from __future__ import annotations

from decimal import Decimal

from app.models import MonthlyAllocation, MonthlyEntryValue, MonthlyIncome, to_decimal


class ComparisonService:
    def month_delta(self, current_month, previous_month) -> dict:
        if previous_month is None:
            zero = Decimal("0.00")
            return {
                "income_delta": zero,
                "cost_delta": zero,
                "remainder_delta": zero,
                "allocation_delta": zero,
            }
        return {
            "income_delta": self._total_income(current_month) - self._total_income(previous_month),
            "cost_delta": self._total_costs(current_month) - self._total_costs(previous_month),
            "remainder_delta": self._remainder(current_month) - self._remainder(previous_month),
            "allocation_delta": self._total_allocations(current_month)
            - self._total_allocations(previous_month),
        }

    def top_entry_changes(self, current_month, previous_month, limit: int = 6) -> list[dict]:
        if previous_month is None:
            return []
        previous_values = {
            item.entry_id: to_decimal(item.planned_amount) for item in previous_month.entry_values
        }
        changes = []
        for item in current_month.entry_values:
            previous_amount = previous_values.get(item.entry_id, Decimal("0.00"))
            current_amount = to_decimal(item.planned_amount)
            delta = current_amount - previous_amount
            if delta:
                changes.append(
                    {
                        "entry_name": item.entry.name,
                        "category_name": item.entry.category.name,
                        "delta": delta,
                        "current_amount": current_amount,
                        "previous_amount": previous_amount,
                    }
                )
        changes.sort(key=lambda item: abs(item["delta"]), reverse=True)
        return changes[:limit]

    def _total_income(self, month) -> Decimal:
        return sum((to_decimal(item.amount) for item in month.incomes), Decimal("0.00"))

    def _total_costs(self, month) -> Decimal:
        return sum((to_decimal(item.planned_amount) for item in month.entry_values), Decimal("0.00"))

    def _total_allocations(self, month) -> Decimal:
        return sum((to_decimal(item.amount) for item in month.allocations), Decimal("0.00"))

    def _remainder(self, month) -> Decimal:
        return self._total_income(month) - self._total_costs(month)