from __future__ import annotations

from datetime import date
from decimal import Decimal
from typing import Iterable

from django.db import transaction
from django.db.models import Q
from django.utils import timezone

from ..models import (
    Organization,
    Material,
    WeighTicket,
    WeighLine,
    MaterialEmissionFactor,
    CarbonEvent,
    CarbonBalance,
)


LB_TO_KG = Decimal("0.45359237")


def _convert_qty(qty: Decimal, from_unit: str, to_unit: str) -> Decimal:
    if from_unit == to_unit:
        return qty
    # Minimal support: lb <-> kg; pcs assumed 1:1 (no conversion)
    if from_unit == Material.UNIT_LB and to_unit == Material.UNIT_KG:
        return (qty * LB_TO_KG).quantize(Decimal("0.000"))
    if from_unit == Material.UNIT_KG and to_unit == Material.UNIT_LB:
        return (qty / LB_TO_KG).quantize(Decimal("0.000"))
    return qty


def get_applicable_factor(org: Organization, material: Material, unit: str, as_of: date | None = None) -> MaterialEmissionFactor | None:
    """Return the most recent factor effective for the given org/material/unit.

    If none matches the unit, try the material's default unit as a fallback.
    """
    as_of = as_of or timezone.now().date()
    qs = (
        MaterialEmissionFactor.objects.filter(
            organization=org,
            material=material,
            unit=unit,
        )
        .filter(Q(effective_from__isnull=True) | Q(effective_from__lte=as_of))
        .order_by("-effective_from", "-id")
    )
    fac = qs.first()
    if fac:
        return fac
    # fallback to default unit
    if unit != material.default_unit:
        qs2 = (
            MaterialEmissionFactor.objects.filter(
                organization=org,
                material=material,
                unit=material.default_unit,
            )
            .filter(Q(effective_from__isnull=True) | Q(effective_from__lte=as_of))
            .order_by("-effective_from", "-id")
        )
        return qs2.first()
    return None


@transaction.atomic
def create_event_for_line(line: WeighLine) -> CarbonEvent | None:
    """Create or update a pending CarbonEvent for a given weigh line.

    Returns the event or None if no factor available.
    """
    ticket = line.ticket
    org = ticket.pickup.organization
    fac = get_applicable_factor(org, line.material, line.unit)
    if not fac:
        # Try convert to factor unit
        fac = get_applicable_factor(org, line.material, fac_unit := line.material.default_unit)
        if not fac:
            return None
        qty = _convert_qty(Decimal(line.quantity), line.unit, fac.unit)
    else:
        qty = Decimal(line.quantity)
    kgco2e = (qty * Decimal(fac.kgco2e_per_unit)).quantize(Decimal("0.000001"))

    ev, _ = CarbonEvent.objects.update_or_create(
        weigh_line=line,
        defaults={
            "organization": org,
            "weigh_ticket": ticket,
            "material": line.material,
            "quantity": qty,
            "unit": fac.unit,
            "kgco2e": kgco2e,
            "event_date": timezone.now(),
            "status": CarbonEvent.STATUS_PENDING,
        },
    )
    return ev


def calculate_events_for_ticket(ticket: WeighTicket) -> list[CarbonEvent]:
    events: list[CarbonEvent] = []
    for ln in ticket.lines.select_related("material"):
        ev = create_event_for_line(ln)
        if ev:
            events.append(ev)
    return events


def approve_events(org: Organization, events: Iterable[CarbonEvent], *, approver) -> int:
    """Approve events and update monthly CarbonBalance aggregates.

    Returns number approved.
    """
    approved = 0
    months: set[tuple[int, int]] = set()
    now = timezone.now()
    for ev in events:
        if ev.organization_id != org.id:
            continue
        if ev.status == CarbonEvent.STATUS_APPROVED:
            continue
        ev.status = CarbonEvent.STATUS_APPROVED
        ev.approved_by = approver
        ev.approved_at = now
        ev.save(update_fields=["status", "approved_by", "approved_at", "updated_at"])
        months.add((ev.event_date.year, ev.event_date.month))
        approved += 1
    # recompute balances for affected months
    for (y, m) in months:
        recompute_balance_for_month(org, y, m)
    return approved


def recompute_balance_for_month(org: Organization, year: int, month: int) -> CarbonBalance:
    qs = CarbonEvent.objects.filter(
        organization=org,
        status=CarbonEvent.STATUS_APPROVED,
        event_date__year=year,
        event_date__month=month,
    )
    # Avoid circular import for Sum; simple loop to sum
    total_val = Decimal("0")
    cnt = 0
    for ev in qs.only("kgco2e"):
        total_val += Decimal(ev.kgco2e)
        cnt += 1
    bal, _ = CarbonBalance.objects.get_or_create(organization=org, year=year, month=month)
    bal.events_count = cnt
    bal.approved_kgco2e = total_val.quantize(Decimal("0.000001"))
    bal.save(update_fields=["events_count", "approved_kgco2e", "updated_at"])
    return bal


def get_totals_for_org(org: Organization) -> dict[str, Decimal]:
    """Convenience totals for dashboards."""
    from django.utils.timezone import now

    today = now().date()
    y = today.year
    m = today.month
    bal = CarbonBalance.objects.filter(organization=org, year=y, month=m).first()
    pending = CarbonEvent.objects.filter(organization=org, status=CarbonEvent.STATUS_PENDING).count()
    return {
        "pending_events": pending,
        "approved_mtd": (bal.approved_kgco2e if bal else Decimal("0")),
    }