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random: Improve RandomMixin::randbits

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The previous randbits code would, when requesting more randomness than available
in its random bits buffer, discard the remaining entropy and generate new.

Benchmarks show that it's usually better to first consume the existing randomness
and only then generate new ones. This adds some complexity to randbits, but it
doesn't weigh up against the reduced need to generate more randomness.
This commit is contained in:
Pieter Wuille
2024-03-10 10:16:30 -04:00
parent 9b14d3d2da
commit 21ce9d8658
3 changed files with 71 additions and 18 deletions
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52
src/test/random_tests.cpp
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@@ -39,9 +39,9 @@ BOOST_AUTO_TEST_CASE(fastrandom_tests)
BOOST_CHECK_EQUAL(7, ctx.rand_uniform_delay(time_point, 9s).time_since_epoch().count());
BOOST_CHECK_EQUAL(-6, ctx.rand_uniform_delay(time_point, -9s).time_since_epoch().count());
BOOST_CHECK_EQUAL(1, ctx.rand_uniform_delay(time_point, 0s).time_since_epoch().count());
BOOST_CHECK_EQUAL(1467825113502396065, ctx.rand_uniform_delay(time_point, 9223372036854775807s).time_since_epoch().count());
BOOST_CHECK_EQUAL(-970181367944767837, ctx.rand_uniform_delay(time_point, -9223372036854775807s).time_since_epoch().count());
BOOST_CHECK_EQUAL(24761, ctx.rand_uniform_delay(time_point, 9h).time_since_epoch().count());
BOOST_CHECK_EQUAL(4652286523065884857, ctx.rand_uniform_delay(time_point, 9223372036854775807s).time_since_epoch().count());
BOOST_CHECK_EQUAL(-8813961240025683129, ctx.rand_uniform_delay(time_point, -9223372036854775807s).time_since_epoch().count());
BOOST_CHECK_EQUAL(26443, ctx.rand_uniform_delay(time_point, 9h).time_since_epoch().count());
}
BOOST_CHECK_EQUAL(ctx1.rand32(), ctx2.rand32());
BOOST_CHECK_EQUAL(ctx1.rand32(), ctx2.rand32());
@@ -103,6 +103,52 @@ BOOST_AUTO_TEST_CASE(fastrandom_randbits)
}
}
/** Verify that RandomMixin::randbits returns 0 and 1 for every requested bit. */
BOOST_AUTO_TEST_CASE(randbits_test)
{
FastRandomContext ctx_lens; //!< RNG for producing the lengths requested from ctx_test.
FastRandomContext ctx_test; //!< The RNG being tested.
int ctx_test_bitsleft{0}; //!< (Assumed value of) ctx_test::bitbuf_len
// Run the entire test 5 times.
for (int i = 0; i < 5; ++i) {
// count (first) how often it has occurred, and (second) how often it was true:
// - for every bit position, in every requested bits count (0 + 1 + 2 + ... + 64 = 2080)
// - for every value of ctx_test_bitsleft (0..63 = 64)
std::vector<std::pair<uint64_t, uint64_t>> seen(2080 * 64);
while (true) {
// Loop 1000 times, just to not continuously check std::all_of.
for (int j = 0; j < 1000; ++j) {
// Decide on a number of bits to request (0 through 64, inclusive; don't use randbits/randrange).
int bits = ctx_lens.rand64() % 65;
// Generate that many bits.
uint64_t gen = ctx_test.randbits(bits);
// Make sure the result is in range.
if (bits < 64) BOOST_CHECK_EQUAL(gen >> bits, 0);
// Mark all the seen bits in the output.
for (int bit = 0; bit < bits; ++bit) {
int idx = bit + (bits * (bits - 1)) / 2 + 2080 * ctx_test_bitsleft;
seen[idx].first += 1;
seen[idx].second += (gen >> bit) & 1;
}
// Update ctx_test_bitself.
if (bits > ctx_test_bitsleft) {
ctx_test_bitsleft = ctx_test_bitsleft + 64 - bits;
} else {
ctx_test_bitsleft -= bits;
}
}
// Loop until every bit position/combination is seen 242 times.
if (std::all_of(seen.begin(), seen.end(), [](const auto& x) { return x.first >= 242; })) break;
}
// Check that each bit appears within 7.78 standard deviations of 50%
// (each will fail with P < 1/(2080 * 64 * 10^9)).
for (const auto& val : seen) {
assert(fabs(val.first * 0.5 - val.second) < sqrt(val.first * 0.25) * 7.78);
}
}
}
/** Does-it-compile test for compatibility with standard library RNG interface. */
BOOST_AUTO_TEST_CASE(stdrandom_test)
{