"""
Double entry accounting system:

A debit is an accounting entry that either increases an asset or expense account,
or decreases a liability or equity account. It is positioned to the left in an accounting entry.
Debit means "left", dividends/expenses/assets/losses increased with debit.

A credit is an accounting entry that either increases a liability or equity account,
or decreases an asset or expense account.
Credit means "right", gains/income/revenues/liabilities/equity increased with credit.
"""
from datetime import datetime, timedelta
from decimal import Decimal
from typing import Optional, Type

from math import floor
from django.core.exceptions import ValidationError
from jacc.helpers import sum_queryset
from django.conf import settings
from django.db import models, transaction
from django.db.models import QuerySet, Q
from django.utils.timezone import now
from jutil.cache import CachedFieldsMixin
from django.utils.translation import gettext_lazy as _

from jutil.format import choices_label
from jutil.modelfields import SafeCharField, SafeTextField

CATEGORY_ANY = ''
CATEGORY_DEBIT = 'D'  # "left", dividends/expenses/assets/losses increased with debit
CATEGORY_CREDIT = 'C'  # "right", gains/income/revenues/liabilities/equity increased with credit

CATEGORY_TYPE = (
    (CATEGORY_ANY, ''),
    (CATEGORY_DEBIT, _('Debit')),
    (CATEGORY_CREDIT, _('Credit')),
)

CURRENCY_TYPE = (
    ('EUR', 'EUR'),
    ('USD', 'USD'),
)

INVOICE_NOT_DUE_YET = 'N'
INVOICE_DUE = 'D'
INVOICE_LATE = 'L'
INVOICE_PAID = 'P'

INVOICE_STATE = (
    (INVOICE_NOT_DUE_YET, _('Not due yet')),
    (INVOICE_DUE, _('Due')),
    (INVOICE_LATE, _('Late')),
    (INVOICE_PAID, _('Paid')),
)

INVOICE_DEFAULT = 'I1'
INVOICE_CREDIT_NOTE = 'I2'

INVOICE_TYPE = (
    (INVOICE_DEFAULT, _('Invoice')),
    (INVOICE_CREDIT_NOTE, _('Credit Note')),
)


class AccountEntrySourceFile(models.Model):
    """
    Account entry source is set for entries based on some event like payment file import
    """
    name = SafeCharField(verbose_name=_('name'), max_length=255, db_index=True, blank=True, default='')
    created = models.DateTimeField(verbose_name=_('created'), default=now, db_index=True, editable=False, blank=True)
    last_modified = models.DateTimeField(verbose_name=_('last modified'), auto_now=True, db_index=True, editable=False, blank=True)

    class Meta:
        verbose_name = _('account entry source file')
        verbose_name_plural = _('account entry source files')

    def __str__(self):
        return '[{}] {}'.format(self.id, self.name)


class EntryType(models.Model):
    code = SafeCharField(verbose_name=_('code'), max_length=64, db_index=True, unique=True)
    identifier = SafeCharField(verbose_name=_('identifier'), max_length=40, db_index=True, blank=True, default='')
    name = SafeCharField(verbose_name=_('name'), max_length=128, db_index=True, blank=True, default='')
    created = models.DateTimeField(verbose_name=_('created'), default=now, db_index=True, editable=False, blank=True)
    last_modified = models.DateTimeField(verbose_name=_('last modified'), auto_now=True, db_index=True, editable=False, blank=True)
    payback_priority = models.SmallIntegerField(verbose_name=_('payback priority'), default=0, blank=True, db_index=True)
    is_settlement = models.BooleanField(verbose_name=_('is settlement'), default=False, blank=True, db_index=True)
    is_payment = models.BooleanField(verbose_name=_('is payment'), default=False, blank=True, db_index=True)

    class Meta:
        verbose_name = _('entry type')
        verbose_name_plural = _('entry types')

    def __str__(self):
        return '{} ({})'.format(self.name, self.code)


class AccountEntryManager(models.Manager):
    pass


class AccountEntry(models.Model):
    """
    Single mutation in account state.
    """
    objects: models.Manager = AccountEntryManager()
    account = models.ForeignKey('Account', verbose_name=_('record account'), related_name='accountentry_set', db_index=True, on_delete=models.PROTECT)
    created = models.DateTimeField(verbose_name=_('created'), default=now, db_index=True, editable=False, blank=True)
    last_modified = models.DateTimeField(verbose_name=_('last modified'), auto_now=True, editable=False, blank=True)
    timestamp = models.DateTimeField(verbose_name=_('timestamp'), default=now, db_index=True, blank=True)
    type = models.ForeignKey(EntryType, verbose_name=_('type'), related_name='+', on_delete=models.PROTECT, null=True, default=None, blank=True)
    description = SafeCharField(verbose_name=_('description'), max_length=256, default='', blank=True)
    amount = models.DecimalField(verbose_name=_('amount'), max_digits=10, decimal_places=2, blank=True, default=None, null=True, db_index=True)
    source_file = models.ForeignKey(AccountEntrySourceFile, verbose_name=_('account entry source file'), related_name='+', null=True, default=None, blank=True, on_delete=models.CASCADE, help_text=_('entry.source.file.help.text'))  # nopep8, pylint: disable=line-too-long
    source_invoice = models.ForeignKey('Invoice', verbose_name=_('source invoice'), null=True, related_name='+', default=None, blank=True, on_delete=models.CASCADE, help_text=_('entry.source.invoice.help.text'))  # nopep8, pylint: disable=line-too-long
    settled_invoice = models.ForeignKey('Invoice', verbose_name=_('settled invoice'), null=True, related_name='+', default=None, blank=True, on_delete=models.PROTECT, help_text=_('entry.settled.invoice.help.text'))  # nopep8, pylint: disable=line-too-long
    settled_item = models.ForeignKey('AccountEntry', verbose_name=_('settled item'), null=True, related_name='settlement_set', default=None, blank=True, on_delete=models.PROTECT, help_text=_('entry.settled.item.help.text'))  # nopep8, pylint: disable=line-too-long
    parent = models.ForeignKey('AccountEntry', verbose_name=_('account.entry.parent'), related_name='child_set', db_index=True, on_delete=models.CASCADE, null=True, default=None, blank=True)  # nopep8, pylint: disable=line-too-long
    archived = models.BooleanField(_('archived'), default=False, blank=True)

    class Meta:
        verbose_name = _('account entry')
        verbose_name_plural = _('account entries')

    def __str__(self):
        return '[{}] {} {} {}'.format(self.id, self.timestamp.date().isoformat() if self.timestamp else '',
                                      self.type if self.type else '', self.amount)

    def clean(self):
        if self.source_invoice and self.settled_invoice:
            raise ValidationError('Both source_invoice ({}) and settled_invoice ({}) cannot be set same time for account entry ({})'.format(
                self.source_invoice, self.settled_invoice, self))

    @property
    def balance(self) -> Decimal:
        """
        Returns account balance after this entry.
        :return: Decimal
        """
        return sum_queryset(AccountEntry.objects.filter(account=self.account, timestamp__lte=self.timestamp).exclude(timestamp=self.timestamp, id__gt=self.id))
    balance.fget.short_description = _('balance')  # type: ignore  # pytype: disable=attribute-error


class AccountType(models.Model):
    code = SafeCharField(verbose_name=_('code'), max_length=32, db_index=True, unique=True)
    name = SafeCharField(verbose_name=_('name'), max_length=64, db_index=True, unique=True)
    is_asset = models.BooleanField(verbose_name=_('asset'))
    created = models.DateTimeField(verbose_name=_('created'), default=now, db_index=True, editable=False, blank=True)
    last_modified = models.DateTimeField(verbose_name=_('last modified'), auto_now=True, db_index=True, editable=False, blank=True)

    class Meta:
        verbose_name = _('account type')
        verbose_name_plural = _('account types')

    def __str__(self):
        return self.name

    @property
    def is_liability(self) -> bool:
        return not self.is_asset
    is_liability.fget.short_description = _('liability')  # type: ignore  # pytype: disable=attribute-error


class Account(models.Model):
    """
    Collects together accounting entries and provides summarizing functionality.
    """
    type = models.ForeignKey(AccountType, verbose_name=_('type'), related_name='+', on_delete=models.PROTECT)
    name = SafeCharField(verbose_name=_('name'), max_length=64, blank=True, default='', db_index=True)
    currency = SafeCharField(verbose_name=_('currency'), max_length=3, default='EUR', choices=CURRENCY_TYPE, blank=True)
    created = models.DateTimeField(verbose_name=_('created'), default=now, db_index=True, editable=False, blank=True)
    last_modified = models.DateTimeField(verbose_name=_('last modified'), auto_now=True, db_index=True, editable=False, blank=True)

    class Meta:
        verbose_name = _('account')
        verbose_name_plural = _('accounts')

    def __str__(self):
        return '[{}] {}'.format(self.id, self.name if self.name else self.type)

    def is_asset(self) -> bool:
        return self.type.is_asset
    is_asset.boolean = True  # type: ignore
    is_asset.short_description = _('asset')  # type: ignore

    def is_liability(self) -> bool:
        return self.type.is_liability
    is_liability.boolean = True  # type: ignore
    is_liability.short_description = _('liability')  # type: ignore

    @property
    def balance(self) -> Decimal:
        return sum_queryset(self.accountentry_set.all())
    balance.fget.short_description = _('balance')  # type: ignore  # pytype: disable=attribute-error

    def get_balance(self, t: datetime):
        """
        Returns account balance before specified datetime (excluding entries on the datetime).
        :param t: datetime
        :return: Decimal
        """
        return sum_queryset(self.accountentry_set.all().filter(timestamp__lt=t))

    def needs_settling(self, e: AccountEntry) -> bool:
        """
        Returns True if all of following conditions are True:
        a) entry has valid amount set
        b) entry type is settlement
        c) entry has been recorded to this account
        d) invoice to be settled has been set
        e) entry has not been settled (=child set empty)
        :param e: AccountEntry (settlement)
        :return: bool
        """
        return bool(e.amount is not None and e.type and e.type.is_settlement and e.account.id == self.id and
                    e.settled_invoice and AccountEntry.objects.filter(parent=e).count() == 0)


class InvoiceManager(models.Manager):
    @transaction.atomic
    def update_cached_fields(self, **kw):
        for obj in self.filter(**kw):
            obj.update_cached_fields()


def get_default_due_date():
    return now() + timedelta(days=settings.DEFAULT_DUE_DATE_DAYS) if hasattr(settings, 'DEFAULT_DUE_DATE_DAYS') else None


class Invoice(models.Model, CachedFieldsMixin):
    """
    Invoice model. Typically used as base model for actual app-specific invoice model.

    Convention for naming date/time variables:
    1) date fields are suffixed with _date if they are either plain date fields or interpreted as such (due_date)
    2) natural datetime fields are in past tense, e.g. created, sent (instead of create_date, send_date)

    Note: It is useful sometimes to have full datetime with timezone even for plain dates like due_date,
    because this to be processing to be independent of server, client and invoice time zones.
    """
    objects: models.Manager = InvoiceManager()
    type = SafeCharField(verbose_name=_('type'), max_length=2, db_index=True, default=INVOICE_DEFAULT, blank=True, choices=INVOICE_TYPE)
    number = SafeCharField(verbose_name=_('invoice number'), max_length=32, default='', blank=True, db_index=True)
    created = models.DateTimeField(verbose_name=_('created'), default=now, db_index=True, editable=False, blank=True)
    last_modified = models.DateTimeField(verbose_name=_('last modified'), auto_now=True, db_index=True, editable=False, blank=True)
    sent = models.DateTimeField(verbose_name=_('sent'), db_index=True, default=None, blank=True, null=True)
    due_date = models.DateTimeField(verbose_name=_('due date'), db_index=True, default=get_default_due_date)
    notes = SafeTextField(verbose_name=_('notes'), blank=True, default='')
    filename = SafeCharField(verbose_name=_('filename'), max_length=255, blank=True, default='', db_index=True)
    amount = models.DecimalField(verbose_name=_('amount'), max_digits=10, decimal_places=2, default=0, blank=True)
    paid_amount = models.DecimalField(verbose_name=_('paid amount'), max_digits=10, decimal_places=2, editable=False, blank=True, null=True, default=None, db_index=True)  # nopep8, pylint: disable=line-too-long
    unpaid_amount = models.DecimalField(verbose_name=_('unpaid amount'), max_digits=10, decimal_places=2, editable=False, blank=True, null=True, default=None, db_index=True)  # nopep8, pylint: disable=line-too-long
    overpaid_amount = models.DecimalField(verbose_name=_('overpaid amount'), max_digits=10, decimal_places=2, editable=False, blank=True, null=True, default=None, db_index=True)  # nopep8, pylint: disable=line-too-long
    close_date = models.DateTimeField(verbose_name=_('close date'), default=None, null=True, blank=True, db_index=True)
    late_days = models.SmallIntegerField(verbose_name=_('late days'), default=None, null=True, blank=True, db_index=True)
    state = SafeCharField(verbose_name=_('state'), max_length=1, blank=True, default='', db_index=True, choices=INVOICE_STATE)
    cached_fields = [
        'amount',
        'paid_amount',
        'unpaid_amount',
        'overpaid_amount',
        'close_date',
        'late_days',
        'state',
    ]

    class Meta:
        verbose_name = _('invoice')
        verbose_name_plural = _('invoices')

    def __str__(self):
        return '[{}] {} {}'.format(self.id, self.due_date.date().isoformat() if self.due_date else '', self.amount)

    @property
    def receivables_account(self) -> Optional[Account]:
        """
        Returns receivables account. Receivables account is assumed to be the one were invoice rows were recorded.
        :return: Account or None
        """
        row = AccountEntry.objects.filter(source_invoice=self).order_by('id').first()
        return row.account if row else None

    @property
    def currency(self) -> str:
        recv = self.receivables_account
        return recv.currency if recv else ''

    def get_entries(self, acc: Account, cls: Type[AccountEntry] = AccountEntry) -> QuerySet:
        """
        Returns entries related to this invoice on specified account.
        :param acc: Account
        :param cls: AccountEntry class
        :return: QuerySet
        """
        return cls.objects.filter(Q(account=acc) & (Q(source_invoice=self) | Q(settled_invoice=self))) if acc else cls.objects.none()

    def get_balance(self, acc: Account) -> Decimal:
        """
        Returns balance of this invoice on specified account.
        :param acc: Account
        :return:
        """
        return sum_queryset(self.get_entries(acc))

    def get_item_balances(self, acc: Account) -> list:
        """
        Returns balances of items of the invoice.
        :param acc: Account
        :return: list (AccountEntry, Decimal) in item id order
        """
        items = []
        entries = self.get_entries(acc)
        for item in entries.filter(source_invoice=self).order_by('id'):
            assert isinstance(item, AccountEntry)
            settlements = sum_queryset(entries.filter(settled_item=item))
            bal = item.amount + settlements if item.amount is not None else settlements
            items.append((item, bal))
        return items

    def get_unpaid_items(self, acc: Account) -> list:
        """
        Returns unpaid items of the invoice in payback priority order.
        :param acc: Account
        :return: list (AccountEntry, Decimal) in payback priority order
        """
        unpaid_items = []
        for item, bal in self.get_item_balances(acc):
            assert isinstance(item, AccountEntry)
            priority = item.type.payback_priority if item.type is not None else 0
            if self.type == INVOICE_DEFAULT:
                if bal > Decimal(0):
                    unpaid_items.append((priority, item, bal))
            elif self.type == INVOICE_CREDIT_NOTE:
                if bal < Decimal(0):
                    unpaid_items.append((priority, item, bal))
            else:
                raise Exception('jacc.models.Invoice.get_unpaid_items() unimplemented for invoice type {}'.format(self.type))
        return [i[1:] for i in sorted(unpaid_items, key=lambda x: x[0])]

    def get_amount(self) -> Decimal:
        return sum_queryset(self.items, 'amount')

    @property
    def receivables(self) -> QuerySet:
        acc = self.receivables_account
        if acc is None:
            return AccountEntry.objects.none()
        return self.get_entries(acc)

    @property
    def items(self) -> QuerySet:
        return self.receivables.filter(source_invoice=self)

    def get_paid_amount(self) -> Decimal:
        return self.get_amount() - self.get_unpaid_amount()

    def get_unpaid_amount(self) -> Decimal:
        return sum_queryset(self.receivables)

    def get_overpaid_amount(self) -> Decimal:
        amt = sum_queryset(self.receivables)
        if self.type == INVOICE_CREDIT_NOTE:
            return max(Decimal('0.00'), amt)
        return max(Decimal('0.00'), -amt)

    @property
    def is_paid(self) -> bool:
        if self.unpaid_amount is None:
            return False
        return self.unpaid_amount >= Decimal('0.00') if self.type == INVOICE_CREDIT_NOTE else self.unpaid_amount <= Decimal('0.00')
    is_paid.fget.short_description = _('is paid')  # type: ignore  # pytype: disable=attribute-error

    @property
    def is_due(self) -> bool:
        return not self.is_paid and now() >= self.due_date
    is_due.fget.short_description = _('is due')  # type: ignore  # pytype: disable=attribute-error

    def get_close_date(self) -> Optional[datetime]:
        recv = self.receivables.order_by('-timestamp', '-id')
        first = recv.first()
        if first is None:
            return None
        total = sum_queryset(recv)
        if self.type == INVOICE_CREDIT_NOTE:
            if total >= Decimal('0.00'):
                return first.timestamp
        else:
            if total <= Decimal('0.00'):
                return first.timestamp
        return None

    def get_late_days(self, t: Optional[datetime] = None) -> int:
        t = self.close_date or t
        if t is None:
            t = now()
        return int(floor((t - self.due_date).total_seconds() / 86400.0))

    @property
    def is_late(self) -> bool:
        if self.late_days is None:
            return False
        return not self.is_paid and self.late_days >= settings.LATE_LIMIT_DAYS

    def get_state(self) -> str:
        if self.is_paid:
            return INVOICE_PAID
        t = now()
        if t - self.due_date >= timedelta(days=settings.LATE_LIMIT_DAYS):
            return INVOICE_LATE
        if t >= self.due_date:
            return INVOICE_DUE
        return INVOICE_NOT_DUE_YET

    def get_state_name(self) -> str:
        return choices_label(INVOICE_STATE, self.get_state())

    @property
    def state_name(self) -> str:
        return choices_label(INVOICE_STATE, self.state)
    state_name.fget.short_description = _('state')  # type: ignore  # pytype: disable=attribute-error


class Contract(models.Model):
    """
    Base class for contracts (e.g. rent contracts, loans, etc.)
    """
    created = models.DateTimeField(verbose_name=_('created'), default=now, db_index=True, editable=False, blank=True)
    last_modified = models.DateTimeField(verbose_name=_('last modified'), auto_now=True, db_index=True, editable=False, blank=True)
    name = SafeCharField(verbose_name=_('name'), max_length=128, default='', blank=True, db_index=True)

    class Meta:
        verbose_name = _('contract')
        verbose_name_plural = _('contracts')

    def __str__(self):
        return '[{}]'.format(self.id)
