Ternary Search
(src/opt/ternary_search.hpp)

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• Last update: 2021-11-23 19:49:23+09:00
• Include: #include "src/opt/ternary_search.hpp"

Code

#pragma once

/**
 * @file ternary_search.hpp
 * @brief Ternary Search
 */

#include <cassert>
#include <type_traits>

namespace workspace {

/**
 * @brief Ternary search on discrete range.
 * @param first Left end, inclusive
 * @param last Right end, exclusive
 * @param comp Compare function
 * @return Local minimal point.
 */
template <class Iter, class Comp>
typename std::enable_if<
    std::is_convertible<decltype(std::declval<Comp>()(std::declval<Iter>(),
                                                      std::declval<Iter>())),
                        bool>::value,
    Iter>::type
ternary_search(Iter first, Iter last, Comp comp) {
  assert(first < last);
  typename std::make_signed<decltype(last - first)>::type dist(last - first);
  while (2 < dist) {
    Iter left(first + dist / 3), right(first + dist * 2 / 3);
    if (comp(left, right))
      last = right, dist = (dist + dist) / 3;
    else
      first = left, dist -= dist / 3;
  }
  if (1 < dist && comp(first + 1, first)) ++first;
  return first;
}

/**
 * @brief Ternary search on discrete range.
 * @param first Left end, inclusive
 * @param last Right end, exclusive
 * @param func Function
 * @return Local minimal point.
 */
template <class Iter, class Func>
typename std::enable_if<
    std::is_same<decltype(std::declval<Func>()(std::declval<Iter>()), nullptr),
                 std::nullptr_t>::value,
    Iter>::type
ternary_search(Iter const &first, Iter const &last, Func func) {
  return ternary_search(first, last, [&](Iter const &__i, Iter const &__j) {
    return func(__i) < func(__j);
  });
}

/**
 * @brief Ternary search on the real number line.
 * @param first Left end
 * @param last Right end
 * @param eps Error tolerance
 * @param comp Compare function
 * @return Local minimal point.
 */
template <class Real, class Comp>
typename std::enable_if<
    std::is_convertible<decltype(std::declval<Comp>()(std::declval<Real>(),
                                                      std::declval<Real>())),
                        bool>::value,
    Real>::type
ternary_search(Real first, Real last, Real const &eps, Comp comp) {
  assert(first < last);
  while (eps < last - first) {
    Real left{(first * 2 + last) / 3}, right{(first + last * 2) / 3};
    if (comp(left, right))
      last = right;
    else
      first = left;
  }
  return first;
}

/**
 * @brief Ternary search on the real number line.
 * @param first Left end
 * @param last Right end
 * @param eps Error tolerance
 * @param func Function
 * @return Local minimal point.
 */
template <class Real, class Func>
typename std::enable_if<
    std::is_same<decltype(std::declval<Func>()(std::declval<Real>()), nullptr),
                 std::nullptr_t>::value,
    Real>::type
ternary_search(Real const &first, Real const &last, Real const &eps,
               Func func) {
  return ternary_search(
      first, last, eps,
      [&](Real const &__i, Real const &__j) { return func(__i) < func(__j); });
}

}  // namespace workspace

#line 2 "src/opt/ternary_search.hpp"

/**
 * @file ternary_search.hpp
 * @brief Ternary Search
 */

#include <cassert>
#include <type_traits>

namespace workspace {

/**
 * @brief Ternary search on discrete range.
 * @param first Left end, inclusive
 * @param last Right end, exclusive
 * @param comp Compare function
 * @return Local minimal point.
 */
template <class Iter, class Comp>
typename std::enable_if<
    std::is_convertible<decltype(std::declval<Comp>()(std::declval<Iter>(),
                                                      std::declval<Iter>())),
                        bool>::value,
    Iter>::type
ternary_search(Iter first, Iter last, Comp comp) {
  assert(first < last);
  typename std::make_signed<decltype(last - first)>::type dist(last - first);
  while (2 < dist) {
    Iter left(first + dist / 3), right(first + dist * 2 / 3);
    if (comp(left, right))
      last = right, dist = (dist + dist) / 3;
    else
      first = left, dist -= dist / 3;
  }
  if (1 < dist && comp(first + 1, first)) ++first;
  return first;
}

/**
 * @brief Ternary search on discrete range.
 * @param first Left end, inclusive
 * @param last Right end, exclusive
 * @param func Function
 * @return Local minimal point.
 */
template <class Iter, class Func>
typename std::enable_if<
    std::is_same<decltype(std::declval<Func>()(std::declval<Iter>()), nullptr),
                 std::nullptr_t>::value,
    Iter>::type
ternary_search(Iter const &first, Iter const &last, Func func) {
  return ternary_search(first, last, [&](Iter const &__i, Iter const &__j) {
    return func(__i) < func(__j);
  });
}

/**
 * @brief Ternary search on the real number line.
 * @param first Left end
 * @param last Right end
 * @param eps Error tolerance
 * @param comp Compare function
 * @return Local minimal point.
 */
template <class Real, class Comp>
typename std::enable_if<
    std::is_convertible<decltype(std::declval<Comp>()(std::declval<Real>(),
                                                      std::declval<Real>())),
                        bool>::value,
    Real>::type
ternary_search(Real first, Real last, Real const &eps, Comp comp) {
  assert(first < last);
  while (eps < last - first) {
    Real left{(first * 2 + last) / 3}, right{(first + last * 2) / 3};
    if (comp(left, right))
      last = right;
    else
      first = left;
  }
  return first;
}

/**
 * @brief Ternary search on the real number line.
 * @param first Left end
 * @param last Right end
 * @param eps Error tolerance
 * @param func Function
 * @return Local minimal point.
 */
template <class Real, class Func>
typename std::enable_if<
    std::is_same<decltype(std::declval<Func>()(std::declval<Real>()), nullptr),
                 std::nullptr_t>::value,
    Real>::type
ternary_search(Real const &first, Real const &last, Real const &eps,
               Func func) {
  return ternary_search(
      first, last, eps,
      [&](Real const &__i, Real const &__j) { return func(__i) < func(__j); });
}

}  // namespace workspace

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