Fix stim.Circuit.flow_generators inverting measurement index order within MPP instructions (#956)

Author: Strilanc

src/stim/util_top/circuit_flow_generators.h

@@ -36,6 +36,7 @@ struct CircuitFlowGeneratorSolver {
     std::vector<size_t> buf_for_rows_with;
     std::vector<int32_t> buf_for_xor_merge;
     std::vector<GateTarget> buf_targets;
+    std::vector<GateTarget> buf_targets_2;
 
     explicit CircuitFlowGeneratorSolver(CircuitStats stats);
     static CircuitFlowGeneratorSolver<W> solver_with_circuit_generators(

src/stim/util_top/circuit_flow_generators.inl

@@ -364,7 +364,12 @@ void CircuitFlowGeneratorSolver<W>::undo_instruction(CircuitInstruction inst) {
                 CircuitInstruction{inst.gate_type, {}, buf_targets, inst.tag},
                 num_qubits,
                 [&](CircuitInstruction sub_inst) {
-                    undo_instruction(sub_inst);
+                    buf_targets_2.clear();
+                    buf_targets_2.insert(buf_targets_2.end(), sub_inst.targets.begin(), sub_inst.targets.end());
+                    if (sub_inst.gate_type == GateType::M) {
+                        std::reverse(buf_targets_2.begin(), buf_targets_2.end());
+                    }
+                    undo_instruction(CircuitInstruction{sub_inst.gate_type, sub_inst.args, buf_targets_2, sub_inst.tag});
                 });
             break;
 

src/stim/util_top/circuit_flow_generators.test.cc

@@ -253,6 +253,21 @@ TEST_EACH_WORD_SIZE_W(circuit_flow_generators, various, {
             Flow<W>::from_str("XXX -> _ZY xor rec[2]"),
             Flow<W>::from_str("Z_X -> _ZX xor rec[3]"),
         }));
+
+    EXPECT_EQ(
+        circuit_flow_generators<W>(Circuit(R"CIRCUIT(
+            MPP Y0*Y1 Y2*Y3
+        )CIRCUIT")),
+        (std::vector<Flow<W>>{
+            Flow<W>::from_str("1 -> __YY xor rec[1]"),
+            Flow<W>::from_str("1 -> YY__ xor rec[0]"),
+            Flow<W>::from_str("___Y -> ___Y"),
+            Flow<W>::from_str("__XZ -> __ZX xor rec[1]"),
+            Flow<W>::from_str("__ZZ -> __ZZ"),
+            Flow<W>::from_str("_Y__ -> _Y__"),
+            Flow<W>::from_str("XZ__ -> ZX__ xor rec[0]"),
+            Flow<W>::from_str("ZZ__ -> ZZ__"),
+        }));
 })
 
 TEST_EACH_WORD_SIZE_W(circuit_flow_generators, all_operations, {

src/stim/util_top/circuit_flow_generators_test.py

@@ -26,6 +26,85 @@ def test_solve_flow_measurements():
     ]) == [[0], [], [0]]
 
 
+def test_solve_flow_generators_measurements_multi_target():
+    assert stim.Circuit("""
+        M 1 2
+    """).flow_generators() == [
+        stim.Flow("1 -> __Z xor rec[1]"),
+        stim.Flow("1 -> _Z_ xor rec[0]"),
+        stim.Flow("__Z -> rec[1]"),
+        stim.Flow("_Z_ -> rec[0]"),
+        stim.Flow("X__ -> X__"),
+        stim.Flow("Z__ -> Z__"),
+    ]
+
+    assert stim.Circuit("""
+        MX 1 2
+    """).flow_generators() == [
+        stim.Flow("1 -> __X xor rec[1]"),
+        stim.Flow("1 -> _X_ xor rec[0]"),
+        stim.Flow("__X -> rec[1]"),
+        stim.Flow("_X_ -> rec[0]"),
+        stim.Flow("X__ -> X__"),
+        stim.Flow("Z__ -> Z__"),
+    ]
+
+    assert stim.Circuit("""
+        MYY 1 2 3 4
+    """).flow_generators() == [
+        stim.Flow("1 -> ___YY xor rec[1]"),
+        stim.Flow("1 -> _YY__ xor rec[0]"),
+        stim.Flow("____Y -> ____Y"),
+        stim.Flow("___XZ -> ___ZX xor rec[1]"),
+        stim.Flow("___ZZ -> ___ZZ"),
+        stim.Flow("__Y__ -> __Y__"),
+        stim.Flow("_XZ__ -> _ZX__ xor rec[0]"),
+        stim.Flow("_ZZ__ -> _ZZ__"),
+        stim.Flow("X____ -> X____"),
+        stim.Flow("Z____ -> Z____"),
+    ]
+    assert stim.Circuit("""
+        MPP Y1*Y2 Y3*Y4
+    """).flow_generators() == [
+        stim.Flow("1 -> ___YY xor rec[1]"),
+        stim.Flow("1 -> _YY__ xor rec[0]"),
+        stim.Flow("____Y -> ____Y"),
+        stim.Flow("___XZ -> ___ZX xor rec[1]"),
+        stim.Flow("___ZZ -> ___ZZ"),
+        stim.Flow("__Y__ -> __Y__"),
+        stim.Flow("_XZ__ -> _ZX__ xor rec[0]"),
+        stim.Flow("_ZZ__ -> _ZZ__"),
+        stim.Flow("X____ -> X____"),
+        stim.Flow("Z____ -> Z____"),
+    ]
+
+
+def test_solve_flow_measurements_multi_target():
+    assert stim.Circuit("""
+        M 1 2
+    """).solve_flow_measurements([
+        stim.Flow("Z1 -> 1"),
+    ]) == [[0]]
+
+    assert stim.Circuit("""
+        MX 1 2
+    """).solve_flow_measurements([
+        stim.Flow("X1 -> 1"),
+    ]) == [[0]]
+
+    assert stim.Circuit("""
+        MYY 1 2 3 4
+    """).solve_flow_measurements([
+        stim.Flow("Y1*Y2 -> 1"),
+    ]) == [[0]]
+
+    assert stim.Circuit("""
+        MPP Y1*Y2 Y3*Y4
+    """).solve_flow_measurements([
+        stim.Flow("Y1*Y2 -> 1"),
+    ]) == [[0]]
+
+
 def test_solve_flow_measurements_fewer_measurements_heuristic():
     assert stim.Circuit("""
         MPP Z0*Z1*Z2*Z3*Z4*Z5*Z6*Z7*Z8