StochasticContractor.h

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

class StochasticContractor : public BaseContractor {
public:
  StochasticContractor() : gen(std::random_device{}()) {}

  double Contract(const TensorNetwork &network, Types::qubit_t qubit) override {
    // algorithm very similar with the 'Algorithm 2' from arXiv:1709.03636v2
    // [quant-ph] 22 Dec 2018 qTorch: The quantum tensor contraction handler
    // there are some changes, though, in picking up the contraction
 
    long long int CostThreshold = -1;
    size_t rejections = 0;
 
    std::vector<Eigen::Index> keys;
    std::unordered_map<Eigen::Index, Eigen::Index> keysKeys;
 
    auto tensors = InitializeTensors(network, qubit, keys, keysKeys);
 
    using TensorPair = std::pair<Eigen::Index, Eigen::Index>;
    std::unordered_set<TensorPair, boost::hash<TensorPair>> visitedPairs;
 
    // while there is more than one tensor...
    while (tensors.size() > 1) {
      // choose a random tensor
      std::uniform_int_distribution<Eigen::Index> tensor1Dist(
          0LL, static_cast<Eigen::Index>(keys.size()) - 1);
      auto pos1 = tensor1Dist(gen);
      Eigen::Index tensor1Id = keys[pos1];
      const auto tensor1 = tensors[tensor1Id];
 
      // then a random tensor it is connected to
      // this favors the ones that have more connection with the chosen one
 
      std::uniform_int_distribution<Eigen::Index> tensor2Dist(
          0LL, static_cast<Eigen::Index>(tensor1->connections.size()) - 1);
      auto pos2 = tensor2Dist(gen);
      Eigen::Index tensor2Id = tensor1->connections[pos2];
 
      auto t1 = tensor1Id;
      auto t2 = tensor2Id;
      if (t1 > t2)
        std::swap(t1, t2);
      if (visitedPairs.find(std::make_pair(t1, t2)) != visitedPairs.end()) {
        ++rejections;
        if (rejections >= MaxRejections) {
          ++CostThreshold;
          rejections = 0;
          visitedPairs.clear();
        }
        continue;
      } else
        visitedPairs.insert(std::make_pair(t1, t2));
 
      const auto tensor2 = tensors[tensor2Id];
 
      const auto keysKeysIt = keysKeys.find(tensor2Id);
      if (keysKeysIt != keysKeys.end())
        pos2 = keysKeysIt->second;
 
      const Eigen::Index resultRank = GetResultRank(tensor1, tensor2);
      // const Eigen::Index Cost = resultRank - std::max(tensor1->GetRank(),
      // tensor2->GetRank());
      const Eigen::Index Cost =
          resultRank - (tensor1->GetRank() + tensor2->GetRank()) / 2;
      if (Cost <= CostThreshold) {
        rejections = 0;
        CostThreshold = -1;
 
        ContractNodes(qubit, tensors, tensor1Id, tensor2Id, resultRank);
 
        visitedPairs.clear();
 
        // also remove them from 'keys' vector
        // actually only one needs to be removed, the other one will be replaced
        // by the result tensor
        if (pos1 == static_cast<Eigen::Index>(keys.size()) - 1)
          pos1 = pos2;
 
        keys[pos2] = keys.back();
        keysKeys[keys[pos2]] = pos2;
        keysKeys.erase(tensor2Id);
        keys.resize(keys.size() - 1);
 
        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
          keys[pos1] = keys.back();
          keysKeys[keys[pos1]] = pos1;
          keysKeys.erase(tensor1Id);
          keys.resize(keys.size() - 1);
 
          tensors.erase(tensor1Id);
        }
      } else {
        ++rejections;
        if (rejections >= MaxRejections) {
          ++CostThreshold;
          rejections = 0;
          visitedPairs.clear();
        }
      }
    }
 
    return std::real(tensors.begin()->second->tensor->atOffset(0));
  }
 
  size_t GetMaxRejections() const { return MaxRejections; }
 
  void SetMaxRejections(size_t maxRejections) { MaxRejections = maxRejections; }

  std::shared_ptr<ITensorContractor> Clone() const override {
    auto cloned = std::make_shared<StochasticContractor>();
 
    cloned->maxTensorRank = maxTensorRank;
    cloned->enableMultithreading = enableMultithreading;
    cloned->MaxRejections = MaxRejections;
 
    return cloned;
  }
 
protected:
  std::mt19937 gen;
  size_t MaxRejections = 15;
};
 
} // namespace TensorNetworks
 
#endif // __STOCHASTIC_CONTRACTOR_H_