//generate a new rule from random existing productions.
public bool InsertRule(Grammar grammar)
{
for (var i = 0; i < NumberOfRetries; i++)
{
var productions = new List<string>();
var randomDaughter = grammar.StartSymbol;
while (randomDaughter == grammar.StartSymbol)
randomDaughter = grammar.GetRandomNonTerminal(); //the first daughter is never the start symbol.
productions.Add(randomDaughter);
if (_rand.NextDouble() < 0.5f)
productions.Add(grammar.GetRandomNonTerminal());
var newRule = new Rule();
newRule.Occurrences = 1;
newRule.Production = productions.Select(x => new NonTerminalObject(x)).ToArray();
newRule.HeadPosition = _rand.Next(newRule.Production.Length);
newRule.ComplementPosition = _rand.Next(newRule.Production.Length);
if (newRule.HeadTerm == grammar.StartSymbol)
//never let the head be the start symbol. the start symbol can only be the second term(see above).
newRule.HeadPosition = 0;
var ruleName = grammar.StartSymbol;
if (_rand.NextDouble() < 0.9f)
//90% probability of projecting regular head stucture. 10% allow to project to the START symbol.
{
try
{
ruleName = grammar.NonTerminalsTypeDictionary[newRule.HeadTerm] + "P";
}
catch
{
throw new Exception(string.Format("rule head term not found", newRule.HeadTerm));
}
}
newRule.Name = new NonTerminalObject(ruleName);
if (grammar.AreHeadRelationsConsistent(newRule))
{
grammar.AddRule(newRule);
return true;
}
}
return false;
}
public bool ChangeHeadOfRule(Grammar grammar)
{
for (var i = 0; i < NumberOfRetries; i++)
{
var rule = grammar.GetRandomRule();
if (!rule.IsInitialRule()) continue; //do not change complements of schematic rules. (push/pop)
if (rule.Production.Length > 1)
{
var newRule = new Rule(rule);
newRule.HeadPosition = (rule.HeadPosition + 1)%rule.Production.Length;
if (grammar.AreHeadRelationsConsistent(newRule))
{
grammar.DeleteRule(rule);
grammar.AddRule(newRule);
return true;
}
}
}
return false;
}