"""Util functions and helpers related to dates and timestamps calculations."""

import datetime

from dateutil.parser import parse as parse_date

from analytics.constants.date import DATE_FIELDS, DATE_FORMAT, END_DATE, START_DATE
from analytics.constants.error import (
    ERROR_INCORRECT_DATE_FORMAT,
    ERROR_REQUIRED_DATE,
    ERROR_START_DATE_GREATER_THAN_END_DATE,
)
from analytics.constants.parameters import ALL_TIME
from analytics.logic import data_availability as data_availability_logic


def get_start_date_end_date(start_date, end_date, default_days, downloads=False):
    """Return populated start and end dates."""
    if start_date == ALL_TIME and not end_date:
        end_date = data_availability_logic.get_max_available_date()
        return start_date, end_date

    if not (start_date and end_date):
        start_date, end_date = data_availability_logic.get_date_range(
            data_availability_logic.HIGHWATERMARK_DATE,
            days=default_days,
            downloads=downloads,
        )
        return start_date, end_date

    else:
        return start_date, end_date


def get_prior_date_intervals(date, count=2, length=7):
    """Get n intervals of length m ending at given date.

    Starts with an interval with oldest dates, ends with an interval with
    the latest dates.

    Args:
        date (datetime.date): End date
        count (int): Number of intervals to get.
        length (int): Number of days in interval.

    Returns:
        list: A list of dicts:
            (dict): Dictionary representing an interval with keys
            start_date (datetime.date): Start date
            end_date (datetime.date): End date

    """
    intervals = []
    for interval_number in range(1, count + 1):
        interval = {
            "start_date": date
            - datetime.timedelta(days=(length * interval_number) - 1),
            "end_date": date - datetime.timedelta(days=length * (interval_number - 1)),
        }
        intervals.append(interval)
    return intervals[::-1]


def validate_dates(start_date, end_date):
    if not start_date:
        raise Exception(ERROR_REQUIRED_DATE, START_DATE)

    if not end_date:
        raise Exception(ERROR_REQUIRED_DATE, END_DATE)

    try:
        datetime.datetime.strptime(start_date, DATE_FORMAT)
        datetime.datetime.strptime(end_date, DATE_FORMAT)
    except Exception:
        raise Exception(ERROR_INCORRECT_DATE_FORMAT, DATE_FIELDS)

    if parse_date(start_date) > parse_date(end_date):
        raise Exception(ERROR_START_DATE_GREATER_THAN_END_DATE, START_DATE)


def generate_date_range_starting_from_date(start_date, days_back=28):
    """Generate a list of dates starting from passed date and going back in time."""
    start_date = datetime.datetime.strptime(start_date, "%Y-%m-%d").date()
    dates = []
    for i in range(days_back):
        dates.append((start_date - datetime.timedelta(days=i)).strftime("%Y-%m-%d"))
    return dates