# coding=utf-8
# Copyright 2022-present, the HuggingFace Inc. team.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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"""Contains utilities to handle datetimes in Huggingface Hub."""
from datetime import datetime, timedelta, timezone


# Local machine offset compared to UTC.
# Taken from https://stackoverflow.com/a/3168394.
# `utcoffset()` returns `None` if no offset -> empty timedelta.
UTC_OFFSET = datetime.now(timezone.utc).astimezone().utcoffset() or timedelta()


def parse_datetime(date_string: str) -> datetime:
    """
    Parses a date_string returned from the server to a datetime object.

    This parser is a weak-parser is the sense that it handles only a single format of
    date_string. It is expected that the server format will never change. The
    implementation depends only on the standard lib to avoid an external dependency
    (python-dateutil). See full discussion about this decision on PR:
    https://github.com/huggingface/huggingface_hub/pull/999.

    Example:
        ```py
        > parse_datetime('2022-08-19T07:19:38.123Z')
        datetime.datetime(2022, 8, 19, 7, 19, 38, 123000, tzinfo=timezone.utc)
        ```

    Args:
        date_string (`str`):
            A string representing a datetime returned by the Hub server.
            String is expected to follow '%Y-%m-%dT%H:%M:%S.%fZ' pattern.

    Returns:
        A python datetime object.

    Raises:
        :class:`ValueError`:
            If `date_string` cannot be parsed.
    """
    try:
        # Datetime ending with a Z means "UTC". Here we parse the date as local machine
        # timezone and then move it to the appropriate UTC timezone.
        # See https://en.wikipedia.org/wiki/ISO_8601#Coordinated_Universal_Time_(UTC)
        # Taken from https://stackoverflow.com/a/3168394.
        if len(date_string) == 30:
            # Means timezoned-timestamp with nanoseconds precision. We need to truncate the last 3 digits.
            date_string = date_string[:-4] + "Z"
        dt = datetime.strptime(date_string, "%Y-%m-%dT%H:%M:%S.%fZ")
        dt += UTC_OFFSET  # By default, datetime is not timezoned -> move to UTC time
        return dt.astimezone(timezone.utc)  # Set explicit timezone
    except ValueError as e:
        raise ValueError(
            f"Cannot parse '{date_string}' as a datetime. Date string is expected to"
            " follow '%Y-%m-%dT%H:%M:%S.%fZ' pattern."
        ) from e