from __future__ import absolute_import
from base64 import b64encode

from six import b, integer_types
from ..exceptions import UnrewindableBodyError

ACCEPT_ENCODING = 'gzip,deflate'
_FAILEDTELL = object()


def make_headers(keep_alive=None, accept_encoding=None, user_agent=None,
                 basic_auth=None, proxy_basic_auth=None, disable_cache=None):
    """
    Shortcuts for generating request headers.

    :param keep_alive:
        If ``True``, adds 'connection: keep-alive' header.

    :param accept_encoding:
        Can be a boolean, list, or string.
        ``True`` translates to 'gzip,deflate'.
        List will get joined by comma.
        String will be used as provided.

    :param user_agent:
        String representing the user-agent you want, such as
        "python-urllib3/0.6"

    :param basic_auth:
        Colon-separated username:password string for 'authorization: basic ...'
        auth header.

    :param proxy_basic_auth:
        Colon-separated username:password string for 'proxy-authorization: basic ...'
        auth header.

    :param disable_cache:
        If ``True``, adds 'cache-control: no-cache' header.

    Example::

        >>> make_headers(keep_alive=True, user_agent="Batman/1.0")
        {'connection': 'keep-alive', 'user-agent': 'Batman/1.0'}
        >>> make_headers(accept_encoding=True)
        {'accept-encoding': 'gzip,deflate'}
    """
    headers = {}
    if accept_encoding:
        if isinstance(accept_encoding, str):
            pass
        elif isinstance(accept_encoding, list):
            accept_encoding = ','.join(accept_encoding)
        else:
            accept_encoding = ACCEPT_ENCODING
        headers['accept-encoding'] = accept_encoding

    if user_agent:
        headers['user-agent'] = user_agent

    if keep_alive:
        headers['connection'] = 'keep-alive'

    if basic_auth:
        headers['authorization'] = 'Basic ' + \
            b64encode(b(basic_auth)).decode('utf-8')

    if proxy_basic_auth:
        headers['proxy-authorization'] = 'Basic ' + \
            b64encode(b(proxy_basic_auth)).decode('utf-8')

    if disable_cache:
        headers['cache-control'] = 'no-cache'

    return headers


def set_file_position(body, pos):
    """
    If a position is provided, move file to that point.
    Otherwise, we'll attempt to record a position for future use.
    """
    if pos is not None:
        rewind_body(body, pos)
    elif getattr(body, 'tell', None) is not None:
        try:
            pos = body.tell()
        except (IOError, OSError):
            # This differentiates from None, allowing us to catch
            # a failed `tell()` later when trying to rewind the body.
            pos = _FAILEDTELL

    return pos


def rewind_body(body, body_pos):
    """
    Attempt to rewind body to a certain position.
    Primarily used for request redirects and retries.

    :param body:
        File-like object that supports seek.

    :param int pos:
        Position to seek to in file.
    """
    body_seek = getattr(body, 'seek', None)
    if body_seek is not None and isinstance(body_pos, integer_types):
        try:
            body_seek(body_pos)
        except (IOError, OSError):
            raise UnrewindableBodyError("An error occurred when rewinding request "
                                        "body for redirect/retry.")
    elif body_pos is _FAILEDTELL:
        raise UnrewindableBodyError("Unable to record file position for rewinding "
                                    "request body during a redirect/retry.")
    else:
        raise ValueError("body_pos must be of type integer, "
                         "instead it was %s." % type(body_pos))
