Implement classically controlled operations

src/ExecutionPathTracer/ExecutionPath.cs

@@ -118,9 +118,6 @@ public class Operation
         /// <summary>
         /// Group of operations for each classical branch.
         /// </summary>
-        /// <remarks>
-        /// Currently not used as this is intended for classically-controlled operations.
-        /// </remarks>
         [JsonProperty("children")]
         public IEnumerable<IEnumerable<Operation>>? Children { get; set; }
 
@@ -130,6 +127,12 @@ public class Operation
         [JsonProperty("isMeasurement")]
         public bool IsMeasurement { get; set; }
 
+        /// <summary>
+        /// True if operation is a classically-controlled operations.
+        /// </summary>
+        [JsonProperty("isConditional")]
+        public bool IsConditional { get; set; }
+
         /// <summary>
         /// True if operation is a controlled operations.
         /// </summary>

src/ExecutionPathTracer/ExecutionPathTracer.cs

@@ -69,9 +69,9 @@ public void OnOperationStartHandler(ICallable operation, IApplyData arguments)
 
             var metadata = operation.GetRuntimeMetadata(arguments);
 
-            // If metadata is a composite operation (i.e. want to trace its components instead),
+            // If metadata is a composite/conditional operation (i.e. want to trace its components instead),
             // we recursively create a tracer that traces its components instead
-            if (metadata != null && metadata.IsComposite)
+            if (metadata != null && (metadata.IsComposite || metadata.IsConditional))
             {
                 var remainingDepth = this.renderDepth - this.currentDepth;
                 this.compositeTracer = new ExecutionPathTracer(remainingDepth);
@@ -81,6 +81,8 @@ public void OnOperationStartHandler(ICallable operation, IApplyData arguments)
                 this.currCompositeOp = operation;
                 // Set currentOperation to null so we don't render higher-depth operations unintentionally.
                 this.currentOperation = null;
+
+                if (metadata.IsConditional) this.currentOperation = this.MetadataToOperation(metadata);
                 return;
             }
 
@@ -179,6 +181,15 @@ private void AddCompositeOperations()
             if (this.compositeTracer == null)
                 throw new NullReferenceException("ERROR: compositeTracer not initialized.");
 
+            if (this.currentOperation != null && this.currentOperation.IsConditional)
+            {
+                var numChildren = this.compositeTracer.operations.Count();
+                if (numChildren != 2) throw new IndexOutOfRangeException($"ERROR: Found only {numChildren} children for conditional operation.");
+                this.currentOperation.Children = this.compositeTracer.operations
+                    .Select(op => new List<Operation>() { op });
+                this.operations.Add(this.currentOperation);
+                return;
+            }
             // The composite tracer has done its job and we retrieve the operations it traced
             this.operations.AddRange(this.compositeTracer.operations);
         }
@@ -202,6 +213,7 @@ private void AddCompositeOperations()
                 Gate = metadata.Label,
                 DisplayArgs = displayArgs,
                 Children = metadata.Children?.Select(child => child.Select(this.MetadataToOperation).WhereNotNull()),
+                IsConditional = metadata.IsConditional,
                 IsControlled = metadata.IsControlled,
                 IsAdjoint = metadata.IsAdjoint,
                 Controls = this.GetQubitRegisters(metadata.Controls),

src/ExecutionPathTracer/Extensions.cs

@@ -3,11 +3,50 @@
 
 #nullable enable
 
+using System;
 using System.Collections.Generic;
 using Microsoft.Quantum.Simulation.Common;
+using Microsoft.Quantum.Simulation.Core;
+using Microsoft.Quantum.Simulation.QuantumProcessor.Extensions;
 
 namespace Microsoft.Quantum.IQSharp.ExecutionPathTracer
 {
+    /// <summary>
+    /// Custom ApplyIfElse used by Tracer to overrides the default behaviour and executes both branches
+    /// of the conditional statement.
+    /// </summary>
+    public class TracerApplyIfElse : ApplyIfElseR<Qubit, Qubit>
+    {
+        private SimulatorBase Simulator { get; }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="TracerApplyIfElse"/> class.
+        /// </summary>
+        public TracerApplyIfElse(SimulatorBase m) : base(m)
+        {
+            this.Simulator = m;
+        }
+
+        /// <inheritdoc />
+        public override Func<(Result, (ICallable, Qubit), (ICallable, Qubit)), QVoid> Body => (q) =>
+        {
+            (Result measurementResult, (ICallable onZero, Qubit one), (ICallable onOne, Qubit two)) = q;
+            onZero.Apply(one);
+            onOne.Apply(two);
+
+            return QVoid.Instance;
+        };
+
+        /// <inheritdoc />
+        public override RuntimeMetadata? GetRuntimeMetadata(IApplyData args)
+        {
+            var metadata = base.GetRuntimeMetadata(args);
+            if (metadata == null) throw new NullReferenceException($"Null RuntimeMetadata found for {this.ToString()}.");
+            metadata.IsComposite = true;
+            return metadata;
+        }
+    }
+
     /// <summary>
     /// Extension methods to be used with and by <see cref="ExecutionPathTracer"/>.
     /// </summary>
@@ -22,6 +61,10 @@ public static T WithExecutionPathTracer<T>(this T sim, ExecutionPathTracer trace
         {
             sim.OnOperationStart += tracer.OnOperationStartHandler;
             sim.OnOperationEnd += tracer.OnOperationEndHandler;
+            sim.Register(
+                typeof(ApplyIfElseR<Qubit, Qubit>),
+                typeof(TracerApplyIfElse)
+            );
             return sim;
         }
 

src/Tests/ExecutionPathTracerTests.cs

@@ -614,6 +614,56 @@ public void BigTest()
             var expected = new ExecutionPath(qubits, operations);
             Assert.AreEqual(expected.ToJson(), path.ToJson());
         }
+
+        [TestMethod]
+        public void IfTest()
+        {
+            var path = GetExecutionPath("IfCirc");
+            var qubits = new QubitDeclaration[] { new QubitDeclaration(0, 1) };
+            var operations = new Operation[]
+            {
+                new Operation()
+                {
+                    Gate = "M",
+                    IsMeasurement = true,
+                    Controls = new List<Register>() { new QubitRegister(0) },
+                    Targets = new List<Register>() { new ClassicalRegister(0, 0) },
+                },
+                new Operation()
+                {
+                    Gate = "ApplyIfElseR",
+                    DisplayArgs = "(Zero, (X), (Z))",
+                    Controls = new List<Register>() { new ClassicalRegister(0, 0) },
+                    Targets = new List<Register>() { new QubitRegister(0) },
+                    Children = new List<List<Operation>>()
+                    {
+                        new List<Operation>()
+                        {
+                            new Operation()
+                            {
+                                Gate = "X",
+                                Targets = new List<Register>() { new QubitRegister(0) },
+                            },
+                        },
+                        new List<Operation>()
+                        {
+                            new Operation()
+                            {
+                                Gate = "Z",
+                                Targets = new List<Register>() { new QubitRegister(0) },
+                            },
+                        },
+                    },
+                },
+                new Operation()
+                {
+                    Gate = "Reset",
+                    Targets = new List<Register>() { new QubitRegister(0) },
+                },
+            };
+            var expected = new ExecutionPath(qubits, operations);
+            Assert.AreEqual(expected.ToJson(), path.ToJson());
+        }
     }
 
     [TestClass]

src/Tests/Workspace.ExecutionPathTracer/Circuits.qs

@@ -4,6 +4,7 @@
 namespace Tests.ExecutionPathTracer {
 
     open Microsoft.Quantum.Intrinsic;
+    open Microsoft.Quantum.Simulation.QuantumProcessor.Extensions;
 
     // Custom operation
     operation Foo(theta : Double, (qubit : Qubit, bar : String)) : Unit
@@ -94,5 +95,13 @@ namespace Tests.ExecutionPathTracer {
             ResetAll(qs);
         }
     }
+
+    operation IfCirc() : Unit {
+        using (q = Qubit()) {
+            let res = M(q);
+            ApplyIfElseR(res, (X, q), (Z, q));
+            Reset(q);
+        }
+    }
 
 }