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Calculator.java
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Calculator.java
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import java.io.InputStream;
import java.io.IOException;
class Calculator {
private int lookaheadToken;
private InputStream in;
public Calculator(InputStream in) throws IOException {
this.in = in;
lookaheadToken = in.read();
}
private void consume(int symbol) throws IOException, ParseError {
if (lookaheadToken != symbol)
throw new ParseError();
lookaheadToken = in.read();
}
private Boolean isEof(int lookaheadToken){
return lookaheadToken == '\n' || lookaheadToken == -1;
}
private Boolean isNumDigit(int lookaheadToken){
return lookaheadToken >= '0' && lookaheadToken <= '9';
}
public int expr() throws IOException, ParseError {
if(isNumDigit(lookaheadToken) || lookaheadToken == '('){
int result = term();
return expr2(result);
}else{
throw new ParseError();
}
}
public int expr2(int value) throws IOException, ParseError {
if(lookaheadToken == '+' || lookaheadToken == '-'){
boolean isPlus = lookaheadToken == '+';
consume(lookaheadToken);
int result = term();
result = (isPlus) ? value + result : value - result;
return expr2(result);
}else if(lookaheadToken == ')' || isEof(lookaheadToken)){
return value;
}else{
throw new ParseError();
}
}
public int term() throws IOException, ParseError {
if(isNumDigit(lookaheadToken) || lookaheadToken == '('){
int result = fac();
return term2(result);
}else{
throw new ParseError();
}
}
public int term2(int value) throws IOException, ParseError {
if(lookaheadToken == '+' || lookaheadToken == '-' || lookaheadToken == ')' || isEof(lookaheadToken)){
return value;
}else if(lookaheadToken == '*' || lookaheadToken == '/'){
boolean isMult = lookaheadToken == '*';
consume(lookaheadToken);
int result = fac();
result = (isMult) ? value * result : value / result;
return term2(result);
}else{
throw new ParseError();
}
}
public int ParseNumber() throws IOException, ParseError {
int number = 0;
while(isNumDigit(lookaheadToken)){
int digit = lookaheadToken - '0';
number = number*10 + digit;
consume(lookaheadToken);
}
return number;
}
public int fac() throws IOException, ParseError {
if(lookaheadToken == '('){
consume('(');
int result = expr();
consume(')');
return result;
}else if(isNumDigit(lookaheadToken)){
return ParseNumber();
}else{
throw new ParseError();
}
}
public int eval() throws IOException, ParseError {
int result = expr();
if (!isEof(lookaheadToken))
throw new ParseError();
return result;
}
}