# Copyright 2020 The Chromium Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.

"""Library containing utilities for handling args in libraries."""

def _sentinel(tag):
  return struct(**{tag: tag})


# A sentinel value indicating a value that will be computed based off of some
# other values
COMPUTE = _sentinel('__compute__')


# A sentinel value to be used as a default so that it can be distinguished
# between an unspecified value and an explicit value of None
DEFAULT = _sentinel('__default__')


def defaults(extends=None, **vars):
  """Define a structure that provides module-level defaults for function
  arguments.

  Arguments:
    * extends - A struct containing `lucicfg.var` attributes to add to the
      resulting struct.
    * vars - Defaults to define. Each entry results in a `lucicfg.var` attribute
      being added to the resulting struct. The name of the attribute is the
      keyword and the default value of the `lucicfg.var` is the keyword's value.

  Returns:
    A struct containing `lucicfg.var` attributes providing the module level
    defaults and the following methods:
    * get_value(name, value) - Gets the value of an argument. If `value` is not
      `DEFAULT`, `value` is returned. Otherwise, the module-level default for
      `name` is returned.
    * get_value_from_kwargs(name, kwargs) - Gets the value of a keyword
      argument. If `name` is in `kwargs`, `kwargs[name]` is returned. Otherwise,
      the module-level default for `name` is returned.
  """
  methods = ['get_value', 'get_value_from_kwargs']
  for m in methods:
    if m in vars:
      fail("{!r} can't be used as the name of a default: it is a method"
           .format(a))

  vars = {k: lucicfg.var(default = v) for k, v in vars.items()}
  for a in dir(extends):
    if a not in methods:
      vars[a] = getattr(extends, a)

  def get_value(name, value):
    if value != DEFAULT:
      return value
    return vars[name].get()

  def get_value_from_kwargs(name, kwargs):
    return get_value(name, kwargs.get(name, DEFAULT))

  return struct(
      get_value = get_value,
      get_value_from_kwargs = get_value_from_kwargs,
      **vars
  )


args = struct(
    COMPUTE = COMPUTE,
    DEFAULT = DEFAULT,
    defaults = defaults,
)