ForestContractor.h

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#pragma once
 
#ifndef __FOREST_CONTRACTOR_H_
#define __FOREST_CONTRACTOR_H_ 1
 
#include "BaseContractor.h"
 
#include <boost/container_hash/hash.hpp>
 
namespace TensorNetworks {

class ForestContractor : public BaseContractor {
public:
  double Contract(const TensorNetwork &network, Types::qubit_t qubit) override {
    std::vector<Eigen::Index> keys;
    std::unordered_map<Eigen::Index, Eigen::Index> keysKeys;
 
    TensorsMap tensors =
        InitializeTensors(network, qubit, keys, keysKeys, false);
    std::map<Eigen::Index, std::shared_ptr<TensorNode>> tensorsMap(
        tensors.begin(), tensors.end());
 
    using TensorPair = std::pair<Eigen::Index, Eigen::Index>;
    std::unordered_set<TensorPair, boost::hash<TensorPair>> visitedPairs;
 
    Eigen::Index maxRank = 4;
    Eigen::Index nextRank = 4;
 
    // the first two initialized just to keep the compiler happy
    Eigen::Index tensor1Id = 0;
    Eigen::Index tensor2Id = 1;
    Eigen::Index resultRank = 4;
    Eigen::Index bestCost = 4;
 
    // while there is more than one tensor...
    while (tensors.size() > 1) {
      // by going like this we pick up the tensors that are closer to the end of
      // the circuit, to contract with the 'super' ones can be very efficient
      // for forest kind of circuits
 
      bool found = false;
      bool nextRankSet = false;
 
      for (auto tensorIt = tensorsMap.rbegin(); tensorIt != tensorsMap.rend();
           ++tensorIt) {
        const auto &tensor = tensorIt->second;
        const auto curTensorId = tensor->GetId();
 
        // despite having a bigger rank, the contraction could lead to a smaller
        // one to match the rank limit, so don't do this
        // if (tensor->GetRank() > maxRank) continue;
 
        for (Eigen::Index ti = 0;
             ti < static_cast<Eigen::Index>(tensor->connections.size()); ++ti) {
          const auto nextTensorId = tensor->connections[ti];
 
          if (nextTensorId != TensorNode::NotConnected) {
            auto t1 = curTensorId;
            auto t2 = nextTensorId;
            if (t1 > t2)
              std::swap(t1, t2);
 
            const auto p = std::make_pair(t1, t2);
            if (visitedPairs.find(p) != visitedPairs.end())
              continue;
            else
              visitedPairs.insert(p);
 
            const Eigen::Index newRank =
                GetResultRank(tensor, tensors[nextTensorId]);
 
            if (newRank <= maxRank) {
              const Eigen::Index Cost =
                  newRank -
                  (tensor->GetRank() + tensors[nextTensorId]->GetRank()) / 2;
 
              if (!found || newRank < resultRank ||
                  (newRank == resultRank && Cost < bestCost)) {
                tensor1Id = curTensorId;
                tensor2Id = nextTensorId;
                resultRank = newRank;
                bestCost = Cost;
                found = true;
              }
            }
 
            if (!found) {
              if (!nextRankSet) {
                nextRankSet = true;
                nextRank = newRank;
              } else
                nextRank = std::min(nextRank, newRank);
            }
          }
        }
      }
 
      visitedPairs.clear();
 
      if (!found) {
        maxRank = nextRank;
        continue;
      }
 
      ContractNodes(qubit, tensors, tensor1Id, tensor2Id, resultRank);
      tensorsMap[tensor1Id] = tensors[tensor1Id];
      tensorsMap.erase(tensor2Id);
 
      if (resultRank == 0) {
        if (tensors.size() == 1 || tensors[tensor1Id]->contractsTheNeededQubit)
          return std::real(tensors[tensor1Id]->tensor->atOffset(0));
 
        // erasing this tensor happens because (not the case anymore, it's
        // avoided) the tensor network might be a disjoint one and a subnetwork
        // is contracted that does not contain the needed qubit
 
        tensors.erase(tensor1Id);
        tensorsMap.erase(tensor1Id);
      }
    }
 
    return std::real(tensors.begin()->second->tensor->atOffset(0));
  }

  std::shared_ptr<ITensorContractor> Clone() const override {
    auto cloned = std::make_shared<ForestContractor>();
 
    cloned->maxTensorRank = maxTensorRank;
    cloned->enableMultithreading = enableMultithreading;
 
    return cloned;
  }
};
 
} // namespace TensorNetworks
 
#endif // __FOREST_CONTRACTOR_H_