal feerate but
36 : : * different size as equivalent. The empty FeeFrac is neither lower or higher in feerate than any
37 : : * other.
38 : : */
39 : : struct FeeFrac
40 : : {
41 : : /** Helper function for 32*64 signed multiplication, returning an unspecified but totally
42 : : * ordered type. This is a fallback version, separate so it can be tested on platforms where
43 : : * it isn't actually needed. */
44 : 101 : static inline std::pair<int64_t, uint32_t> MulFallback(int64_t a, int32_t b) noexcept
45 : : {
46 : 101 : int64_t low = int64_t{static_cast<uint32_t>(a)} * b;
47 : 101 : int64_t high = (a >> 32) * b;
48 [ - + ]: 101 : return {high + (low >> 32), static_cast<uint32_t>(low)};
49 : : }
50 : :
51 : : /** Helper function for 96/32 signed division, rounding towards negative infinity (if
52 : : * round_down) or positive infinity (if !round_down). This is a fallback version, separate so
53 : : * that it can be tested on platforms where it isn't actually needed.
54 : : *
55 : : * The exact behavior with negative n does not really matter, but this implementation chooses
56 : : * to be consistent for testability reasons.
57 : : *
58 : : * The result must fit in an int64_t, and d must be strictly positive. */
59 : 83 : static inline int64_t DivFallback(std::pair<int64_t, uint32_t> n, int32_t d, bool round_down) noexcept
60 : : {
61 : 83 : Assume(d > 0);
62 : : // Compute quot_high = n.first / d, so the result becomes
63 : : // (n.second + (n.first - quot_high * d) * 2**32) / d + (quot_high * 2**32), or
64 : : // (n.second + (n.first % d) * 2**32) / d + (quot_high * 2**32).
65 : 83 : int64_t quot_high = n.first / d;
66 : : // Evaluate the parenthesized expression above, so the result becomes
67 : : // n_low / d + (quot_high * 2**32)
68 : 83 : int64_t n_low = ((n.first % d) << 32) + n.second;
69 : : // Evaluate the division so the result becomes quot_low + quot_high * 2**32. It is possible
70 : : // that the / operator here rounds in the wrong direction (if n_low is not a multiple of
71 : : // size, and is (if round_down) negative, or (if !round_down) positive). If so, make a
72 : : // correction.
73 : 83 : int64_t quot_low = n_low / d;
74 : 83 : int32_t mod_low = n_low % d;
75 : 83 : quot_low += (mod_low > 0) - (mod_low && round_down);
76 : : // Combine and return the result
77 : 83 : return (quot_high << 32) + quot_low;
78 : : }
79 : :
80 : : #ifdef __SIZEOF_INT128__
81 : : /** Helper function for 32*64 signed multiplication, returning an unspecified but totally
82 : : * ordered type. This is a version relying on __int128. */
83 : 146312000 : static inline __int128 Mul(int64_t a, int32_t b) noexcept
84 : : {
85 [ + + ]: 144967941 : return __int128{a} * b;
86 : : }
87 : :
88 : : /** Helper function for 96/32 signed division, rounding towards negative infinity (if
89 : : * round_down), or towards positive infinity (if !round_down). This is a
90 : : * version relying on __int128.
91 : : *
92 : : * The result must fit in an int64_t, and d must be strictly positive. */
93 : 76 : static inline int64_t Div(__int128 n, int32_t d, bool round_down) noexcept
94 : : {
95 : 76 : Assume(d > 0);
96 : : // Compute the division.
97 : 76 : int64_t quot = n / d;
98 : 76 : int32_t mod = n % d;
99 : : // Correct result if the / operator above rounded in the wrong direction.
100 : 76 : return quot + ((mod > 0) - (mod && round_down));
101 : : }
102 : : #else
103 : : static constexpr auto Mul = MulFallback;
104 : : static constexpr auto Div = DivFallback;
105 : : #endif
106 : :
107 : : int64_t fee;
108 : : int32_t size;
109 : :
110 : : /** Construct an IsEmpty() FeeFrac. */
111 : 346375 : constexpr inline FeeFrac() noexcept : fee{0}, size{0} {}
112 : :
113 : : /** Construct a FeeFrac with specified fee and size. */
114 [ + + ]: 102453933 : constexpr inline FeeFrac(int64_t f, int32_t s) noexcept : fee{f}, size{s} {}
[ + + + + ]
115 : :
116 : : constexpr inline FeeFrac(const FeeFrac&) noexcept = default;
117 : : constexpr inline FeeFrac& operator=(const FeeFrac&) noexcept = default;
118 : :
119 : : /** Check if this is empty (size and fee are 0). */
120 : 2560502 : bool inline IsEmpty() const noexcept {
121 [ - + - + : 2560502 : return size == 0;
+ + - + ]
[ + + + -
+ + + + +
+ + + + +
+ + + + +
+ ][ - + +
+ + + + -
+ - + + ]
122 : : }
123 : :
124 : : /** Add fee and size of another FeeFrac to this one. */
125 : 96238802 : void inline operator+=(const FeeFrac& other) noexcept
126 : : {
127 : 96238802 : fee += other.fee;
128 [ + + + + ]: 95094395 : size += other.size;
129 : : }
130 : :
131 : : /** Subtract fee and size of another FeeFrac from this one. */
132 : 50445 : void inline operator-=(const FeeFrac& other) noexcept
133 : : {
134 : 50445 : fee -= other.fee;
135 : 50445 : size -= other.size;
136 : : }
137 : :
138 : : /** Sum fee and size. */
139 : 2089415 : friend inline FeeFrac operator+(const FeeFrac& a, const FeeFrac& b) noexcept
140 : : {
141 [ + - ]: 2089415 : return {a.fee + b.fee, a.size + b.size};
142 : : }
143 : :
144 : : /** Subtract both fee and size. */
145 : 1081469 : friend inline FeeFrac operator-(const FeeFrac& a, const FeeFrac& b) noexcept
146 : : {
147 [ + + ]: 1081469 : return {a.fee - b.fee, a.size - b.size};
148 : : }
149 : :
150 : : /** Check if two FeeFrac objects are equal (both same fee and same size). */
151 : 50319022 : friend inline bool operator==(const FeeFrac& a, const FeeFrac& b) noexcept
152 : : {
153 [ + - + - : 50318923 : return a.fee == b.fee && a.size == b.size;
+ - - + +
- - + + -
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+ - - + +
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+ ][ + - +
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+ + # # #
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# # # # #
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# ]
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154 : : }
155 : :
156 : : /** Compare two FeeFracs just by feerate. */
157 : 1343980 : friend inline std::weak_ordering FeeRateCompare(const FeeFrac& a, const FeeFrac& b) noexcept
158 : : {
159 : 1343980 : auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);
160 [ + + + + ]: 1343980 : return cross_a <=> cross_b;
161 : : }
162 : :
163 : : /** Check if a FeeFrac object has strictly lower feerate than another. */
164 : 722641 : friend inline bool operator<<(const FeeFrac& a, const FeeFrac& b) noexcept
165 : : {
166 : 722641 : auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);
167 [ + + ]: 722641 : return cross_a < cross_b;
[ - + - + ]
168 : : }
169 : :
170 : : /** Check if a FeeFrac object has strictly higher feerate than another. */
171 : 2695182 : friend inline bool operator>>(const FeeFrac& a, const FeeFrac& b) noexcept
172 : : {
173 : 2695182 : auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);
174 [ + + + + : 2695182 : return cross_a > cross_b;
- + ][ + +
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+ + - + -
+ + + ][ +
+ # # # #
# # # # #
# # # # #
# # # # ]
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175 : : }
176 : :
177 : : /** Compare two FeeFracs. <, >, <=, and >= are auto-generated from this. */
178 : 141550121 : friend inline std::strong_ordering operator<=>(const FeeFrac& a, const FeeFrac& b) noexcept
179 : : {
180 : 141550121 : auto cross_a = Mul(a.fee, b.size), cross_b = Mul(b.fee, a.size);
181 [ + + + + : 141550121 : if (cross_a == cross_b) return b.size <=> a.size;
+ + ]
182 [ + + ]: 66336126 : return cross_a <=> cross_b;
183 : : }
184 : :
185 : : /** Swap two FeeFracs. */
186 : 4951618 : friend inline void swap(FeeFrac& a, FeeFrac& b) noexcept
187 : : {
188 : 303131 : std::swap(a.fee, b.fee);
189 : 303131 : std::swap(a.size, b.size);
190 : : }
191 : :
192 : : /** Compute the fee for a given size `at_size` using this object's feerate.
193 : : *
194 : : * This effectively corresponds to evaluating (this->fee * at_size) / this->size, with the
195 : : * result rounded towards negative infinity (if RoundDown) or towards positive infinity
196 : : * (if !RoundDown).
197 : : *
198 : : * Requires this->size > 0, at_size >= 0, and that the correct result fits in a int64_t. This
199 : : * is guaranteed to be the case when 0 <= at_size <= this->size.
200 : : */
201 : : template<bool RoundDown>
202 : 38 : int64_t EvaluateFee(int32_t at_size) const noexcept
203 : : {
204 : 38 : Assume(size > 0);
205 : 38 : Assume(at_size >= 0);
206 [ + + ]: 38 : if (fee >= 0 && fee < 0x200000000) [[likely]] {
207 : : // Common case where (this->fee * at_size) is guaranteed to fit in a uint64_t.
208 : : if constexpr (RoundDown) {
209 : 2 : return (uint64_t(fee) * at_size) / uint32_t(size);
210 : : } else {
211 : 9 : return (uint64_t(fee) * at_size + size - 1U) / uint32_t(size);
212 : : }
213 : : } else {
214 : : // Otherwise, use Mul and Div.
215 : 27 : return Div(Mul(fee, at_size), size, RoundDown);
216 : : }
217 : : }
218 : :
219 : : public:
220 : : /** Compute the fee for a given size `at_size` using this object's feerate, rounding down. */
221 : 6 : int64_t EvaluateFeeDown(int32_t at_size) const noexcept { return EvaluateFee<true>(at_size); }
222 : : /** Compute the fee for a given size `at_size` using this object's feerate, rounding up. */
223 : 32 : int64_t EvaluateFeeUp(int32_t at_size) const noexcept { return EvaluateFee<false>(at_size); }
224 : : };
225 : :
226 : : /** Compare the feerate diagrams implied by the provided sorted chunks data.
227 : : *
228 : : * The implied diagram for each starts at (0, 0), then contains for each chunk the cumulative fee
229 : : * and size up to that chunk, and then extends infinitely to the right with a horizontal line.
230 : : *
231 : : * The caller must guarantee that the sum of the FeeFracs in either of the chunks' data set do not
232 : : * overflow (so sum fees < 2^63, and sum sizes < 2^31).
233 : : */
234 : : std::partial_ordering CompareChunks(std::span<const FeeFrac> chunks0, std::span<const FeeFrac> chunks1);
235 : :
236 : : /** Tagged wrapper around FeeFrac to avoid unit confusion. */
237 : : template<typename Tag>
238 : 93261 : struct FeePerUnit : public FeeFrac
239 : : {
240 : : // Inherit FeeFrac constructors.
241 : 243403 : using FeeFrac::FeeFrac;
242 : :
243 : : /** Convert a FeeFrac to a FeePerUnit. */
244 : 158800 : static FeePerUnit FromFeeFrac(const FeeFrac& feefrac) noexcept
245 : : {
246 : 63199 : return {feefrac.fee, feefrac.size};
247 : : }
248 : : };
249 : :
250 : : // FeePerUnit instance for satoshi / vbyte.
251 : : struct VSizeTag {};
252 : : using FeePerVSize = FeePerUnit<VSizeTag>;
253 : :
254 : : // FeePerUnit instance for satoshi / WU.
255 : : struct WeightTag {};
256 : : using FeePerWeight = FeePerUnit<WeightTag>;
257 : :
258 : : #endif // BITCOIN_UTIL_FEEFRAC_H
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