work_assignment.cc

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// 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.
 
#include "ortools/sat/work_assignment.h"
 
#include <algorithm>
#include <array>
#include <cmath>
#include <deque>
#include <functional>
#include <limits>
#include <memory>
#include <optional>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
 
#include "absl/container/flat_hash_set.h"
#include "absl/log/check.h"
#include "absl/strings/str_cat.h"
#include "absl/synchronization/mutex.h"
#include "absl/types/span.h"
#include "ortools/base/logging.h"
#include "ortools/sat/cp_model_mapping.h"
#include "ortools/sat/cp_model_utils.h"
#include "ortools/sat/integer.h"
#include "ortools/sat/integer_base.h"
#include "ortools/sat/integer_search.h"
#include "ortools/sat/model.h"
#include "ortools/sat/restart.h"
#include "ortools/sat/sat_base.h"
#include "ortools/sat/sat_decision.h"
#include "ortools/sat/sat_parameters.pb.h"
#include "ortools/sat/sat_solver.h"
#include "ortools/sat/synchronization.h"
#include "ortools/sat/util.h"
#include "ortools/util/strong_integers.h"
#include "ortools/util/time_limit.h"
 
namespace operations_research::sat {
namespace {
 
// We restart the shared tree 10 times after 2 restarts per worker. After that
// we restart when the tree reaches the maximum allowable number of nodes, but
// still at most once per 2 restarts per worker.
const int kSyncsPerWorkerPerRestart = 2;
const int kNumInitialRestarts = 10;
 
// If you build a tree by expanding the nodes with minimal depth+discrepancy,
// the number of leaves when all nodes with a given value have been split
// follows the fibonacci sequence:
// num_leaves(0) := 2;
// num_leaves(1) := 3;
// num_leaves(n) := num_leaves(n-1) + num_leaves(n-2)
// This function returns f(n) := min({i | num_leaves(i) >= n})
int MaxAllowedDiscrepancyPlusDepth(int num_leaves) {
  int i = 0;
  int a = 1;
  int b = 2;
  while (b < num_leaves) {
    std::tie(a, b) = std::make_pair(b, a + b);
    ++i;
  }
  return i;
}
}  // namespace
 
Literal ProtoLiteral::Decode(CpModelMapping* mapping,
                             IntegerEncoder* encoder) const {
  DCHECK_LT(proto_var_, mapping->NumProtoVariables());
  if (mapping->IsBoolean(proto_var_)) {
    return mapping->Literal(proto_var_);
  }
  return encoder->GetOrCreateAssociatedLiteral(DecodeInteger(mapping));
}
 
IntegerLiteral ProtoLiteral::DecodeInteger(CpModelMapping* mapping) const {
  const int positive_var = PositiveRef(proto_var_);
  if (!mapping->IsInteger(positive_var)) {
    return IntegerLiteral();
  }
  if (proto_var_ < 0) {
    return IntegerLiteral::LowerOrEqual(mapping->Integer(positive_var), -lb_);
  }
  return IntegerLiteral::GreaterOrEqual(mapping->Integer(positive_var), lb_);
}
 
std::optional<ProtoLiteral> ProtoLiteral::EncodeInteger(
    IntegerLiteral literal, CpModelMapping* mapping) {
  IntegerVariable positive_var = PositiveVariable(literal.var);
  const int model_var =
      mapping->GetProtoVariableFromIntegerVariable(positive_var);
  if (model_var == -1) {
    return std::nullopt;
  }
  ProtoLiteral result{
      literal.var == positive_var ? model_var : NegatedRef(model_var),
      literal.bound};
  DCHECK_EQ(result.DecodeInteger(mapping), literal);
  DCHECK_EQ(result.Negated().DecodeInteger(mapping), literal.Negated());
  return result;
}
std::optional<ProtoLiteral> ProtoLiteral::Encode(Literal literal,
                                                 CpModelMapping* mapping,
                                                 IntegerEncoder* encoder) {
  const std::optional<ProtoLiteral> result = EncodeLiteral(literal, mapping);
  if (result.has_value()) return result;
 
  for (auto int_lit : encoder->GetIntegerLiterals(literal)) {
    auto result = EncodeInteger(int_lit, mapping);
    if (result.has_value()) {
      DCHECK_EQ(result->DecodeInteger(mapping), int_lit);
      DCHECK_EQ(result->Negated().DecodeInteger(mapping), int_lit.Negated());
      return result;
    }
  }
  return std::nullopt;
}
 
std::optional<ProtoLiteral> ProtoLiteral::EncodeLiteral(
    Literal literal, CpModelMapping* mapping) {
  if (literal.Index() == kNoLiteralIndex) {
    return std::nullopt;
  }
  int model_var =
      mapping->GetProtoVariableFromBooleanVariable(literal.Variable());
  if (model_var == -1) {
    return std::nullopt;
  }
  DCHECK(mapping->IsBoolean(model_var));
  ProtoLiteral result{literal.IsPositive() ? model_var : NegatedRef(model_var),
                      literal.IsPositive() ? 1 : 0};
  return result;
}
 
ProtoTrail::ProtoTrail() { target_phase_.reserve(kMaxPhaseSize); }
 
void ProtoTrail::PushLevel(const ProtoLiteral& decision,
                           IntegerValue objective_lb, int node_id) {
  CHECK_GT(node_id, 0);
  decision_indexes_.push_back(literals_.size());
  literals_.push_back(decision);
  node_ids_.push_back(node_id);
  implications_.push_back({});
  if (!level_to_objective_lbs_.empty()) {
    objective_lb = std::max(level_to_objective_lbs_.back(), objective_lb);
  }
  level_to_objective_lbs_.push_back(objective_lb);
}
 
void ProtoTrail::SetLevelImplied(int level) {
  DCHECK_GE(level, 1);
  DCHECK_LE(level, decision_indexes_.size());
  DCHECK_LE(level, implications_.size());
  SetObjectiveLb(level - 1, ObjectiveLb(level));
  const ProtoLiteral decision = Decision(level);
  implication_level_[decision] = level - 1;
  // We don't store implications for level 0, so only move implications up to
  // the parent if we are removing level 2 or greater.
  if (level >= 2) {
    MutableImplications(level - 1).push_back(decision);
  }
  for (const ProtoLiteral& implication : Implications(level)) {
    implication_level_[implication] = level - 1;
    if (level >= 2) {
      MutableImplications(level - 1).push_back(implication);
    }
  }
  // implications_[level-1] stores the implications for level, which are now
  // stored in the parent's implications, so we can delete them.
  implications_.erase(implications_.begin() + level - 1);
  decision_indexes_.erase(decision_indexes_.begin() + level - 1);
  level_to_objective_lbs_.erase(level_to_objective_lbs_.begin() + level - 1);
}
 
void ProtoTrail::Clear() {
  decision_indexes_.clear();
  literals_.clear();
  level_to_objective_lbs_.clear();
  node_ids_.clear();
  target_phase_.clear();
  implication_level_.clear();
  implications_.clear();
}
 
void ProtoTrail::SetObjectiveLb(int level, IntegerValue objective_lb) {
  if (level == 0) return;
  level_to_objective_lbs_[level - 1] =
      std::max(objective_lb, level_to_objective_lbs_[level - 1]);
}
 
absl::Span<const int> ProtoTrail::NodeIds(int level) const {
  DCHECK_LE(level, decision_indexes_.size());
  int start = level == 0 ? 0 : decision_indexes_[level - 1];
  int end = level == decision_indexes_.size() ? node_ids_.size()
                                              : decision_indexes_[level];
  return absl::MakeSpan(node_ids_.data() + start, end - start);
}
 
absl::Span<const ProtoLiteral> ProtoTrail::Implications(int level) const {
  if (level > implications_.size() || level <= 0) {
    return absl::MakeSpan(literals_.data(), 0);
  }
  return absl::MakeSpan(implications_[level - 1]);
}
 
SharedTreeManager::SharedTreeManager(Model* model)
    : params_(*model->GetOrCreate<SatParameters>()),
      num_workers_(params_.shared_tree_num_workers()),
      shared_response_manager_(model->GetOrCreate<SharedResponseManager>()),
      num_splits_wanted_(
          num_workers_ * params_.shared_tree_open_leaves_per_worker() - 1),
      max_nodes_(
          params_.shared_tree_max_nodes_per_worker() >=
                  std::numeric_limits<int>::max() / std::max(num_workers_, 1)
              ? std::numeric_limits<int>::max()
              : num_workers_ * params_.shared_tree_max_nodes_per_worker()) {
  CHECK_GE(num_workers_, 0);
  // Create the root node with a fake literal.
  nodes_.push_back(
      {.literal = ProtoLiteral(),
       .objective_lb = shared_response_manager_->GetInnerObjectiveLowerBound(),
       .trail_info = std::make_unique<NodeTrailInfo>()});
  unassigned_leaves_.reserve(num_workers_);
  unassigned_leaves_.push_back(&nodes_.back());
}
 
int SharedTreeManager::NumNodes() const {
  absl::MutexLock mutex_lock(&mu_);
  return nodes_.size();
}
 
bool SharedTreeManager::SyncTree(ProtoTrail& path) {
  absl::MutexLock mutex_lock(&mu_);
  std::vector<std::pair<Node*, int>> nodes = GetAssignedNodes(path);
  if (!IsValid(path)) {
    path.Clear();
    return false;
  }
  // We don't rely on these being empty, but we expect them to be.
  DCHECK(to_close_.empty());
  DCHECK(to_update_.empty());
  int prev_level = -1;
  for (const auto& [node, level] : nodes) {
    if (level == prev_level) {
      to_close_.push_back(GetSibling(node));
    } else if (level > 0 && node->objective_lb < path.ObjectiveLb(level)) {
      node->objective_lb = path.ObjectiveLb(level);
      to_update_.push_back(node->parent);
    }
    if (level > 0 && !node->closed) {
      NodeTrailInfo* trail_info = GetTrailInfo(node);
      for (const ProtoLiteral& implication : path.Implications(level)) {
        auto it = trail_info->implications
                      .emplace(implication.proto_var(), implication.lb())
                      .first;
        if (it->second < implication.lb()) {
          it->second = implication.lb();
        }
      }
    }
    prev_level = level;
  }
  ProcessNodeChanges();
  if (nodes.back().first->closed) {
    path.Clear();
    return false;
  }
  // Restart after processing updates - we might learn a new objective bound.
  if (++num_syncs_since_restart_ / num_workers_ > kSyncsPerWorkerPerRestart &&
      num_restarts_ < kNumInitialRestarts) {
    RestartLockHeld();
    path.Clear();
    return false;
  }
  // Sync lower bounds and implications from the shared tree to `path`.
  AssignLeaf(path, nodes.back().first);
  return true;
}
 
void SharedTreeManager::ProposeSplit(ProtoTrail& path, ProtoLiteral decision) {
  absl::MutexLock mutex_lock(&mu_);
  if (!IsValid(path)) return;
  std::vector<std::pair<Node*, int>> nodes = GetAssignedNodes(path);
  if (nodes.back().first->closed) {
    VLOG(2) << "Cannot split closed node";
    return;
  }
  if (nodes.back().first->children[0] != nullptr) {
    LOG_IF(WARNING, nodes.size() > 1)
        << "Cannot resplit previously split node @ " << nodes.back().second
        << "/" << nodes.size();
    return;
  }
  if (nodes_.size() + 2 > max_nodes_) {
    VLOG(2) << "Too many nodes to accept split";
    return;
  }
  if (num_splits_wanted_ <= 0) {
    VLOG(2) << "Enough splits for now";
    return;
  }
  const int num_desired_leaves =
      params_.shared_tree_open_leaves_per_worker() * num_workers_;
  if (params_.shared_tree_split_strategy() ==
          SatParameters::SPLIT_STRATEGY_DISCREPANCY ||
      params_.shared_tree_split_strategy() ==
          SatParameters::SPLIT_STRATEGY_AUTO) {
    int discrepancy = 0;
    for (const auto& [node, level] : nodes) {
      if (node->parent == nullptr || node->implied) continue;
      IntegerValue sibling_bound = GetSibling(node)->objective_lb;
      discrepancy += (node->objective_lb == sibling_bound
                          ? node != node->parent->children[0]
                          : node->objective_lb > sibling_bound);
    }
    // TODO(user): Need to write up the shape this creates.
    // This rule will allow twice as many leaves in the preferred subtree.
    if (discrepancy + path.MaxLevel() >
        MaxAllowedDiscrepancyPlusDepth(num_desired_leaves) +
            params_.shared_tree_balance_tolerance()) {
      VLOG(2) << "Too high discrepancy to accept split";
      return;
    }
  } else if (params_.shared_tree_split_strategy() ==
             SatParameters::SPLIT_STRATEGY_OBJECTIVE_LB) {
    if (nodes.back().first->objective_lb > nodes.front().first->objective_lb) {
      VLOG(2) << "Can only split nodes with minimum objective lb, "
              << nodes.back().first->objective_lb << " > "
              << nodes.front().first->objective_lb;
      return;
    }
  } else if (params_.shared_tree_split_strategy() ==
             SatParameters::SPLIT_STRATEGY_BALANCED_TREE) {
    if (path.MaxLevel() + 1 >
        log2(num_desired_leaves) + params_.shared_tree_balance_tolerance()) {
      VLOG(2) << "Tree too unbalanced to accept split";
      return;
    }
  }
  VLOG_EVERY_N(2, 10) << unassigned_leaves_.size() << " unassigned leaves, "
                      << nodes_.size() << " subtrees, " << num_splits_wanted_
                      << " splits wanted";
  Split(nodes, decision);
  auto [new_leaf, level] = nodes.back();
  path.PushLevel(new_leaf->literal, new_leaf->objective_lb, new_leaf->id);
}
 
void SharedTreeManager::ReplaceTree(ProtoTrail& path) {
  absl::MutexLock mutex_lock(&mu_);
  std::vector<std::pair<Node*, int>> nodes = GetAssignedNodes(path);
  if (nodes.back().first->children[0] == nullptr &&
      !nodes.back().first->closed && nodes.size() > 1) {
    Node* leaf = nodes.back().first;
    VLOG(2) << "Returning leaf to be replaced";
    GetTrailInfo(leaf)->phase.assign(path.TargetPhase().begin(),
                                     path.TargetPhase().end());
    unassigned_leaves_.push_back(leaf);
  }
  path.Clear();
  while (!unassigned_leaves_.empty()) {
    const int i = num_leaves_assigned_++ % unassigned_leaves_.size();
    std::swap(unassigned_leaves_[i], unassigned_leaves_.back());
    Node* leaf = unassigned_leaves_.back();
    unassigned_leaves_.pop_back();
    if (!leaf->closed && leaf->children[0] == nullptr) {
      AssignLeaf(path, leaf);
      path.SetTargetPhase(GetTrailInfo(leaf)->phase);
      return;
    }
  }
  VLOG(2) << "Assigning root because no unassigned leaves are available";
  // TODO(user): Investigate assigning a random leaf so workers can still
  // improve shared tree bounds.
}
 
SharedTreeManager::NodeTrailInfo* SharedTreeManager::GetTrailInfo(Node* node) {
  CHECK(node != nullptr && !node->closed);
  while (node->trail_info == nullptr) {
    node = node->parent;
  }
  CHECK_NE(node, nullptr);
  return node->trail_info.get();
}
 
SharedTreeManager::Node* SharedTreeManager::GetSibling(Node* node) {
  if (node == nullptr || node->parent == nullptr) return nullptr;
  if (node->parent->children[0] != node) {
    return node->parent->children[0];
  }
  return node->parent->children[1];
}
 
void SharedTreeManager::Split(std::vector<std::pair<Node*, int>>& nodes,
                              ProtoLiteral lit) {
  const auto [parent, level] = nodes.back();
  DCHECK(parent->children[0] == nullptr);
  DCHECK(parent->children[1] == nullptr);
  parent->children[0] = MakeSubtree(parent, lit);
  parent->children[1] = MakeSubtree(parent, lit.Negated());
  NodeTrailInfo* trail_info = GetTrailInfo(parent);
  if (trail_info != nullptr) {
    parent->children[0]->trail_info = std::make_unique<NodeTrailInfo>(
        NodeTrailInfo{.phase = trail_info->phase});
    parent->children[1]->trail_info = std::make_unique<NodeTrailInfo>(
        NodeTrailInfo{.phase = std::move(trail_info->phase)});
  }
  nodes.push_back(std::make_pair(parent->children[0], level + 1));
  unassigned_leaves_.push_back(parent->children[1]);
  --num_splits_wanted_;
}
 
SharedTreeManager::Node* SharedTreeManager::MakeSubtree(Node* parent,
                                                        ProtoLiteral literal) {
  nodes_.push_back(
      Node{.literal = literal,
           .objective_lb = parent->objective_lb,
           .parent = parent,
           .id = static_cast<int>(nodes_.size() + node_id_offset_)});
  return &nodes_.back();
}
 
void SharedTreeManager::ProcessNodeChanges() {
  int num_newly_closed = 0;
  while (!to_close_.empty()) {
    Node* node = to_close_.back();
    CHECK_NE(node, nullptr);
    to_close_.pop_back();
    // Iterate over open parents while each sibling is closed.
    while (node != nullptr && !node->closed) {
      ++num_newly_closed;
      ++num_closed_nodes_;
      node->closed = true;
      // Keep the root trail_info so GetTrailInfo never returns nullptr.
      if (node->parent != nullptr) node->trail_info.reset();
      node->objective_lb = kMaxIntegerValue;
      // If we are closing a leaf, try to maintain the same number of leaves;
      num_splits_wanted_ += (node->children[0] == nullptr);
      for (Node* child : node->children) {
        if (child == nullptr || child->closed) continue;
        to_close_.push_back(child);
      }
      Node* sibling = GetSibling(node);
      if (sibling != nullptr) {
        sibling->implied = true;
        if (!sibling->closed) {
          break;
        }
      }
      node = node->parent;
    }
    DCHECK(node == nullptr || node->closed);
    if (node == nullptr) {
      shared_response_manager_->NotifyThatImprovingProblemIsInfeasible(
          ShortStatus());
    } else if (node->parent != nullptr) {
      to_update_.push_back(node->parent);
    }
  }
  if (num_newly_closed > 0) {
    shared_response_manager_->LogMessageWithThrottling(
        "Tree", absl::StrCat("nodes:", nodes_.size(), "/", max_nodes_,
                             " closed:", num_closed_nodes_,
                             " unassigned:", unassigned_leaves_.size(),
                             " restarts:", num_restarts_));
  }
  // TODO(user): We could do resolution here by moving implications that
  // are true in each child to the parent.
  bool root_updated = false;
  while (!to_update_.empty()) {
    Node* node = to_update_.back();
    to_update_.pop_back();
    // Iterate over parents while the lower bound can be improved.
    while (node != nullptr && !node->closed) {
      DCHECK(node->children[0] != nullptr);
      DCHECK(node->children[1] != nullptr);
      NodeTrailInfo* trail_info = GetTrailInfo(node);
      for (Node* child : node->children) {
        if (child->implied && child->trail_info != nullptr) {
          trail_info->implications.merge(child->trail_info->implications);
          child->trail_info.reset();
        }
      }
      IntegerValue child_bound = std::min(node->children[0]->objective_lb,
                                          node->children[1]->objective_lb);
      if (child_bound <= node->objective_lb) break;
      node->objective_lb = child_bound;
      node = node->parent;
    }
    if (node == nullptr) root_updated = true;
  }
  if (root_updated) {
    shared_response_manager_->UpdateInnerObjectiveBounds(
        ShortStatus(), nodes_[0].objective_lb, kMaxIntegerValue);
  }
  // These are shared via SharedBoundsManager, don't duplicate here.
  nodes_[0].trail_info->implications.clear();
}
 
std::vector<std::pair<SharedTreeManager::Node*, int>>
SharedTreeManager::GetAssignedNodes(const ProtoTrail& path) {
  std::vector<std::pair<Node*, int>> nodes({std::make_pair(&nodes_[0], 0)});
  if (!IsValid(path)) {
    // Restart has happened, nodes in this path are no longer valid, but the
    // root is equivalent.
    return nodes;
  }
  for (int i = 0; i <= path.MaxLevel(); ++i) {
    for (int id : path.NodeIds(i)) {
      const int index = id - node_id_offset_;
      CHECK_GE(index, 0) << " in path.NodeIds(" << i
                         << "), max_level=" << path.MaxLevel();
      CHECK_LT(index, nodes_.size());
      DCHECK_EQ(nodes.back().first, nodes_[index].parent);
      nodes.push_back(std::make_pair(&nodes_[index], i));
    }
  }
  return nodes;
}
 
void SharedTreeManager::CloseTree(ProtoTrail& path, int level) {
  absl::MutexLock mutex_lock(&mu_);
  const int node_id_to_close = path.NodeIds(level).front();
  path.Clear();
  if (node_id_to_close < node_id_offset_) return;
  Node* node = &nodes_[node_id_to_close - node_id_offset_];
  VLOG(2) << "Closing subtree at level " << level;
  DCHECK(to_close_.empty());
  to_close_.push_back(node);
  ProcessNodeChanges();
}
 
void SharedTreeManager::AssignLeaf(ProtoTrail& path, Node* leaf) {
  path.Clear();
  std::vector<Node*> reversed_path;
  while (leaf != &nodes_[0]) {
    reversed_path.push_back(&nodes_[leaf->id - node_id_offset_]);
    leaf = leaf->parent;
  }
  while (!reversed_path.empty()) {
    Node* leaf = reversed_path.back();
    reversed_path.pop_back();
    path.PushLevel(leaf->literal, leaf->objective_lb, leaf->id);
    if (leaf->implied) {
      path.SetLevelImplied(path.MaxLevel());
    }
    if (params_.shared_tree_worker_enable_trail_sharing() &&
        leaf->trail_info != nullptr) {
      for (const auto& [var, lb] : leaf->trail_info->implications) {
        path.AddImplication(path.MaxLevel(), ProtoLiteral(var, lb));
      }
    }
  }
}
 
bool SharedTreeManager::IsValid(const ProtoTrail& path) const {
  auto node_ids = path.NodeIds(path.MaxLevel());
  if (node_ids.empty()) return true;
  if (node_ids.back() < node_id_offset_) return false;
  return true;
}
 
void SharedTreeManager::RestartLockHeld() {
  node_id_offset_ += nodes_.size();
  nodes_.resize(1);
  nodes_[0].id = node_id_offset_;
  nodes_[0].children = {nullptr, nullptr};
  unassigned_leaves_.clear();
  num_splits_wanted_ =
      num_workers_ * params_.shared_tree_open_leaves_per_worker() - 1;
  num_closed_nodes_ = 0;
  num_restarts_ += 1;
  num_syncs_since_restart_ = 0;
}
 
std::string SharedTreeManager::ShortStatus() const {
  return absl::StrCat("shared_tree_manager(r=", num_restarts_,
                      " n=", nodes_.size(), ")");
}
 
SharedTreeWorker::SharedTreeWorker(Model* model)
    : parameters_(model->GetOrCreate<SatParameters>()),
      shared_response_(model->GetOrCreate<SharedResponseManager>()),
      time_limit_(model->GetOrCreate<TimeLimit>()),
      manager_(model->GetOrCreate<SharedTreeManager>()),
      mapping_(model->GetOrCreate<CpModelMapping>()),
      sat_solver_(model->GetOrCreate<SatSolver>()),
      trail_(model->GetOrCreate<Trail>()),
      integer_trail_(model->GetOrCreate<IntegerTrail>()),
      encoder_(model->GetOrCreate<IntegerEncoder>()),
      objective_(model->Get<ObjectiveDefinition>()),
      random_(model->GetOrCreate<ModelRandomGenerator>()),
      helper_(model->GetOrCreate<IntegerSearchHelper>()),
      heuristics_(model->GetOrCreate<SearchHeuristics>()),
      decision_policy_(model->GetOrCreate<SatDecisionPolicy>()),
      restart_policy_(model->GetOrCreate<RestartPolicy>()),
      level_zero_callbacks_(model->GetOrCreate<LevelZeroCallbackHelper>()),
      reversible_int_repository_(model->GetOrCreate<RevIntRepository>()),
      assigned_tree_lbds_(/*window_size=*/8) {}
 
const std::vector<Literal>& SharedTreeWorker::DecisionReason(int level) {
  CHECK_LE(level, assigned_tree_literals_.size());
  reason_.clear();
  for (int i = 0; i < level; ++i) {
    reason_.push_back(assigned_tree_literals_[i].Negated());
  }
  return reason_;
}
 
bool SharedTreeWorker::AddDecisionImplication(Literal lit, int level) {
  CHECK_NE(lit.Index(), kNoLiteralIndex);
  CHECK(!sat_solver_->Assignment().LiteralIsTrue(lit));
  if (sat_solver_->Assignment().LiteralIsFalse(lit)) {
    VLOG(2) << "Closing subtree via impl at " << level + 1
            << " assigned=" << assigned_tree_.MaxLevel();
    integer_trail_->ReportConflict(DecisionReason(level), {});
    manager_->CloseTree(assigned_tree_, level);
    assigned_tree_literals_.clear();
    return false;
  }
  integer_trail_->EnqueueLiteral(lit, DecisionReason(level), {});
  VLOG(2) << "Learned shared clause";
  return true;
}
 
bool SharedTreeWorker::AddImplications() {
  const int level = sat_solver_->CurrentDecisionLevel();
  // Level 0 implications are unit clauses and are synced elsewhere.
  if (level == 0) return false;
  if (level > assigned_tree_.MaxLevel()) {
    return false;
  }
  rev_num_processed_implications_.resize(level + 1, 0);
  auto& num_processed_implications = rev_num_processed_implications_[level];
  reversible_int_repository_->SaveState(&num_processed_implications);
  absl::Span<const Literal> implied_literals =
      absl::MakeConstSpan(assigned_tree_implications_[level - 1])
          .subspan(num_processed_implications);
  bool added_clause = false;
  for (Literal impl : implied_literals) {
    ++num_processed_implications;
    if (sat_solver_->Assignment().LiteralIsTrue(impl)) continue;
    added_clause = true;
    if (!AddDecisionImplication(impl, level)) return true;
  }
  if (objective_ != nullptr &&
      objective_->objective_var != kNoIntegerVariable) {
    const IntegerValue obj_lb =
        integer_trail_->LowerBound(objective_->objective_var);
    assigned_tree_.SetObjectiveLb(level, obj_lb);
    const Literal obj_lit =
        encoder_->GetOrCreateAssociatedLiteral(IntegerLiteral::GreaterOrEqual(
            objective_->objective_var, assigned_tree_.ObjectiveLb(level)));
    if (!sat_solver_->Assignment().LiteralIsTrue(obj_lit)) {
      AddDecisionImplication(obj_lit, level);
      return true;
    }
  }
  return added_clause;
}
 
bool SharedTreeWorker::SyncWithLocalTrail() {
  while (true) {
    if (!sat_solver_->FinishPropagation()) return false;
    // Ensure we are at fixed point w.r.t. implications in the tree up to the
    // current level.
    if (AddImplications()) continue;
 
    if (!helper_->BeforeTakingDecision()) return false;
    const int level = sat_solver_->CurrentDecisionLevel();
    if (parameters_->shared_tree_worker_enable_trail_sharing() && level > 0 &&
        level <= assigned_tree_.MaxLevel()) {
      // Add implications from the local trail to share with other workers.
      reversible_int_repository_->SaveState(&reversible_trail_index_);
      for (int i = trail_->Index() - 1; i >= reversible_trail_index_; --i) {
        const Literal lit = (*trail_)[i];
        if (trail_->AssignmentType(lit.Variable()) ==
            AssignmentType::kSearchDecision) {
          break;
        }
        std::optional<ProtoLiteral> encoded = EncodeDecision(lit);
        if (!encoded.has_value()) continue;
        assigned_tree_.AddImplication(level, *encoded);
      }
      reversible_trail_index_ = trail_->Index();
    }
    if (level >= assigned_tree_.MaxLevel()) break;
    // The next decision is assigned, make sure it makes sense.
    const Literal next_decision = assigned_tree_literals_[level];
    if (!sat_solver_->Assignment().LiteralIsAssigned(next_decision)) break;
    if (sat_solver_->Assignment().LiteralIsFalse(next_decision)) {
      // Next assigned decision is impossible.
      VLOG(2) << "Closing subtree at " << level + 1
              << " assigned=" << assigned_tree_.MaxLevel();
      manager_->CloseTree(assigned_tree_, level + 1);
      assigned_tree_literals_.clear();
      assigned_tree_implications_.clear();
      sat_solver_->Backtrack(0);
    } else {
      // The next level is implied by the current one.
      assigned_tree_.SetLevelImplied(level + 1);
      if (level > 0) {
        assigned_tree_implications_[level - 1].insert(
            assigned_tree_implications_[level - 1].end(),
            assigned_tree_implications_[level].begin(),
            assigned_tree_implications_[level].end());
      }
      assigned_tree_implications_.erase(assigned_tree_implications_.begin() +
                                        level);
      assigned_tree_literals_.erase(assigned_tree_literals_.begin() + level);
    }
  }
  return true;
}
 
bool SharedTreeWorker::NextDecision(LiteralIndex* decision_index) {
  const auto& decision_policy =
      heuristics_->decision_policies[heuristics_->policy_index];
  const int next_level = sat_solver_->CurrentDecisionLevel() + 1;
  new_split_available_ = next_level == assigned_tree_.MaxLevel() + 1;
 
  CHECK_EQ(assigned_tree_literals_.size(), assigned_tree_.MaxLevel());
  if (next_level <= assigned_tree_.MaxLevel()) {
    VLOG(2) << "Following shared trail depth=" << next_level << " "
            << parameters_->name();
    const Literal decision = assigned_tree_literals_[next_level - 1];
    CHECK(!sat_solver_->Assignment().LiteralIsFalse(decision))
        << " at depth " << next_level << " " << parameters_->name();
    CHECK(!sat_solver_->Assignment().LiteralIsTrue(decision));
    *decision_index = decision.Index();
    return true;
  }
  return helper_->GetDecision(decision_policy, decision_index);
}
 
void SharedTreeWorker::MaybeProposeSplit() {
  if (!new_split_available_ ||
      sat_solver_->CurrentDecisionLevel() != assigned_tree_.MaxLevel() + 1) {
    return;
  }
  new_split_available_ = false;
  const Literal split_decision =
      sat_solver_->Decisions()[assigned_tree_.MaxLevel()].literal;
  const std::optional<ProtoLiteral> encoded = EncodeDecision(split_decision);
  if (encoded.has_value()) {
    CHECK_EQ(assigned_tree_literals_.size(), assigned_tree_.MaxLevel());
    manager_->ProposeSplit(assigned_tree_, *encoded);
    if (assigned_tree_.MaxLevel() > assigned_tree_literals_.size()) {
      assigned_tree_literals_.push_back(split_decision);
      assigned_tree_implications_.push_back({});
    }
    CHECK_EQ(assigned_tree_literals_.size(), assigned_tree_.MaxLevel());
  }
}
 
bool SharedTreeWorker::ShouldReplaceSubtree() {
  // If we have no assignment, try to get one.
  if (assigned_tree_.MaxLevel() == 0) return true;
  if (restart_policy_->NumRestarts() <
          parameters_->shared_tree_worker_min_restarts_per_subtree() ||
      time_limit_->GetElapsedDeterministicTime() <
          earliest_replacement_dtime_) {
    return false;
  }
  return assigned_tree_lbds_.WindowAverage() <
         restart_policy_->LbdAverageSinceReset();
}
 
bool SharedTreeWorker::SyncWithSharedTree() {
  manager_->SyncTree(assigned_tree_);
  if (ShouldReplaceSubtree()) {
    ++num_trees_;
    VLOG(2) << parameters_->name() << " acquiring tree #" << num_trees_
            << " after " << restart_policy_->NumRestarts() << " restarts"
            << " prev depth: " << assigned_tree_.MaxLevel()
            << " target: " << assigned_tree_lbds_.WindowAverage()
            << " lbd: " << restart_policy_->LbdAverageSinceReset();
    if (parameters_->shared_tree_worker_enable_phase_sharing() &&
        // Only save the phase if we've done a non-trivial amount of work on
        // this subtree.
        FinishedMinRestarts() &&
        !decision_policy_->GetBestPartialAssignment().empty()) {
      assigned_tree_.ClearTargetPhase();
      for (Literal lit : decision_policy_->GetBestPartialAssignment()) {
        // Only set the phase for booleans to avoid creating literals on other
        // workers.
        auto encoded = ProtoLiteral::EncodeLiteral(lit, mapping_);
        if (!encoded.has_value()) continue;
        if (!assigned_tree_.AddPhase(*encoded)) break;
      }
    }
    manager_->ReplaceTree(assigned_tree_);
    assigned_tree_lbds_.Add(restart_policy_->LbdAverageSinceReset());
    restart_policy_->Reset();
    earliest_replacement_dtime_ = 0;
    if (parameters_->shared_tree_worker_enable_phase_sharing()) {
      VLOG(2) << "Importing phase of length: "
              << assigned_tree_.TargetPhase().size();
      decision_policy_->ClearBestPartialAssignment();
      for (const ProtoLiteral& lit : assigned_tree_.TargetPhase()) {
        decision_policy_->SetTargetPolarity(DecodeDecision(lit));
      }
    }
  }
  // If we commit to this subtree, keep it for at least 1s of dtime.
  // This allows us to replace obviously bad subtrees quickly, and not replace
  // too frequently overall.
  if (FinishedMinRestarts() && earliest_replacement_dtime_ >=
                                   time_limit_->GetElapsedDeterministicTime()) {
    earliest_replacement_dtime_ =
        time_limit_->GetElapsedDeterministicTime() + 1;
    // Treat this as reassigning the same tree.
    assigned_tree_lbds_.Add(restart_policy_->LbdAverageSinceReset());
  }
  VLOG(2) << "Assigned level: " << assigned_tree_.MaxLevel() << " "
          << parameters_->name();
  assigned_tree_literals_.clear();
  assigned_tree_implications_.clear();
  for (int i = 1; i <= assigned_tree_.MaxLevel(); ++i) {
    assigned_tree_literals_.push_back(
        DecodeDecision(assigned_tree_.Decision(i)));
    std::vector<Literal> implications;
    for (const ProtoLiteral& impl : assigned_tree_.Implications(i)) {
      implications.push_back(DecodeDecision(impl));
    }
    assigned_tree_implications_.push_back(std::move(implications));
  }
  return true;
}
 
SatSolver::Status SharedTreeWorker::Search(
    const std::function<void()>& feasible_solution_observer) {
  // Inside GetAssociatedLiteral if a literal becomes fixed at level 0 during
  // Search, the code CHECKs it is at level 0 when decoding the literal, but
  // the fixed literals are cached, so we can create them now to avoid a
  // crash.
  sat_solver_->Backtrack(0);
  encoder_->GetTrueLiteral();
  encoder_->GetFalseLiteral();
  level_zero_callbacks_->callbacks.push_back(
      [this]() { return SyncWithSharedTree(); });
  const bool has_objective =
      objective_ != nullptr && objective_->objective_var != kNoIntegerVariable;
  while (!time_limit_->LimitReached()) {
    if (!sat_solver_->FinishPropagation()) {
      return sat_solver_->UnsatStatus();
    }
    if (heuristics_->restart_policies[heuristics_->policy_index]()) {
      heuristics_->policy_index = restart_policy_->NumRestarts() %
                                  heuristics_->decision_policies.size();
      sat_solver_->Backtrack(0);
    }
    if (!SyncWithLocalTrail()) return sat_solver_->UnsatStatus();
    LiteralIndex decision_index;
    if (!NextDecision(&decision_index)) continue;
    if (time_limit_->LimitReached()) return SatSolver::LIMIT_REACHED;
    if (decision_index == kNoLiteralIndex) {
      feasible_solution_observer();
      if (!has_objective) return SatSolver::FEASIBLE;
      const IntegerValue objective =
          integer_trail_->LowerBound(objective_->objective_var);
      sat_solver_->Backtrack(0);
      if (!integer_trail_->Enqueue(
              IntegerLiteral::LowerOrEqual(objective_->objective_var,
                                           objective - 1),
              {}, {})) {
        return SatSolver::INFEASIBLE;
      }
 
      continue;
    }
    const Literal decision(decision_index);
    CHECK(!sat_solver_->Assignment().LiteralIsFalse(decision));
    CHECK(!sat_solver_->Assignment().LiteralIsTrue(decision));
    if (!helper_->TakeDecision(decision)) {
      return sat_solver_->UnsatStatus();
    }
    MaybeProposeSplit();
  }
 
  return SatSolver::LIMIT_REACHED;
}
 
Literal SharedTreeWorker::DecodeDecision(ProtoLiteral lit) {
  return lit.Decode(mapping_, encoder_);
}
 
std::optional<ProtoLiteral> SharedTreeWorker::EncodeDecision(Literal decision) {
  return ProtoLiteral::Encode(decision, mapping_, encoder_);
}
 
}  // namespace operations_research::sat