自制语法分析器

自制语法分析器

代码如下：

#include "stdio.h"
#include "stdlib.h"
#include "token.h"

#define LINE_BUF_SIZE (1024)
static Token st_look_ahead_token;
static int st_look_ahead_token_exists;

static void my_get_token(Token* token)
{
	if (st_look_ahead_token_exists)
	{
		*token = st_look_ahead_token;
		st_look_ahead_token_exists = 0;
	}
	else
	{
		get_token(token);
	}
}

static void unget_token(Token* token)
{
	st_look_ahead_token = *token;
	st_look_ahead_token_exists = 1;
}

double parse_expression(void);

static double parse_primary_expression()
{
	Token token;
	my_get_token(&token);
	if (token.kind == NUMBER_TOKEN)
	{
		return token.value;
	}	
	return 0.0;
}

static double parse_term()
{
	double v1;
	double v2;
	Token token;
	v1 = parse_primary_expression();
	for(;;)
	{		
		my_get_token(&token);
		if(token.kind !=  MUL_OPERATOR_TOKEN && token.kind !=  DIV_OPERATOR_TOKEN)
		{
			unget_token(&token);
			break;
		}	

		v2 = parse_primary_expression();

		if(token.kind ==  MUL_OPERATOR_TOKEN)		
		{
			v1 *= v2;
		}
		else if( token.kind ==  DIV_OPERATOR_TOKEN)
		{
			v1 /= v2;
		}
	}

	

	return v1;
}
	
double parse_expression()
{
	double v1;
	double v2;
	Token token;


	v1 = parse_term();
	for (;;)
	{
		
		my_get_token(&token);
		if(token.kind !=  ADD_OPERATOR_TOKEN && token.kind !=  SUB_OPERATOR_TOKEN)
		{
			unget_token(&token);
			break;
		}
		v2 = parse_term();

		if(token.kind ==  ADD_OPERATOR_TOKEN)		
		{
			v1 += v2;
		}
		else if( token.kind ==  SUB_OPERATOR_TOKEN)
		{
			v1 -= v2;
		}
		else
		{
			unget_token(&token);
		}
		
	}

	return v1;
}

double parse_line(void)
{
	double value;
	st_look_ahead_token_exists = 0;
	value = parse_expression();

	return value;
}

int main(int argc,char ** argv)
{
	char line[LINE_BUF_SIZE];
	double value;
	while (fgets(line,LINE_BUF_SIZE,stdin)!=NULL)
	{
		set_line(line);
		value = parse_line();
		printf(">>%f\n",value);
	}

	return 0;
}

说明：它会去解释每一行，先去解释加减法，判断加减法中有乘除的，会对乘除算式取得值再来计算加减。