jlox/src/main/java/com/craftinginterpreters/lox/Scanner.java

/* Copyright (C) 2019-2023 Hangzhou HSH Co. Ltd.
 * All right reserved.*/
package com.craftinginterpreters.lox;

import java.util.ArrayList;
import java.util.List;

import static com.craftinginterpreters.lox.TokenType.*;

/**
 * @author simon
 * @date 2023-06-04 16:30
 * @desc
 */
public class Scanner {
    private final String source;
    private final List<Token> tokens = new ArrayList<>();
    private int start = 0;  //  points to the first character in the lexeme being scanned
    private int current = 0; // points at the character currently being considered.
    private int line = 1;

    public Scanner(String source) {
        this.source = source;
    }

    public List<Token> scanTokens() {
        while (!isAtEnd()) {
            // We are at the beginning of the next lexeme.
            start = current;
            scanToken();
        }

        // add finish token to the end of list
        tokens.add(new Token(EOF, "", null, line));
        return tokens;
    }

    private void scanToken() {
        char c = advance();
        switch (c) {
            case '(' -> addToken(LEFT_PARAM);
            case ')' -> addToken(RIGHT_PARAM);
            case '{' -> addToken(LEFT_BRACE);
            case '}' -> addToken(RIGHT_BRACE);
            case ',' -> addToken(COMMA);
            case '.' -> addToken(DOT);
            case '-' -> addToken(MINUS);
            case '+' -> addToken(PLUS);
            case ';' -> addToken(SEMICOLON);
            case '*' -> addToken(STAR);
            // 两个字符 != == >= <=
            case '!' -> addToken(match('=') ? BANG_EQUAL : BANG);
            case '=' -> addToken(match('=') ? EQUAL_EQUAL : EQUAL);
            case '<' -> addToken(match('=') ? LESS_EQUAL : LESS);
            case '>' -> addToken(match('=') ? GRATER_EQUAL : GRATER);
            case '/' -> {
                if (match('/')) {
                    // A comment goes until the end of the line.
                    while (peek() != '\n' && !isAtEnd()) advance();
                } else {
                    addToken(SLASH);
                }
            }

            // Ignore whitespace
            case ' ', '\r', '\t' -> {
            }
            case '\n' -> line++;

            default -> Lox.error(line, "Unexpected character: " + c);
        }
    }

    private char peek() {
        if (isAtEnd()) return '\0';
        return source.charAt(current);
    }

    private void addToken(TokenType type) {
        addToken(type, null);
    }

    private void addToken(TokenType type, Object literal) {
        String text = source.substring(start, current);
        tokens.add(new Token(type, text, literal, line));
    }

    /**
     * consumes the next character in the source file and returns it
     *
     * @return
     */
    private char advance() {
        return source.charAt(current++);
    }

    private boolean isAtEnd() {
        return current >= source.length();
    }

    private boolean match(char expected) {
        if (isAtEnd()) return false;
        if (source.charAt(current) != expected) return false;

        current++;
        return true;
    }
}