<?php

namespace Amp\Dns;

use Amp\Cache\ArrayCache;
use Amp\Cache\Cache;
use Amp\Loop;
use Amp\MultiReasonException;
use Amp\Promise;
use Amp\Success;
use LibDNS\Messages\Message;
use LibDNS\Messages\MessageTypes;
use LibDNS\Records\Question;
use LibDNS\Records\QuestionFactory;
use function Amp\call;

class BasicResolver implements Resolver {
    const CACHE_PREFIX = "amphp.dns.";

    /** @var \Amp\Dns\ConfigLoader */
    private $configLoader;

    /** @var \LibDNS\Records\QuestionFactory */
    private $questionFactory;

    /** @var \Amp\Dns\Config|null */
    private $config;

    /** @var Cache */
    private $cache;

    /** @var Server[] */
    private $servers = [];

    /** @var Promise[] */
    private $pendingServers = [];

    /** @var Promise[] */
    private $pendingQueries = [];

    /** @var string */
    private $gcWatcher;

    public function __construct(Cache $cache = null, ConfigLoader $configLoader = null) {
        $this->cache = $cache ?? new ArrayCache;
        $this->configLoader = $configLoader ?? \stripos(PHP_OS, "win") === 0
                ? new WindowsConfigLoader
                : new UnixConfigLoader;

        $this->questionFactory = new QuestionFactory;

        $this->gcWatcher = Loop::repeat(5000, function () {
            $now = \time();

            foreach ($this->servers as $server) {
                if ($server->getLastActivity() < $now - 60) {
                    unset($this->servers);
                }
            }
        });

        Loop::unreference($this->gcWatcher);
    }

    public function __destruct() {
        Loop::cancel($this->gcWatcher);
    }

    /** @inheritdoc */
    public function resolve(string $name, int $typeRestriction = null): Promise {
        if ($typeRestriction !== null && $typeRestriction !== Record::A && $typeRestriction !== Record::AAAA) {
            throw new \Error("Invalid value for parameter 2: null|Record::A|Record::AAAA expected");
        }

        return call(function () use ($name, $typeRestriction) {
            if (!$this->config) {
                $this->config = yield $this->configLoader->loadConfig();
            }

            $inAddr = @\inet_pton($name);

            if ($inAddr !== false) {
                // It's already a valid IP, don't query, immediately return
                if ($typeRestriction) {
                    if ($typeRestriction === Record::A && isset($inAddr[4])) {
                        throw new ResolutionException("Got an IPv6 address, but type is restricted to IPv4");
                    }

                    if ($typeRestriction === Record::AAAA && !isset($inAddr[4])) {
                        throw new ResolutionException("Got an IPv4 address, but type is restricted to IPv6");
                    }
                }

                return [
                    new Record($name, isset($inAddr[4]) ? Record::AAAA : Record::A, null),
                ];
            }

            $name = normalizeDnsName($name);

            if ($records = $this->queryHosts($name, $typeRestriction)) {
                return $records;
            }

            for ($redirects = 0; $redirects < 5; $redirects++) {
                try {
                    if ($typeRestriction) {
                        $records = yield $this->query($name, $typeRestriction);
                    } else {
                        try {
                            list(, $records) = yield Promise\some([
                                $this->query($name, Record::A),
                                $this->query($name, Record::AAAA),
                            ]);

                            $records = \array_merge(...$records);
                        } catch (MultiReasonException $e) {
                            foreach ($e->getReasons() as $reason) {
                                if ($reason instanceof NoRecordException) {
                                    throw $reason;
                                }
                            }

                            throw new ResolutionException("All query attempts failed", 0, $e);
                        }
                    }
                } catch (NoRecordException $e) {
                    try {
                        /** @var Record[] $cnameRecords */
                        $cnameRecords = yield $this->query($name, Record::CNAME);
                        $name = $cnameRecords[0]->getValue();
                        continue;
                    } catch (NoRecordException $e) {
                        /** @var Record[] $dnameRecords */
                        $dnameRecords = yield $this->query($name, Record::DNAME);
                        $name = $dnameRecords[0]->getValue();
                        continue;
                    }
                }
            }

            return $records;
        });
    }

    private function queryHosts(string $name, int $typeRestriction = null): array {
        $hosts = $this->config->getKnownHosts();
        $records = [];

        $returnIPv4 = $typeRestriction === null || $typeRestriction === Record::A;
        $returnIPv6 = $typeRestriction === null || $typeRestriction === Record::AAAA;

        if ($returnIPv4 && isset($hosts[Record::A][$name])) {
            $records[] = new Record($hosts[Record::A][$name], Record::A, null);
        }

        if ($returnIPv6 && isset($hosts[Record::AAAA][$name])) {
            $records[] = new Record($hosts[Record::AAAA][$name], Record::AAAA, null);
        }

        return $records;
    }

    /** @inheritdoc */
    public function query(string $name, int $type): Promise {
        if (isset($this->pendingQueries[$type . " " . $name])) {
            return $this->pendingQueries[$type . " " . $name];
        }

        $promise = call(function () use ($name, $type) {
            if (!$this->config) {
                $this->config = yield $this->configLoader->loadConfig();
            }

            $name = $this->normalizeName($name, $type);
            $question = $this->createQuestion($name, $type);

            if (null !== $cachedValue = yield $this->cache->get($this->getCacheKey($name, $type))) {
                return $this->decodeCachedResult($name, $type, $cachedValue);
            }

            $nameservers = $this->config->getNameservers();
            $attempts = $this->config->getAttempts();
            $receivedTruncatedResponse = false;

            for ($attempt = 0; $attempt < $attempts; ++$attempt) {
                $i = $attempt % \count($nameservers);
                $protocol = $receivedTruncatedResponse ? "tcp" : "udp";

                /** @var \Amp\Dns\Server $server */
                $server = yield $this->getServer($protocol . "://" . $nameservers[$i]);

                if (!$server->isAlive()) {
                    unset($this->servers[$protocol . "://" . $nameservers[$i]]);

                    /** @var \Amp\Dns\Server $server */
                    $server = yield $this->getServer($protocol . "://" . $nameservers[$i]);
                }

                /** @var Message $response */
                $response = yield $server->ask($question, $this->config->getTimeout());
                $this->assertAcceptableResponse($response);

                if ($response->isTruncated()) {
                    if (!$receivedTruncatedResponse) {
                        // Retry with TCP, don't count attempt
                        $receivedTruncatedResponse = true;
                        $attempt--;
                        continue;
                    }

                    throw new ResolutionException("Server returned truncated response");
                }

                $answers = $response->getAnswerRecords();
                $result = [];
                $ttls = [];

                /** @var \LibDNS\Records\Resource $record */
                foreach ($answers as $record) {
                    $recordType = $record->getType();

                    $result[$recordType][] = (string) $record->getData();
                    $ttls[$recordType] = \min($ttls[$recordType] ?? \PHP_INT_MAX, $record->getTTL());
                }

                foreach ($result as $recordType => $records) {
                    // We don't care here whether storing in the cache fails
                    $this->cache->set($this->getCacheKey($name, $recordType), \json_encode($records), $ttls[$recordType]);
                }

                if (!isset($result[$type])) {
                    // "it MUST NOT cache it for longer than five (5) minutes" per RFC 2308 section 7.1
                    $this->cache->set($this->getCacheKey($name, $type), \json_encode([]), 300);
                    throw new NoRecordException("No records returned for {$name}");
                }

                return \array_map(function ($data) use ($type, $ttls) {
                    return new Record($data, $type, $ttls[$type]);
                }, $result[$type]);
            }

            throw new ResolutionException("No response from any nameserver after {$attempts} attempts");
        });

        $this->pendingQueries[$type . " " . $name] = $promise;
        $promise->onResolve(function () use ($name, $type) {
            unset($this->pendingQueries[$type . " " . $name]);
        });

        return $promise;
    }

    /**
     * Reloads the configuration in the background.
     *
     * Once it's finished, the configuration will be used for new requests.
     *
     * @return Promise
     */
    public function reloadConfig(): Promise {
        return call(function () {
            $this->config = yield $this->configLoader->loadConfig();
        });
    }

    /**
     * @param string $name
     * @param int    $type
     *
     * @return \LibDNS\Records\Question
     */
    private function createQuestion(string $name, int $type): Question {
        if (0 > $type || 0xffff < $type) {
            $message = \sprintf('%d does not correspond to a valid record type (must be between 0 and 65535).', $type);
            throw new \Error($message);
        }

        $question = $this->questionFactory->create($type);
        $question->setName($name);

        return $question;
    }

    private function getCacheKey(string $name, int $type): string {
        return self::CACHE_PREFIX . $name . "#" . $type;
    }

    private function decodeCachedResult(string $name, string $type, string $encoded) {
        $decoded = \json_decode($encoded, true);

        if (!$decoded) {
            throw new NoRecordException("No records returned for {$name} (cached result)");
        }

        $result = [];

        foreach ($decoded as $data) {
            $result[] = new Record($data, $type);
        }

        return $result;
    }

    private function normalizeName(string $name, int $type) {
        if ($type === Record::PTR) {
            if (($packedIp = @inet_pton($name)) !== false) {
                if (isset($packedIp[4])) { // IPv6
                    $name = \wordwrap(\strrev(\bin2hex($packedIp)), 1, ".", true) . ".ip6.arpa";
                } else { // IPv4
                    $name = \inet_ntop(\strrev($packedIp)) . ".in-addr.arpa";
                }
            }
        } elseif (\in_array($type, [Record::A, Record::AAAA])) {
            $name = normalizeDnsName($name);
        }

        return $name;
    }

    private function getServer($uri): Promise {
        if (isset($this->servers[$uri])) {
            return new Success($this->servers[$uri]);
        }

        if (isset($this->pendingServers[$uri])) {
            return $this->pendingServers[$uri];
        }

        if (\substr($uri, 0, 3) === "udp") {
            $server = UdpServer::connect($uri);
        } else {
            $server = TcpServer::connect($uri);
        }

        $server->onResolve(function ($error, $server) use ($uri) {
            if ($error) {
                unset($this->pendingServers[$uri]);
            } else {
                $this->servers[$uri] = $server;
            }
        });

        return $server;
    }

    private function assertAcceptableResponse(Message $response) {
        if ($response->getResponseCode() !== 0) {
            throw new ResolutionException(\sprintf("Server returned error code: %d", $response->getResponseCode()));
        }

        if ($response->getType() !== MessageTypes::RESPONSE) {
            throw new ResolutionException("Invalid server reply; expected RESPONSE but received QUERY");
        }
    }
}