from __future__ import annotations

import json
import re
from dataclasses import dataclass

from fastapi import Request
from sqlalchemy import select
from sqlalchemy.orm import Session

from app.modules.core.audit.models import AuditLog
from app.modules.core.tenancy.year_control import is_financial_year_locked, is_row_financial_year_locked
from app.modules.documents.models import EngagementDocument
from app.modules.services.client_services import assessment_year_from_financial_year, period_choices_for_service
from app.modules.services.due_dates import apply_due_date_rule_to_subscription
from app.modules.services.models import (
    ClientServiceSubscription,
    ClientServiceTaskInstance,
    EngagementClosureChecklist,
)

_FINANCIAL_YEAR_RE = re.compile(r"^(\d{4})-(\d{2})$")
_CLOSED_ENGAGEMENT_STATUSES = {"completed", "cancelled", "inactive"}


@dataclass(frozen=True)
class EngagementFYCorrectionResult:
    subscription_id: int
    old_financial_year: str
    new_financial_year: str
    old_period_label: str
    new_period_label: str
    task_count: int
    document_count: int


def validate_financial_year(value: str | None) -> str:
    raw = (value or "").strip()
    match = _FINANCIAL_YEAR_RE.fullmatch(raw)
    if not match:
        raise ValueError("Financial year must be in YYYY-YY format, for example 2025-26.")
    start_year = int(match.group(1))
    expected_suffix = str(start_year + 1)[-2:]
    if match.group(2) != expected_suffix:
        raise ValueError("Financial year end year does not match the start year.")
    return f"{start_year:04d}-{expected_suffix}"


def _remap_period_label(row: ClientServiceSubscription, target_financial_year: str) -> str:
    recurrence = (getattr(getattr(row, "catalogue", None), "recurrence_type", None) or "one_time").strip().lower()
    current = (getattr(row, "period_label", None) or "").strip()
    choices = period_choices_for_service(target_financial_year, recurrence)
    valid_codes = [code for code, _label in choices]

    if recurrence == "monthly":
        # Period codes are YYYY-MM. Keep the same month while moving it to the
        # corresponding month inside the corrected financial year.
        try:
            month = int(current.rsplit("-", 1)[1])
        except (ValueError, IndexError):
            raise ValueError("The existing monthly period is invalid and cannot be remapped automatically.")
        target_start = int(target_financial_year.split("-", 1)[0])
        target_year = target_start if month >= 4 else target_start + 1
        candidate = f"{target_year:04d}-{month:02d}"
        if candidate not in valid_codes:
            raise ValueError("The monthly period cannot be mapped to the corrected financial year.")
        return candidate

    if recurrence == "quarterly":
        if current not in valid_codes:
            raise ValueError("The existing quarter is invalid for the corrected financial year.")
        return current

    return ""


def _engagement_has_finalised_documents(db: Session, subscription_id: int) -> bool:
    rows = db.execute(
        select(EngagementDocument).where(
            EngagementDocument.engagement_id == subscription_id,
            EngagementDocument.is_deleted.is_(False),
        )
    ).scalars().all()
    for document in rows:
        if (getattr(document, "final_release_status", None) or "draft").strip().lower() == "released":
            return True
        if (getattr(document, "udin_number", None) or "").strip():
            return True
    return False


def validate_engagement_can_change_financial_year(
    db: Session,
    *,
    row: ClientServiceSubscription,
    target_financial_year: str,
) -> None:
    if getattr(row, "is_locked", False):
        raise ValueError("Locked engagements cannot be moved to another financial year.")
    if is_row_financial_year_locked(db, row):
        raise ValueError(f"Source FY {row.financial_year} is locked and cannot be changed.")
    if is_financial_year_locked(db, tenant_id=row.tenant_id, year_code=target_financial_year):
        raise ValueError(f"Target FY {target_financial_year} is locked and cannot receive an engagement.")
    if not getattr(row, "is_active", True):
        raise ValueError("Inactive engagements cannot be moved to another financial year.")
    status = (getattr(row, "status", None) or "active").strip().lower()
    if status in _CLOSED_ENGAGEMENT_STATUSES:
        raise ValueError("Only open engagements can have their financial year corrected.")

    closure = db.execute(
        select(EngagementClosureChecklist).where(
            EngagementClosureChecklist.subscription_id == row.id
        )
    ).scalar_one_or_none()
    if closure and (closure.closure_status or "").strip().lower() == "approved":
        raise ValueError("The engagement closure is already approved. Reopen it before correcting the financial year.")
    if _engagement_has_finalised_documents(db, row.id):
        raise ValueError("A released final document or UDIN exists. Reopen/correct the finalisation workflow before changing FY.")


def _duplicate_engagement(
    db: Session,
    *,
    row: ClientServiceSubscription,
    target_financial_year: str,
    target_period_label: str,
) -> ClientServiceSubscription | None:
    return db.execute(
        select(ClientServiceSubscription).where(
            ClientServiceSubscription.tenant_id == row.tenant_id,
            ClientServiceSubscription.service_catalogue_id == row.service_catalogue_id,
            ClientServiceSubscription.scope_key == row.scope_key,
            ClientServiceSubscription.financial_year == target_financial_year,
            ClientServiceSubscription.period_label == target_period_label,
            ClientServiceSubscription.id != row.id,
        )
    ).scalar_one_or_none()


def _add_audit_log(
    db: Session,
    *,
    request: Request | None,
    row: ClientServiceSubscription,
    actor,
    old_financial_year: str,
    new_financial_year: str,
    old_period_label: str,
    new_period_label: str,
    reason: str,
    task_count: int,
    document_count: int,
) -> None:
    ip_address = request.client.host if request and request.client else None
    user_agent = request.headers.get("user-agent") if request else None
    client_name = getattr(getattr(row, "client", None), "client_name", None) or f"Client {row.client_id}"
    service_name = getattr(getattr(row, "catalogue", None), "service_name", None) or f"Service {row.service_catalogue_id}"
    db.add(
        AuditLog(
            actor_user_id=getattr(actor, "id", None),
            actor_email=getattr(actor, "email", None),
            actor_tenant_id=getattr(actor, "tenant_id", None),
            actor_branch_id=getattr(actor, "branch_id", None),
            action="engagement.financial_year_corrected",
            entity_type="ClientServiceSubscription",
            entity_id=str(row.id),
            entity_name=f"{client_name} - {service_name}",
            status="success",
            target_tenant_id=row.tenant_id,
            target_branch_id=row.branch_id,
            ip_address=ip_address,
            user_agent=user_agent,
            details_json=json.dumps(
                {
                    "old_financial_year": old_financial_year,
                    "new_financial_year": new_financial_year,
                    "old_assessment_year": assessment_year_from_financial_year(old_financial_year),
                    "new_assessment_year": assessment_year_from_financial_year(new_financial_year),
                    "old_period_label": old_period_label,
                    "new_period_label": new_period_label,
                    "reason": reason,
                    "task_instances_updated": task_count,
                    "engagement_documents_updated": document_count,
                    "physical_document_paths_moved": False,
                },
                ensure_ascii=False,
                sort_keys=True,
            ),
        )
    )


def correct_engagement_financial_year(
    db: Session,
    *,
    row: ClientServiceSubscription,
    target_financial_year: str,
    actor,
    reason: str,
    request: Request | None = None,
) -> EngagementFYCorrectionResult:
    target_fy = validate_financial_year(target_financial_year)
    reason_text = (reason or "").strip()
    if not reason_text:
        raise ValueError("A correction reason is required for the audit trail.")

    old_fy = validate_financial_year(row.financial_year)
    if target_fy == old_fy:
        raise ValueError("The corrected financial year is the same as the existing financial year.")

    validate_engagement_can_change_financial_year(db, row=row, target_financial_year=target_fy)
    old_period = (row.period_label or "").strip()
    new_period = _remap_period_label(row, target_fy)

    duplicate = _duplicate_engagement(
        db,
        row=row,
        target_financial_year=target_fy,
        target_period_label=new_period,
    )
    if duplicate:
        raise ValueError(
            f"A matching engagement already exists in FY {target_fy}"
            + (f" for period {new_period}" if new_period else "")
            + "."
        )

    # Pre-flight the task unique key before mutating anything. A partially
    # inconsistent legacy subscription must be repaired manually rather than
    # losing execution/review history.
    target_task = db.execute(
        select(ClientServiceTaskInstance.id).where(
            ClientServiceTaskInstance.subscription_id == row.id,
            ClientServiceTaskInstance.financial_year == target_fy,
        ).limit(1)
    ).scalar_one_or_none()
    if target_task is not None:
        raise ValueError(
            f"This engagement already has a task instance tagged to FY {target_fy}. "
            "The mixed-year task data must be reviewed before FY correction."
        )

    new_ay = assessment_year_from_financial_year(target_fy)
    tasks = db.execute(
        select(ClientServiceTaskInstance).where(
            ClientServiceTaskInstance.subscription_id == row.id
        )
    ).scalars().all()
    documents = db.execute(
        select(EngagementDocument).where(
            EngagementDocument.engagement_id == row.id,
            EngagementDocument.is_deleted.is_(False),
        )
    ).scalars().all()

    row.financial_year = target_fy
    row.assessment_year = new_ay
    row.period_label = new_period
    row.updated_by_user_id = getattr(actor, "id", None)

    for task in tasks:
        task.financial_year = target_fy
        task.assessment_year = new_ay
        task.period_label = new_period
        task.updated_by_user_id = getattr(actor, "id", None)

    # Document metadata follows the corrected engagement. Existing physical
    # revision paths are deliberately preserved so no already-uploaded evidence
    # is orphaned; future uploads use the corrected engagement FY.
    for document in documents:
        document.financial_year = target_fy
        document.assessment_year = new_ay
        document.updated_by_user_id = getattr(actor, "id", None)

    apply_due_date_rule_to_subscription(db, row, force=True)
    _add_audit_log(
        db,
        request=request,
        row=row,
        actor=actor,
        old_financial_year=old_fy,
        new_financial_year=target_fy,
        old_period_label=old_period,
        new_period_label=new_period,
        reason=reason_text,
        task_count=len(tasks),
        document_count=len(documents),
    )
    db.flush()

    return EngagementFYCorrectionResult(
        subscription_id=row.id,
        old_financial_year=old_fy,
        new_financial_year=target_fy,
        old_period_label=old_period,
        new_period_label=new_period,
        task_count=len(tasks),
        document_count=len(documents),
    )