fix BasicAer sampling bug (Qiskit#1624)

* fix BasicAer sampling bug * changelog * remove legacysimulator from test_compile
edasgupta · Dec 29, 2018 · 5f7b2e7 · 5f7b2e7
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Showing 3 changed files with 10 additions and 5 deletions.
diff --git a/CHANGELOG.rst b/CHANGELOG.rst
@@ -30,6 +30,12 @@ Changed
 - The snapshot instruction now takes ``label`` and ``snap_type`` instead of
   ``slot`` (#1615).
 
+Fixed
+-----
+
+- Fixed a bug with measurement sampling optimization in BasicAer
+  qasm_simulator (#1624).
+
 Removed
 -------
 

diff --git a/qiskit/providers/builtinsimulators/qasm_simulator.py b/qiskit/providers/builtinsimulators/qasm_simulator.py
@@ -206,8 +206,8 @@ def _add_sample_measure(self, measure_params, num_samples):
         memory = []
         for sample in samples:
             classical_state = self._classical_state
-            for qubit, cbit in measure_params:
-                qubit_outcome = int((sample & (1 << qubit)) >> qubit)
+            for count, (qubit, cbit) in enumerate(sorted(measure_params)):
+                qubit_outcome = int((sample & (1 << count)) >> count)
                 bit = 1 << cbit
                 classical_state = (classical_state & (~bit)) | (qubit_outcome << cbit)
             value = bin(classical_state)[2:]

diff --git a/test/python/tools/test_compiler.py b/test/python/tools/test_compiler.py
@@ -22,7 +22,7 @@
 from qiskit.providers.ibmq import least_busy
 from .._mockutils import FakeBackend
 from ..common import QiskitTestCase
-from ..common import requires_qe_access, requires_cpp_simulator
+from ..common import requires_qe_access
 
 
 class TestCompiler(QiskitTestCase):
@@ -472,13 +472,12 @@ def test_compile_coupling_map(self):
         threshold = 0.05 * shots
         self.assertDictAlmostEqual(counts, target, threshold)
 
-    @requires_cpp_simulator
     def test_example_swap_bits(self):
         """Test a toy example swapping a set bit around.
 
         Uses the mapper. Pass if results are correct.
         """
-        backend = qiskit.Aer.get_backend('qasm_simulator')
+        backend = qiskit.BasicAer.get_backend('qasm_simulator')
         coupling_map = [[0, 1], [0, 8], [1, 2], [1, 9], [2, 3], [2, 10],
                         [3, 4], [3, 11], [4, 5], [4, 12], [5, 6], [5, 13],
                         [6, 7], [6, 14], [7, 15], [8, 9], [9, 10], [10, 11],