lexer.java

Java编写的编译器前端

package lexer;		// File Lexer.java
import java.io.*; import java.util.*; 
public class Lexer {
	public static int line = 1;
	char peek = ' ';
	Hashtable words = new Hashtable();
	void reserve(Word w) { words.put(w.lexeme, w); }
	public Lexer() {
		reserve( new Word("if",    Tag.IF)    );
	 	reserve( new Word("else",  Tag.ELSE)  );
		reserve( new Word("while", Tag.WHILE) );
		reserve( new Word("do",    Tag.DO)    );
		reserve( new Word("break", Tag.BREAK) );
		reserve( Word.True ); reserve( Word.False );
		reserve( Type.Int  ); reserve( Type.Char  );
		reserve( Type.Bool ); reserve( Type.Double );
	}
	void readch() throws IOException { peek = (char)System.in.read(); }
	boolean readch(char c) throws IOException {
		readch();
		if( peek != c ) return false;
		peek = ' ';
		return true;
	}
	public Token scan() throws IOException {
		for( ; ; readch() ){
			if( peek == ' ' || peek == '\t' ) continue;
			else if( peek == '\n' ) line++;
			else break;
		}
		switch( peek ) {
		case '&':
			if( readch('&') ) return Word.and; else return new Token('&');
		case '|':
			if( readch('|') ) return Word.or;  else return new Token('|');
		case '=':
			if( readch('=') ) return Word.eq;  else return new Token('=');
		case '!':
                        if( readch('=') ) return Word.ne;  else return new Token('!');
		case '<':
                        if( readch('=') ) return Word.le;  else return new Token('<');
		case '>':
                        if( readch('=') ) return Word.ge;  else return new Token('>');
		}
		if( Character.isDigit(peek) ) {
			int v = 0;
			do {
				v = 10*v + Character.digit(peek, 10); readch();
			}while( Character.isDigit(peek) );
			if( peek != '.') return new Num(v);
			double x = v; double d = 10;
			for(;;){
				readch();
				if( !Character.isDigit(peek) ) break;
				x = x + Character.digit(peek,10) / d; d *= 10;
			}
			return new Real(x);
		}
		if( Character.isLetter(peek) ) {
			StringBuffer b = new StringBuffer();
			do {
				b.append(peek); readch();
			} while( Character.isLetterOrDigit(peek) );
			String s = b.toString();
			Word w = (Word)words.get(s);
			if( w != null ) return w;
			w = new Word(s, Tag.ID);
			words.put(s, w);
			return w;
		}
		Token tok = new Token(peek); peek = ' ';
		return tok;
	}
}