/// <summary>
/// Compacts transitions by eliminating trailing null tansitions.
/// I.e if the last couple characters in the alphabet, for one state don't lead to another state, these cells can be
/// eliminated from the table.
/// Transitions that point to a later state are adjusted by the amount of eliminated cells.
/// </summary>
public TransitionCompacter(IStateMachineBuilder builder)
{
// Build a set of indices that can be skipped and a table of offsets for adjusting the remaining transitions.
var skippedIndices = new HashSet <int>();
var offsetMap = new int[builder.Transitions.Count];
var skipTotal = 0;
for (var i = 0; i < builder.Transitions.Count; i += builder.AlphabetSize)
{
// Count the trailing nulls.
var transitionsToKeep = builder.AlphabetSize;
for (; transitionsToKeep > 0; transitionsToKeep--)
{
var transition = i + transitionsToKeep - 1;
if (!builder.IsNull(transition) || builder.IsResult(transition))
// Results and normal transitions can't be skipped.
{
break;
}
skippedIndices.Add(transition);
}
// Build the set and table.
var toSkip = builder.AlphabetSize - transitionsToKeep;
skipTotal += toSkip;
for (var j = 0; j < builder.AlphabetSize; ++j)
{
var index = i + builder.AlphabetSize + j;
if (index >= offsetMap.Length)
{
break;
}
offsetMap[index] = skipTotal;
}
}
// Adjust the remaining transitions.
var clone = builder.Transitions.ToList();
for (var i = 0; i < clone.Count; ++i)
{
if (builder.IsNull(i) || builder.IsResult(i)) // nulls and results are not adjusted.
{
continue;
}
clone[i] -= offsetMap[clone[i]];
}
// Copy into a new list while skipping eliminated cells.
Transitions = clone.Where((t, i) => !skippedIndices.Contains(i)).ToList();
// Create offsets.
var offsets = new List <int>();
for (var i = 0; i < builder.Transitions.Count; i += builder.AlphabetSize)
{
offsets.Add(offsetMap[i]);
}
Offsets = offsets;
}
/// <summary>
/// Counts the number of null transitions per character in the alphabet.
/// </summary>
private static IEnumerable <int> CountNullTransitions(IStateMachineBuilder builder)
{
var nullTransitions = new int[builder.AlphabetSize];
for (var i = 0; i < builder.Transitions.Count; i++)
{
if (builder.IsResult(i))
{
continue;
}
if (builder.IsNull(i))
{
nullTransitions[i % builder.AlphabetSize] += 1;
}
}
return(nullTransitions);
}