DumbContractor.h

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

class DumbContractor : 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);
 
    Eigen::Index minId = std::numeric_limits<Eigen::Index>::max();
 
    if (contractTheLowestTensorId) {
      for (const auto &tensor : tensors) {
        if (tensor.first < minId)
          minId = tensor.first;
      }
    }
 
    // auto it = tensors.begin();
 
    // while there is more than one tensor...
    while (tensors.size() > 1) {
      Eigen::Index tensor1Id;
      Eigen::Index tensor2Id;
 
      if (contractTheLowestTensorId) {
        tensor1Id = minId;
        auto it = tensors.find(tensor1Id);
        ++it;
        if (it == tensors.end())
          it = tensors.begin();
 
        tensor2Id = it->first;
      } else {
        auto it = tensors.begin();
        tensor1Id = it->first;
        ++it;
        tensor2Id = it->first;
      }
 
      // you could even comment out this loop and it should work
      // the unconnected tensors will still be contracted, the contraction
      // result is the outer product this is a good chance to test the
      // contraction that involves outer products, as the other contractors
      // avoid it
 
      const auto &tensor = tensors[tensor1Id];
      Eigen::Index resultRank = std::numeric_limits<Eigen::Index>::max();
      for (Eigen::Index ti = 0;
           ti < static_cast<Eigen::Index>(tensor->connections.size()); ++ti) {
        const auto nextTensorId = tensor->connections[ti];
        if (nextTensorId != TensorNode::NotConnected) {
          tensor2Id = nextTensorId;
 
          const Eigen::Index newRank =
              GetResultRank(tensor, tensors[nextTensorId]);
 
          if (newRank <= resultRank) {
            if (newRank < resultRank) {
              tensor2Id = nextTensorId;
              resultRank = newRank;
            }
          }
        }
      }
 
      // it = tensors.find(tensor2Id);
      //++it;
      // if (it == tensors.end())
      //    it = tensors.begin();
 
      ContractNodes(qubit, tensors, tensor1Id, tensor2Id, resultRank);
 
      if (resultRank == 0) {
        if (tensors.size() == 1 || tensor->contractsTheNeededQubit)
          return std::real(tensor->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);
 
        if (contractTheLowestTensorId) {
          minId = std::numeric_limits<Eigen::Index>::max();
          for (const auto &tensor : tensors) {
            if (tensor.first < minId)
              minId = tensor.first;
          }
        }
      }
    }
 
    return std::real(tensors.begin()->second->tensor->atOffset(0));
  }
 
  void SetContractTheLowestTensorId(bool c) { contractTheLowestTensorId = c; }
 
  bool GetContractTheLowestTensorId() const {
    return contractTheLowestTensorId;
  }

  std::shared_ptr<ITensorContractor> Clone() const override {
    auto cloned = std::make_shared<DumbContractor>();
 
    cloned->maxTensorRank = maxTensorRank;
    cloned->enableMultithreading = enableMultithreading;
    cloned->contractTheLowestTensorId = contractTheLowestTensorId;
 
    return cloned;
  }
 
private:
  bool contractTheLowestTensorId = true;
};
 
} // namespace TensorNetworks
 
#endif // __DUMB_CONTRACTOR_H_