private CflParse LeftDerivative(CflParse[] tokens, string input)
{
var length = tokens.Length;
var dv = new LinkedList <CflParse>();
var at = -1;
while (at < length)
{
while (dv.Count >= 2)
{
var left = dv.Last.Previous.Value;
var right = dv.Last.Value;
var reductions = Grammar.Lookup(left.Productions, right.Productions);
if (reductions != null)
{
var last = new CflParse(reductions, left, right);
dv.RemoveLast();
dv.RemoveLast();
dv.AddLast(last);
}
else
{
break;
}
}
at++;
if (at < length)
{
dv.AddLast(tokens[at]);
}
}
return(dv.Count == 1 ? dv.Last.Value : Error(input));
}
public override CflParse Parse(string input)
{
var tokens = Tokenize(input);
var length = tokens.Length;
var matrix = new CflParse[length, length];
for (var s = 1; s <= length; s++)
{
matrix[0, s - 1] = tokens[s - 1];
}
for (var l = 2; l <= length; l++)
{
for (var s = 1; s <= length - l + 1; s++)
{
for (var p = 1; p <= l - 1; p++)
{
var left = matrix[p - 1, s - 1];
var right = matrix[l - p - 1, s + p - 1];
if ((left != null) && (right != null))
{
var reductions = Grammar.Lookup(left.Productions, right.Productions);
if (reductions != null)
{
if (matrix[l - 1, s - 1] != null)
{
matrix[l - 1, s - 1].Productions.UnionWith(reductions);
}
else
{
matrix[l - 1, s - 1] = new CflParse(reductions, left, right);
}
}
}
}
}
}
matrix[length - 1, 0] = matrix[length - 1, 0] ?? Error(input);
return(matrix[length - 1, 0]);
}
private CflParse RightDerivative(CflParse[] tokens, string input)
{
var length = tokens.Length;
var dv = new LinkedList <CflParse>();
var at = length;
while (at >= 0)
{
while (dv.Count >= 2)
{
// Rewriting via R2
// Lc Rc ~> Lc (U head(Rc) head(tail(Rc))) tail(tail(Rc))
var left = dv.First.Value;
var right = dv.First.Next.Value;
var reductions = Grammar.Lookup(left.Productions, right.Productions);
if (reductions != null)
{
var first = new CflParse(reductions, left, right);
dv.RemoveFirst();
dv.RemoveFirst();
dv.AddFirst(first);
}
else
{
break;
}
}
at--;
if (at >= 0)
{
// Rewriting via R1
// Lc Rc ~> (Lc \ w) (U w) Rc
dv.AddFirst(tokens[at]);
}
}
return(dv.Count == 1 ? dv.First.Value : Error(input));
}