"""Fail2ban jail configuration file parser and activator.

Parses the full set of fail2ban jail configuration files
(``jail.conf``, ``jail.local``, ``jail.d/*.conf``, ``jail.d/*.local``)
to discover all defined jails — both active and inactive — and provides
functions to activate or deactivate them by writing ``.local`` override
files.

Merge order (fail2ban convention):
    1. ``jail.conf``
    2. ``jail.local``
    3. ``jail.d/*.conf`` (alphabetical)
    4. ``jail.d/*.local`` (alphabetical)

Security note: the ``activate_jail`` and ``deactivate_jail`` callers must
supply a validated jail name.  This module validates the name against an
allowlist pattern before constructing any filesystem paths to prevent
directory traversal.
"""

from __future__ import annotations

import asyncio
import configparser
import contextlib
import os
import re
import tempfile
from pathlib import Path
from typing import Any

import structlog

from app.models.config import (
    ActivateJailRequest,
    FilterConfig,
    FilterListResponse,
    InactiveJail,
    InactiveJailListResponse,
    JailActivationResponse,
)
from app.services import conffile_parser, jail_service
from app.utils.fail2ban_client import Fail2BanClient, Fail2BanConnectionError

log: structlog.stdlib.BoundLogger = structlog.get_logger()

# ---------------------------------------------------------------------------
# Constants
# ---------------------------------------------------------------------------

_SOCKET_TIMEOUT: float = 10.0

# Allowlist pattern for jail names used in path construction.
_SAFE_JAIL_NAME_RE: re.Pattern[str] = re.compile(
    r"^[A-Za-z0-9][A-Za-z0-9._-]{0,127}$"
)

# Sections that are not jail definitions.
_META_SECTIONS: frozenset[str] = frozenset({"INCLUDES", "DEFAULT"})

# True-ish values for the ``enabled`` key.
_TRUE_VALUES: frozenset[str] = frozenset({"true", "yes", "1"})

# False-ish values for the ``enabled`` key.
_FALSE_VALUES: frozenset[str] = frozenset({"false", "no", "0"})


# ---------------------------------------------------------------------------
# Custom exceptions
# ---------------------------------------------------------------------------


class JailNotFoundInConfigError(Exception):
    """Raised when the requested jail name is not defined in any config file."""

    def __init__(self, name: str) -> None:
        """Initialise with the jail name that was not found.

        Args:
            name: The jail name that could not be located.
        """
        self.name: str = name
        super().__init__(f"Jail not found in config files: {name!r}")


class JailAlreadyActiveError(Exception):
    """Raised when trying to activate a jail that is already active."""

    def __init__(self, name: str) -> None:
        """Initialise with the jail name.

        Args:
            name: The jail that is already active.
        """
        self.name: str = name
        super().__init__(f"Jail is already active: {name!r}")


class JailAlreadyInactiveError(Exception):
    """Raised when trying to deactivate a jail that is already inactive."""

    def __init__(self, name: str) -> None:
        """Initialise with the jail name.

        Args:
            name: The jail that is already inactive.
        """
        self.name: str = name
        super().__init__(f"Jail is already inactive: {name!r}")


class JailNameError(Exception):
    """Raised when a jail name contains invalid characters."""


class ConfigWriteError(Exception):
    """Raised when writing a ``.local`` override file fails."""


# ---------------------------------------------------------------------------
# Internal helpers
# ---------------------------------------------------------------------------


def _safe_jail_name(name: str) -> str:
    """Validate *name* and return it unchanged or raise :class:`JailNameError`.

    Args:
        name: Proposed jail name.

    Returns:
        The name unchanged if valid.

    Raises:
        JailNameError: If *name* contains unsafe characters.
    """
    if not _SAFE_JAIL_NAME_RE.match(name):
        raise JailNameError(
            f"Jail name {name!r} contains invalid characters. "
            "Only alphanumeric characters, hyphens, underscores, and dots are "
            "allowed; must start with an alphanumeric character."
        )
    return name


def _ordered_config_files(config_dir: Path) -> list[Path]:
    """Return all jail config files in fail2ban merge order.

    Args:
        config_dir: The fail2ban configuration root directory.

    Returns:
        List of paths in ascending priority order (later entries override
        earlier ones).
    """
    files: list[Path] = []

    jail_conf = config_dir / "jail.conf"
    if jail_conf.is_file():
        files.append(jail_conf)

    jail_local = config_dir / "jail.local"
    if jail_local.is_file():
        files.append(jail_local)

    jail_d = config_dir / "jail.d"
    if jail_d.is_dir():
        files.extend(sorted(jail_d.glob("*.conf")))
        files.extend(sorted(jail_d.glob("*.local")))

    return files


def _build_parser() -> configparser.RawConfigParser:
    """Create a :class:`configparser.RawConfigParser` for fail2ban configs.

    Returns:
        Parser with interpolation disabled and case-sensitive option names.
    """
    parser = configparser.RawConfigParser(interpolation=None, strict=False)
    # fail2ban keys are lowercase but preserve case to be safe.
    parser.optionxform = str  # type: ignore[assignment]
    return parser


def _is_truthy(value: str) -> bool:
    """Return ``True`` if *value* is a fail2ban boolean true string.

    Args:
        value: Raw string from config (e.g. ``"true"``, ``"yes"``, ``"1"``).

    Returns:
        ``True`` when the value represents enabled.
    """
    return value.strip().lower() in _TRUE_VALUES


def _parse_int_safe(value: str) -> int | None:
    """Parse *value* as int, returning ``None`` on failure.

    Args:
        value: Raw string to parse.

    Returns:
        Integer value, or ``None``.
    """
    try:
        return int(value.strip())
    except (ValueError, AttributeError):
        return None


def _parse_multiline(raw: str) -> list[str]:
    """Split a multi-line INI value into individual non-blank lines.

    Args:
        raw: Raw multi-line string from configparser.

    Returns:
        List of stripped, non-empty, non-comment strings.
    """
    result: list[str] = []
    for line in raw.splitlines():
        stripped = line.strip()
        if stripped and not stripped.startswith("#"):
            result.append(stripped)
    return result


def _resolve_filter(raw_filter: str, jail_name: str, mode: str) -> str:
    """Resolve fail2ban variable placeholders in a filter string.

    Handles the common default ``%(__name__)s[mode=%(mode)s]`` pattern that
    fail2ban uses so the filter name displayed to the user is readable.

    Args:
        raw_filter: Raw ``filter`` value from config (may contain ``%()s``).
        jail_name: The jail's section name, used to substitute ``%(__name__)s``.
        mode: The jail's ``mode`` value, used to substitute ``%(mode)s``.

    Returns:
        Human-readable filter string.
    """
    result = raw_filter.replace("%(__name__)s", jail_name)
    result = result.replace("%(mode)s", mode)
    return result


def _parse_jails_sync(
    config_dir: Path,
) -> tuple[dict[str, dict[str, str]], dict[str, str]]:
    """Synchronously parse all jail configs and return merged definitions.

    This is a CPU-bound / IO-bound sync function; callers must dispatch to
    an executor for async use.

    Args:
        config_dir: The fail2ban configuration root directory.

    Returns:
        A two-tuple ``(jails, source_files)`` where:

        - ``jails``: ``{jail_name: {key: value}}`` – merged settings for each
          jail with DEFAULT values already applied.
        - ``source_files``: ``{jail_name: str(path)}`` – path of the file that
          last defined each jail section (for display in the UI).
    """
    parser = _build_parser()
    files = _ordered_config_files(config_dir)

    # Track which file each section came from (last write wins).
    source_files: dict[str, str] = {}
    for path in files:
        try:
            single = _build_parser()
            single.read(str(path), encoding="utf-8")
            for section in single.sections():
                if section not in _META_SECTIONS:
                    source_files[section] = str(path)
        except (configparser.Error, OSError) as exc:
            log.warning("jail_config_read_error", path=str(path), error=str(exc))

    # Full merged parse: configparser applies DEFAULT values to every section.
    try:
        parser.read([str(p) for p in files], encoding="utf-8")
    except configparser.Error as exc:
        log.warning("jail_config_parse_error", error=str(exc))

    jails: dict[str, dict[str, str]] = {}
    for section in parser.sections():
        if section in _META_SECTIONS:
            continue
        try:
            # items() merges DEFAULT values automatically.
            jails[section] = dict(parser.items(section))
        except configparser.Error as exc:
            log.warning(
                "jail_section_parse_error", section=section, error=str(exc)
            )

    log.debug("jails_parsed", count=len(jails), config_dir=str(config_dir))
    return jails, source_files


def _build_inactive_jail(
    name: str,
    settings: dict[str, str],
    source_file: str,
) -> InactiveJail:
    """Construct an :class:`~app.models.config.InactiveJail` from raw settings.

    Args:
        name: Jail section name.
        settings: Merged key→value dict (DEFAULT values already applied).
        source_file: Path of the file that last defined this section.

    Returns:
        Populated :class:`~app.models.config.InactiveJail`.
    """
    raw_filter = settings.get("filter", "")
    mode = settings.get("mode", "normal")
    filter_name = _resolve_filter(raw_filter, name, mode) if raw_filter else name

    raw_action = settings.get("action", "")
    actions = _parse_multiline(raw_action) if raw_action else []

    raw_logpath = settings.get("logpath", "")
    logpath = _parse_multiline(raw_logpath) if raw_logpath else []

    enabled_raw = settings.get("enabled", "false")
    enabled = _is_truthy(enabled_raw)

    maxretry_raw = settings.get("maxretry", "")
    maxretry = _parse_int_safe(maxretry_raw)

    return InactiveJail(
        name=name,
        filter=filter_name,
        actions=actions,
        port=settings.get("port") or None,
        logpath=logpath,
        bantime=settings.get("bantime") or None,
        findtime=settings.get("findtime") or None,
        maxretry=maxretry,
        source_file=source_file,
        enabled=enabled,
    )


async def _get_active_jail_names(socket_path: str) -> set[str]:
    """Fetch the set of currently running jail names from fail2ban.

    Returns an empty set gracefully if fail2ban is unreachable.

    Args:
        socket_path: Path to the fail2ban Unix domain socket.

    Returns:
        Set of active jail names, or empty set on connection failure.
    """
    try:
        client = Fail2BanClient(socket_path=socket_path, timeout=_SOCKET_TIMEOUT)

        def _to_dict_inner(pairs: Any) -> dict[str, Any]:
            if not isinstance(pairs, (list, tuple)):
                return {}
            result: dict[str, Any] = {}
            for item in pairs:
                try:
                    k, v = item
                    result[str(k)] = v
                except (TypeError, ValueError):
                    pass
            return result

        def _ok(response: Any) -> Any:
            code, data = response
            if code != 0:
                raise ValueError(f"fail2ban error {code}: {data!r}")
            return data

        status_raw = _ok(await client.send(["status"]))
        status_dict = _to_dict_inner(status_raw)
        jail_list_raw: str = str(status_dict.get("Jail list", "") or "").strip()
        if not jail_list_raw:
            return set()
        return {j.strip() for j in jail_list_raw.split(",") if j.strip()}
    except Fail2BanConnectionError:
        log.warning("fail2ban_unreachable_during_inactive_list")
        return set()
    except Exception as exc:  # noqa: BLE001
        log.warning(
            "fail2ban_status_error_during_inactive_list", error=str(exc)
        )
        return set()


def _write_local_override_sync(
    config_dir: Path,
    jail_name: str,
    enabled: bool,
    overrides: dict[str, Any],
) -> None:
    """Write a ``jail.d/{name}.local`` file atomically.

    Always writes to ``jail.d/{jail_name}.local``.  If the file already
    exists it is replaced entirely.  The write is atomic: content is
    written to a temp file first, then renamed into place.

    Args:
        config_dir: The fail2ban configuration root directory.
        jail_name: Validated jail name (used as filename stem).
        enabled: Value to write for ``enabled =``.
        overrides: Optional setting overrides (bantime, findtime, maxretry,
                   port, logpath).

    Raises:
        ConfigWriteError: If writing fails.
    """
    jail_d = config_dir / "jail.d"
    try:
        jail_d.mkdir(parents=True, exist_ok=True)
    except OSError as exc:
        raise ConfigWriteError(
            f"Cannot create jail.d directory: {exc}"
        ) from exc

    local_path = jail_d / f"{jail_name}.local"

    lines: list[str] = [
        "# Managed by BanGUI — do not edit manually",
        "",
        f"[{jail_name}]",
        "",
        f"enabled = {'true' if enabled else 'false'}",
    ]

    if overrides.get("bantime") is not None:
        lines.append(f"bantime  = {overrides['bantime']}")
    if overrides.get("findtime") is not None:
        lines.append(f"findtime = {overrides['findtime']}")
    if overrides.get("maxretry") is not None:
        lines.append(f"maxretry = {overrides['maxretry']}")
    if overrides.get("port") is not None:
        lines.append(f"port     = {overrides['port']}")
    if overrides.get("logpath"):
        paths: list[str] = overrides["logpath"]
        if paths:
            lines.append(f"logpath  = {paths[0]}")
            for p in paths[1:]:
                lines.append(f"           {p}")

    content = "\n".join(lines) + "\n"

    try:
        with tempfile.NamedTemporaryFile(
            mode="w",
            encoding="utf-8",
            dir=jail_d,
            delete=False,
            suffix=".tmp",
        ) as tmp:
            tmp.write(content)
            tmp_name = tmp.name
        os.replace(tmp_name, local_path)
    except OSError as exc:
        # Clean up temp file if rename failed.
        with contextlib.suppress(OSError):
            os.unlink(tmp_name)  # noqa: F821 — only reachable when tmp_name is set
        raise ConfigWriteError(
            f"Failed to write {local_path}: {exc}"
        ) from exc

    log.info(
        "jail_local_written",
        jail=jail_name,
        path=str(local_path),
        enabled=enabled,
    )


# ---------------------------------------------------------------------------
# Public API
# ---------------------------------------------------------------------------


async def list_inactive_jails(
    config_dir: str,
    socket_path: str,
) -> InactiveJailListResponse:
    """Return all jails defined in config files that are not currently active.

    Parses ``jail.conf``, ``jail.local``, and ``jail.d/`` following the
    fail2ban merge order.  A jail is considered inactive when:

    - Its merged ``enabled`` value is ``false`` (or absent, which defaults to
      ``false`` in fail2ban), **or**
    - Its ``enabled`` value is ``true`` in config but fail2ban does not report
      it as running.

    Args:
        config_dir: Absolute path to the fail2ban configuration directory.
        socket_path: Path to the fail2ban Unix domain socket.

    Returns:
        :class:`~app.models.config.InactiveJailListResponse` with all
        inactive jails.
    """
    loop = asyncio.get_event_loop()
    parsed_result: tuple[dict[str, dict[str, str]], dict[str, str]] = (
        await loop.run_in_executor(None, _parse_jails_sync, Path(config_dir))
    )
    all_jails, source_files = parsed_result
    active_names: set[str] = await _get_active_jail_names(socket_path)

    inactive: list[InactiveJail] = []
    for jail_name, settings in sorted(all_jails.items()):
        if jail_name in active_names:
            # fail2ban reports this jail as running — skip it.
            continue

        source = source_files.get(jail_name, config_dir)
        inactive.append(_build_inactive_jail(jail_name, settings, source))

    log.info(
        "inactive_jails_listed",
        total_defined=len(all_jails),
        active=len(active_names),
        inactive=len(inactive),
    )
    return InactiveJailListResponse(jails=inactive, total=len(inactive))


async def activate_jail(
    config_dir: str,
    socket_path: str,
    name: str,
    req: ActivateJailRequest,
) -> JailActivationResponse:
    """Enable an inactive jail and reload fail2ban.

    Writes ``enabled = true`` (plus any override values from *req*) to
    ``jail.d/{name}.local`` and then triggers a full fail2ban reload so the
    jail starts immediately.

    Args:
        config_dir: Absolute path to the fail2ban configuration directory.
        socket_path: Path to the fail2ban Unix domain socket.
        name: Name of the jail to activate.  Must exist in the parsed config.
        req: Optional override values to write alongside ``enabled = true``.

    Returns:
        :class:`~app.models.config.JailActivationResponse`.

    Raises:
        JailNameError: If *name* contains invalid characters.
        JailNotFoundInConfigError: If *name* is not defined in any config file.
        JailAlreadyActiveError: If fail2ban already reports *name* as running.
        ConfigWriteError: If writing the ``.local`` file fails.
        ~app.utils.fail2ban_client.Fail2BanConnectionError: If the fail2ban
            socket is unreachable during reload.
    """
    _safe_jail_name(name)

    loop = asyncio.get_event_loop()
    all_jails, _source_files = await loop.run_in_executor(
        None, _parse_jails_sync, Path(config_dir)
    )

    if name not in all_jails:
        raise JailNotFoundInConfigError(name)

    active_names = await _get_active_jail_names(socket_path)
    if name in active_names:
        raise JailAlreadyActiveError(name)

    overrides: dict[str, Any] = {
        "bantime": req.bantime,
        "findtime": req.findtime,
        "maxretry": req.maxretry,
        "port": req.port,
        "logpath": req.logpath,
    }

    await loop.run_in_executor(
        None,
        _write_local_override_sync,
        Path(config_dir),
        name,
        True,
        overrides,
    )

    try:
        await jail_service.reload_all(socket_path)
    except Exception as exc:  # noqa: BLE001
        log.warning("reload_after_activate_failed", jail=name, error=str(exc))

    log.info("jail_activated", jail=name)
    return JailActivationResponse(
        name=name,
        active=True,
        message=f"Jail {name!r} activated successfully.",
    )


async def deactivate_jail(
    config_dir: str,
    socket_path: str,
    name: str,
) -> JailActivationResponse:
    """Disable an active jail and reload fail2ban.

    Writes ``enabled = false`` to ``jail.d/{name}.local`` and triggers a
    full fail2ban reload so the jail stops immediately.

    Args:
        config_dir: Absolute path to the fail2ban configuration directory.
        socket_path: Path to the fail2ban Unix domain socket.
        name: Name of the jail to deactivate.  Must exist in the parsed config.

    Returns:
        :class:`~app.models.config.JailActivationResponse`.

    Raises:
        JailNameError: If *name* contains invalid characters.
        JailNotFoundInConfigError: If *name* is not defined in any config file.
        JailAlreadyInactiveError: If fail2ban already reports *name* as not
            running.
        ConfigWriteError: If writing the ``.local`` file fails.
        ~app.utils.fail2ban_client.Fail2BanConnectionError: If the fail2ban
            socket is unreachable during reload.
    """
    _safe_jail_name(name)

    loop = asyncio.get_event_loop()
    all_jails, _source_files = await loop.run_in_executor(
        None, _parse_jails_sync, Path(config_dir)
    )

    if name not in all_jails:
        raise JailNotFoundInConfigError(name)

    active_names = await _get_active_jail_names(socket_path)
    if name not in active_names:
        raise JailAlreadyInactiveError(name)

    await loop.run_in_executor(
        None,
        _write_local_override_sync,
        Path(config_dir),
        name,
        False,
        {},
    )

    try:
        await jail_service.reload_all(socket_path)
    except Exception as exc:  # noqa: BLE001
        log.warning("reload_after_deactivate_failed", jail=name, error=str(exc))

    log.info("jail_deactivated", jail=name)
    return JailActivationResponse(
        name=name,
        active=False,
        message=f"Jail {name!r} deactivated successfully.",
    )


# ---------------------------------------------------------------------------
# Filter discovery helpers (Task 2.1)
# ---------------------------------------------------------------------------

# Allowlist pattern for filter names used in path construction.
_SAFE_FILTER_NAME_RE: re.Pattern[str] = re.compile(
    r"^[A-Za-z0-9][A-Za-z0-9._-]{0,127}$"
)


class FilterNotFoundError(Exception):
    """Raised when the requested filter name is not found in ``filter.d/``."""

    def __init__(self, name: str) -> None:
        """Initialise with the filter name that was not found.

        Args:
            name: The filter name that could not be located.
        """
        self.name: str = name
        super().__init__(f"Filter not found: {name!r}")


def _extract_filter_base_name(filter_raw: str) -> str:
    """Extract the base filter name from a raw fail2ban filter string.

    fail2ban jail configs may specify a filter with an optional mode suffix,
    e.g. ``sshd``, ``sshd[mode=aggressive]``, or
    ``%(__name__)s[mode=%(mode)s]``.  This function strips the ``[…]`` mode
    block and any leading/trailing whitespace to return just the file-system
    base name used to look up ``filter.d/{name}.conf``.

    Args:
        filter_raw: Raw ``filter`` value from a jail config (already
                    with ``%(__name__)s`` substituted by the caller).

    Returns:
        Base filter name, e.g. ``"sshd"``.
    """
    bracket = filter_raw.find("[")
    if bracket != -1:
        return filter_raw[:bracket].strip()
    return filter_raw.strip()


def _build_filter_to_jails_map(
    all_jails: dict[str, dict[str, str]],
    active_names: set[str],
) -> dict[str, list[str]]:
    """Return a mapping of filter base name → list of active jail names.

    Iterates over every jail whose name is in *active_names*, resolves its
    ``filter`` config key, and records the jail against the base filter name.

    Args:
        all_jails: Merged jail config dict — ``{jail_name: {key: value}}``.
        active_names: Set of jail names currently running in fail2ban.

    Returns:
        ``{filter_base_name: [jail_name, …]}``.
    """
    mapping: dict[str, list[str]] = {}
    for jail_name, settings in all_jails.items():
        if jail_name not in active_names:
            continue
        raw_filter = settings.get("filter", "")
        mode = settings.get("mode", "normal")
        resolved = _resolve_filter(raw_filter, jail_name, mode) if raw_filter else jail_name
        base = _extract_filter_base_name(resolved)
        if base:
            mapping.setdefault(base, []).append(jail_name)
    return mapping


def _parse_filters_sync(
    filter_d: Path,
) -> list[tuple[str, str, str, bool]]:
    """Synchronously scan ``filter.d/`` and return per-filter tuples.

    Each tuple contains:

    - ``name``          — filter base name (``"sshd"``).
    - ``filename``      — actual filename (``"sshd.conf"``).
    - ``content``       — merged file content (``conf`` overridden by ``local``).
    - ``has_local``     — whether a ``.local`` override exists.

    Args:
        filter_d: Path to the ``filter.d`` directory.

    Returns:
        List of ``(name, filename, content, has_local)`` tuples, sorted by name.
    """
    if not filter_d.is_dir():
        log.warning("filter_d_not_found", path=str(filter_d))
        return []

    results: list[tuple[str, str, str, bool]] = []
    for conf_path in sorted(filter_d.glob("*.conf")):
        if not conf_path.is_file():
            continue
        name = conf_path.stem
        filename = conf_path.name
        local_path = conf_path.with_suffix(".local")
        has_local = local_path.is_file()

        try:
            content = conf_path.read_text(encoding="utf-8")
        except OSError as exc:
            log.warning(
                "filter_read_error", name=name, path=str(conf_path), error=str(exc)
            )
            continue

        if has_local:
            try:
                local_content = local_path.read_text(encoding="utf-8")
                # Append local content after conf so configparser reads local
                # values last (higher priority).
                content = content + "\n" + local_content
            except OSError as exc:
                log.warning(
                    "filter_local_read_error",
                    name=name,
                    path=str(local_path),
                    error=str(exc),
                )

        results.append((name, filename, content, has_local))

    log.debug("filters_scanned", count=len(results), filter_d=str(filter_d))
    return results


# ---------------------------------------------------------------------------
# Public API — filter discovery (Task 2.1)
# ---------------------------------------------------------------------------


async def list_filters(
    config_dir: str,
    socket_path: str,
) -> FilterListResponse:
    """Return all available filters from ``filter.d/`` with active/inactive status.

    Scans ``{config_dir}/filter.d/`` for ``.conf`` files, merges any
    corresponding ``.local`` overrides, parses each file into a
    :class:`~app.models.config.FilterConfig`, and cross-references with the
    currently running jails to determine which filters are active.

    A filter is considered *active* when its base name matches the ``filter``
    field of at least one currently running jail.

    Args:
        config_dir: Absolute path to the fail2ban configuration directory.
        socket_path: Path to the fail2ban Unix domain socket.

    Returns:
        :class:`~app.models.config.FilterListResponse` with all filters
        sorted alphabetically, active ones carrying non-empty
        ``used_by_jails`` lists.
    """
    filter_d = Path(config_dir) / "filter.d"
    loop = asyncio.get_event_loop()

    # Run the synchronous scan in a thread-pool executor.
    raw_filters: list[tuple[str, str, str, bool]] = await loop.run_in_executor(
        None, _parse_filters_sync, filter_d
    )

    # Fetch active jail names and their configs concurrently.
    all_jails_result, active_names = await asyncio.gather(
        loop.run_in_executor(None, _parse_jails_sync, Path(config_dir)),
        _get_active_jail_names(socket_path),
    )
    all_jails, _source_files = all_jails_result

    filter_to_jails = _build_filter_to_jails_map(all_jails, active_names)

    filters: list[FilterConfig] = []
    for name, filename, content, has_local in raw_filters:
        conf_path = filter_d / filename
        cfg = conffile_parser.parse_filter_file(
            content, name=name, filename=filename
        )
        used_by = sorted(filter_to_jails.get(name, []))
        filters.append(
            FilterConfig(
                name=cfg.name,
                filename=cfg.filename,
                before=cfg.before,
                after=cfg.after,
                variables=cfg.variables,
                prefregex=cfg.prefregex,
                failregex=cfg.failregex,
                ignoreregex=cfg.ignoreregex,
                maxlines=cfg.maxlines,
                datepattern=cfg.datepattern,
                journalmatch=cfg.journalmatch,
                active=len(used_by) > 0,
                used_by_jails=used_by,
                source_file=str(conf_path),
                has_local_override=has_local,
            )
        )

    log.info("filters_listed", total=len(filters), active=sum(1 for f in filters if f.active))
    return FilterListResponse(filters=filters, total=len(filters))


async def get_filter(
    config_dir: str,
    socket_path: str,
    name: str,
) -> FilterConfig:
    """Return a single filter from ``filter.d/`` with active/inactive status.

    Reads ``{config_dir}/filter.d/{name}.conf``, merges any ``.local``
    override, and enriches the parsed :class:`~app.models.config.FilterConfig`
    with ``active``, ``used_by_jails``, ``source_file``, and
    ``has_local_override``.

    Args:
        config_dir: Absolute path to the fail2ban configuration directory.
        socket_path: Path to the fail2ban Unix domain socket.
        name: Filter base name (e.g. ``"sshd"`` or ``"sshd.conf"``).

    Returns:
        :class:`~app.models.config.FilterConfig` with status fields populated.

    Raises:
        FilterNotFoundError: If no ``{name}.conf`` file exists in
            ``filter.d/``.
    """
    # Normalise — strip extension if provided.
    base_name = name[:-5] if name.endswith(".conf") else name

    filter_d = Path(config_dir) / "filter.d"
    conf_path = filter_d / f"{base_name}.conf"
    local_path = conf_path.with_suffix(".local")
    loop = asyncio.get_event_loop()

    def _read() -> tuple[str, bool]:
        if not conf_path.is_file():
            raise FilterNotFoundError(base_name)
        content = conf_path.read_text(encoding="utf-8")
        has_local = local_path.is_file()
        if has_local:
            try:
                content += "\n" + local_path.read_text(encoding="utf-8")
            except OSError as exc:
                log.warning(
                    "filter_local_read_error",
                    name=base_name,
                    path=str(local_path),
                    error=str(exc),
                )
        return content, has_local

    content, has_local = await loop.run_in_executor(None, _read)

    cfg = conffile_parser.parse_filter_file(
        content, name=base_name, filename=f"{base_name}.conf"
    )

    all_jails_result, active_names = await asyncio.gather(
        loop.run_in_executor(None, _parse_jails_sync, Path(config_dir)),
        _get_active_jail_names(socket_path),
    )
    all_jails, _source_files = all_jails_result
    filter_to_jails = _build_filter_to_jails_map(all_jails, active_names)

    used_by = sorted(filter_to_jails.get(base_name, []))
    log.info("filter_fetched", name=base_name, active=len(used_by) > 0)
    return FilterConfig(
        name=cfg.name,
        filename=cfg.filename,
        before=cfg.before,
        after=cfg.after,
        variables=cfg.variables,
        prefregex=cfg.prefregex,
        failregex=cfg.failregex,
        ignoreregex=cfg.ignoreregex,
        maxlines=cfg.maxlines,
        datepattern=cfg.datepattern,
        journalmatch=cfg.journalmatch,
        active=len(used_by) > 0,
        used_by_jails=used_by,
        source_file=str(conf_path),
        has_local_override=has_local,
    )