import uuid
import datetime as dt
from functools import reduce

class Datagram(object):
    def __init__(self, val=0):
        self.val = val

    def op(self, other):
        return Datagram(self.val+other.val)

class PrimitiveItem(object):
    def __init__(self, logical_id, datagram, date_created, logical_date):
        self.uuid = uuid.UUID()

        self.logical_id = logical_id

        self.datagram = datagram
        self.logical_date = logical_date
        self.date_created = date_created

class RollupItem(object):
    def __init__(self, logical_id, uuid_of_previous_rollup_item, datagram, date_created):
        self.uuid = uuid.UUID()

        self.logical_id = logical_id
        self.uuid_of_previous_rollup_item = uuid_of_previous_rollup_item

        self.datagram = datagram
        self.created = date_created

def get_rollup_item_head_for(logical_id):
    # query data souce for roll up item by logical_id for most recent date,
    # there should be only one snapshot by date
    # or
    # return identity, RollUpItem(uuid.UUID(), None, datagram.identity, dt.today())
    pass

def get_primitive_items_for(logical_id, date_created):
    # query data source for records by (logical_id, date) -> [items]
    pass

def combine_rollup_items_for(logical_id, date_created):
    primitive_items = get_primitive_items_for(logical_id, date_created-1)
    head_of_previous_rollup = get_rollup_item_head_for(logical_id)
    acc = reduce(lambda x, y: x.op(y), \
            [primitive_item.datagram for primitive_item in primitive_items])
    # write rollupitem to datasource with date_created
    return RollupItem(logical_id, head_of_previous_rollup, \
            acc.val+head_of_previous_rollup.datagram.val, dt.today())

if __name__ == '__main__':
    acc = reduce(lambda x, y: x.op(y), \
            [datagram for datagram in [Datagram(1), Datagram(2)]])
    acc = reduce(lambda x, y: x.op(y), \
            [datagram for datagram in [Datagram(1), Datagram(2), Datagram(3)]])
    print(acc.val)