"""Apply and recover guarded renames (US04-03, US04-04).

This is the only module that moves a photo library's files. Every mutation is
bracketed by the journal (US04-02): intent is recorded **before** the move, and the
journal plus the real files on disk are the sole basis for recovery afterwards. The
two halves live together because they are one crash-safe transaction — apply writes
exactly the evidence recovery reads.

Per operation the sequence is:

```
journal.begin (moving)      ← intent persisted BEFORE any disk change
recheck preconditions       ← immediately before the mutation, never trusting the plan
move                        ← same-fs rename | staged case-only | copy-verify-delete
journal → moved
update assets.current_path + path occurrences (one transaction)
journal → database_updated
verify bytes and paths on disk
journal → verified → complete
```

Safety rules that are never relaxed:

- An unexpected occupant at the destination is **never** overwritten; the operation
  fails and the source is left untouched.
- Cross-filesystem moves are copy → verify hash → delete source, so the bytes exist
  in two places until they are proven identical.
- A case-only rename goes through a staged temporary name, because on a
  case-insensitive filesystem source and destination are the same directory entry.
- Recovery never guesses: a ``manual`` verdict blocks unrelated mutations instead of
  retrying. Resume and rollback revalidate the recorded hashes and refuse a path
  whose bytes changed.
- Recovery and rollback are idempotent — replaying them after repeated restarts
  converges on the same state.
"""

from __future__ import annotations

import json
import os
import shutil
import uuid
from datetime import datetime, timezone
from pathlib import Path

from sqlalchemy import select
from sqlalchemy.orm import sessionmaker

from photo_pipeline.faults import maybe_fault
from photo_pipeline.models import Asset, AssetPath, RenamePlan
from photo_pipeline.services import hashing
from photo_pipeline.services.rename_journal import (
    ALLOWED_TRANSITIONS,
    MANUAL,
    RESUMABLE,
    JournalState,
    RenameJournal,
)

# ``applied`` is included so a repeated apply is a safe no-op rather than an error:
# every operation is already complete and simply skips (idempotent confirmation).
# ``planned`` is what recovery leaves behind after resetting a resumable operation,
# so excluding it would make "resumable" impossible to actually resume. An invalid
# plan is still refused — by the separate blockers check, not by state.
APPLYABLE_PLAN_STATES = frozenset({"planned", "validated", "applying", "failed", "applied"})


class ApplyError(RuntimeError):
    """The request cannot be applied (unknown plan, invalid state, blocked)."""


class ApplyConflict(RuntimeError):
    """Optimistic confirmation failed; the plan changed since it was previewed."""


class PreconditionFailed(RuntimeError):
    """A precondition rechecked immediately before mutation no longer holds."""

    def __init__(self, code: str, message: str) -> None:
        super().__init__(message)
        self.code = code


def _now() -> datetime:
    return datetime.now(timezone.utc)


class RenameApplyService:
    def __init__(self, session_factory: sessionmaker, *, library_roots: tuple = ()) -> None:
        self._session_factory = session_factory
        self._roots = tuple(Path(root) for root in library_roots)
        self.journal = RenameJournal(session_factory)

    # ── apply ─────────────────────────────────────────────────────────────────

    def apply(
        self,
        plan_id: str,
        *,
        expected_version: int,
        expected_checksum: str | None = None,
        worker_id: str = "apply",
    ) -> dict:
        """Apply a confirmed plan. Requires the current version (and optionally the
        checksum) so a stale browser confirmation cannot run a changed plan."""
        plan = self._require_plan(plan_id)
        if plan["version"] != expected_version:
            raise ApplyConflict(
                f"plan {plan_id} is at version {plan['version']}, expected {expected_version}"
            )
        if expected_checksum is not None and plan["checksum"] != expected_checksum:
            raise ApplyConflict(f"plan {plan_id} checksum changed since it was previewed")
        if plan["state"] not in APPLYABLE_PLAN_STATES:
            raise ApplyError(f"plan {plan_id} is {plan['state']} and cannot be applied")
        if plan["blockers"]:
            raise ApplyError(f"plan {plan_id} has unresolved blockers: {plan['blockers']}")

        # Exclusive mutation lease: refuse while any other rename may have the
        # library half-moved (concept §16 lock hierarchy).
        blocking = [row for row in self.journal.incomplete() if row["plan_id"] != plan_id]
        if blocking:
            raise ApplyError(
                f"another rename is unresolved ({blocking[0]['id']}); recover it first"
            )

        token = self._claim_plan(plan_id)
        applied = failed = skipped = 0
        for operation in self.journal.operations(plan_id):
            if operation["journal_state"] == JournalState.COMPLETE:
                skipped += 1  # repeated apply is a no-op for finished work
                continue
            try:
                self._apply_one(operation, token=token, worker_id=worker_id)
                applied += 1
            except PreconditionFailed as error:
                self._record_failure(operation["id"], token, error.code, str(error))
                failed += 1
            except Exception as error:  # unexpected: record and stop touching disk
                self._record_failure(operation["id"], token, "apply_error", str(error))
                failed += 1
        state = self.journal.sync_plan_state(plan_id)
        return {
            "plan_id": plan_id,
            "applied": applied,
            "failed": failed,
            "skipped": skipped,
            "state": state,
        }

    def _record_failure(self, operation_id: str, token: int, code: str, message: str) -> None:
        """Record a failed operation in a state its journal can actually reach.

        ``failed`` only makes sense while nothing has moved. Once the folder is at
        its destination — a postcondition failure such as bytes edited during the
        move — the operation is not "failed and forgotten": the disk changed and
        the database followed, so it becomes ``rollback_required`` and waits for a
        human (US07-04). Guessing an unreachable transition used to raise out of
        ``apply`` and lose the record entirely.
        """
        current = self.journal.get(operation_id)["journal_state"]
        target = (
            JournalState.FAILED
            if JournalState.FAILED in ALLOWED_TRANSITIONS.get(current, set())
            else JournalState.ROLLBACK_REQUIRED
        )
        self.journal.transition(
            operation_id, target, fencing_token=token, error=(code, message)
        )

    def _apply_one(self, operation: dict, *, token: int, worker_id: str) -> None:
        source = Path(operation["source_path"])
        destination = Path(operation["destination_path"])

        # 1. Intent first — after this point a crash is recoverable from evidence.
        self.journal.begin(operation["id"], worker_id=worker_id, fencing_token=token)
        maybe_fault(JournalState.MOVING)

        # 2. Recheck preconditions immediately before mutating, never trusting the
        #    plan's snapshot: files can change between preview and confirmation.
        self._recheck(operation, source, destination)

        # 3. The move itself.
        if operation["case_only"]:
            self._staged_case_rename(source, destination)
        elif operation["same_filesystem"] is False:
            self._copy_verify_delete(source, destination)
        else:
            os.rename(source, destination)

        self.journal.transition(operation["id"], JournalState.MOVED, fencing_token=token)
        maybe_fault(JournalState.MOVED)

        # 4. Database: stable IDs keep their identity, paths are re-pointed and the
        #    old occurrence is closed — all in one transaction.
        self._reconcile_paths(operation, source, destination)
        self.journal.transition(operation["id"], JournalState.DATABASE_UPDATED, fencing_token=token)
        maybe_fault(JournalState.DATABASE_UPDATED)

        # 5. Postconditions: the bytes really are at the new paths.
        self._verify(operation, destination)
        self.journal.transition(operation["id"], JournalState.VERIFIED, fencing_token=token)
        maybe_fault(JournalState.VERIFIED)

        self.journal.transition(operation["id"], JournalState.COMPLETE, fencing_token=token)
        maybe_fault(JournalState.COMPLETE)

    def _recheck(self, operation: dict, source: Path, destination: Path) -> None:
        if not source.exists():
            raise PreconditionFailed("source_missing", f"source {source} disappeared")
        if source.is_symlink() or destination.is_symlink():
            raise PreconditionFailed("symlink", "refusing to rename through a symlink")
        # An unexpected occupant is never overwritten. For a case-only rename the
        # destination resolves to the same entry as the source, so its existence is
        # expected and not a collision.
        if destination.exists() and not operation["case_only"]:
            raise PreconditionFailed("destination_exists", f"destination {destination} is occupied")
        if not self._within_roots(source) or not self._within_roots(destination):
            raise PreconditionFailed("root_escape", "path is outside the library roots")

        expected = operation.get("expected_sha256") or {}
        with self._session_factory() as session:
            assets = session.scalars(
                select(Asset).where(Asset.id.in_(operation["asset_ids"]))
            ).all()
            current = {asset.id: asset.current_path for asset in assets}
        for asset_id, path in current.items():
            file_path = Path(path) if path else None
            if file_path is None or not file_path.exists():
                raise PreconditionFailed("source_changed", f"asset {asset_id} is missing")
            wanted = expected.get(asset_id)
            if wanted and hashing.sha256_file(file_path) != wanted:
                raise PreconditionFailed(
                    "source_changed", f"asset {asset_id} changed since the plan was built"
                )

    def _within_roots(self, path: Path) -> bool:
        if not self._roots:
            return False
        return any(path == root or root in path.parents for root in self._roots)

    # ── move procedures ───────────────────────────────────────────────────────

    @staticmethod
    def _staged_case_rename(source: Path, destination: Path) -> None:
        """Case-only rename via a temporary name: on a case-insensitive filesystem
        source and destination are the same entry, so a direct rename is a no-op or
        an error depending on the platform."""
        staging = source.with_name(f".rename-{uuid.uuid4().hex[:8]}")
        os.rename(source, staging)
        try:
            os.rename(staging, destination)
        except OSError:
            os.rename(staging, source)  # put it back rather than leave it staged
            raise

    @staticmethod
    def _copy_verify_delete(source: Path, destination: Path) -> None:
        """Cross-filesystem move: the bytes exist in both places until they are
        proven identical, and only then is the source removed."""
        staging = destination.with_name(f".rename-{uuid.uuid4().hex[:8]}")
        shutil.copytree(source, staging)
        for original in sorted(p for p in source.rglob("*") if p.is_file()):
            copied = staging / original.relative_to(source)
            if not copied.exists():
                shutil.rmtree(staging, ignore_errors=True)
                raise PreconditionFailed("copy_incomplete", f"{original} was not copied")
            if hashing.sha256_file(original) != hashing.sha256_file(copied):
                shutil.rmtree(staging, ignore_errors=True)
                raise PreconditionFailed("copy_mismatch", f"{original} copied with wrong bytes")
        os.rename(staging, destination)
        shutil.rmtree(source)

    # ── database reconciliation ───────────────────────────────────────────────

    def _reconcile_paths(self, operation: dict, source: Path, destination: Path) -> None:
        """Re-point every asset in the operation at its new path, keeping the stable
        ID and recording the path history."""
        now = _now()
        with self._session_factory() as session:
            assets = session.scalars(
                select(Asset).where(Asset.id.in_(operation["asset_ids"]))
            ).all()
            for asset in assets:
                if not asset.current_path:
                    continue
                old = Path(asset.current_path)
                try:
                    relative = old.relative_to(source)
                except ValueError:
                    continue  # already re-pointed by an earlier partial run
                new_path = destination / relative
                for row in session.scalars(
                    select(AssetPath).where(
                        AssetPath.asset_id == asset.id,
                        AssetPath.path == asset.current_path,
                        AssetPath.valid_until.is_(None),
                    )
                ):
                    row.valid_until = now
                session.add(
                    AssetPath(
                        asset_id=asset.id,
                        path=str(new_path),
                        valid_from=now,
                        reason="rename",
                    )
                )
                asset.current_path = str(new_path)
                asset.state_version += 1
                asset.updated_at = now
            session.commit()

    def _verify(self, operation: dict, destination: Path) -> None:
        """Postcondition: every asset's recorded path exists and its bytes are the
        ones the plan expected."""
        expected = operation.get("expected_sha256") or {}
        with self._session_factory() as session:
            assets = session.scalars(
                select(Asset).where(Asset.id.in_(operation["asset_ids"]))
            ).all()
            paths = {asset.id: asset.current_path for asset in assets}
        for asset_id, path in paths.items():
            if not path or not Path(path).exists():
                raise PreconditionFailed("verify_missing", f"asset {asset_id} is not at {path}")
            wanted = expected.get(asset_id)
            if wanted and hashing.sha256_file(path) != wanted:
                raise PreconditionFailed("verify_bytes", f"asset {asset_id} bytes changed")

    # ── recovery (US04-04) ────────────────────────────────────────────────────

    def recover(self, *, worker_id: str = "recovery") -> dict:
        """Resolve every incomplete operation from journal + disk evidence.

        Idempotent: running it repeatedly converges. Ambiguous (``manual``) work is
        left untouched and keeps blocking unrelated mutations.
        """
        results = {"resumed": 0, "completed": 0, "manual": 0}
        touched_plans: set[str] = set()
        for verdict in self.journal.classify_all():
            operation = self.journal.get(verdict["operation_id"])
            touched_plans.add(operation["plan_id"])
            token = (operation["fencing_token"] or 0) + 1
            if verdict["classification"] == MANUAL:
                results["manual"] += 1
                continue
            if verdict["classification"] == RESUMABLE:
                # Nothing moved: return the operation to the planned state so a
                # later apply can retry it cleanly.
                self._to_planned(operation)
                results["resumed"] += 1
                continue
            # rollback_safe: the content is at the destination. Finish the remaining
            # bookkeeping deterministically rather than moving anything again.
            self._finish_moved(operation, token=token, worker_id=worker_id)
            results["completed"] += 1
        for plan_id in touched_plans:
            self.journal.sync_plan_state(plan_id)
        return results

    def _to_planned(self, operation: dict) -> None:
        state = operation["journal_state"]
        if state == JournalState.PLANNED:
            return
        if state in (JournalState.MOVING, JournalState.FAILED):
            self.journal.transition(operation["id"], JournalState.PLANNED)
        elif state == JournalState.ROLLBACK_REQUIRED:
            self.journal.transition(operation["id"], JournalState.ROLLED_BACK)

    def _finish_moved(self, operation: dict, *, token: int, worker_id: str) -> None:
        """Drive an operation whose bytes are already at the destination through the
        remaining states, re-running the idempotent bookkeeping."""
        operation_id = operation["id"]
        source = Path(operation["source_path"])
        destination = Path(operation["destination_path"])
        state = operation["journal_state"]

        if state == JournalState.ROLLBACK_REQUIRED:
            self.rollback_operation(operation_id, token=token)
            return

        if state == JournalState.MOVING:
            self.journal.transition(operation_id, JournalState.MOVED, fencing_token=token)
            state = JournalState.MOVED
        if state == JournalState.MOVED:
            self._reconcile_paths(operation, source, destination)
            self.journal.transition(
                operation_id, JournalState.DATABASE_UPDATED, fencing_token=token
            )
            state = JournalState.DATABASE_UPDATED
        if state == JournalState.DATABASE_UPDATED:
            self._verify(operation, destination)
            self.journal.transition(operation_id, JournalState.VERIFIED, fencing_token=token)
            state = JournalState.VERIFIED
        if state == JournalState.VERIFIED:
            self.journal.transition(operation_id, JournalState.COMPLETE, fencing_token=token)

    # ── rollback ──────────────────────────────────────────────────────────────

    def rollback_operation(self, operation_id: str, *, token: int | None = None) -> dict:
        """Move an operation's content back to its source, revalidating the recorded
        hashes first. Refuses when the source is occupied or the bytes changed."""
        operation = self.journal.get(operation_id)
        if operation is None:
            raise ApplyError(f"unknown rename operation {operation_id!r}")
        source = Path(operation["source_path"])
        destination = Path(operation["destination_path"])
        token = token if token is not None else (operation["fencing_token"] or 0) + 1

        if operation["journal_state"] not in (
            JournalState.MOVED,
            JournalState.DATABASE_UPDATED,
            JournalState.VERIFIED,
            JournalState.ROLLBACK_REQUIRED,
            JournalState.MOVING,
        ):
            raise ApplyError(
                f"operation {operation_id} is {operation['journal_state']}; nothing to roll back"
            )
        if source.exists() and not operation["case_only"]:
            raise ApplyError(f"source {source} is occupied; refusing to overwrite it")
        if not destination.exists():
            raise ApplyError(f"destination {destination} is missing; cannot roll back")

        # Revalidate before moving anything back: a changed file is not ours to move.
        expected = operation.get("expected_sha256") or {}
        if expected:
            for original in sorted(p for p in destination.rglob("*") if p.is_file()):
                digest = hashing.sha256_file(original)
                if expected and digest not in expected.values():
                    raise ApplyError(
                        f"{original} does not match the recorded hashes; manual recovery required"
                    )

        if operation["journal_state"] != JournalState.ROLLBACK_REQUIRED:
            self.journal.transition(
                operation_id, JournalState.ROLLBACK_REQUIRED, fencing_token=token
            )
        if operation["case_only"]:
            self._staged_case_rename(destination, source)
        elif operation["same_filesystem"] is False:
            self._copy_verify_delete(destination, source)
        else:
            os.rename(destination, source)

        self._reconcile_paths(operation, destination, source)
        self.journal.transition(operation_id, JournalState.ROLLED_BACK, fencing_token=token)
        self.journal.sync_plan_state(operation["plan_id"])
        return self.journal.get(operation_id)

    def rollback_plan(self, plan_id: str) -> dict:
        """Roll back every applied operation of a plan, newest first."""
        rolled = 0
        for operation in reversed(self.journal.operations(plan_id)):
            if operation["journal_state"] in (
                JournalState.MOVED,
                JournalState.DATABASE_UPDATED,
                JournalState.VERIFIED,
                JournalState.COMPLETE,
                JournalState.ROLLBACK_REQUIRED,
            ):
                if operation["journal_state"] == JournalState.COMPLETE:
                    # A completed operation must be reopened before it can move back.
                    continue
                self.rollback_operation(operation["id"])
                rolled += 1
        state = self.journal.sync_plan_state(plan_id)
        return {"plan_id": plan_id, "rolled_back": rolled, "state": state}

    # ── helpers ───────────────────────────────────────────────────────────────

    def _require_plan(self, plan_id: str) -> dict:
        with self._session_factory() as session:
            plan = session.get(RenamePlan, plan_id)
            if plan is None:
                raise ApplyError(f"unknown rename plan {plan_id!r}")
            return {
                "id": plan.id,
                "state": plan.state,
                "version": plan.version,
                "checksum": plan.checksum,
                "blockers": json.loads(plan.blockers or "[]"),
            }

    def _claim_plan(self, plan_id: str) -> int:
        """Bump the plan version and use it as this attempt's fencing token, so a
        worker from a superseded attempt cannot commit."""
        with self._session_factory() as session:
            plan = session.get(RenamePlan, plan_id)
            plan.version += 1
            plan.state = "applying"
            plan.updated_at = _now()
            token = plan.version
            session.commit()
        return token