"""Immutable double-entry ledger — the financial source of truth (V2 §7).

Balances are derived from immutable entries. Journals must balance
(sum debits == sum credits) before they commit; historical rows are never
edited or deleted — corrections use reversal/compensating journals.
"""

from uuid import uuid4

from django.db import models
from django.db.models import Q


class LedgerAccount(models.Model):
    class Type(models.TextChoices):
        ASSET = "asset", "Asset"
        LIABILITY = "liability", "Liability"
        REVENUE = "revenue", "Revenue"
        EXPENSE = "expense", "Expense"
        EQUITY = "equity", "Equity"

    id = models.UUIDField(primary_key=True, default=uuid4, editable=False)
    code = models.CharField(max_length=20, unique=True, db_index=True)
    name = models.CharField(max_length=120)
    type = models.CharField(max_length=12, choices=Type.choices)
    is_active = models.BooleanField(default=True)
    created_at = models.DateTimeField(auto_now_add=True)

    class Meta:
        db_table = "ledger_accounts"
        ordering = ["code"]

    def __str__(self):
        return f"{self.code} {self.name}"


class LedgerJournal(models.Model):
    """A balanced business transaction grouping one or more entries."""

    id = models.UUIDField(primary_key=True, default=uuid4, editable=False)
    reference = models.CharField(max_length=120, unique=True, db_index=True)
    description = models.CharField(max_length=255, blank=True, default="")
    deal = models.ForeignKey(
        "escrow.Deal", on_delete=models.SET_NULL, related_name="journals", null=True, blank=True
    )
    idempotency_key = models.CharField(max_length=120, unique=True, null=True, blank=True)
    created_by = models.CharField(max_length=120, blank=True, default="")  # staff operator or "system"
    created_at = models.DateTimeField(auto_now_add=True, db_index=True)

    class Meta:
        db_table = "ledger_journals"
        ordering = ["-created_at"]

    def __str__(self):
        return self.reference


class LedgerEntry(models.Model):
    """Immutable debit/credit line. Exactly one of debit/credit is non-zero."""

    id = models.UUIDField(primary_key=True, default=uuid4, editable=False)
    journal = models.ForeignKey(LedgerJournal, on_delete=models.CASCADE, related_name="entries")
    account = models.ForeignKey(LedgerAccount, on_delete=models.PROTECT, related_name="entries")
    debit = models.DecimalField(max_digits=18, decimal_places=2, default=0)
    credit = models.DecimalField(max_digits=18, decimal_places=2, default=0)
    memo = models.CharField(max_length=255, blank=True, default="")
    created_at = models.DateTimeField(auto_now_add=True)

    class Meta:
        db_table = "ledger_entries"
        # Per-line invariants (non-negative; exactly one of debit/credit
        # non-zero) are enforced here at the DB level as a last line of
        # defense. The balanced-journal invariant (sum(debit)=sum(credit)
        # per journal) can't be expressed as a single-row CHECK constraint,
        # so that still relies on ledger.services.post_journal being the
        # only sanctioned writer.
        constraints = [
            models.CheckConstraint(
                check=Q(debit__gte=0) & Q(credit__gte=0),
                name="ledger_entry_amounts_non_negative",
            ),
            models.CheckConstraint(
                check=(
                    (Q(debit__gt=0) & Q(credit=0))
                    | (Q(debit=0) & Q(credit__gt=0))
                ),
                name="ledger_entry_exactly_one_side",
            ),
        ]

    def __str__(self):
        side = f"DR {self.debit}" if self.debit else f"CR {self.credit}"
        return f"{self.account.code} {side}"