Qiskit · mergify · Mar 13, 2020 · Mar 9, 2020 · Mar 12, 2020 · Mar 13, 2020
diff --git a/qiskit/circuit/__init__.py b/qiskit/circuit/__init__.py
@@ -43,6 +43,7 @@
    Reset
    Instruction
    InstructionSet
+   EquivalenceLibrary
 
 Parametric Quantum Circuits
 ===========================
@@ -75,3 +76,4 @@
 from .parameter import Parameter
 from .parametervector import ParameterVector
 from .parameterexpression import ParameterExpression
+from .equivalence import EquivalenceLibrary
diff --git a/qiskit/circuit/equivalence.py b/qiskit/circuit/equivalence.py
@@ -0,0 +1,184 @@
+# -*- coding: utf-8 -*-
+
+# This code is part of Qiskit.
+#
+# (C) Copyright IBM 2020.
+#
+# This code is licensed under the Apache License, Version 2.0. You may
+# obtain a copy of this license in the LICENSE.txt file in the root directory
+# of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
+#
+# Any modifications or derivative works of this code must retain this
+# copyright notice, and modified files need to carry a notice indicating
+# that they have been altered from the originals.
+
+"""Gate equivalence library."""
+
+from collections import namedtuple
+
+from .exceptions import CircuitError
+from .parameterexpression import ParameterExpression
+
+Key = namedtuple('Key', ['name',
+                         'num_qubits'])
+
+Entry = namedtuple('Entry', ['search_base',
+                             'equivalences'])
+
+Equivalence = namedtuple('Equivalence', ['params',  # Ordered to match Gate.params
+                                         'circuit'])
+
+
+class EquivalenceLibrary():
+    """A library providing a one-way mapping of Gates to their equivalent
+    implementations as QuantumCircuits."""
+
+    def __init__(self, *, base=None):
+        """Create a new equivalence library.
+
+        Args:
+            base (Optional[EquivalenceLibrary]):  Base equivalence library to
+                will be referenced if an entry is not found in this library.
+        """
+        self._base = base
+
+        self._map = {}
+
+    def add_equivalence(self, gate, equivalent_circuit):
+        """Add a new equivalence to the library. Future queries for the Gate
+        will include the given circuit, in addition to all existing equivalences
+        (including those from base).
+
+        Parameterized Gates (those including `qiskit.circuit.Parameters` in their
+        `Gate.params`) can be marked equivalent to parameterized circuits,
+        provided the parameters match.
+
+        Args:
+            gate (Gate): A Gate instance.
+            equivalent_circuit (QuantumCircuit): A circuit equivalently
+                implementing the given Gate.
+        """
+
+        _raise_if_shape_mismatch(gate, equivalent_circuit)
+        _raise_if_param_mismatch(gate.params, equivalent_circuit.parameters)
+
+        key = Key(name=gate.name,
+                  num_qubits=gate.num_qubits)
+
+        equiv = Equivalence(params=gate.params.copy(),
+                            circuit=equivalent_circuit.copy())
+
+        if key not in self._map:
+            self._map[key] = Entry(search_base=True, equivalences=[])
+
+        self._map[key].equivalences.append(equiv)
+
+    def set_entry(self, gate, entry):
+        """Set the equivalence record for a Gate. Future queries for the Gate
+        will return only the circuits provided.
+
+        Parameterized Gates (those including `qiskit.circuit.Parameters` in their
+        `Gate.params`) can be marked equivalent to parameterized circuits,
+        provided the parameters match.
+
+        Args:
+            gate (Gate): A Gate instance.
+            entry (List['QuantumCircuit']) : A list of QuantumCircuits, each
+                equivalently implementing the given Gate.
+        """
+
+        for equiv in entry:
+            _raise_if_shape_mismatch(gate, equiv)
+            _raise_if_param_mismatch(gate.params, equiv.parameters)
+
+        key = Key(name=gate.name,
+                  num_qubits=gate.num_qubits)
+
+        equivs = [Equivalence(params=gate.params.copy(),
+                              circuit=equiv.copy())
+                  for equiv in entry]
+
+        self._map[key] = Entry(search_base=False,
+                               equivalences=equivs)
+
+    def get_entry(self, gate):
+        """Gets the set of QuantumCircuits circuits from the library which
+        equivalently implement the given Gate.
+
+        Parameterized circuits will have their parameters replaced with the
+        corresponding entries from Gate.params.
+
+        Args:
+            gate (Gate) - Gate: A Gate instance.
+
+        Returns:
+            List[QuantumCircuit]: A list of equivalent QuantumCircuits. If empty,
+                library contains no known decompositions of Gate.
+
+                Returned circuits will be ordered according to their insertion in
+                the library, from earliest to latest, from top to base. The
+                ordering of the StandardEquivalenceLibrary will not generally be
+                consistent across Qiskit versions.
+        """
+
+        key = Key(name=gate.name,
+                  num_qubits=gate.num_qubits)
+
+        query_params = gate.params
+
+        if key in self._map:
+            entry = self._map[key]
+            search_base, equivs = entry
+
+            rtn = [_rebind_equiv(equiv, query_params) for equiv in equivs]
+
+            if search_base and self._base is not None:
+                return rtn + self._base.get_entry(gate)
+            return rtn
+
+        if self._base is None:
+            return []
+
+        return self._base.get_entry(gate)
+
+
+def _raise_if_param_mismatch(gate_params, circuit_parameters):
+    gate_parameters = [p for p in gate_params
+                       if isinstance(p, ParameterExpression)]
+
+    if set(gate_parameters) != circuit_parameters:
+        raise CircuitError('Cannot add equivalence between circuit and gate '
+                           'of different parameters. Gate params: {}. '
+                           'Circuit params: {}.'.format(
+                               gate_parameters,
+                               circuit_parameters))
+
+
+def _raise_if_shape_mismatch(gate, circuit):
+    if (gate.num_qubits != circuit.n_qubits
+            or gate.num_clbits != circuit.n_clbits):
+        raise CircuitError('Cannot add equivalence between circuit and gate '
+                           'of different shapes. Gate: {} qubits and {} clbits. '
+                           'Circuit: {} qubits and {} clbits.'.format(
+                               gate.num_qubits, gate.num_clbits,
+                               circuit.n_qubits, circuit.n_clbits))
+
+
+def _rebind_equiv(equiv, query_params):
+    equiv_params, equiv_circuit = equiv
+
+    param_map = dict(zip(equiv_params, query_params))
+
+    symbolic_param_map, numeric_param_map = _partition_dict(
+        param_map,
+        lambda k, v: isinstance(v, ParameterExpression))
+
+    equiv = equiv_circuit.bind_parameters(numeric_param_map)
+    equiv._substitute_parameters(symbolic_param_map)
+
+    return equiv
+
+
+def _partition_dict(dict_, predicate):
+    return ({k: v for k, v in dict_.items() if predicate(k, v)},
+            {k: v for k, v in dict_.items() if not predicate(k, v)})
diff --git a/qiskit/circuit/quantumcircuit.py b/qiskit/circuit/quantumcircuit.py
@@ -969,6 +969,13 @@ def n_qubits(self):
             qubits += reg.size
         return qubits
 
+    @property
+    def n_clbits(self):
+        """
+        Return number of classical bits.
+        """
+        return sum(len(reg) for reg in self.cregs)
+
     def count_ops(self):
         """Count each operation kind in the circuit.