/// <summary>
/// 计算所有可能推导出null的结点。
/// </summary>
/// <param name="grammar"></param>
/// <returns></returns>
public static void GetNullableDict(
this RegulationList grammar, out Dictionary <TreeNodeType, bool> nullableDict)
{
nullableDict = new Dictionary <TreeNodeType, bool>();
// 初始化nullable
List <TreeNodeType> allNodeTypes = grammar.GetAllTreeNodeTypes();
foreach (var item in allNodeTypes)
{
nullableDict.Add(item, false);
}
// 迭代到不动点
bool changed = false;
do
{
changed = false;
foreach (var regulation in grammar)
{
// 如果RightPart的结点全部可为null，就说明此regulation.Left是可推导出"null"的。
if (Nullable(regulation.RightPart, 0, regulation.RightPart.Count(), nullableDict))
{
if (!nullableDict[regulation.Left])
{
nullableDict[regulation.Left] = true;
changed = true;
}
}
}
} while (changed);
}
