import ast
import json

def parse_acl_formula(acl_formula):
  """
  Parse an ACL formula expression into a parse tree that we can interpret in JS, e.g.
  "rec.office == 'Seattle' and user.email in ['sally@', 'xie@']".

  The idea is to support enough to express ACL rules flexibly, but we don't need to support too
  much, since rules should be reasonably simple.

  The returned tree has the form [NODE_TYPE, arguments...], with these NODE_TYPEs supported:
    And|Or                  ...values
    Add|Sub|Mult|Div|Mod    left, right
    Not                     operand
    Eq|NotEq|Lt|LtE|Gt|GtE  left, right
    Is|IsNot|In|NotIn       left, right
    List                    ...elements
    Const                   value (number, string, bool)
    Name                    name (string)
    Attr                    node, attr_name
  """
  try:
    tree = ast.parse(acl_formula, mode='eval')
    return _TreeConverter().visit(tree)
  except SyntaxError as err:
    # In case of an error, include line and offset.
    raise SyntaxError("%s on line %s col %s" % (err.args[0], err.lineno, err.offset))


def parse_acl_formula_json(acl_formula):
  """
  As parse_acl_formula(), but stringifies the result, and converts empty string to empty string.
  """
  return json.dumps(parse_acl_formula(acl_formula)) if acl_formula else ""


named_constants = {
  'True': True,
  'False': False,
  'None': None,
}

class _TreeConverter(ast.NodeVisitor):
  # AST nodes are documented here: https://docs.python.org/2/library/ast.html#abstract-grammar
  # pylint:disable=no-self-use

  def visit_Expression(self, node):
    return self.visit(node.body)

  def visit_BoolOp(self, node):
    return [node.op.__class__.__name__] + [self.visit(v) for v in node.values]

  def visit_BinOp(self, node):
    if not isinstance(node.op, (ast.Add, ast.Sub, ast.Mult, ast.Div, ast.Mod)):
      return self.generic_visit(node)
    return [node.op.__class__.__name__, self.visit(node.left), self.visit(node.right)]

  def visit_UnaryOp(self, node):
    if not isinstance(node.op, (ast.Not)):
      return self.generic_visit(node)
    return [node.op.__class__.__name__, self.visit(node.operand)]

  def visit_Compare(self, node):
    # We don't try to support chained comparisons like "1 < 2 < 3" (though it wouldn't be hard).
    if len(node.ops) != 1 or len(node.comparators) != 1:
      raise ValueError("Can't use chained comparisons")
    return [node.ops[0].__class__.__name__, self.visit(node.left), self.visit(node.comparators[0])]

  def visit_Name(self, node):
    if node.id in named_constants:
      return ["Const", named_constants[node.id]]
    return ["Name", node.id]

  def visit_Attribute(self, node):
    return ["Attr", self.visit(node.value), node.attr]

  def visit_Num(self, node):
    return ["Const", node.n]

  def visit_Str(self, node):
    return ["Const", node.s]

  def visit_List(self, node):
    return ["List"] + [self.visit(e) for e in node.elts]

  def visit_Tuple(self, node):
    return self.visit_List(node)    # We don't distinguish tuples and lists

  def generic_visit(self, node):
    raise ValueError("Unsupported syntax at %s:%s" % (node.lineno, node.col_offset + 1))