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Fix bug in Equihash.IntListMultimap & adjust constants
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Fix a trivial bug in the iterator returned by 'IntListMultimap::get',
caused by mistaken use of the iterator index in place of the key when
doing lookups into the overspill map. This was causing puzzle solutions
to be invalid about 3% of the time, as well as substantially reducing
the average number of solutions found per nonce.

As the fix increases the mean solution count per nonce to the correct
value of 2.0 predicted by the paper (regardless of puzzle params k & n),
inline the affected constants to simplify 'Equihash::adjustDifficulty'.
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@stejbac
stejbac committed Nov 30, 2021
1 parent e383edc commit 0ee6175
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Showing 3 changed files with 34 additions and 37 deletions.
12 changes: 5 additions & 7 deletions common/src/main/java/bisq/common/crypto/Equihash.java
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Original file line number Diff line number Diff line change
Expand Up @@ -81,10 +81,8 @@
@SuppressWarnings("UnstableApiUsage")
public class Equihash {
private static final int HASH_BIT_LENGTH = 256;
/** Observed mean solution count per nonce for Equihash-n-4 puzzles with unit difficulty. */
public static final double EQUIHASH_n_4_MEAN_SOLUTION_COUNT_PER_NONCE = 1.63;
/** Observed mean solution count per nonce for Equihash-n-5 puzzles with unit difficulty. */
public static final double EQUIHASH_n_5_MEAN_SOLUTION_COUNT_PER_NONCE = 1.34;
/** Mean solution count per nonce for Equihash puzzles with unit difficulty. */
private static final double MEAN_SOLUTION_COUNT_PER_NONCE = 2.0;

private final int k, N;
private final int tableCapacity;
Expand Down Expand Up @@ -127,8 +125,8 @@ private static BigInteger inverseDifficultyMinusOne(double difficulty) {
/** Adjust the provided difficulty to take the variable number of puzzle solutions per
* nonce into account, so that the expected number of attempts needed to solve a given
* puzzle equals the reciprocal of the provided difficulty. */
public static double adjustDifficulty(double realDifficulty, double meanSolutionCountPerNonce) {
return Math.max(-meanSolutionCountPerNonce / Math.log1p(-1.0 / Math.max(realDifficulty, 1.0)), 1.0);
public static double adjustDifficulty(double realDifficulty) {
return Math.max(-MEAN_SOLUTION_COUNT_PER_NONCE / Math.log1p(-1.0 / Math.max(realDifficulty, 1.0)), 1.0);
}

public Puzzle puzzle(byte[] seed) {
Expand Down Expand Up @@ -339,7 +337,7 @@ PrimitiveIterator.OfInt get(int key) {

private Iterator<Integer> overspillIterator() {
if (overspillIterator == null) {
overspillIterator = overspillMultimap.get(i).iterator();
overspillIterator = overspillMultimap.get(key).iterator();
}
return overspillIterator;
}
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3 changes: 1 addition & 2 deletions common/src/main/java/bisq/common/crypto/EquihashProofOfWorkService.java
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Original file line number Diff line number Diff line change
Expand Up @@ -70,7 +70,6 @@ boolean verify(ProofOfWork proofOfWork) {
}

private static double scaledDifficulty(double difficulty) {
return Equihash.adjustDifficulty(DIFFICULTY_SCALE_FACTOR * difficulty,
Equihash.EQUIHASH_n_5_MEAN_SOLUTION_COUNT_PER_NONCE);
return Equihash.adjustDifficulty(DIFFICULTY_SCALE_FACTOR * difficulty);
}
}
56 changes: 28 additions & 28 deletions common/src/test/java/bisq/common/crypto/EquihashTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -33,7 +33,6 @@
import org.junit.Ignore;
import org.junit.Test;

import static bisq.common.crypto.Equihash.EQUIHASH_n_5_MEAN_SOLUTION_COUNT_PER_NONCE;
import static java.lang.Double.POSITIVE_INFINITY;
import static org.junit.Assert.assertEquals;
import static org.junit.Assert.assertTrue;
Expand All @@ -56,16 +55,17 @@ public void testHashUpperBound() {

@Test
public void testAdjustDifficulty() {
assertEquals(1.0, Equihash.adjustDifficulty(0.0, 1.34), 0.0001);
assertEquals(1.0, Equihash.adjustDifficulty(0.5, 1.34), 0.0001);
assertEquals(1.0, Equihash.adjustDifficulty(1.0, 1.34), 0.0001);
assertEquals(1.0, Equihash.adjustDifficulty(1.2, 1.34), 0.0001);
assertEquals(1.22, Equihash.adjustDifficulty(1.5, 1.34), 0.01);
assertEquals(1.93, Equihash.adjustDifficulty(2.0, 1.34), 0.01);
assertEquals(2.62, Equihash.adjustDifficulty(2.5, 1.34), 0.01);
assertEquals(3.30, Equihash.adjustDifficulty(3.0, 1.34), 0.01);
assertEquals(134.0, Equihash.adjustDifficulty(100.0, 1.34), 1.0);
assertEquals(Equihash.adjustDifficulty(POSITIVE_INFINITY, 1.34), POSITIVE_INFINITY, 1.0);
assertEquals(1.0, Equihash.adjustDifficulty(0.0), 0.0001);
assertEquals(1.0, Equihash.adjustDifficulty(0.5), 0.0001);
assertEquals(1.0, Equihash.adjustDifficulty(1.0), 0.0001);
assertEquals(1.0, Equihash.adjustDifficulty(1.1), 0.0001);
assertEquals(1.12, Equihash.adjustDifficulty(1.2), 0.01);
assertEquals(1.83, Equihash.adjustDifficulty(1.5), 0.01);
assertEquals(2.89, Equihash.adjustDifficulty(2.0), 0.01);
assertEquals(3.92, Equihash.adjustDifficulty(2.5), 0.01);
assertEquals(4.93, Equihash.adjustDifficulty(3.0), 0.01);
assertEquals(200.0, Equihash.adjustDifficulty(100.0), 1.5);
assertEquals(Equihash.adjustDifficulty(POSITIVE_INFINITY), POSITIVE_INFINITY, 1.0);
}

@Test
Expand All @@ -87,12 +87,12 @@ public void testFindSolution() {
public void benchmarkFindSolution() {
// On Intel Core i3 CPU M 330 @ 2.13GHz ...
//
// For Equihash-90-5 with real difficulty 2.0, adjusted difficulty 1.933211354791211 ...
// Total elapsed solution time: 292789 ms
// Mean time to solve one puzzle: 292 ms
// Puzzle solution time per unit difficulty: 146 ms
// For Equihash-90-5 with real difficulty 2.0, adjusted difficulty 2.8853900817779268 ...
// Total elapsed solution time: 279583 ms
// Mean time to solve one puzzle: 279 ms
// Puzzle solution time per unit difficulty: 139 ms
//
double adjustedDifficulty = Equihash.adjustDifficulty(2.0, EQUIHASH_n_5_MEAN_SOLUTION_COUNT_PER_NONCE);
double adjustedDifficulty = Equihash.adjustDifficulty(2.0);
Equihash equihash = new Equihash(90, 5, adjustedDifficulty);

Stopwatch stopwatch = Stopwatch.createStarted();
Expand Down Expand Up @@ -140,24 +140,24 @@ public void benchmarkVerify() {
@Ignore
public void solutionCountPerNonceStats() {
// For Equihash-60-4...
// Got puzzle solution count mean: 1.6161
// Got expected count stats: [0 x 1987, 1 x 3210, 2 x 2595, 3 x 1398, 4 x 564, 5 x 183, 6 x 49, 7 x 11, 8 x 3]
// Got actual count stats: [0 x 2014, 1 x 3230, 2 x 2546, 3 x 1395, 4 x 543, 5 x 191, 6 x 50, 7 x 24, 8 x 4, 9 x 3]
// Got puzzle solution count mean: 1.9797
// Got expected count stats: [0 x 1381, 1 x 2734, 2 x 2707, 3 x 1786, 4 x 884, 5 x 350, 6 x 115, 7 x 33, 8 x 8, 9 x 2]
// Got actual count stats: [0 x 1413, 1 x 2704, 2 x 2699, 3 x 1813, 4 x 866, 5 x 330, 6 x 115, 7 x 35, 8 x 19, 9 x 5, 10]
//
// For Equihash-70-4...
// Got puzzle solution count mean: 1.6473
// Got expected count stats: [0 x 1926, 1 x 3172, 2 x 2613, 3 x 1434, 4 x 591, 5 x 195, 6 x 53, 7 x 13, 8 x 2, 9]
// Got actual count stats: [0 x 1958, 1 x 3172, 2 x 2584, 3 x 1413, 4 x 585, 5 x 204, 6 x 61, 7 x 17, 8 x 5, 9]
// Got puzzle solution count mean: 1.9988
// Got expected count stats: [0 x 1355, 1 x 2708, 2 x 2707, 3 x 1803, 4 x 902, 5 x 360, 6 x 120, 7 x 34, 8 x 9, 9 x 2]
// Got actual count stats: [0 x 1362, 1 x 2690, 2 x 2720, 3 x 1826, 4 x 870, 5 x 353, 6 x 129, 7 x 41, 8 x 7, 9 x 2]
//
// For Equihash-90-5...
// Got puzzle solution count mean: 1.3419
// Got expected count stats: [0 x 2613, 1 x 3508, 2 x 2353, 3 x 1052, 4 x 353, 5 x 95, 6 x 21, 7 x 4, 8]
// Got actual count stats: [0 x 2698, 1 x 3446, 2 x 2311, 3 x 1045, 4 x 352, 5 x 104, 6 x 33, 7 x 5, 8 x 3, 9, 10, 12]
// Got puzzle solution count mean: 1.9921
// Got expected count stats: [0 x 1364, 1 x 2717, 2 x 2707, 3 x 1797, 4 x 896, 5 x 356, 6 x 119, 7 x 33, 8 x 9, 9 x 2]
// Got actual count stats: [0 x 1379, 1 x 2709, 2 x 2729, 3 x 1750, 4 x 900, 5 x 362, 6 x 119, 7 x 39, 8 x 11, 9, 10]
//
// For Equihash-96-5...
// Got puzzle solution count mean: 1.3363
// Got expected count stats: [0 x 2628, 1 x 3512, 2 x 2347, 3 x 1045, 4 x 349, 5 x 93, 6 x 21, 7 x 4, 8]
// Got actual count stats: [0 x 2708, 1 x 3409, 2 x 2344, 3 x 1048, 4 x 368, 5 x 94, 6 x 23, 7 x 6]
// Got puzzle solution count mean: 1.9997
// Got expected count stats: [0 x 1354, 1 x 2707, 2 x 2707, 3 x 1804, 4 x 902, 5 x 360, 6 x 121, 7 x 34, 8 x 9, 9 x 2]
// Got actual count stats: [0 x 1405, 1 x 2621, 2 x 2733, 3 x 1802, 4 x 928, 5 x 342, 6 x 123, 7 x 29, 8 x 13, 9 x 3, 10]
//
Equihash equihash = new Equihash(90, 5, 1.0);
byte[] seed = new byte[32];
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