jlox/src/lox/Parser.java

package lox;

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

import static lox.TokenType.*;

class Parser {
  private static class ParseError extends RuntimeException {}

  private final List<Token> tokens;
  private int current = 0;

  Parser(List<Token> tokens) {
    this.tokens = tokens;
  }

  List<Stmt> parse() {
    List<Stmt> statements = new ArrayList<>();
    while (!isAtEnd()) {
      statements.add(declaration());
    }
    return statements;
  }

  private Stmt declaration() {
    try {
      if (match(VAR)) {
        return varDeclaration();
      }
      return statement();
    } catch (ParseError error) {
      synchronize();
      return null;
    }
  }

  private Stmt varDeclaration() {
    Token name = consume(IDENTIFIER, "Expect variable name.");

    Expr initializer = null;
    if (match(EQUAL)) {
      initializer = expression();
    }
    consume(SEMICOLON, "Expect semicolon.");
    return new Stmt.Var(name, initializer);
  }

  private Stmt statement() {
    if (match(PRINT)) {
      return printStatement();
    }
    return expressionStatement();
  }

  private Stmt printStatement() {
    Expr value = expression();
    consume(SEMICOLON, "Expect ';' after value.");
    return new Stmt.Print(value);
  }

  private Stmt expressionStatement() {
    Expr value = expression();
    consume(SEMICOLON, "Expect ';' after value.");
    return new Stmt.Expression(value);
  }

  private Expr expression() {
    return equality();
  }

  private Expr equality() {
    Expr expr = comparison();
    while (match(BANG_EQUAL, EQUAL_EQUAL)) {
      Token operator = previous();
      Expr right = comparison();
      expr = new Expr.Binary(expr, operator, right);
    }
    return expr;
  }

  private Expr comparison() {
    Expr expr = term();
    while (match(GREATER, GREATER_EQUAL, LESS, LESS_EQUAL)) {
      Token operator = previous();
      Expr right = term();
      expr = new Expr.Binary(expr, operator, right);
    }
    return expr;
  }

  private Expr term() {
    Expr expr = factor();
    while (match(PLUS, MINUS)) {
      Token operator = previous();
      Expr right = factor();
      expr = new Expr.Binary(expr, operator, right);
    }
    return expr;
  }

  private Expr factor() {
    Expr expr = unary();
    while (match(STAR, SLASH)) {
      Token operator = previous();
      Expr right = unary();
      expr = new Expr.Binary(expr, operator, right);
    }
    return expr;
  }

  private Expr unary() {
    if (match(BANG, MINUS)) {
      Token operator = previous();
      Expr right = unary();
      return new Expr.Unary(operator, right);
    }
    return primary();
  }

  private Expr primary() {
    if (match(FALSE)) {
      return new Expr.Literal(false);
    }
    if (match(TRUE)) {
      return new Expr.Literal(true);
    }
    if (match(NIL)) {
      return new Expr.Literal(null);
    }
    if (match(NUMBER, STRING)) {
      return new Expr.Literal(previous().literal);
    }
    if (match(LEFT_PAREN)) {
      Expr expr = expression();
      consume(RIGHT_PAREN, "Expect ')' after expression.");
      return new Expr.Grouping(expr);
    }
    if (match(IDENTIFIER)) {
      return new Expr.Variable(previous());
    }

    throw error(peek(), "Expected expression.");
  }

  private boolean match(TokenType... types) {
    for (TokenType type : types) {
      if (check(type)) {
        advance();
        return true;
      }
    }
    return false;
  }

  private Token consume(TokenType type, String message) {
    if (check(type)) {
      return advance();
    }
    throw error(peek(), message);
  }

  private boolean check(TokenType type) {
    if (isAtEnd()) {
      return false;
    }
    return peek().type == type;
  }

  private Token advance() {
    if (!isAtEnd()) {
      current++;
    }
    return previous();
  }

  private boolean isAtEnd() {
    return peek().type == EOF;
  }

  private Token peek() {
    return tokens.get(current);
  }

  private Token previous() {
    return tokens.get(current - 1);
  }

  private ParseError error(Token token, String message) {
    Lox.error(token, message);
    return new ParseError();
  }

  private void synchronize() {
    advance();
    while (!isAtEnd()) {
      if (previous().type == SEMICOLON) {
        return;
      }
      switch (peek().type) {
        case CLASS, FOR, FUN, IF, PRINT, RETURN, VAR, WHILE:
          return;
      }
    }
    advance();
  }
}