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package Interpreter;
import java.util.ArrayList;
import java.util.List;
public class TokenScanner {
private String sourceCode;
List<Token> tokens = new ArrayList<>();
private int tokenStart=0;
private int currentLoc=0;
//Extract tokens from the source code by reading character by character
List<Token> extractTokens(String sourceCode){
this.sourceCode=sourceCode;
while (!checkEOF()){
tokenStart=currentLoc;
readToken();
}
return tokens;
}
//Extract a single token
private void readToken(){
char checkChar = sourceCode.charAt(currentLoc);
switch(checkChar){
case '(': createTokenNull(TokenType.LEFT_PAREN); break;
case ')': createTokenNull(TokenType.RIGHT_PAREN); break;
case '+': createTokenNull(TokenType.PLUS); break;
case '-': createTokenNull(TokenType.MINUS); break;
case '*': createTokenNull(TokenType.STAR); break;
case '/': createTokenNull(TokenType.SLASH); break;
//Some tokens are multiple characters long (==, <=) etc
//so need to check next char as well
case '=':
if (checkNextChar('=')){
createTokenNull(TokenType.EQUALITY);
break;
} else {
createTokenNull(TokenType.EQUALS);
break;
}
case '<':
if (checkNextChar('=')){
createTokenNull(TokenType.LESS_EQUAL);
break;
} else {
createTokenNull(TokenType.LESS);
break;
}
case '>':
if (checkNextChar('=')){
createTokenNull(TokenType.GREATER_EQUAL);
break;
} else {
createTokenNull(TokenType.GREATER);
break;
}
default:
//Check for numer
if (checkIsDigit(checkChar)){
while (checkIsDigit(lookAhead())){
currentLoc++;
}
//Check if number contains a decimal point
if (lookAhead()=='.' && checkIsDigit(lookTwoAhead())){
currentLoc++;
while (checkIsDigit(lookAhead())){
currentLoc++;
}
}
createToken(TokenType.NUMBER, Double.parseDouble(sourceCode.substring(tokenStart, currentLoc+1)));
}
}
currentLoc++;
}
//Test for end of file
private boolean checkEOF(){
return currentLoc>=sourceCode.length();
}
//Create a token without a value
private void createTokenNull(TokenType type){
createToken(type, null);
}
//Create token
private void createToken(TokenType type, Object value){
String tokenText = sourceCode.substring(tokenStart, currentLoc+1);
tokens.add(new Token(type, tokenText, value));
}
//Check if the next char matches a given char
private boolean checkNextChar(char matchChar){
if (checkEOF()){
return false;
}
if (sourceCode.charAt(currentLoc+1)==matchChar){
currentLoc++;
return true;
}
return false;
}
//Look at the next char in the source code
private char lookAhead(){
if (currentLoc+1>=sourceCode.length()){
return ' ';
} else {
return sourceCode.charAt(currentLoc+1);
}
}
//Look 2 chars ahead in the source code
private char lookTwoAhead(){
if (currentLoc+2>=sourceCode.length()){
return ' ';
} else {
return sourceCode.charAt(currentLoc+2);
}
}
//Check if a given char is a digit
private boolean checkIsDigit(char checkChar){
return checkChar>='0' && checkChar<='9';
}
}
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