Made Width and Depth compatible in current version of the tracer

src/Simulation/EntryPointDriver.Tests/Tests.fs

@@ -486,6 +486,7 @@ let private resourceSummary =
      T               0   0
      Depth           0   0
      Width           1   1
+     QubitCount      1   1
      BorrowedWidth   0   0"
 
 [<Fact>]

src/Simulation/QCTraceSimulator/DepthCounter.cs

@@ -15,19 +15,23 @@ namespace Microsoft.Quantum.Simulation.QCTraceSimulatorRuntime
 {
     public class DepthCounter : IQCTraceSimulatorListener, ICallGraphStatistics
     {
-        class OperationCallRecord
+        private class OperationCallRecord
         {
             public HashedString OperationName;
             public OperationFunctor FunctorSpecialization;
             public double MinOperationStartTime;
             public double MaxOperationStartTime;
             public double ReleasedQubitsAvailableTime;
             public double ReturnedQubitsAvailableTime;
+            public long MaxQubitIdAtStart = -1;
             public QubitTimeMetrics[] InputQubitMetrics;
         }
 
         StatisticsCollector<CallGraphEdge> stats;
         Stack<OperationCallRecord> operationCallStack;
+        QubitAvailabilityTimeTracker qubitAvailabilityTime = new QubitAvailabilityTimeTracker(
+            initialCapacity: 128, // Reasonable number to preallocate.
+            defaultAvailabilityTime: 0.0);
 
         public IStatisticCollectorResults<CallGraphEdge> Results => stats;
 
@@ -44,25 +48,30 @@ public DepthCounter(IDoubleStatistic[] statisticsToCollect = null)
             operationCallStack.Push(opRec);
         }
 
-        public class Metrics
+        public static class Metrics
         {
             public const string Depth = "Depth";
             public const string StartTimeDifference = "StartTimeDifference";
+            public const string Width = "Width";
         }
 
-        public double[] StatisticsRecord(double Depth, double StartTimeDifference)
+        public double[] StatisticsRecord(double Depth, double StartTimeDifference, double Width)
         {
-            return new double[] { Depth, StartTimeDifference };
+            return new double[] { Depth, StartTimeDifference, Width };
         }
 
         #region IQCTraceSimulatorListener implementation 
         public object NewTracingData(long qubitId)
         {
-            return new QubitTimeMetrics();
+            return new QubitTimeMetrics(qubitId);
         }
 
         public void OnAllocate(object[] qubitsTraceData)
         {
+            foreach (QubitTimeMetrics metric in qubitsTraceData.Cast<QubitTimeMetrics>())
+            {
+                qubitAvailabilityTime.MarkQubitIdUsed(metric.QubitId);
+            }
         }
 
         public void OnBorrow(object[] qubitsTraceData, long newQubitsAllocated)
@@ -74,12 +83,11 @@ public void OnOperationEnd(object[] returnedQubitsTraceData)
             double maxReturnedQubitsAvailableTime = 0;
             if ( returnedQubitsTraceData != null )
             {
-                QubitTimeMetrics[] qubitsMetrics = Utils.UnboxAs<QubitTimeMetrics>(returnedQubitsTraceData);
-                maxReturnedQubitsAvailableTime = QubitsMetricsUtils.MaxQubitAvailableTime(qubitsMetrics);
+                maxReturnedQubitsAvailableTime = MaxAvailableTime(returnedQubitsTraceData.Cast<QubitTimeMetrics>());
             }
             OperationCallRecord opRec = operationCallStack.Pop();
             Debug.Assert(operationCallStack.Count != 0, "Operation call stack must never get empty");
-            double inputQubitsAvailableTime = QubitsMetricsUtils.MaxQubitAvailableTime(opRec.InputQubitMetrics);
+            double inputQubitsAvailableTime = MaxAvailableTime(opRec.InputQubitMetrics);
             double operationEndTime =
                 Max(
                     Max(maxReturnedQubitsAvailableTime, opRec.ReturnedQubitsAvailableTime),
@@ -93,7 +101,8 @@ public void OnOperationEnd(object[] returnedQubitsTraceData)
             double[] metrics =
                 StatisticsRecord( 
                     Depth : operationEndTime - opRec.MaxOperationStartTime,
-                    StartTimeDifference: opRec.MaxOperationStartTime - opRec.MinOperationStartTime );
+                    StartTimeDifference: opRec.MaxOperationStartTime - opRec.MinOperationStartTime,
+                    Width: qubitAvailabilityTime.GetMaxQubitId() - opRec.MaxQubitIdAtStart );
 
             stats.AddSample(new CallGraphEdge(opRec.OperationName, callerName,opRec.FunctorSpecialization, caller.FunctorSpecialization), metrics);
         }
@@ -105,33 +114,59 @@ public void OnOperationStart(HashedString name, OperationFunctor functorSpeciali
             opRec.FunctorSpecialization = functorSpecialization;
             opRec.OperationName = name;
             opRec.InputQubitMetrics = Utils.UnboxAs<QubitTimeMetrics>(qubitsTraceData);
-            opRec.MaxOperationStartTime = QubitsMetricsUtils.MaxQubitAvailableTime(opRec.InputQubitMetrics);
-            opRec.MinOperationStartTime = QubitsMetricsUtils.MinQubitAvailableTime(opRec.InputQubitMetrics);
+            opRec.MaxOperationStartTime = MaxAvailableTime(opRec.InputQubitMetrics);
+            opRec.MinOperationStartTime = MinAvailableTime(opRec.InputQubitMetrics);
+            opRec.MaxQubitIdAtStart = qubitAvailabilityTime.GetMaxQubitId();
             operationCallStack.Push(opRec);
         }
 
+        private double MinAvailableTime(IEnumerable<QubitTimeMetrics> qubitTimeMetrics)
+        {
+            Debug.Assert(qubitTimeMetrics != null);
+            double min = Double.MaxValue;
+            foreach (QubitTimeMetrics metric in qubitTimeMetrics)
+            {
+                min = Min(min, qubitAvailabilityTime[metric.QubitId]);
+            }
+            return min != Double.MaxValue ? min : 0;
+        }
+
+        private double MaxAvailableTime(IEnumerable<QubitTimeMetrics> qubitTimeMetrics)
+        {
+            Debug.Assert(qubitTimeMetrics != null);
+            double max = 0;
+            foreach (QubitTimeMetrics metric in qubitTimeMetrics)
+            {
+                max = Max(max, qubitAvailabilityTime[metric.QubitId]);
+            }
+            return max;
+        }
+
         public void OnPrimitiveOperation(int id, object[] qubitsTraceData, double primitiveOperationDuration)
         {
-            QubitTimeMetrics[] qubitsMetrics = Utils.UnboxAs<QubitTimeMetrics>(qubitsTraceData);
-            double startTime = QubitsMetricsUtils.MaxQubitAvailableTime(qubitsMetrics);
-            foreach (QubitTimeMetrics q in qubitsMetrics )
+            IEnumerable<QubitTimeMetrics> qubitsMetrics = qubitsTraceData.Cast<QubitTimeMetrics>();
+
+            double startTime = MaxAvailableTime(qubitsMetrics);
+            foreach (QubitTimeMetrics q in qubitsMetrics)
             {
-                q.RecordQubitUsage(startTime, primitiveOperationDuration);
+                qubitAvailabilityTime[q.QubitId] = startTime + primitiveOperationDuration;
             }
         }
 
         public void OnRelease(object[] qubitsTraceData)
         {
             OperationCallRecord opRec = operationCallStack.Peek();
-            QubitTimeMetrics[] qubitsMetrics = Utils.UnboxAs<QubitTimeMetrics>(qubitsTraceData);
-            opRec.ReleasedQubitsAvailableTime = Max(opRec.ReleasedQubitsAvailableTime, QubitsMetricsUtils.MaxQubitAvailableTime(qubitsMetrics));
+            opRec.ReleasedQubitsAvailableTime = Max(
+                opRec.ReleasedQubitsAvailableTime,
+                MaxAvailableTime(qubitsTraceData.Cast<QubitTimeMetrics>()));
         }
 
         public void OnReturn(object[] qubitsTraceData, long qubitReleased)
         {
             OperationCallRecord opRec = operationCallStack.Peek();
-            QubitTimeMetrics[] qubitsMetrics = Utils.UnboxAs<QubitTimeMetrics>(qubitsTraceData);
-            opRec.ReturnedQubitsAvailableTime = Max(opRec.ReturnedQubitsAvailableTime, QubitsMetricsUtils.MaxQubitAvailableTime(qubitsMetrics));
+            opRec.ReturnedQubitsAvailableTime = Max(
+                opRec.ReturnedQubitsAvailableTime,
+                MaxAvailableTime(qubitsTraceData.Cast<QubitTimeMetrics>()));
         }
 
         /// <summary>

src/Simulation/QCTraceSimulator/QCTraceSimulatorCore.cs

@@ -30,6 +30,13 @@ public class QCTraceSimulatorCoreConfiguration
         /// of the call graph.
         /// </summary>
         public uint CallStackDepthLimit = uint.MaxValue;
+
+        /// <summary>
+        /// Controls if depth or width optimization is favored.
+        /// If set to true, resulting circuit is optimized for depth by discouraging qubit reuse.
+        /// If set to false, resulting circuit is optimized for width by encouraging qubit reuse.
+        /// </summary>
+        public bool OptimizeDepth = false;
     }
 
     /// <summary>
@@ -101,7 +108,7 @@ QCTraceSimulatorCoreConfiguration config
                 tracingDataInQubitsIsNeeded = listenerNeedsTracingData[i] || tracingDataInQubitsIsNeeded;
             }
 
-            qubitManager = (IQubitManager) new TraceableQubitManager(qubitDataInitializers);
+            qubitManager = (IQubitManager) new TraceableQubitManager(qubitDataInitializers, configuration.OptimizeDepth);
             callStackDepthLimit = Math.Max( 1, configuration.CallStackDepthLimit );
         }
 

src/Simulation/QCTraceSimulator/QubitAvailabilityTimeTracker.cs

@@ -0,0 +1,78 @@
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+
+namespace Microsoft.Quantum.Simulation.QCTraceSimulatorRuntime
+{
+    /// <summary>
+    /// Tracks time when qubits were last used and therefore, time when qubits become available.
+    /// Tracking is done by qubit id, which survives during reuse of qubit.
+    /// </summary>
+    internal class QubitAvailabilityTimeTracker
+    {
+        /// <summary>
+        /// Availability time of all qubits starts at 0.
+        /// </summary>
+        private double DefaultAvailabilityTime = 0.0;
+
+        /// <summary>
+        /// This tracks time when a qubit was last used, indexed by qubit id.
+        /// </summary>
+        private List<double> QubitAvailableAt;
+
+        /// <summary>
+        /// Maximum qubit id seen so far.
+        /// </summary>
+        private long MaxQubitId = -1;
+
+        internal QubitAvailabilityTimeTracker(int initialCapacity, double defaultAvailabilityTime)
+        {
+            DefaultAvailabilityTime = defaultAvailabilityTime;
+            QubitAvailableAt = new List<double>(initialCapacity);
+        }
+
+        internal double this[long qubitId]
+        {
+            get
+            {
+                if (qubitId < QubitAvailableAt.Count)
+                {
+                    return QubitAvailableAt[(int)qubitId];
+                }
+                return DefaultAvailabilityTime;
+            }
+            set
+            {
+                if (qubitId == QubitAvailableAt.Count)
+                {
+                    QubitAvailableAt.Add(value);
+                    return;
+                }
+                else if (qubitId >= int.MaxValue)
+                {
+                    throw new IndexOutOfRangeException("Too many qubits to track.");
+                }
+                else if (qubitId > QubitAvailableAt.Count)
+                {
+                    QubitAvailableAt.AddRange(Enumerable.Repeat(DefaultAvailabilityTime, (int)qubitId - QubitAvailableAt.Count + 1));
+                }
+                QubitAvailableAt[(int)qubitId] = value;
+            }
+        }
+
+        internal long GetMaxQubitId()
+        {
+            return MaxQubitId;
+        }
+
+        internal void MarkQubitIdUsed(long qubitId)
+        {
+            MaxQubitId = System.Math.Max(MaxQubitId, qubitId);
+        }
+
+    }
+
+}

src/Simulation/QCTraceSimulator/QubitMetrics.cs

@@ -8,24 +8,14 @@ namespace Microsoft.Quantum.Simulation.QCTraceSimulatorRuntime
     /// </summary>
     public class QubitTimeMetrics
     {
-        /// <summary>
-        /// Time when the qubit becomes available
-        /// </summary>
-        public double AvailableAt { get; private set; } = 0;
+        // TODO: Qubit Ids are already available in qubits, but DepthCounter doesn't have access to it
+        // in OnPrimitiveOperation because it's not part of IQCTraceSimulatorListener interface.
+        // Consider changing architecture to pass qubits rather than metrics in IQCTraceSimulatorListener.
+        public long QubitId { get; }
 
-        public QubitTimeMetrics()
+        public QubitTimeMetrics(long qubitId)
         {
-        }
-
-        /// <param name="timeAt">Beginning of the execution of the primitive operation on the qubit</param>
-        /// <param name="duration">Duration of the primitive operation</param>
-        public void RecordQubitUsage(double timeAt, double duration)
-        {
-            if (timeAt < AvailableAt)
-            {
-                throw new QubitTimeMetricsException();
-            }
-            AvailableAt = timeAt + duration;
+            QubitId = qubitId;
         }
     }
-}
+}

src/Simulation/QCTraceSimulator/TraceableQubitManager.cs

@@ -23,8 +23,8 @@ class TraceableQubitManager : QubitManagerTrackingScope
         /// The qubit manager makes sure that trace data array for qubits 
         /// is initialized with objects created by qubitTraceDataInitializers callbacks
         /// </summary>
-        public TraceableQubitManager( Func<long,object>[] qubitTraceDataInitializers ) 
-            : base(NumQubits, mayExtendCapacity : true, disableBorrowing : false)
+        public TraceableQubitManager( Func<long,object>[] qubitTraceDataInitializers, bool optimizeDepth ) 
+            : base(NumQubits, mayExtendCapacity : true, disableBorrowing : false, encourageReuse: !optimizeDepth)
         {
             this.qubitTraceDataInitializers = qubitTraceDataInitializers.Clone() as
                 Func<long, object>[];

src/Simulation/QCTraceSimulator/WidthCounter.cs

@@ -45,7 +45,7 @@ public WidthCounter( IDoubleStatistic[] statisticsToCollect = null )
             AddToCallStack(CallGraphEdge.CallGraphRootHashed, OperationFunctor.Body);
         }
 
-        public class Metrics
+        public static class Metrics
         {
             public const string InputWidth = "InputWidth";
             public const string ExtraWidth = "ExtraWidth";