Change transitions range representation in Automaton.StateData (#140)

* Pair (FirstTranstiionIndex, LastTransitionIndex) changed to pair
  (FirstTransitionIndex, TransitionsCount).
* Introduced Automaton.Builder.LinkedStateData struct which
  mirrors Automaton.StateData but represents transitions as linked list.
  Previously Automaton.StateData was reused for this purpose.
  That was confusing.

src/Runtime/Distributions/Automata/Automaton.Builder.cs

@@ -20,7 +20,7 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
             /// <summary>
             /// States created so far.
             /// </summary>
-            private readonly List<StateData> states;
+            private readonly List<LinkedStateData> states;
 
             /// <summary>
             /// Transitions created so far.
@@ -29,8 +29,7 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
             /// Unlike in <see cref="StateCollection.transitions"/>, transitions
             /// for single entity are not represented by contiguous segment of array, but rather as a linked
             /// list. It is done this way, because transitions can be added at any moment and inserting
-            /// transition into a middle of array is not feasible. <see cref="StateData.FirstTransition"/>
-            /// references head of list and <see cref="StateData.LastTransition"/> references last element of list.
+            /// transition into a middle of array is not feasible.
             /// </remarks>
             private readonly List<LinkedTransitionNode> transitions;
 
@@ -45,7 +44,7 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
             /// </summary>
             public Builder(int startStateCount = 1)
             {
-                this.states = new List<StateData>();
+                this.states = new List<LinkedStateData>();
                 this.transitions = new List<LinkedTransitionNode>();
                 this.AddStates(startStateCount);
             }
@@ -120,10 +119,10 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
 
                 var index = this.states.Count;
                 this.states.Add(
-                    new StateData
+                    new LinkedStateData
                     {
-                        FirstTransition = -1,
-                        LastTransition = -1,
+                        FirstTransitionIndex = -1,
+                        LastTransitionIndex = -1,
                         EndWeight = Weight.Zero,
                     });
                 return new StateBuilder(this, index);
@@ -357,7 +356,7 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
                     for (var i = 0; i < oldStateCount; ++i)
                     {
                         var state = this.states[i];
-                        if (state.CanEnd && state.FirstTransition != -1)
+                        if (!state.EndWeight.IsZero && state.FirstTransitionIndex != -1)
                         {
                             return false;
                         }
@@ -397,9 +396,8 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
 
                 for (var i = 0; i < resultStates.Length; ++i)
                 {
-                    var state = this.states[i];
-                    var transitionIndex = state.FirstTransition;
-                    state.FirstTransition = nextResultTransitionIndex;
+                    var firstResultTransitionIndex = nextResultTransitionIndex;
+                    var transitionIndex = this.states[i].FirstTransitionIndex;
                     while (transitionIndex != -1)
                     {
                         var node = this.transitions[transitionIndex];
@@ -415,8 +413,10 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
                         transitionIndex = node.Next;
                     }
 
-                    state.LastTransition = nextResultTransitionIndex;
-                    resultStates[i] = state;
+                    resultStates[i] = new StateData(
+                        firstResultTransitionIndex,
+                        nextResultTransitionIndex - firstResultTransitionIndex,
+                        this.states[i].EndWeight);
                 }
 
                 Debug.Assert(
@@ -470,7 +470,7 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
                 /// <summary>
                 /// Gets a value indicating whether the ending weight of this state is greater than zero.
                 /// </summary>
-                public bool CanEnd => this.builder.states[this.Index].CanEnd;
+                public bool CanEnd => !this.builder.states[this.Index].EndWeight.IsZero;
 
                 /// <summary>
                 /// Gets or sets the ending weight of the state.
@@ -486,7 +486,7 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
                 /// Gets <see cref="TransitionIterator"/> over transitions of this state.
                 /// </summary>
                 public TransitionIterator TransitionIterator =>
-                    new TransitionIterator(this.builder, this.Index, this.builder.states[this.Index].FirstTransition);
+                    new TransitionIterator(this.builder, this.Index, this.builder.states[this.Index].FirstTransitionIndex);
 
                 /// <summary>
                 /// Initializes a new instance of <see cref="StateBuilder"/> struct.
@@ -525,22 +525,22 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
                         {
                             Transition = transition,
                             Next = -1,
-                            Prev = state.LastTransition,
+                            Prev = state.LastTransitionIndex,
                         });
 
-                    if (state.LastTransition != -1)
+                    if (state.LastTransitionIndex != -1)
                     {
                         // update "next" field in old tail
-                        var oldTail = this.builder.transitions[state.LastTransition];
+                        var oldTail = this.builder.transitions[state.LastTransitionIndex];
                         oldTail.Next = transitionIndex;
-                        this.builder.transitions[state.LastTransition] = oldTail;
+                        this.builder.transitions[state.LastTransitionIndex] = oldTail;
                     }
                     else
                     {
-                        state.FirstTransition = transitionIndex;
+                        state.FirstTransitionIndex = transitionIndex;
                     }
 
-                    state.LastTransition = transitionIndex;
+                    state.LastTransitionIndex = transitionIndex;
                     this.builder.states[this.Index] = state;
 
                     
@@ -759,16 +759,16 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
                     }
 
                     // update references in state
-                    if (state.FirstTransition == this.index || state.LastTransition == this.index)
+                    if (state.FirstTransitionIndex == this.index || state.LastTransitionIndex == this.index)
                     {
-                        if (state.FirstTransition == this.index)
+                        if (state.FirstTransitionIndex == this.index)
                         {
-                            state.FirstTransition = node.Next;
+                            state.FirstTransitionIndex = node.Next;
                         }
 
-                        if (state.LastTransition == this.index)
+                        if (state.LastTransitionIndex == this.index)
                         {
-                            state.LastTransition = node.Prev;
+                            state.LastTransitionIndex = node.Prev;
                         }
 
                         this.builder.states[this.stateIndex] = state;
@@ -793,6 +793,29 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
 
             }
 
+            /// <summary>
+            /// Version of <see cref="StateData"/> that is used during automaton constructions. Unlike
+            /// regular <see cref="StateData"/> transitions are stored as a linked list over
+            /// <see cref="Builder.transitions"/> array.
+            /// </summary>
+            private struct LinkedStateData
+            {
+                /// <summary>
+                /// Index of the head of transitions list in <see cref="Builder.transitions"/>.
+                /// </summary>
+                public int FirstTransitionIndex { get; internal set; }
+
+                /// <summary>
+                /// Index of the tail of transitions list in <see cref="Builder.transitions"/>.
+                /// </summary>
+                public int LastTransitionIndex { get; internal set; }
+
+                /// <summary>
+                /// Ending weight of the state.
+                /// </summary>
+                public Weight EndWeight { get; internal set; }
+            }
+
             /// <summary>
             /// Linked list node for representing transitions for state.
             /// </summary>

src/Runtime/Distributions/Automata/Automaton.DataContainer.cs

@@ -108,12 +108,13 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
 
                 foreach (var state in this.States)
                 {
-                    if (state.FirstTransition < 0 || state.LastTransition > this.Transitions.Count)
+                    var lastTransitionIndex = state.FirstTransitionIndex + state.TransitionsCount;
+                    if (state.FirstTransitionIndex < 0 || lastTransitionIndex > this.Transitions.Count)
                     {
                         return false;
                     }
 
-                    for (var i = state.FirstTransition; i < state.LastTransition; ++i)
+                    for (var i = state.FirstTransitionIndex; i < lastTransitionIndex; ++i)
                     {
                         var transition = this.Transitions[i];
                         if (transition.DestinationStateIndex < 0 ||

src/Runtime/Distributions/Automata/Automaton.State.cs

@@ -67,8 +67,8 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
             public ReadOnlyArraySegment<Transition> Transitions =>
                 new ReadOnlyArraySegment<Transition>(
                     this.transitions,
-                    this.Data.FirstTransition,
-                    this.Data.LastTransition - this.Data.FirstTransition);
+                    this.Data.FirstTransitionIndex,
+                    this.Data.TransitionsCount);
 
             internal StateData Data => this.states[this.Index];
 

src/Runtime/Distributions/Automata/Automaton.StateData.cs

@@ -29,20 +29,15 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
             /// this property contains index of the head of the linked-list of transitions.
             /// </remarks>
             [DataMember]
-            public int FirstTransition { get; internal set; }
+            public int FirstTransitionIndex { get; internal set; }
 
             /// <summary>
-            /// Gets or sets index of the first transition in <see cref="DataContainer.Transitions"/> after
-            /// <see cref="FirstTransition"/> which does not belong to this state. All transitions for
+            /// Gets or sets count of transition in <see cref="DataContainer.Transitions"/> after
+            /// <see cref="FirstTransitionIndex"/> which belong to this state. All transitions for
             /// the same state are stored as a contiguous block.
             /// </summary>
-            /// <remarks>
-            /// During automaton construction <see cref="Automaton{TSequence,TElement,TElementDistribution,TSequenceManipulator,TThis}.Builder"/>
-            /// stores transitions as linked-list instead of contiguous block. So, during construction
-            /// this property contains index of the tail of the linked-list of transitions.
-            /// </remarks>
             [DataMember]
-            public int LastTransition { get; internal set; }
+            public int TransitionsCount { get; internal set; }
 
             /// <summary>
             /// Gets or sets ending weight of the state.
@@ -53,11 +48,11 @@ namespace Microsoft.ML.Probabilistic.Distributions.Automata
             /// <summary>
             /// Initializes a new instance of the <see cref="StateData"/> struct.
             /// </summary>
-            [Construction("FirstTransition", "LastTransition", "EndWeight")]
-            public StateData(int firstTransition, int lastTransition, Weight endWeight)
+            [Construction("FirstTransitionIndex", "TransitionsCount", "EndWeight")]
+            public StateData(int firstTransitionIndex, int transitionsCount, Weight endWeight)
             {
-                this.FirstTransition = firstTransition;
-                this.LastTransition = lastTransition;
+                this.FirstTransitionIndex = firstTransitionIndex;
+                this.TransitionsCount = transitionsCount;
                 this.EndWeight = endWeight;
             }
 

test/Tests/Strings/AutomatonTests.cs

@@ -864,7 +864,7 @@ namespace Microsoft.ML.Probabilistic.Tests
                     new[]
                         {
                             new StringAutomaton.StateData(0, 1, Weight.One),
-                            new StringAutomaton.StateData(1, 1, Weight.One),
+                            new StringAutomaton.StateData(1, 0, Weight.One),
                         },
                     new[] { new StringAutomaton.Transition(DiscreteChar.PointMass('a'), Weight.One, 1) }));