parse.cpp

一个简单的词法分析器

#include "global.h"
#include "lexer.h"
#include "emit.h"
#include "error.h"
#include "parse.h"

int lookahead;

void parse(){
	lookahead=lexan();
	while(lookahead!=DONE){
		expr();printf(";\n");match(';');
	}
}
void expr(){
	int t;
	term();
	while(1)
		switch(lookahead){
		case '+':case '-':
			t=lookahead;
			match(lookahead);emit(t,NONE);term();
			continue;
		default:
			return;
	}
}
void term(){
	int t;
	factor();
	while(1)
         switch(lookahead){
		 case '*':case '/':case DIV:case MOD:case '=':
			t=lookahead;
			match(lookahead);emit(t,NONE);factor();
			continue;
	case '(':
			printf("(\n");
			match('(');expr();match(')');
			printf(")\n");
			break;
			continue;
		default:
			return;
	}
}
void factor(){
	switch(lookahead){
		case '(':
			printf("(\n");
			match('(');expr();match(')');
			printf(")\n");
			break;
		case NUM:
			emit(NUM,tokenval);match(NUM);break;
		case ID:
			emit(ID,tokenval);match(ID);break;
		case SIN:
			emit(SIN,NONE);match(SIN);break;
		default:
			error("syntax error");
	}
}
void match(int t){
	if(lookahead==t)
		lookahead=lexan();
	else
		error("syntax error");
}