[libc++] Use correct rand.eng.mers all-zeroes seed sequence fallback
Summary:
When a seed sequence would lead to having no non-zero significant bits
in the initial state of a `mersenne_twister_engine`, the fallback is to
flip the most significant bit of the first value that appears in the
textual representation of the initial state.
rand.eng.mers describes this as setting the value to be 2 to the power
of one less than w; the previous value encoded in the implementation,
namely one less than "2 to the power of w", is replaced by the correct
value in this patch.
Reviewers: mclow.lists, EricWF, jasonliu
Reviewed By: mclow.lists
Subscribers: mclow.lists, jasonliu, EricWF, christof, ldionne, cfe-commits
Differential Revision: https://reviews.llvm.org/D50736
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@339969 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/random b/include/random
index 89664a6..724bd0f 100644
--- a/include/random
+++ b/include/random
@@ -2337,7 +2337,7 @@
for (size_t __i = 1; __i < __n; ++__i)
if (__x_[__i] != 0)
return;
- __x_[0] = _Max;
+ __x_[0] = result_type(1) << (__w - 1);
}
}
@@ -2363,7 +2363,7 @@
for (size_t __i = 1; __i < __n; ++__i)
if (__x_[__i] != 0)
return;
- __x_[0] = _Max;
+ __x_[0] = result_type(1) << (__w - 1);
}
}
diff --git a/test/std/numerics/rand/rand.eng/rand.eng.mers/ctor_sseq_all_zero.pass.cpp b/test/std/numerics/rand/rand.eng/rand.eng.mers/ctor_sseq_all_zero.pass.cpp
new file mode 100644
index 0000000..4599348
--- /dev/null
+++ b/test/std/numerics/rand/rand.eng/rand.eng.mers/ctor_sseq_all_zero.pass.cpp
@@ -0,0 +1,81 @@
+//===----------------------------------------------------------------------===//
+//
+// The LLVM Compiler Infrastructure
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+// <random>
+
+// template <class UIntType, size_t w, size_t n, size_t m, size_t r,
+// UIntType a, size_t u, UIntType d, size_t s,
+// UIntType b, size_t t,
+// UIntType c, size_t l, UIntType f>
+// class mersenne_twister_engine;
+
+// template <class Sseq> explicit mersenne_twister_engine(Sseq &q);
+//
+// [ ... ] Finally, if the most significant $w-r$ bits of $X_{-n}$ are zero,
+// and if each of the other resulting $X_i$ is $0$, changes $X_{-n}$ to
+// $ 2^{w-1} $.
+
+#include <random>
+
+#include <algorithm>
+#include <cassert>
+#include <cstddef>
+#if TEST_STD_VER >= 11
+#include <initializer_list>
+#endif
+
+struct all_zero_seed_seq {
+ typedef unsigned int result_type;
+
+ all_zero_seed_seq() {}
+
+ template <typename InputIterator>
+ all_zero_seed_seq(InputIterator, InputIterator) {}
+#if TEST_STD_VER >= 11
+ all_zero_seed_seq(std::initializer_list<result_type>) {}
+#endif
+
+ template <typename RandomAccessIterator>
+ void generate(RandomAccessIterator rb, RandomAccessIterator re) {
+ std::fill(rb, re, 0u);
+ }
+
+ std::size_t size() const { return 0u; }
+ template <typename OutputIterator> void param(OutputIterator) const {}
+};
+
+template <typename result_type, std::size_t word_size>
+void test(void) {
+ const std::size_t state_size = 1u;
+ const std::size_t shift_size = 1u;
+ const std::size_t tempering_l = word_size;
+
+ all_zero_seed_seq q;
+ std::mersenne_twister_engine<result_type, word_size, state_size,
+ shift_size,
+ 0u,
+ 0x0,
+ 0u, 0x0, 0u, 0x0, 0u, 0x0,
+ tempering_l,
+ 0u>
+ e(q);
+
+ const result_type Xneg1 = result_type(1) << (word_size - 1);
+ const result_type Y = Xneg1;
+ const result_type X0 = Xneg1 ^ (Y >> 1);
+ assert(e() == X0);
+}
+
+int main() {
+ // Test for k == 1: word_size <= 32.
+ test<unsigned short, 3u>();
+
+ // Test for k == 2: (32 < word_size <= 64).
+ test<unsigned long long, 33u>();
+}
