Revert deletion of graph algorithms

ortools/base/BUILD.bazel

@@ -548,6 +548,12 @@ cc_library(
     ],
 )
 
+cc_library(
+    name = "top_n",
+    hdrs = ["top_n.h"],
+    deps = [":logging"],
+)
+
 cc_library(
     name = "types",
     hdrs = ["types.h"],

ortools/base/top_n.h

@@ -0,0 +1,327 @@
+// Copyright 2010-2025 Google LLC
+// 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
+//
+//     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.
+
+// ref:
+// https://github.com/tensorflow/tensorflow/blob/master/tensorflow/core/lib/gtl/top_n.h
+// This simple class finds the top n elements of an incrementally provided set
+// of elements which you push one at a time.  If the number of elements exceeds
+// n, the lowest elements are incrementally dropped.  At the end you get
+// a vector of the top elements sorted in descending order (through Extract() or
+// ExtractNondestructive()), or a vector of the top elements but not sorted
+// (through ExtractUnsorted() or ExtractUnsortedNondestructive()).
+//
+// The value n is specified in the constructor.  If there are p elements pushed
+// altogether:
+//   The total storage requirements are O(min(n, p)) elements
+//   The running time is O(p * log(min(n, p))) comparisons
+// If n is a constant, the total storage required is a constant and the running
+// time is linear in p.
+
+#ifndef ORTOOLS_BASE_TOP_N_H_
+#define ORTOOLS_BASE_TOP_N_H_
+
+#include <stddef.h>
+
+#include <algorithm>
+#include <functional>
+#include <vector>
+
+namespace operations_research {
+namespace gtl {
+// Cmp is an stl binary predicate.  Note that Cmp is the "greater" predicate,
+// not the more commonly used "less" predicate.
+//
+// If you use a "less" predicate here, the TopN will pick out the bottom N
+// elements out of the ones passed to it, and it will return them sorted in
+// ascending order.
+//
+// TopN is rule-of-zero copyable and movable if its members are.
+template <class T, class Cmp = std::greater<T> >
+class TopN {
+ public:
+  // The TopN is in one of the three states:
+  //
+  //  o UNORDERED: this is the state an instance is originally in,
+  //    where the elements are completely orderless.
+  //
+  //  o BOTTOM_KNOWN: in this state, we keep the invariant that there
+  //    is at least one element in it, and the lowest element is at
+  //    position 0. The elements in other positions remain
+  //    unsorted. This state is reached if the state was originally
+  //    UNORDERED and a peek_bottom() function call is invoked.
+  //
+  //  o HEAP_SORTED: in this state, the array is kept as a heap and
+  //    there are exactly (limit_+1) elements in the array. This
+  //    state is reached when at least (limit_+1) elements are
+  //    pushed in.
+  //
+  //  The state transition graph is at follows:
+  //
+  //             peek_bottom()                (limit_+1) elements
+  //  UNORDERED --------------> BOTTOM_KNOWN --------------------> HEAP_SORTED
+  //      |                                                           ^
+  //      |                      (limit_+1) elements                  |
+  //      +-----------------------------------------------------------+
+  enum State { UNORDERED, BOTTOM_KNOWN, HEAP_SORTED };
+  using UnsortedIterator = typename std::vector<T>::const_iterator;
+  // 'limit' is the maximum number of top results to return.
+  explicit TopN(size_t limit) : TopN(limit, Cmp()) {}
+  TopN(size_t limit, const Cmp& cmp) : limit_(limit), cmp_(cmp) {}
+  size_t limit() const { return limit_; }
+  // Number of elements currently held by this TopN object.  This
+  // will be no greater than 'limit' passed to the constructor.
+  size_t size() const { return std::min(elements_.size(), limit_); }
+  bool empty() const { return size() == 0; }
+  // If you know how many elements you will push at the time you create the
+  // TopN object, you can call reserve to preallocate the memory that TopN
+  // will need to process all 'n' pushes.  Calling this method is optional.
+  void reserve(size_t n) { elements_.reserve(std::min(n, limit_ + 1)); }
+  // Push 'v'.  If the maximum number of elements was exceeded, drop the
+  // lowest element and return it in 'dropped' (if given). If the maximum is not
+  // exceeded, 'dropped' will remain unchanged. 'dropped' may be omitted or
+  // nullptr, in which case it is not filled in.
+  // Requires: T is CopyAssignable, Swappable
+  void push(const T& v) { push(v, nullptr); }
+  void push(const T& v, T* dropped) { PushInternal(v, dropped); }
+  // Move overloads of push.
+  // Requires: T is MoveAssignable, Swappable
+  void push(T&& v) {  // NOLINT(build/c++11)
+    push(std::move(v), nullptr);
+  }
+  void push(T&& v, T* dropped) {  // NOLINT(build/c++11)
+    PushInternal(std::move(v), dropped);
+  }
+  // Peeks the bottom result without calling Extract()
+  const T& peek_bottom();
+  // Destructively extract the elements as a vector, sorted in descending order.
+  // Leaves TopN in an empty state.
+  std::vector<T> Take();
+  // Extract the elements as a vector sorted in descending order.  The caller
+  // assumes ownership of the vector and must delete it when done.  This is a
+  // destructive operation.  The only method that can be called immediately
+  // after Extract() is Reset().
+  std::vector<T>* Extract();
+  // Similar to Extract(), but makes no guarantees the elements are in sorted
+  // order.  As with Extract(), the caller assumes ownership of the vector and
+  // must delete it when done.  This is a destructive operation.  The only
+  // method that can be called immediately after ExtractUnsorted() is Reset().
+  std::vector<T>* ExtractUnsorted();
+  // A non-destructive version of Extract(). Copy the elements in a new vector
+  // sorted in descending order and return it.  The caller assumes ownership of
+  // the new vector and must delete it when done.  After calling
+  // ExtractNondestructive(), the caller can continue to push() new elements.
+  std::vector<T>* ExtractNondestructive() const;
+  // A non-destructive version of Extract(). Copy the elements to a given
+  // vector sorted in descending order. After calling
+  // ExtractNondestructive(), the caller can continue to push() new elements.
+  // Note:
+  //  1. The given argument must to be allocated.
+  //  2. Any data contained in the vector prior to the call will be deleted
+  //     from it. After the call the vector will contain only the elements
+  //     from the data structure.
+  void ExtractNondestructive(std::vector<T>* output) const;
+  // A non-destructive version of ExtractUnsorted(). Copy the elements in a new
+  // vector and return it, with no guarantees the elements are in sorted order.
+  // The caller assumes ownership of the new vector and must delete it when
+  // done.  After calling ExtractUnsortedNondestructive(), the caller can
+  // continue to push() new elements.
+  std::vector<T>* ExtractUnsortedNondestructive() const;
+  // A non-destructive version of ExtractUnsorted(). Copy the elements into
+  // a given vector, with no guarantees the elements are in sorted order.
+  // After calling ExtractUnsortedNondestructive(), the caller can continue
+  // to push() new elements.
+  // Note:
+  //  1. The given argument must to be allocated.
+  //  2. Any data contained in the vector prior to the call will be deleted
+  //     from it. After the call the vector will contain only the elements
+  //     from the data structure.
+  void ExtractUnsortedNondestructive(std::vector<T>* output) const;
+  // Return an iterator to the beginning (end) of the container,
+  // with no guarantees about the order of iteration. These iterators are
+  // invalidated by mutation of the data structure.
+  UnsortedIterator unsorted_begin() const { return elements_.begin(); }
+  UnsortedIterator unsorted_end() const { return elements_.begin() + size(); }
+  // Accessor for comparator template argument.
+  Cmp* comparator() { return &cmp_; }
+  // This removes all elements.  If Extract() or ExtractUnsorted() have been
+  // called, this will put it back in an empty but useable state.
+  void Reset();
+
+ private:
+  template <typename U>
+  void PushInternal(
+      U&& v,
+      T* dropped);  // NOLINT(build/c++11)
+                    // elements_ can be in one of two states:
+                    //   elements_.size() <= limit_:  elements_ is an unsorted
+                    //   vector of elements
+                    //      pushed so far.
+                    //   elements_.size() > limit_:  The last element of
+                    //   elements_ is unused;
+                    //      the other elements of elements_ are an stl heap
+                    //      whose size is exactly limit_.  In this case
+                    //      elements_.size() is exactly one greater than limit_,
+                    //      but don't use "elements_.size() == limit_ + 1" to
+                    //      check for that because you'll get a false positive
+                    //      if limit_ == size_t(-1).
+  std::vector<T> elements_;
+  size_t limit_;  // Maximum number of elements to find
+  Cmp cmp_;       // Greater-than comparison function
+  State state_ = UNORDERED;
+};
+// ----------------------------------------------------------------------
+// Implementations of non-inline functions
+template <class T, class Cmp>
+template <typename U>
+void TopN<T, Cmp>::PushInternal(U&& v, T* dropped) {  // NOLINT(build/c++11)
+  if (limit_ == 0) {
+    if (dropped) *dropped = std::forward<U>(v);  // NOLINT(build/c++11)
+    return;
+  }
+  if (state_ != HEAP_SORTED) {
+    elements_.push_back(std::forward<U>(v));  // NOLINT(build/c++11)
+    if (state_ == UNORDERED || cmp_(elements_.back(), elements_.front())) {
+      // Easy case: we just pushed the new element back
+    } else {
+      // To maintain the BOTTOM_KNOWN state, we need to make sure that
+      // the element at position 0 is always the smallest. So we put
+      // the new element at position 0 and push the original bottom
+      // element in the back.
+      // Warning: this code is subtle.
+      using std::swap;
+      swap(elements_.front(), elements_.back());
+    }
+    if (elements_.size() == limit_ + 1) {
+      // Transition from unsorted vector to a heap.
+      std::make_heap(elements_.begin(), elements_.end(), cmp_);
+      if (dropped) *dropped = std::move(elements_.front());
+      std::pop_heap(elements_.begin(), elements_.end(), cmp_);
+      state_ = HEAP_SORTED;
+    }
+  } else {
+    // Only insert the new element if it is greater than the least element.
+    if (cmp_(v, elements_.front())) {
+      // Store new element in the last slot of elements_.  Remember from the
+      // comments on elements_ that this last slot is unused, so we don't
+      // overwrite anything useful.
+      elements_.back() = std::forward<U>(
+          v);  // NOLINT(build/c++11)
+               // stp::pop_heap() swaps elements_.front() and elements_.back()
+               // and rearranges elements from [elements_.begin(),
+               // elements_.end() - 1) such that they are a heap according to
+               // cmp_.  Net effect: remove elements_.front() from the heap, and
+               // add the new element instead.  For more info, see
+               // https://en.cppreference.com/w/cpp/algorithm/pop_heap.
+      std::pop_heap(elements_.begin(), elements_.end(), cmp_);
+      if (dropped) *dropped = std::move(elements_.back());
+    } else {
+      if (dropped) *dropped = std::forward<U>(v);  // NOLINT(build/c++11)
+    }
+  }
+}
+template <class T, class Cmp>
+const T& TopN<T, Cmp>::peek_bottom() {
+  CHECK(!empty());
+  if (state_ == UNORDERED) {
+    // We need to do a linear scan to find out the bottom element
+    int min_candidate = 0;
+    for (size_t i = 1; i < elements_.size(); ++i) {
+      if (cmp_(elements_[min_candidate], elements_[i])) {
+        min_candidate = i;
+      }
+    }
+    // By swapping the element at position 0 and the minimal
+    // element, we transition to the BOTTOM_KNOWN state
+    if (min_candidate != 0) {
+      using std::swap;
+      swap(elements_[0], elements_[min_candidate]);
+    }
+    state_ = BOTTOM_KNOWN;
+  }
+  return elements_.front();
+}
+template <class T, class Cmp>
+std::vector<T> TopN<T, Cmp>::Take() {
+  std::vector<T> out = std::move(elements_);
+  if (state_ != State::HEAP_SORTED) {
+    std::sort(out.begin(), out.end(), cmp_);
+  } else {
+    out.pop_back();
+    std::sort_heap(out.begin(), out.end(), cmp_);
+  }
+  Reset();
+  return out;
+}
+
+template <class T, class Cmp>
+std::vector<T>* TopN<T, Cmp>::Extract() {
+  auto out = new std::vector<T>;
+  out->swap(elements_);
+  if (state_ != HEAP_SORTED) {
+    std::sort(out->begin(), out->end(), cmp_);
+  } else {
+    out->pop_back();
+    std::sort_heap(out->begin(), out->end(), cmp_);
+  }
+  return out;
+}
+template <class T, class Cmp>
+std::vector<T>* TopN<T, Cmp>::ExtractUnsorted() {
+  auto out = new std::vector<T>;
+  out->swap(elements_);
+  if (state_ == HEAP_SORTED) {
+    // Remove the limit_+1'th element.
+    out->pop_back();
+  }
+  return out;
+}
+template <class T, class Cmp>
+std::vector<T>* TopN<T, Cmp>::ExtractNondestructive() const {
+  auto out = new std::vector<T>;
+  ExtractNondestructive(out);
+  return out;
+}
+template <class T, class Cmp>
+void TopN<T, Cmp>::ExtractNondestructive(std::vector<T>* output) const {
+  CHECK(output);
+  *output = elements_;
+  if (state_ != HEAP_SORTED) {
+    std::sort(output->begin(), output->end(), cmp_);
+  } else {
+    output->pop_back();
+    std::sort_heap(output->begin(), output->end(), cmp_);
+  }
+}
+template <class T, class Cmp>
+std::vector<T>* TopN<T, Cmp>::ExtractUnsortedNondestructive() const {
+  auto elements = new std::vector<T>;
+  ExtractUnsortedNondestructive(elements);
+  return elements;
+}
+template <class T, class Cmp>
+void TopN<T, Cmp>::ExtractUnsortedNondestructive(std::vector<T>* output) const {
+  CHECK(output);
+  *output = elements_;
+  if (state_ == HEAP_SORTED) {
+    // Remove the limit_+1'th element.
+    output->pop_back();
+  }
+}
+template <class T, class Cmp>
+void TopN<T, Cmp>::Reset() {
+  elements_.clear();
+  state_ = UNORDERED;
+}
+}  // namespace gtl
+}  // namespace operations_research
+#endif  // ORTOOLS_BASE_TOP_N_H_