gistfile1.cpp

#include <iostream>
#include <cstdio>
namespace jelta
{
	enum TokenType {
		LPAREN,
		RPAREN,
		INTEGER,
		SYMBOL,
		UNUSED_LAST
	};
	struct Token
	{
		TokenType type;
		std::string val;
		Token(TokenType t, std::string v) : type(t), val(v) {}
	};
	class Lexer
	{
		char last_char;
		inline void next() { last_char = getchar(); }

	public:
		Lexer() : last_char(' ') {}

		Token get_next_token()
		{
			while(isspace(last_char))
				next();

			switch(last_char)
			{
			case '(': next(); return Token(LPAREN, "(");
			case ')': next(); return Token(RPAREN, ")");
			}
			
			if(isdigit(last_char))
			{
				std::string n;
				while(isdigit(last_char))
				{
					n += last_char;
					next();
				}
				return Token(INTEGER, n);
			}
			if(isalpha(last_char))
			{
				std::string s;
				while(isalnum(last_char))
				{
					s += last_char;
					next();
				}
				return Token(SYMBOL, s);
			}
			std::string x;
			x+= last_char; 
			next();
			return Token(SYMBOL, x);
		}

	static std::string get_token_name(Token t)
	{
		switch(t.type)
		{
		case LPAREN: return "LPAREN";
		case RPAREN: return "RPAREN";
		case INTEGER: return "INTEGER";
		case SYMBOL: return "SYMBOL";
		default: return "UNKNOWN";
		}
	}
	};

	struct AST { virtual void p(){}; };
	struct Pair : AST { AST *a, *b; Pair(AST *_a, AST *_b) : a(_a), b(_b) {}; 
		virtual void p(){ std::cout << "("; a->p(); std::cout << " "; b->p(); std::cout << ")"; }
	};
	struct Symbol : AST { std::string val; Symbol(std::string v) : val(v) {};
		virtual void p(){ std::cout << val; }	
	};

	class Parser
	{
		Lexer lex;
	public:
		AST *parse()
		{
			jelta::Token t = lex.get_next_token();
 			//std::cout << jelta::Lexer::get_token_name(t) << " " << t.val << std::endl;
			switch(t.type)
			{
			case LPAREN: {
				Pair *p = new Pair(parse(), parse()); lex.get_next_token();  return p;
			}
			case SYMBOL: return new Symbol(t.val);			
			}
			return new AST();
		}
	};
}

int main()
{
 jelta::Parser p;
 //for(;;){
 	std::cout << "input: ";
 	jelta::AST *a = p.parse();
	std::cout << std::endl << "output: ";
	a->p();
	std::cout << std::endl;
 //}
}