/// <summary>
/// Removes states that do not have an possible input transitions to a
/// terminating state
/// </summary>
/// <param name="partial"></param>
/// <returns></returns>
public PartialDFA RemoveDeadStates(
PartialDFA partial)
{
var terminable = partial.Terminals.ToSet(Comparer);
var unknown = partial.States.Where(s => !terminable.Contains(s)).ToSet(Comparer);
var queue = new Queue <TState>(terminable);
while (queue.Any())
{
var current = queue.Dequeue();
var sourceStates = partial.Transitions
.Where(t => terminable.Contains(t.Target))
.Where(t => unknown.Contains(t.Source))
.Select(t => t.Source);
foreach (var state in sourceStates)
{
terminable.Add(state);
unknown.Remove(state);
queue.Enqueue(state);
}
}
if (!unknown.Any())
{
return(partial);
}
var transitions =
from transition in partial.Transitions
where !unknown.Contains(transition.Source)
where !unknown.Contains(transition.Target)
select transition;
return(new PartialDFA(
partial.Initial,
partial.Terminals.ToList(),
transitions.ToList()));
}
/// <summary>
/// Uses the equivalence minimization algorithm to reduce duplicate states
/// </summary>
/// <param name="partial"></param>
/// <returns></returns>
public PartialDFA EquivalenceMinimization(
PartialDFA partial)
{
// Initialize with two groups from terminals and everything else
var counter = 0;
var groups = new Dictionary <Set <TState>, int> {
{ partial.Terminals.ToSet(Comparer), counter++ },
{ partial.States.Where(s => !partial.Terminals.Contains(s)).ToSet(Comparer), counter++ }
};
var marked = new Set <Set <TState> >();
var unmarked = new Queue <Set <TState> >(groups.Keys);
// Continue until all sets are marked
while (unmarked.Any())
{
var current = unmarked.Dequeue();
if (current.Count == 1)
{
// Can't reduce a set of one
marked.Add(current);
continue;
}
var table =
from state in current
let transitons = (
from input in partial.Inputs
let targets = partial.Transitions.Where(t => t.Source.Equals(state) && t.Input.Equals(input)).ToList()
let targetGroup = targets.Any() ? groups.Keys.Single(g => g.Contains(targets.Single().Target)) : null
let targetGroupId = !(targetGroup is null) ? groups[targetGroup] : -1
select targetGroupId
).ToSet()
select(
State: state,
Transitions: transitons,
Hash: transitons.GetSequentialHashCode()
);
var partitions =
from row in table
group row by row.Hash into rowsByHash
select rowsByHash;
var newGroups = partitions
.Select(grp => new Set <TState>(grp.Select(g => g.State), Comparer))
.ToList();
if (newGroups.Count == 1)
{
marked.Add(current);
continue;
}
// Move all marked groups back to unmarked
foreach (var markedGroup in marked)
{
unmarked.Enqueue(markedGroup);
}
marked.Clear();
// Current group is getting partitioned, to remove it
groups.Remove(current);
foreach (var newGroup in newGroups)
{
groups.Add(newGroup, counter++);
unmarked.Enqueue(newGroup);
}
}
var terminals =
from terminal in partial.Terminals
select groups.Keys.Single(g => g.Contains(terminal));
var transitions =
from sourceGroup in groups.Keys
from input in partial.Inputs
let source = sourceGroup.First()
where partial.Transitions.Any(t => t.Source.Equals(source) && t.Input.Equals(input))
let target = partial.Transitions.Single(t => t.Source.Equals(source) && t.Input.Equals(input)).Target
let targetGroup = groups.Keys.Single(g => g.Contains(target))
select GroupTransition(sourceGroup, input, targetGroup);
return(Reduce(
groups.Keys.Single(g => g.Contains(partial.Initial)),
terminals,
transitions
));
}