// 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_EXPLICIT_SEED_SEQ_H_
#define ABSL_RANDOM_INTERNAL_EXPLICIT_SEED_SEQ_H_

#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <initializer_list>
#include <iterator>
#include <vector>

#include "absl/base/config.h"
#include "absl/base/internal/endian.h"

namespace absl {
ABSL_NAMESPACE_BEGIN
namespace random_internal {

// This class conforms to the C++ Standard "Seed Sequence" concept
// [rand.req.seedseq].
//
// An "ExplicitSeedSeq" is meant to provide a conformant interface for
// forwarding pre-computed seed material to the constructor of a class
// conforming to the "Uniform Random Bit Generator" concept. This class makes no
// attempt to mutate the state provided by its constructor, and returns it
// directly via ExplicitSeedSeq::generate().
//
// If this class is asked to generate more seed material than was provided to
// the constructor, then the remaining bytes will be filled with deterministic,
// nonrandom data.
class ExplicitSeedSeq {
 public:
  using result_type = uint32_t;

  ExplicitSeedSeq() : state_() {}

  // Copy and move both allowed.
  ExplicitSeedSeq(const ExplicitSeedSeq& other) = default;
  ExplicitSeedSeq& operator=(const ExplicitSeedSeq& other) = default;
  ExplicitSeedSeq(ExplicitSeedSeq&& other) = default;
  ExplicitSeedSeq& operator=(ExplicitSeedSeq&& other) = default;

  template <typename Iterator>
  ExplicitSeedSeq(Iterator begin, Iterator end) {
    for (auto it = begin; it != end; it++) {
      state_.push_back(*it & 0xffffffff);
    }
  }

  template <typename T>
  ExplicitSeedSeq(std::initializer_list<T> il)
      : ExplicitSeedSeq(il.begin(), il.end()) {}

  size_t size() const { return state_.size(); }

  template <typename OutIterator>
  void param(OutIterator out) const {
    std::copy(std::begin(state_), std::end(state_), out);
  }

  template <typename OutIterator>
  void generate(OutIterator begin, OutIterator end) {
    for (size_t index = 0; begin != end; begin++) {
      *begin = state_.empty() ? 0 : little_endian::FromHost32(state_[index++]);
      if (index >= state_.size()) {
        index = 0;
      }
    }
  }

 protected:
  std::vector<uint32_t> state_;
};

}  // namespace random_internal
ABSL_NAMESPACE_END
}  // namespace absl

#endif  // ABSL_RANDOM_INTERNAL_EXPLICIT_SEED_SEQ_H_
