random/internal/uniform_helper.h

// Copyright 2019 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_UNIFORM_HELPER_H_
#define ABSL_RANDOM_INTERNAL_UNIFORM_HELPER_H_

#include <cmath>
#include <limits>
#include <type_traits>

#include "absl/base/config.h"
#include "absl/meta/type_traits.h"
#include "absl/random/internal/traits.h"

namespace absl {
ABSL_NAMESPACE_BEGIN

template <typename IntType>
class uniform_int_distribution;

template <typename RealType>
class uniform_real_distribution;

// Interval tag types which specify whether the interval is open or closed
// on either boundary.

namespace random_internal {
template <typename T>
struct TagTypeCompare {};

template <typename T>
constexpr bool operator==(TagTypeCompare<T>, TagTypeCompare<T>) {
  // Tags are mono-states. They always compare equal.
  return true;
}
template <typename T>
constexpr bool operator!=(TagTypeCompare<T>, TagTypeCompare<T>) {
  return false;
}

}  // namespace random_internal

struct IntervalClosedClosedTag
    : public random_internal::TagTypeCompare<IntervalClosedClosedTag> {};
struct IntervalClosedOpenTag
    : public random_internal::TagTypeCompare<IntervalClosedOpenTag> {};
struct IntervalOpenClosedTag
    : public random_internal::TagTypeCompare<IntervalOpenClosedTag> {};
struct IntervalOpenOpenTag
    : public random_internal::TagTypeCompare<IntervalOpenOpenTag> {};

namespace random_internal {

// In the absence of an explicitly provided return-type, the template
// "uniform_inferred_return_t<A, B>" is used to derive a suitable type, based on
// the data-types of the endpoint-arguments {A lo, B hi}.
//
// Given endpoints {A lo, B hi}, one of {A, B} will be chosen as the
// return-type, if one type can be implicitly converted into the other, in a
// lossless way. The template "is_widening_convertible" implements the
// compile-time logic for deciding if such a conversion is possible.
//
// If no such conversion between {A, B} exists, then the overload for
// absl::Uniform() will be discarded, and the call will be ill-formed.
// Return-type for absl::Uniform() when the return-type is inferred.
template <typename A, typename B>
using uniform_inferred_return_t =
    absl::enable_if_t<absl::disjunction<is_widening_convertible<A, B>,
                                        is_widening_convertible<B, A>>::value,
                      typename std::conditional<
                          is_widening_convertible<A, B>::value, B, A>::type>;

// The functions
//    uniform_lower_bound(tag, a, b)
// and
//    uniform_upper_bound(tag, a, b)
// are used as implementation-details for absl::Uniform().
//
// Conceptually,
//    [a, b] == [uniform_lower_bound(IntervalClosedClosed, a, b),
//               uniform_upper_bound(IntervalClosedClosed, a, b)]
//    (a, b) == [uniform_lower_bound(IntervalOpenOpen, a, b),
//               uniform_upper_bound(IntervalOpenOpen, a, b)]
//    [a, b) == [uniform_lower_bound(IntervalClosedOpen, a, b),
//               uniform_upper_bound(IntervalClosedOpen, a, b)]
//    (a, b] == [uniform_lower_bound(IntervalOpenClosed, a, b),
//               uniform_upper_bound(IntervalOpenClosed, a, b)]
//
template <typename IntType, typename Tag>
typename absl::enable_if_t<
    absl::conjunction<
        std::is_integral<IntType>,
        absl::disjunction<std::is_same<Tag, IntervalOpenClosedTag>,
                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
    IntType>
uniform_lower_bound(Tag, IntType a, IntType) {
  return a < (std::numeric_limits<IntType>::max)() ? (a + 1) : a;
}

template <typename FloatType, typename Tag>
typename absl::enable_if_t<
    absl::conjunction<
        std::is_floating_point<FloatType>,
        absl::disjunction<std::is_same<Tag, IntervalOpenClosedTag>,
                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
    FloatType>
uniform_lower_bound(Tag, FloatType a, FloatType b) {
  return std::nextafter(a, b);
}

template <typename NumType, typename Tag>
typename absl::enable_if_t<
    absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
                      std::is_same<Tag, IntervalClosedOpenTag>>::value,
    NumType>
uniform_lower_bound(Tag, NumType a, NumType) {
  return a;
}

template <typename IntType, typename Tag>
typename absl::enable_if_t<
    absl::conjunction<
        std::is_integral<IntType>,
        absl::disjunction<std::is_same<Tag, IntervalClosedOpenTag>,
                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
    IntType>
uniform_upper_bound(Tag, IntType, IntType b) {
  return b > (std::numeric_limits<IntType>::min)() ? (b - 1) : b;
}

template <typename FloatType, typename Tag>
typename absl::enable_if_t<
    absl::conjunction<
        std::is_floating_point<FloatType>,
        absl::disjunction<std::is_same<Tag, IntervalClosedOpenTag>,
                          std::is_same<Tag, IntervalOpenOpenTag>>>::value,
    FloatType>
uniform_upper_bound(Tag, FloatType, FloatType b) {
  return b;
}

template <typename IntType, typename Tag>
typename absl::enable_if_t<
    absl::conjunction<
        std::is_integral<IntType>,
        absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
                          std::is_same<Tag, IntervalOpenClosedTag>>>::value,
    IntType>
uniform_upper_bound(Tag, IntType, IntType b) {
  return b;
}

template <typename FloatType, typename Tag>
typename absl::enable_if_t<
    absl::conjunction<
        std::is_floating_point<FloatType>,
        absl::disjunction<std::is_same<Tag, IntervalClosedClosedTag>,
                          std::is_same<Tag, IntervalOpenClosedTag>>>::value,
    FloatType>
uniform_upper_bound(Tag, FloatType, FloatType b) {
  return std::nextafter(b, (std::numeric_limits<FloatType>::max)());
}

// Returns whether the bounds are valid for the underlying distribution.
// Inputs must have already been resolved via uniform_*_bound calls.
//
// The c++ standard constraints in [rand.dist.uni.int] are listed as:
//    requires: lo <= hi.
//
// In the uniform_int_distrubtion, {lo, hi} are closed, closed. Thus:
// [0, 0] is legal.
// [0, 0) is not legal, but [0, 1) is, which translates to [0, 0].
// (0, 1) is not legal, but (0, 2) is, which translates to [1, 1].
// (0, 0] is not legal, but (0, 1] is, which translates to [1, 1].
//
// The c++ standard constraints in [rand.dist.uni.real] are listed as:
//    requires: lo <= hi.
//    requires: (hi - lo) <= numeric_limits<T>::max()
//
// In the uniform_real_distribution, {lo, hi} are closed, open, Thus:
// [0, 0] is legal, which is [0, 0+epsilon).
// [0, 0) is legal.
// (0, 0) is not legal, but (0-epsilon, 0+epsilon) is.
// (0, 0] is not legal, but (0, 0+epsilon] is.
//
template <typename FloatType>
absl::enable_if_t<std::is_floating_point<FloatType>::value, bool>
is_uniform_range_valid(FloatType a, FloatType b) {
  return a <= b && std::isfinite(b - a);
}

template <typename IntType>
absl::enable_if_t<std::is_integral<IntType>::value, bool>
is_uniform_range_valid(IntType a, IntType b) {
  return a <= b;
}

// UniformDistribution selects either absl::uniform_int_distribution
// or absl::uniform_real_distribution depending on the NumType parameter.
template <typename NumType>
using UniformDistribution =
    typename std::conditional<std::is_integral<NumType>::value,
                              absl::uniform_int_distribution<NumType>,
                              absl::uniform_real_distribution<NumType>>::type;

// UniformDistributionWrapper is used as the underlying distribution type
// by the absl::Uniform template function. It selects the proper Abseil
// uniform distribution and provides constructor overloads that match the
// expected parameter order as well as adjusting distribtuion bounds based
// on the tag.
template <typename NumType>
struct UniformDistributionWrapper : public UniformDistribution<NumType> {
  template <typename TagType>
  explicit UniformDistributionWrapper(TagType, NumType lo, NumType hi)
      : UniformDistribution<NumType>(
            uniform_lower_bound<NumType>(TagType{}, lo, hi),
            uniform_upper_bound<NumType>(TagType{}, lo, hi)) {}

  explicit UniformDistributionWrapper(NumType lo, NumType hi)
      : UniformDistribution<NumType>(
            uniform_lower_bound<NumType>(IntervalClosedOpenTag(), lo, hi),
            uniform_upper_bound<NumType>(IntervalClosedOpenTag(), lo, hi)) {}

  explicit UniformDistributionWrapper()
      : UniformDistribution<NumType>(std::numeric_limits<NumType>::lowest(),
                                     (std::numeric_limits<NumType>::max)()) {}
};

}  // namespace random_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_INTERNAL_UNIFORM_HELPER_H_