#!/usr/bin/env python3

# © Copyright 2021-2022, Scott Gasch

"""A caching layer around the kernel's network mapping between IPs and MACs"""


import datetime
import logging
import os
import warnings
from typing import Any, Optional

from overrides import overrides

import argparse_utils
import config
import exec_utils
import file_utils
import persistent
import site_config
import string_utils
from collect.bidict import BiDict

logger = logging.getLogger(__name__)

cfg = config.add_commandline_args(
    f'MAC <--> IP Address mapping table cache ({__file__})',
    'Commandline args related to MAC <--> IP Address mapping',
)
cfg.add_argument(
    '--arper_cache_location',
    default=site_config.get_config().arper_cache_file,
    metavar='FILENAME',
    help='Where to cache the kernel ARP table',
)
cfg.add_argument(
    '--arper_supplimental_cache_location',
    default=site_config.get_config(site_config.other_location()).arper_cache_file,
    metavar='FILENAME',
    help='Where someone else is caching the kernel ARP table',
)
cfg.add_argument(
    '--arper_cache_max_staleness',
    type=argparse_utils.valid_duration,
    default=datetime.timedelta(seconds=60 * 30),
    metavar='DURATION',
    help='Max acceptable age of the kernel arp table cache',
)
cfg.add_argument(
    '--arper_min_entries_to_be_valid',
    type=int,
    default=site_config.get_config().arper_minimum_device_count,
    help='Min number of arp entries to bother persisting.',
)


@persistent.persistent_autoloaded_singleton()  # type: ignore
class Arper(persistent.Persistent):
    """A caching layer around the kernel's network mapping between IPs and
    MACs.  This class restores persisted state that expires
    periodically (see --arper_cache_max_staleness) at program startup
    time.  If it's unable to use the file's contents, it queries the
    kernel (via arp) and uses an auxillary utility called arp-scan to
    query the network.  If it has to do this there's a latency hit but
    it persists the collected data in the cache file.  Either way, the
    class behaves as a global singleton hosting this data thereafter.

    """

    def __init__(
        self,
        cached_local_state: Optional[BiDict] = None,
        cached_supplimental_state: Optional[BiDict] = None,
    ) -> None:
        """For most purposes, ignore the arguments.  Because this is a
        Persistent subclass the decorator will handle invoking our load
        and save methods to read/write persistent state transparently.

        Args:
            cached_local_state: local state to initialize mapping
            cached_supplimental_state: remote state to initialize mapping
        """

        self.state = BiDict()
        if cached_local_state is not None:
            logger.debug('Loading Arper map from cached local state.')
            self.state = cached_local_state
        else:
            logger.debug('No usable cached state; calling /usr/sbin/arp')
            self._update_from_arp_scan()
            self._update_from_arp()
        if len(self.state) < config.config['arper_min_entries_to_be_valid']:
            raise Exception(f'Arper didn\'t find enough entries; only got {len(self.state)}.')
        if cached_supplimental_state is not None:
            logger.debug('Also added %d supplimental entries.', len(cached_supplimental_state))
            for mac, ip in cached_supplimental_state.items():
                self.state[mac] = ip
        for mac, ip in self.state.items():
            logger.debug('%s <-> %s', mac, ip)

    def _update_from_arp_scan(self):
        """Internal method to initialize our state via a call to arp-scan."""

        network_spec = site_config.get_config().network
        try:
            output = exec_utils.cmd(
                f'/usr/local/bin/arp-scan --retry=6 --timeout 350 --backoff=1.4 --random --numeric --plain --ignoredups {network_spec}',
                timeout_seconds=10.0,
            )
        except Exception as e:
            logger.exception(e)
            return
        for line in output.split('\n'):
            ip = string_utils.extract_ip_v4(line)
            mac = string_utils.extract_mac_address(line)
            if ip is not None and mac is not None and mac != 'UNKNOWN' and ip != 'UNKNOWN':
                mac = mac.lower()
                logger.debug('ARPER: %s => %s', mac, ip)
                self.state[mac] = ip

    def _update_from_arp(self):
        """Internal method to initialize our state via a call to arp."""

        try:
            output = exec_utils.cmd('/usr/sbin/arp -a', timeout_seconds=10.0)
        except Exception as e:
            logger.exception(e)
            return
        for line in output.split('\n'):
            ip = string_utils.extract_ip_v4(line)
            mac = string_utils.extract_mac_address(line)
            if ip is not None and mac is not None and mac != 'UNKNOWN' and ip != 'UNKNOWN':
                mac = mac.lower()
                logger.debug('ARPER: %s => %s', mac, ip)
                self.state[mac] = ip

    def get_ip_by_mac(self, mac: str) -> Optional[str]:
        """Given a MAC address, see if we know it's IP address and, if so,
        return it.  If not, return None.

        Args:
            mac: the MAC address to lookup.  Should be formatted like
                 ab:cd:ef:g1:23:45.

        Returns:
            The IPv4 address associated with that MAC address (as a string)
            or None if it's not known.
        """
        m = string_utils.extract_mac_address(mac)
        if not m:
            return None
        m = m.lower()
        if not string_utils.is_mac_address(m):
            return None
        return self.state.get(m, None)

    def get_mac_by_ip(self, ip: str) -> Optional[str]:
        """Given an IPv4 address (as a string), check to see if we know what
        MAC address is associated with it and, if so, return it.  If not,
        return None.

        Args:
            ip: the IPv4 address to look up.

        Returns:
            The associated MAC address, if known.  Or None if not.
        """
        return self.state.inverse.get(ip, None)

    @classmethod
    def _load_state(
        cls,
        cache_file: str,
        freshness_threshold_sec: int,
        state: BiDict,
    ):
        """Internal helper method behind load."""

        if not file_utils.file_is_readable(cache_file):
            logger.debug('Can\'t read %s', cache_file)
            return
        if persistent.was_file_written_within_n_seconds(
            cache_file,
            freshness_threshold_sec,
        ):
            logger.debug('Loading state from %s', cache_file)
            count = 0
            with open(cache_file, 'r') as rf:
                contents = rf.readlines()
                for line in contents:
                    line = line[:-1]
                    logger.debug('ARPER:%s> %s', cache_file, line)
                    (mac, ip) = line.split(',')
                    mac = mac.strip()
                    mac = mac.lower()
                    ip = ip.strip()
                    state[mac] = ip
                    count += 1
        else:
            logger.debug('%s is too stale.', cache_file)

    @classmethod
    @overrides
    def load(cls) -> Any:
        """Internal helper method to fulfull Persistent requirements."""

        local_state: BiDict = BiDict()
        cache_file = config.config['arper_cache_location']
        max_staleness = config.config['arper_cache_max_staleness'].total_seconds()
        logger.debug('Trying to load main arper cache from %s...', cache_file)
        cls._load_state(cache_file, max_staleness, local_state)
        if len(local_state) <= config.config['arper_min_entries_to_be_valid']:
            msg = f'{cache_file} is invalid: only {len(local_state)} entries.  Deleting it.'
            logger.warning(msg)
            warnings.warn(msg, stacklevel=2)
            try:
                os.remove(cache_file)
            except Exception:
                pass

        supplimental_state: BiDict = BiDict()
        cache_file = config.config['arper_supplimental_cache_location']
        max_staleness = config.config['arper_cache_max_staleness'].total_seconds()
        logger.debug('Trying to suppliment arper state from %s', cache_file)
        cls._load_state(cache_file, max_staleness, supplimental_state)
        if len(local_state) > 0:
            return cls(local_state, supplimental_state)
        return None

    @overrides
    def save(self) -> bool:
        """Internal helper method to fulfull Persistent requirements."""

        if len(self.state) > config.config['arper_min_entries_to_be_valid']:
            logger.debug('Persisting state to %s', config.config["arper_cache_location"])
            with file_utils.FileWriter(config.config['arper_cache_location']) as wf:
                for (mac, ip) in self.state.items():
                    mac = mac.lower()
                    print(f'{mac}, {ip}', file=wf)
            return True
        else:
            logger.warning(
                'Only saw %d entries; needed at least %d to bother persisting.',
                len(self.state),
                config.config["arper_min_entries_to_be_valid"],
            )
            return False