theta/include/theta_a_not_b_impl.hpp

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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
 *
 *   http://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 THETA_A_NOT_B_IMPL_HPP_
#define THETA_A_NOT_B_IMPL_HPP_

#include <algorithm>

#include "conditional_back_inserter.hpp"

namespace datasketches {

/*
 * author Alexander Saydakov
 * author Lee Rhodes
 * author Kevin Lang
 */

template<typename A>
theta_a_not_b_alloc<A>::theta_a_not_b_alloc(uint64_t seed):
seed_hash_(theta_sketch_alloc<A>::get_seed_hash(seed))
{}

template<typename A>
compact_theta_sketch_alloc<A> theta_a_not_b_alloc<A>::compute(const theta_sketch_alloc<A>& a, const theta_sketch_alloc<A>& b, bool ordered) const {
  if (a.is_empty() || a.get_num_retained() == 0 || b.is_empty()) return compact_theta_sketch_alloc<A>(a, ordered);
  if (a.get_seed_hash() != seed_hash_) throw std::invalid_argument("A seed hash mismatch");
  if (b.get_seed_hash() != seed_hash_) throw std::invalid_argument("B seed hash mismatch");

  const uint64_t theta = std::min(a.get_theta64(), b.get_theta64());
  vector_u64<A> keys;
  bool is_empty = a.is_empty();

  if (b.get_num_retained() == 0) {
    std::copy_if(a.begin(), a.end(), std::back_inserter(keys), less_than(theta));
  } else {
    if (a.is_ordered() && b.is_ordered()) { // sort-based
      std::set_difference(a.begin(), a.end(), b.begin(), b.end(), conditional_back_inserter(keys, less_than(theta)));
    } else { // hash-based
      const uint8_t lg_size = lg_size_from_count(b.get_num_retained(), update_theta_sketch_alloc<A>::REBUILD_THRESHOLD);
      vector_u64<A> b_hash_table(1 << lg_size, 0);
      for (auto key: b) {
        if (key < theta) {
          update_theta_sketch_alloc<A>::hash_search_or_insert(key, b_hash_table.data(), lg_size);
        } else if (b.is_ordered()) {
          break; // early stop
        }
      }

      // scan A lookup B
      for (auto key: a) {
        if (key < theta) {
          if (!update_theta_sketch_alloc<A>::hash_search(key, b_hash_table.data(), lg_size)) keys.push_back(key);
        } else if (a.is_ordered()) {
          break; // early stop
        }
      }
    }
  }
  if (keys.empty() && theta == theta_sketch_alloc<A>::MAX_THETA) is_empty = true;
  if (ordered && !a.is_ordered()) std::sort(keys.begin(), keys.end());
  return compact_theta_sketch_alloc<A>(is_empty, theta, std::move(keys), seed_hash_, a.is_ordered() || ordered);
}

} /* namespace datasketches */

# endif