import json
from collections import defaultdict
from functools import wraps
from operator import attrgetter

VALID_HTTP_METHODS = ['GET', 'POST']


class User:
    def __init__(self, name):
        self.name = name
        self.owes = defaultdict(float)
        self.owed_by = defaultdict(float)
        self.balance = 0.0

    def lend(self, borrower, amount):

        total_owed = self.owed_by[borrower] - self.owes[borrower] + amount
        if total_owed > 0:
            self.owed_by[borrower] = total_owed
            self.owes.pop(borrower, None)
        elif total_owed < 0:
            self.owes[borrower] = -total_owed
            self.owed_by.pop(borrower, None)
        else:
            self.owed_by.pop(borrower, None)
            self.owes.pop(borrower, None)

        self.balance += amount

    def borrow(self, lender, amount):
        self.lend(lender, -amount)

    @classmethod
    def from_dict(cls, d):
        user = cls(d['name'])
        user.owed_by.update(d['owed_by'])
        user.owes.update(d['owes'])
        user.balance = d['balance']
        return user


class MethodRegistry:
    def __init__(self):
        self.methods = {k: {} for k in VALID_HTTP_METHODS }

    def register_url(self, url, http_method):

        def actual_decorator(f):

            @wraps(f)
            def _wrapper(f_self, payload):

                if payload:
                    payload = json.loads(payload)
                    #It would be essential to validate JSON schema here

                f_result = f(f_self, payload)

                return json.dumps(f_result, default=lambda obj: obj.__dict__, sort_keys=True)

            self.methods[http_method][url] = _wrapper

        return actual_decorator


class RestAPI:

    registry = MethodRegistry()

    def __init__(self, database=None):
        self._users = {user['name'] : User.from_dict(user) for user in database['users']} if database else {}

    def _process_request(self, method, url, payload):
        methods = self.registry.methods[method]

        #It would be natural to handle exceptions here
        if url in methods.keys():
            return methods[url](self, payload)

    def get(self, url, payload=None):
        return self._process_request('GET', url, payload)

    def post(self, url, payload=None):
        return self._process_request('POST', url, payload)

    @registry.register_url('/users', 'GET')
    def users(self, payload=None):
        if payload:
            requested_users = payload['users']
            response = [user for user in self._users.values() if user.name in requested_users]
        else:
            response = [user for user in self._users.values()]

        return {'users': response}

    @registry.register_url('/add', 'POST')
    def add(self, payload=None):

        name = payload['user']
        user = User(name)
        self._users[user.name] = user
        return user

    @registry.register_url('/iou', 'POST')
    def iou(self, payload=None):

        lender = payload['lender']
        borrower = payload['borrower']
        amount = payload['amount']

        self._users[lender].lend(borrower, amount)
        self._users[borrower].borrow(lender, amount)

        return {'users': sorted([self._users[lender], self._users[borrower]], key=attrgetter('name'))}