add qubits-truncate optimization (Qiskit#164)

src/base/state.hpp

@@ -309,8 +309,6 @@ void State<state_t>::add_creg_to_data(OutputData &data) const {
     data.add_register_singleshot(creg_.register_hex());
   }
 }
-
-
 //-------------------------------------------------------------------------
 } // end namespace Base
 //-------------------------------------------------------------------------

src/framework/circuitopt.hpp

@@ -17,8 +17,7 @@
 #include <string>
 #include <vector>
 
-#include "base/controller.hpp"
-#include "base/state.hpp"
+#include "framework/operations.hpp"
 
 namespace AER {
 

src/framework/data.hpp

@@ -227,7 +227,10 @@ template <typename T>
 void OutputData::add_additional_data(const std::string &key, const T &data) {
   if (return_additional_data_) {
     json_t js = data; // use implicit to_json conversion function for T
-    additional_data_[key] = js;
+    if (JSON::check_key(key, additional_data_))
+      additional_data_[key].update(js.begin(), js.end());
+    else
+      additional_data_[key] = js;
   }
 }
 

src/framework/operations.hpp

@@ -121,7 +121,14 @@ struct Op {
 };
 
 std::ostream& operator<<(std::ostream& s, const Op& op) {
-  s << op.name;
+  s << op.name << "[";
+  bool first = true;
+  for (size_t qubit: op.qubits) {
+    if (!first) s << ",";
+    s << qubit;
+    first = false;
+  }
+  s << "]";
   return s;
 }
 

src/simulators/qasm/basic_optimization.hpp

@@ -16,17 +16,18 @@ using uint_t = uint_t;
 using op_t = Operations::Op;
 using optype_t = Operations::OpType;
 using oplist_t = std::vector<op_t>;
+using opset_t = Operations::OpSet;
 using reg_t = std::vector<uint_t>;
 
 class ReduceNop : public CircuitOptimization {
 public:
   void optimize_circuit(Circuit& circ,
-                        const Operations::OpSet &opset,
+                        const opset_t &opset,
                         OutputData &data) const override;
 };
 
 void ReduceNop::optimize_circuit(Circuit& circ,
-                                 const Operations::OpSet &allowed_opset,
+                                 const opset_t &allowed_opset,
                                  OutputData &data) const {
 
   oplist_t::iterator it = circ.ops.begin();
@@ -41,12 +42,12 @@ void ReduceNop::optimize_circuit(Circuit& circ,
 class Debug : public CircuitOptimization {
 public:
   void optimize_circuit(Circuit& circ,
-                        const Operations::OpSet &opset,
+                        const opset_t &opset,
                         OutputData &data) const override;
 };
 
 void Debug::optimize_circuit(Circuit& circ,
-                             const Operations::OpSet &allowed_opset,
+                             const opset_t &allowed_opset,
                              OutputData &data) const {
 
   oplist_t::iterator it = circ.ops.begin();
@@ -63,7 +64,7 @@ class Fusion : public CircuitOptimization {
   void set_config(const json_t &config) override;
 
   void optimize_circuit(Circuit& circ,
-                        const Operations::OpSet &opset,
+                        const opset_t &opset,
                         OutputData &data) const override;
 
   bool can_ignore(const op_t& op) const;
@@ -155,7 +156,7 @@ void Fusion::set_config(const json_t &config) {
 #ifdef DEBUG
 void Fusion::dump(const Circuit& circuit) const {
   int idx = 0;
-  for (const Operations::Op& op : circuit.ops) {
+  for (const op_t& op : circuit.ops) {
     std::cout << "  " << idx++ << ":\t" << op.name << " " << op.qubits << std::endl;
     for (const cmatrix_t&  mat: op.mats) {
       const uint_t row = mat.GetRows();
@@ -174,12 +175,12 @@ void Fusion::dump(const Circuit& circuit) const {
 #endif
 
 void Fusion::optimize_circuit(Circuit& circ,
-                              const Operations::OpSet &allowed_opset,
+                              const opset_t &allowed_opset,
                               OutputData &data) const {
 
   if (circ.num_qubits < threshold_
       || !active_
-      || allowed_opset.optypes.find(Operations::OpType::matrix_sequence) == allowed_opset.optypes.end())
+      || allowed_opset.optypes.find(optype_t::matrix_sequence) == allowed_opset.optypes.end())
     return;
 
   bool ret = false;
@@ -463,6 +464,124 @@ cmatrix_t Fusion::matrix(const op_t& op) const {
     throw std::runtime_error("Fusion: unexpected operation type");
   }
 }
+
+class TruncateQubits : public CircuitOptimization {
+public:
+
+  void set_config(const json_t &config) override;
+
+  // truncate unnecessary qubits
+  void optimize_circuit(Circuit& circ,
+                        const opset_t &opset,
+                        OutputData &data) const override;
+
+private:
+  // check this optimization can be applied
+  bool can_apply(const Circuit& circ) const;
+
+  // check this optimization can be applied
+  bool can_apply(const op_t& op) const;
+
+  // generate a new mapping. a value of reg_t is original and its index is the new mapping
+  reg_t generate_mapping(const Circuit& circ) const;
+
+  // remap qubits in an operation
+  op_t remap_qubits(const op_t op, const reg_t new_mapping)const;
+
+  // show debug info
+  bool verbose_ = false;
+
+  // disabled in config
+  bool active_ = true;
+};
+
+void TruncateQubits::set_config(const json_t &config) {
+
+  CircuitOptimization::set_config(config);
+
+  if (JSON::check_key("truncate_verbose", config_))
+    JSON::get_value(verbose_, "truncate_verbose", config_);
+
+  if (JSON::check_key("truncate_enable", config_))
+    JSON::get_value(active_, "truncate_enable", config_);
+
+  if (JSON::check_key("initial_statevector", config_))
+    active_ = false;
+
+}
+
+void TruncateQubits::optimize_circuit(Circuit& circ,
+                             const opset_t &allowed_opset,
+                             OutputData &data) const {
+
+  if (!active_ || !can_apply(circ))
+    return;
+
+  reg_t new_mapping = generate_mapping(circ);
+
+  if (new_mapping.size() == circ.num_qubits)
+    return;
+
+  oplist_t new_ops;
+  for (const op_t& old_op: circ.ops)
+    new_ops.push_back(remap_qubits(old_op, new_mapping));
+
+  circ.ops = new_ops;
+  circ.num_qubits = new_mapping.size();
+
+  if (verbose_) {
+    data.add_additional_data("metadata",
+                             json_t::object({{"truncate_verbose", new_mapping}}));
+  }
+
+}
+
+reg_t TruncateQubits::generate_mapping(const Circuit& circ) const {
+  size_t not_used = circ.num_qubits + 1;
+  reg_t mapping = reg_t(circ.num_qubits, not_used);
+
+  for (const op_t& op: circ.ops)
+    for (size_t qubit: op.qubits)
+      mapping[qubit] = qubit;
+
+  mapping.erase(std::remove(mapping.begin(), mapping.end(), not_used), mapping.end());
+
+  return mapping;
+}
+
+op_t TruncateQubits::remap_qubits(const op_t op, const reg_t new_mapping) const {
+  op_t new_op = op;
+  new_op.qubits.clear();
+
+  for (const size_t qubit: op.qubits) {
+    size_t new_qubit = std::distance(new_mapping.begin(), find(new_mapping.begin(), new_mapping.end(), qubit));
+    new_op.qubits.push_back(new_qubit);
+  }
+  return new_op;
+
+}
+
+bool TruncateQubits::can_apply(const Circuit& circ) const {
+
+  for (const op_t& op: circ.ops)
+    if (!can_apply(op))
+      return false;
+
+  return true;
+}
+
+bool TruncateQubits::can_apply(const op_t& op) const {
+  switch (op.type) {
+  case optype_t::matrix_sequence: //TODO
+  case optype_t::kraus: //TODO
+  case optype_t::snapshot:
+  case optype_t::noise_switch:
+    return false;
+  default:
+    return true;
+  }
+}
+
 //-------------------------------------------------------------------------
 } // end namespace AER
 //-------------------------------------------------------------------------

src/simulators/qasm/qasm_controller.hpp

@@ -236,6 +236,7 @@ class QasmController : public Base::Controller {
 QasmController::QasmController() {
   add_circuit_optimization(ReduceNop());
   add_circuit_optimization(Fusion());
+  add_circuit_optimization(TruncateQubits());
 }
 
 //-------------------------------------------------------------------------