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authorGravatar Abseil Team <absl-team@google.com>2020-02-25 22:27:31 +0100
committerGravatar CJ Johnson <johnsoncj@google.com>2020-02-25 17:56:58 -0500
commitb832dce8489ef7b6231384909fd9b68d5a5ff2b7 (patch)
tree3ad4be9a9a4105366be714da9458e076a77be18f /absl/random/internal/generate_real.h
parentaa844899c937bde5d2b24f276b59997e5b668bde (diff)
Creation of LTS branch "lts_2020_02_25"20200225
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+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
+#define ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_
+
+// This file contains some implementation details which are used by one or more
+// of the absl random number distributions.
+
+#include <cstdint>
+#include <cstring>
+#include <limits>
+#include <type_traits>
+
+#include "absl/base/internal/bits.h"
+#include "absl/meta/type_traits.h"
+#include "absl/random/internal/fastmath.h"
+#include "absl/random/internal/traits.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace random_internal {
+
+// Tristate tag types controlling the output of GenerateRealFromBits.
+struct GeneratePositiveTag {};
+struct GenerateNegativeTag {};
+struct GenerateSignedTag {};
+
+// GenerateRealFromBits generates a single real value from a single 64-bit
+// `bits` with template fields controlling the output.
+//
+// The `SignedTag` parameter controls whether positive, negative,
+// or either signed/unsigned may be returned.
+// When SignedTag == GeneratePositiveTag, range is U(0, 1)
+// When SignedTag == GenerateNegativeTag, range is U(-1, 0)
+// When SignedTag == GenerateSignedTag, range is U(-1, 1)
+//
+// When the `IncludeZero` parameter is true, the function may return 0 for some
+// inputs, otherwise it never returns 0.
+//
+// When a value in U(0,1) is required, use:
+// Uniform64ToReal<double, PositiveValueT, true>;
+//
+// When a value in U(-1,1) is required, use:
+// Uniform64ToReal<double, SignedValueT, false>;
+//
+// This generates more distinct values than the mathematical equivalent
+// `U(0, 1) * 2.0 - 1.0`.
+//
+// Scaling the result by powers of 2 (and avoiding a multiply) is also possible:
+// GenerateRealFromBits<double>(..., -1); => U(0, 0.5)
+// GenerateRealFromBits<double>(..., 1); => U(0, 2)
+//
+template <typename RealType, // Real type, either float or double.
+ typename SignedTag = GeneratePositiveTag, // Whether a positive,
+ // negative, or signed
+ // value is generated.
+ bool IncludeZero = true>
+inline RealType GenerateRealFromBits(uint64_t bits, int exp_bias = 0) {
+ using real_type = RealType;
+ using uint_type = absl::conditional_t<std::is_same<real_type, float>::value,
+ uint32_t, uint64_t>;
+
+ static_assert(
+ (std::is_same<double, real_type>::value ||
+ std::is_same<float, real_type>::value),
+ "GenerateRealFromBits must be parameterized by either float or double.");
+
+ static_assert(sizeof(uint_type) == sizeof(real_type),
+ "Mismatched unsinged and real types.");
+
+ static_assert((std::numeric_limits<real_type>::is_iec559 &&
+ std::numeric_limits<real_type>::radix == 2),
+ "RealType representation is not IEEE 754 binary.");
+
+ static_assert((std::is_same<SignedTag, GeneratePositiveTag>::value ||
+ std::is_same<SignedTag, GenerateNegativeTag>::value ||
+ std::is_same<SignedTag, GenerateSignedTag>::value),
+ "");
+
+ static constexpr int kExp = std::numeric_limits<real_type>::digits - 1;
+ static constexpr uint_type kMask = (static_cast<uint_type>(1) << kExp) - 1u;
+ static constexpr int kUintBits = sizeof(uint_type) * 8;
+
+ int exp = exp_bias + int{std::numeric_limits<real_type>::max_exponent - 2};
+
+ // Determine the sign bit.
+ // Depending on the SignedTag, this may use the left-most bit
+ // or it may be a constant value.
+ uint_type sign = std::is_same<SignedTag, GenerateNegativeTag>::value
+ ? (static_cast<uint_type>(1) << (kUintBits - 1))
+ : 0;
+ if (std::is_same<SignedTag, GenerateSignedTag>::value) {
+ if (std::is_same<uint_type, uint64_t>::value) {
+ sign = bits & uint64_t{0x8000000000000000};
+ }
+ if (std::is_same<uint_type, uint32_t>::value) {
+ const uint64_t tmp = bits & uint64_t{0x8000000000000000};
+ sign = static_cast<uint32_t>(tmp >> 32);
+ }
+ // adjust the bits and the exponent to account for removing
+ // the leading bit.
+ bits = bits & uint64_t{0x7FFFFFFFFFFFFFFF};
+ exp++;
+ }
+ if (IncludeZero) {
+ if (bits == 0u) return 0;
+ }
+
+ // Number of leading zeros is mapped to the exponent: 2^-clz
+ // bits is 0..01xxxxxx. After shifting, we're left with 1xxx...0..0
+ int clz = base_internal::CountLeadingZeros64(bits);
+ bits <<= (IncludeZero ? clz : (clz & 63)); // remove 0-bits.
+ exp -= clz; // set the exponent.
+ bits >>= (63 - kExp);
+
+ // Construct the 32-bit or 64-bit IEEE 754 floating-point value from
+ // the individual fields: sign, exp, mantissa(bits).
+ uint_type val =
+ (std::is_same<SignedTag, GeneratePositiveTag>::value ? 0u : sign) |
+ (static_cast<uint_type>(exp) << kExp) |
+ (static_cast<uint_type>(bits) & kMask);
+
+ // bit_cast to the output-type
+ real_type result;
+ memcpy(static_cast<void*>(&result), static_cast<const void*>(&val),
+ sizeof(result));
+ return result;
+}
+
+} // namespace random_internal
+ABSL_NAMESPACE_END
+} // namespace absl
+
+#endif // ABSL_RANDOM_INTERNAL_GENERATE_REAL_H_