// math operations.
// input: L<S> (list of numbers)
// output: S (number)
static class LMath extends LearnerImpl {
  static boolean debug;
  String allOperations = "+ - * /";
  new (Tree)Set<String> possibleOperations;
  
  LMath() {
    possibleOperations.addAll(Arrays.asList(allOperations.split(" +")));
  }
  
  public void processInOut(Object _in, Object _out) {
    List in = (List) _in;
    String out = (String) _out;
    BigInteger[] inNumbers = makeBigIntArray(in);
    BigInteger[] outNumbers = new BigInteger[] {bigint(out)};
    findOperation(inNumbers, outNumbers);
    if (debug)
      System.out.println("Operations: " + possibleOperations);
  }
  
  public void findOperation(BigInteger[] in, BigInteger[] out) {
    filterOperations(in, out);
    if (possibleOperations.isEmpty())
      fail("tilt");
  }
    
  public void filterOperations(BigInteger[] in, BigInteger[] out) {
    for (Iterator<String> i = possibleOperations.iterator(); i.hasNext(); ) {
      String op = i.next();
      BigInteger[] out2 = doOperation(op, in);
      if (out2 == null || !arraysEqual(out, out2))
        i.remove(); // keep only matching operations
    }
  }
  
  public BigInteger[] doOperation(String op, BigInteger[] in) {
    op = op.intern();
    try {
      if (in.length == 2) {
        BigInteger a = in[0], b = in[1], x = null;
        if (op == "+")
          x = a.add(b);
        else if (op == "-")
          x = a.subtract(b);
        else if (op == "*")
          x = a.multiply(b);
        else if (op == "/")
          x = a.divide(b);
        return x != null ? new BigInteger[] {x} : null;
      }
      return null;
    } catch (Throwable e) {
      return null;
    }
  }
  
  public String processIn(Object _in) {
    L<S> in = cast _in;
    String op = possibleOperations.iterator().next();
    if (debug)
      System.out.println("op: " + op);
    BigInteger[] inNumbers = makeBigIntArray(in);
    BigInteger[] outNumbers = doOperation(op, inNumbers);
    return outNumbers[0].toString();
  }
  
  String BI = BigInteger.class.getName();
  
  public void toJava(Code code) {
    String op = possibleOperations.iterator().next();
    String a = "new " + BI + "((String) " + code.list() + ".get(0))";
    String b = "new " + BI + "((String) " + code.list() + ".get(1))";
    if (op.equals("+"))
      code.assign(a + ".add(" + b + ").toString()");
    else
      todo();
  }
}