public MultiRepresentationWeightFunction <TDictionary> Repeat(int minTimes = 1, int?maxTimes = null)
{
Argument.CheckIfInRange(minTimes >= 0, nameof(minTimes), "The minimum number of repetitions must be non-negative.");
Argument.CheckIfValid(!maxTimes.HasValue || maxTimes.Value >= minTimes, "The maximum number of repetitions must not be less than the minimum number.");
if (weightFunction is PointMassWeightFunction pointMass && maxTimes.HasValue && maxTimes - minTimes < MaxDictionarySize)
{
var newSequenceElements = new List <TElement>(SequenceManipulator.GetLength(pointMass.Point) * maxTimes.Value);
for (int i = 0; i < minTimes; ++i)
{
newSequenceElements.AddRange(pointMass.Point);
}
if (minTimes == maxTimes)
{
return(FromPoint(SequenceManipulator.ToSequence(newSequenceElements)));
}
else
{
Weight uniformWeight = Weight.FromValue(1.0 / (maxTimes.Value - minTimes));
Dictionary <TSequence, Weight> dict = new Dictionary <TSequence, Weight>(maxTimes.Value - minTimes + 1);
dict.Add(SequenceManipulator.ToSequence(newSequenceElements), uniformWeight);
for (int i = minTimes + 1; i <= maxTimes.Value; ++i)
{
newSequenceElements.AddRange(pointMass.Point);
dict.Add(SequenceManipulator.ToSequence(newSequenceElements), uniformWeight);
}
return(FromDictionary(DictionaryWeightFunction <TDictionary> .FromDistinctWeights(dict)));
}
}
if (weightFunction is TDictionary dictionary && maxTimes.HasValue)
{
var resultSupportSize = ResultSupportSize(dictionary.Dictionary.Count, minTimes, maxTimes.Value);
if (resultSupportSize <= MaxDictionarySize)
{
return(FromDictionary(dictionary.Repeat(minTimes, maxTimes.Value, (int)resultSupportSize + 1)));
}
}
return(FromAutomaton(AsAutomaton().Repeat(minTimes, maxTimes)));
double ResultSupportSize(int sourceSupportSize, int minReps, int maxReps)
{
return(Math.Pow(sourceSupportSize, minReps) * (1 - Math.Pow(sourceSupportSize, maxReps - minReps + 1)) / (1 - sourceSupportSize));
}
}
/// <summary>
/// Enumerate support of this automaton without elimination of duplicate elements
/// </summary>
/// <param name="maxTraversedPaths">
/// Maximum number of paths in the automaton this function
/// is allowed to traverse before stopping. Defaults to <see cref="int.MaxValue"/>.
/// Can be used to limit the performance impact of this call in cases when
/// the support is useful only if it can be obtained quickly.
/// </param>
/// <param name="stopOnNonPointMassElementDistribution">
/// When set to true, the enumeration is canceled upon encountering a non-point mass
/// element distribution on a transition and a <see langword="null"/> value is yielded.
/// </param>
/// <returns>
/// The sequences supporting this automaton. Sequences may be non-distinct if
/// automaton is not determinized. A <see langword="null"/> value in enumeration means that
/// an infinite loop was reached or that the enumeration was stopped because
/// a condition set by one of this method's parameters was met.
/// Public <see cref="EnumerateSupport(int)"/> /
/// <see cref="TryEnumerateSupport(int, out IEnumerable{TSequence}, int, bool)"/>
/// methods handle null value differently.
/// </returns>
/// <remarks>
/// Conceptually enumerating support is just depth-first traversal of automaton from start
/// state recording the elements met on path. Real implementation is a little hairy because
/// of the following reasons:
/// - Recursion can not be used, instead an explicit stack is used
/// - Each transition can have a distribution with a lot of support, so it has to be
/// enumerated lazily, making state stored on stack large
/// - Some highly-branching paths can have no end state in them. This fact has to be
/// tracked to void spending exponential time traversing states which produce no output
/// - Loops have to be tracked: some loops make automaton non-enumerable, some don't
/// - An fast-path for non-branchy automata is implemented. It makes traversing those 10x
/// faster by skipping some boilerplate for tracking traversal state in these cases.
/// </remarks>
private IEnumerable <TSequence> EnumerateSupportInternalWithDuplicates(
int maxTraversedPaths = int.MaxValue,
bool stopOnNonPointMassElementDistribution = false)
{
// Sequence of elements on path to current state in automaton
var sequence = new List <TElement>();
// Stack of states for backtracking during depth-first traversal
var stack = new Stack <StateEnumerationState>();
// Stores 2 bits of data about state:
// - Is this state a dead end. I.e. end state is not reachable through any path starting
// from this state. This flag is computed incrementally most of the time. But if
// automaton contains loops, a special procedure `ComputeEndStatesReachability` is
// invoked which computes it efficiently for whole graph.
// - Whether this state is being visited now and if yes - how long is the sequence from
// root to this state. This is used for (non-empty) loop detection. We store length + 1,
// because 0 value of flags has special meaning of "this state has not been visited yet".
var flags = new StateEnumerationFlags[this.States.Count];
// Number of sequences produced by this time. By comparing value of this counter
// before entering the state and when leaving it is easy to detect whether the state
// is a dead end.
var producedCount = 0;
// Number of paths traversed by this time. Used in early break condition.
var traversedPathsCount = 0;
// Enumeration state for current state. A top of traversal stack, materialized in local
// variable for convenience.
var current = InitEnumerationState(this.Start.Index, 0, this.Start.CanEnd ? 0 : -1);
// If true, we can assume that `IsDeadEnd` flag is computed for all states
var endStateReachabilityComputed = false;
// Mark the first state as visited
flags[this.Start.Index] = (StateEnumerationFlags)1;
if (this.Start.CanEnd)
{
// We do not ever "enter" the start state via a transition, so need to handle
// "start state is also an end state" case explicitly
yield return(SequenceManipulator.ToSequence(sequence));
}
while (true)
{
if (!TryBacktrackIfNeed())
{
// Nowhere to backtrack, enumerated everything
yield break;
}
// In theory, this operation could be called only when `current.PathLength`
// decreases. But that happens in multiple places, so it's easier to truncate path
// on each iteration, even if it is a noop.
sequence.RemoveRange(current.PathLength, sequence.Count - current.PathLength);
var transitionAdvancementResult = AdvanceToNextTransition(out var nextStateIndex, out var nextElement);
if (transitionAdvancementResult == TransitionAdvancementResult.NoMoreTransitions)
{
// Failed to go to next transition: destination state is unreachable
continue;
}
if (transitionAdvancementResult == TransitionAdvancementResult.ShouldStopEnumeration)
{
yield return(null);
yield break;
}
if (nextElement.HasValue)
{
sequence.Add(nextElement.Value);
}
nextStateIndex = TraverseTrivialStates(nextStateIndex);
var nextState = this.Data.States[nextStateIndex];
if (flags[nextStateIndex] != 0)
{
++traversedPathsCount;
if (traversedPathsCount > maxTraversedPaths)
{
// Hit the limit on the number of returned sequences.
yield return(null);
yield break;
}
// This states is either dead end or a loop. If this is a dead end, then it
// will not be traversed at all.
if ((flags[nextStateIndex] & StateEnumerationFlags.IsDeadEnd) != 0)
{
continue;
}
// This is a loop, so let's check if it can produce anything.
// If it produces anything - then support is in-enumerable.
// If it doesn't, there's no point in entering this state at all
if (!endStateReachabilityComputed)
{
ComputeEndStateReachability(flags);
endStateReachabilityComputed = true;
}
if ((flags[nextStateIndex] & StateEnumerationFlags.IsDeadEnd) != 0 ||
sequence.Count + 1 == (int)(flags[nextStateIndex] & StateEnumerationFlags.DepthMask))
{
// Just skip this loop
// - Either no end state is reachable from this loop. So it can "produce"
// as many elements as it wants, this does not matter because this
// sequence would never be output.
// - Or this loop produces 0 elements (i.e. consists of epsilon transitions
// only), because path was extended by 0 elements after traversing the loop.
continue;
}
// A non-empty loop that can produce something was found. Signal to outer call
// via returning null.
yield return(null);
yield break;
}
var producedBeforeThisState = producedCount;
if (nextState.CanEnd && sequence.Count > current.LongestOutputInPath)
{
// Output sequence only if it is longer then any sequence produced on the path
// to this state. This is an important optimization in some real automata.
// Simplified example: graph `a -> b -> c -> d -> e`. Where both states `a` and
// `e` are marked as end states and every arrow corresponds to 2 epsilon
// transitions. There are 2^4 = 16 unique paths from `a` to `e`. We need to
// detect that `e` is practically a dead end even though it has `CanEnd` flag.
// By noticing that `e` and `a` states produce sequences of the same length,
// we can assume that `e` is effectively not an end state because it does not
// produce any new sequences.
++producedCount;
++traversedPathsCount;
if (traversedPathsCount > maxTraversedPaths)
{
// Hit the limit on the number of returned sequences.
yield return(null);
yield break;
}
yield return(SequenceManipulator.ToSequence(sequence));
}
if (nextState.TransitionsCount == 0)
{
// Fast path: there is no need to enter this state at all because we would
// backtrack immediately to current state on next iteration.
if (!nextState.CanEnd)
{
flags[nextStateIndex] = StateEnumerationFlags.IsDeadEnd;
}
}
else
{
// Slow path: store current state on stack and move to next state
flags[nextStateIndex] = (StateEnumerationFlags)(sequence.Count + 1);
stack.Push(current);
current = InitEnumerationState(
nextStateIndex,
producedBeforeThisState,
nextState.CanEnd ? sequence.Count : current.LongestOutputInPath);
}
}
StateEnumerationState InitEnumerationState(
int index, int producedBeforeThisState, int longestOutputInPath)
{
var state = this.Data.States[index];
return(new StateEnumerationState
{
StateIndex = index,
ProducedCount = producedBeforeThisState,
PathLength = sequence.Count,
LongestOutputInPath = longestOutputInPath,
TransitionIndex = state.FirstTransitionIndex - 1,
RemainingTransitionsCount = state.TransitionsCount,
ElementEnumerator = null,
});
}
// Backtracks according to traversal stack if that is needed.
// Returns false if automaton has been fully enumerated.
//
// If backtracking is needed (i.e. no more non-processed transitions left in current
// state), replaces `current` with the current top of the stack if that's possible.
// Also updates the `flags` with information whether path through this state produces
// any output.
bool TryBacktrackIfNeed()
{
// While no more transitions are left in current state - backtrack
while (current.ElementEnumerator == null && current.RemainingTransitionsCount == 0)
{
if (stack.Count == 0)
{
// Nowhere to backtrack, this automaton has been fully enumerated
if (this.Data.IsEnumerable == null)
{
this.Data = this.Data.With(isEnumerable: true);
}
return(false);
}
// If upon leaving current state, number of produced sequences is equal to
// the number upon entrance then this is a dead end.
flags[current.StateIndex] = current.ProducedCount == producedCount
? StateEnumerationFlags.IsDeadEnd
: 0;
current = stack.Pop();
}
return(true);
}
// Updates `current` enumeration state by moving to next element/state index reachable
// from current state.
TransitionAdvancementResult AdvanceToNextTransition(out int nextStateIndex, out Option <TElement> nextElement)
{
Debug.Assert(
current.ElementEnumerator != null || current.RemainingTransitionsCount != 0,
"TryBacktrack() must skip all states with no transitions left");
if (current.ElementEnumerator != null)
{
// Advance to next element in current transition
nextStateIndex = this.Data.Transitions[current.TransitionIndex].DestinationStateIndex;
if ((flags[nextStateIndex] & StateEnumerationFlags.IsDeadEnd) != 0)
{
// While enumerating current transition we learned that it leads to a
// dead end. Stop enumerating elements on this transition, because will
// waste time on enumerating dead end.
current.ElementEnumerator = null;
}
else
{
nextElement = current.ElementEnumerator.Current;
if (!current.ElementEnumerator.MoveNext())
{
// Element done, move to next transition on next iteration
current.ElementEnumerator = null;
}
return(TransitionAdvancementResult.Success);
}
}
// Advance to next transition
while (current.RemainingTransitionsCount != 0)
{
++current.TransitionIndex;
--current.RemainingTransitionsCount;
var transition = this.Data.Transitions[current.TransitionIndex];
nextStateIndex = transition.DestinationStateIndex;
if (transition.Weight.IsZero ||
(flags[nextStateIndex] & StateEnumerationFlags.IsDeadEnd) != 0)
{
// Do not follow paths which produce nothing and try next transition
continue;
}
if (transition.IsEpsilon)
{
nextElement = Option.None;
}
else
{
var elementDistribution = transition.ElementDistribution.Value;
if (elementDistribution.IsPointMass)
{
nextElement = elementDistribution.Point;
}
else
{
if (stopOnNonPointMassElementDistribution)
{
nextStateIndex = -1;
nextElement = Option.None;
return(TransitionAdvancementResult.ShouldStopEnumeration);
}
if (!(elementDistribution is CanEnumerateSupport <TElement> supportEnumerator))
{
this.Data = this.Data.With(isEnumerable: false);
throw new InvalidOperationException();
}
var enumerator = supportEnumerator.EnumerateSupport().GetEnumerator();
if (!enumerator.MoveNext())
{
// This transition is not marked as epsilon (i.e. contains
// distribution on it). But this distribution contains no support.
// Go to next transition.
continue;
}
nextElement = enumerator.Current;
current.ElementEnumerator = enumerator.MoveNext() ? enumerator : null;
}
}
return(TransitionAdvancementResult.Success);
}
nextStateIndex = -1;
nextElement = Option.None;
return(TransitionAdvancementResult.NoMoreTransitions);
}
// Traverses all trivial states - non-terminal states with only one non-epsilon forward
// transition with non-zero weight.
//
// It is safe (and a lot faster) to traverse these states without involving
// backtracking logic or complicated code for enumerating transitions. The only tricky
// part is loop detection - because these states are not backtracked, flags on the
// won't be updated. But that is safe because loop involves at least 1 backward
// transition, and that one won't be fast-tracked.
//
// This procedure is fast-path optimization for automata with very low uncertainty.
// Most automata in practice belong to this category.
int TraverseTrivialStates(int currentStateIndex)
{
while (true)
{
var state = this.Data.States[currentStateIndex];
if (flags[currentStateIndex] != 0 ||
state.CanEnd ||
state.TransitionsCount != 1)
{
return(currentStateIndex);
}
var transition = this.Data.Transitions[state.FirstTransitionIndex];
if (transition.Weight.IsZero ||
transition.IsEpsilon ||
transition.DestinationStateIndex <= currentStateIndex)
{
return(currentStateIndex);
}
var dist = transition.ElementDistribution.Value;
if (!dist.IsPointMass)
{
return(currentStateIndex);
}
sequence.Add(dist.Point);
currentStateIndex = transition.DestinationStateIndex;
}
}
}