package org.nutz.el.arithmetic;
import java.io.IOException;
import java.util.LinkedList;
import java.util.Queue;
import org.nutz.el.Operator;
import org.nutz.el.opt.arithmetic.LBracketOpt;
import org.nutz.el.opt.arithmetic.RBracketOpt;
import org.nutz.el.opt.logic.QuestionOpt;
import org.nutz.el.opt.logic.QuestionSelectOpt;
import org.nutz.el.parse.Converter;
/**
* Shunting yard算法是一个用于将中缀表达式转换为后缀表达式的经典算法，由艾兹格·迪杰斯特拉引入，因其操作类似于火车编组场而得名。<br/>
* 参考:
* <a href='http://zh.wikipedia.org/wiki/Shunting_yard%E7%AE%97%E6%B3%95'>Shunting yard算法</a>
*
* @author juqkai(juqkai@gmail.com)
*
*/
public class ShuntingYard {
private LinkedList<Operator> opts;
private Queue<Object> rpn;
/**
* 转换操作符.
* 根据 ShuntingYard 算法进行操作
* @param current
*/
private void parseOperator(Operator current){
//空,直接添加进操作符队列
if(opts.isEmpty()){
opts.addFirst(current);
return;
}
//左括号
if(current instanceof LBracketOpt){
opts.addFirst(current);
return;
}
//遇到右括号
if(current instanceof RBracketOpt){
while(!(opts.peek() instanceof LBracketOpt)){
rpn.add(opts.poll());
}
opts.poll();
return;
}
//符号队列top元素优先级大于当前,则直接添加到
if(!opts.isEmpty() && opts.peek().fetchPriority() > current.fetchPriority()){
opts.addFirst(current);
return;
}
//一般情况,即优先级小于栈顶,那么直接弹出来,添加到逆波兰表达式中
while(!opts.isEmpty() && opts.peek().fetchPriority() <= current.fetchPriority()){
//三元表达式嵌套的特殊处理
if(opts.peek() instanceof QuestionOpt && current instanceof QuestionOpt){
break;
}
if(opts.peek() instanceof QuestionOpt && current instanceof QuestionSelectOpt){
rpn.add(opts.poll());
break;
}
rpn.add(opts.poll());
}
opts.addFirst(current);
}
/**
* 转换成 逆波兰表示法（Reverse Polish notation，RPN，或逆波兰记法）
* @param val
* @throws IOException
*/
public Queue<Object> parseToRPN(String val) {
rpn = new LinkedList<Object>();
opts = new LinkedList<Operator>();
Converter converter = new Converter(val);
converter.initItems();
while(!converter.isEnd()){
Object item = converter.fetchItem();
if(item instanceof Operator){
parseOperator((Operator) item);
continue;
}
rpn.add(item);
}
while(!opts.isEmpty()){
rpn.add(opts.poll());
}
return rpn;
}
}