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package ch.bfh.lexer;
// Lexer for classical arithmetic expressions with identifiers and assignments.
// Scans a source string char by char.
public class CalculatorLexer {
private String src; // source string for lexical analysis
private int idx; // current index in source
private int len; // length of source
public CalculatorLexer() { }
public void initLexer(String source) {
this.src = source;
idx = 0;
len = src.length();
}
// Consumes letters only and builds an identifier
private String identifier() {
StringBuffer s = new StringBuffer();
do {
s.append(src.charAt(idx));
idx++;
} while (idx < len && Character.isLetter(src.charAt(idx)));
return s.toString();
}
// Consumes digits and convert integer part and decimal part
// Convert characters using the formula
// "3456.253" = [(((3+0)*10+4)*10+5)*10+6]+[0.1*2+0.01*5+0.001*3]
private double number() throws LexerException {
double v = 0; // accumulates the result
double factor = 0.1; // factor for decimal part
do { // integer part
v = v * 10 + Character.digit(src.charAt(idx),30);
idx++;
} while (idx < len && Character.isDigit(src.charAt(idx)));
if (idx < len && src.charAt(idx) == '.') { // decimal point
idx++;
if (idx < len && Character.isDigit(src.charAt(idx))) { // decimal part
while (idx < len && Character.isDigit(src.charAt(idx))) {
v = v + (factor * Character.digit(src.charAt(idx),30));
factor = factor * 0.1;
idx++;
}
}
else throw new LexerException("Illegal number: decimal part missing");
}
return v;
}
// Skips blanks, tabs, newlines
private void skip() {
char c;
while (idx < len) {
c = src.charAt(idx);
if (c==' ' || c=='\t' || c=='\n') idx++;
else break;
}
}
// returns next token
public Token nextToken() throws LexerException {
Token tok = new Token();
skip();
if (idx>=len) {
tok.str="EOL";
tok.type=Token.EOL;
}
else
// is it a positive number?
if (Character.isDigit(src.charAt(idx))) {
tok.value = number();
tok.type = Token.NUM;
tok.str = Double.toString(tok.value);
}
else
if (Character.isLetter(src.charAt(idx))) {
tok.value = 0;
tok.type = Token.ID;
tok.str = identifier();
if (tok.str.compareTo("let")==0) tok.type = Token.LET;
if (tok.str.compareTo("exit")==0) tok.type = Token.END;
}
else {
switch (src.charAt(idx)) {
case '+': tok.type = Token.ADD; tok.str = "+"; break;
case '-': tok.type = Token.SUB; tok.str = "-"; break;
case '*': tok.type = Token.MUL; tok.str = "*"; break;
case '/': tok.type = Token.DIV; tok.str = "/"; break;
case '(': tok.type = Token.PAL; tok.str = "("; break;
case ')': tok.type = Token.PAR; tok.str = ")"; break;
case '=': tok.type = Token.EQU; tok.str = "="; break;
default : throw new LexerException("Illegal Token: '" + src.charAt(idx) + "'");
}
idx++;
}
return tok;
}
}
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