"""Utility module to access to DynamoDB."""

import datetime
import itertools
import os

import boto3
from botocore import config as botocore_config

DDB_WRITE_MAX_ATTEMPTS = int(os.environ.get('DDB_WRITE_MAX_ATTEMPTS', 10))


def get_dynamodb(max_attempts=None):
    """Get Boto DynamoDB resource.

    Args:
        max_attempts (int): optional max retry attempts

    Returns:
        boto3.resources.factory.dynamodb.ServiceResource: DynamoDB resource

    """
    if max_attempts is None:
        max_attempts = DDB_WRITE_MAX_ATTEMPTS

    config_instance = botocore_config.Config(
        retries={'max_attempts': max_attempts})
    return boto3.resource('dynamodb', config=config_instance)


def get_dynamodb_table(table_name):
    """Get the DynamoDB table.

    Args:
        table_name (str): DynamoDB table name

    Returns:
        boto3.resources.factory.dynamodb.Table: DynamoDB table

    """
    dynamodb = get_dynamodb()
    return dynamodb.Table(table_name)


def get_table_count(table_name):
    """Get the total count of table items.

    Args:
        table_name (str): DynamoDB table name

    Returns:
        int: table item count

    """
    table = get_dynamodb_table(table_name)
    resp = full_scan(table, Select='COUNT')
    return resp.get('Count')


def get_ttl_value_for_item(time_delta):
    """Calculate the TTL value for the item.

    Args:
        time_delta (datetime.timedelta): expected time to live

    Returns:
         int: TTL in seconds from epoch

    """
    ttl_datetime = datetime.datetime.now() + time_delta
    return int(ttl_datetime.timestamp())


def get_scan_results(table, scan_kwargs):
    """Yield scan responses with respect to LastEvaluatedKey.

    Args:
        table (boto3.resources.factory.dynamodb.Table): DynamoDB table to scan
        scan_kwargs (dict): scan key word arguments

    Returns:
        dict: individual scan response

    """
    key = True
    while key:
        resp = table.scan(**scan_kwargs)
        key = resp.get('LastEvaluatedKey')
        if key:
            scan_kwargs['ExclusiveStartKey'] = key
        yield resp


def full_scan(table, **scan_kwargs):
    """Do a full table scan, handle the pagination.

    Args:
        table (boto3.resources.factory.dynamodb.Table): DynamoDB table to scan
        scan_kwargs (kwargs): scan key word arguments

    Returns:
        dict: combined response similar to original boto3 response

    """
    result = {
        'Count': 0,
        'ScannedCount': 0}
    items = []
    for resp in get_scan_results(table, scan_kwargs):
        result['Count'] += resp['Count']
        result['ScannedCount'] += resp['ScannedCount']
        if 'Items' in resp:
            items.append(resp['Items'])
    if items:
        result['Items'] = list(itertools.chain.from_iterable(items))
    return result