skip WHITESPACE '\s+'

token NUM    '[0-9]+'
token VAR    '[A-Za-z_][A-Za-z0-9_]*'
token OP     '\+|-'
token MOP    '\*|/'
token IN     '==>'
token OUT    '<=='
token ASSIGN '->'
token SEMI   ';'

%

<program>  ::= <stmtlist>
<stmtlist> **= <stmt>

<stmt>InputStmt  ::= <IN> <VAR> <SEMI>
<stmt>OutputStmt ::= <OUT> <exp> <SEMI>
<stmt>AssignStmt ::= <VAR> <ASSIGN> <exp> <SEMI>

<exp>      ::= <mult> <exprest>
<exprest>  **= <OP> <mult>
<mult>     ::= <value> <multrest>
<multrest> **= <MOP> <value>
<value>NUMValue ::= <NUM>
<value>VARValue ::= <VAR>

%

Program:import
%%%
import java.util.*;
%%%

Program
%%%
  public static Map<String, Integer> hshmap = new HashMap<>();
  public static Scanner scanner = new Scanner(System.in);

  public void $run() { stmtlist.execute(); }
%%%

Stmtlist
%%%
  public void execute() {
    for (Stmt s : stmtList) s.execute();
  }
%%%

Stmt
%%%
  abstract public void execute();
%%%

InputStmt
%%%
  public void execute() {
    System.out.print("? ");
    Program.hshmap.put(var.toString(), Program.scanner.nextInt());
  }
%%%

OutputStmt
%%%
  public void execute() { System.out.println(exp.compute()); }
%%%

AssignStmt
%%%
  public void execute() { Program.hshmap.put(var.toString(), exp.compute()); }
%%%

Exp
%%%
  public int compute() {
    int v = mult.compute();
    for (int i = 0; i < exprest.opList.size(); i++)
      v += exprest.opList.get(i).toString().equals("+")
           ? exprest.multList.get(i).compute()
           : -exprest.multList.get(i).compute();
    return v;
  }
%%%

Mult
%%%
  public int compute() {
    int v = value.compute();
    for (int i = 0; i < multrest.mopList.size(); i++)
      v = multrest.mopList.get(i).toString().equals("*")
          ? v * multrest.valueList.get(i).compute()
          : v / multrest.valueList.get(i).compute();
    return v;
  }
%%%

Value
%%%
  abstract public int compute();
%%%

NUMValue
%%%
  public int compute() { return Integer.parseInt(num.toString()); }
%%%

VARValue
%%%
  public int compute() {
    if (!Program.hshmap.containsKey(var.toString()))
      throw new RuntimeException("Undefined variable: " + var);
    return Program.hshmap.get(var.toString());
  }
%%%