// SPDX-License-Identifier: MIT
// Copyright © 2018 WireGuard LLC. All Rights Reserved.

import Network
import Foundation

class DNSResolver {
    let endpoints: [Endpoint?]
    let dispatchGroup: DispatchGroup
    var dispatchWorkItems: [DispatchWorkItem]
    static var cache = NSCache<NSString, NSString>()

    init(endpoints: [Endpoint?]) {
        self.endpoints = endpoints
        self.dispatchWorkItems = []
        self.dispatchGroup = DispatchGroup()
    }

    func resolveWithoutNetworkRequests() -> [Endpoint?]? {
        var resolvedEndpoints: [Endpoint?] = Array<Endpoint?>(repeating: nil, count: endpoints.count)
        for (i, endpoint) in self.endpoints.enumerated() {
            guard let endpoint = endpoint else { continue }
            if let resolvedEndpointStringInCache = DNSResolver.cache.object(forKey: endpoint.stringRepresentation() as NSString),
                let resolvedEndpointInCache = Endpoint(from: resolvedEndpointStringInCache as String) {
                resolvedEndpoints[i] = resolvedEndpointInCache
            } else {
                return nil
            }
        }
        return resolvedEndpoints
    }

    func resolve(completionHandler: @escaping ([Endpoint?]?) -> Void) {
        let endpoints = self.endpoints
        let dispatchGroup = self.dispatchGroup
        dispatchWorkItems = []
        var resolvedEndpoints: [Endpoint?] = Array<Endpoint?>(repeating: nil, count: endpoints.count)
        var isResolvedByDNSRequest: [Bool] = Array<Bool>(repeating: false, count: endpoints.count)
        for (i, endpoint) in self.endpoints.enumerated() {
            guard let endpoint = endpoint else { continue }
            if let resolvedEndpointStringInCache = DNSResolver.cache.object(forKey: endpoint.stringRepresentation() as NSString),
                let resolvedEndpointInCache = Endpoint(from: resolvedEndpointStringInCache as String) {
                resolvedEndpoints[i] = resolvedEndpointInCache
            } else {
                let workItem = DispatchWorkItem {
                    resolvedEndpoints[i] = DNSResolver.resolveSync(endpoint: endpoint)
                    isResolvedByDNSRequest[i] = true
                }
                dispatchWorkItems.append(workItem)
                DispatchQueue.global(qos: .userInitiated).async(group: dispatchGroup, execute: workItem)
            }
        }
        dispatchGroup.notify(queue: .main) {
            assert(endpoints.count == resolvedEndpoints.count)
            for (i, endpoint) in endpoints.enumerated() {
                guard let endpoint = endpoint, let resolvedEndpoint = resolvedEndpoints[i] else { return }
                if (isResolvedByDNSRequest[i]) {
                    DNSResolver.cache.setObject(resolvedEndpoint.stringRepresentation() as NSString,
                                                forKey: endpoint.stringRepresentation() as NSString)
                }
            }
            let numberOfEndpointsToResolve = endpoints.compactMap { $0 }.count
            let numberOfResolvedEndpoints = resolvedEndpoints.compactMap { $0 }.count
            if (numberOfResolvedEndpoints < numberOfEndpointsToResolve) {
                completionHandler(nil)
            } else {
                completionHandler(resolvedEndpoints)
            }
        }
    }

    func cancel() {
        for workItem in dispatchWorkItems {
            workItem.cancel()
        }
    }

    deinit {
        cancel()
    }
}

extension DNSResolver {
    // Based on DNS resolution code by Jason Donenfeld <jason@zx2c4.com>
    // in parse_endpoint() in src/tools/config.c in the WireGuard codebase
    private static func resolveSync(endpoint: Endpoint) -> Endpoint? {
        var hints = addrinfo(
            ai_flags: 0,
            ai_family: AF_UNSPEC,
            ai_socktype: SOCK_DGRAM, // WireGuard is UDP-only
            ai_protocol: IPPROTO_UDP, // WireGuard is UDP-only
            ai_addrlen: 0,
            ai_canonname: nil,
            ai_addr: nil,
            ai_next: nil)
        var resultPointer = UnsafeMutablePointer<addrinfo>(OpaquePointer(bitPattern: 0))
        switch (endpoint.host) {
        case .name(let name, _):
            // The endpoint is a hostname and needs DNS resolution
            let returnValue = getaddrinfo(
                name.cString(using: .utf8), // Hostname
                "\(endpoint.port)".cString(using: .utf8), // Port
                &hints,
                &resultPointer)
            if (returnValue == 0) {
                // getaddrinfo succeeded
                let ipv4Buffer = UnsafeMutablePointer<Int8>.allocate(capacity: Int(INET_ADDRSTRLEN))
                let ipv6Buffer = UnsafeMutablePointer<Int8>.allocate(capacity: Int(INET6_ADDRSTRLEN))
                var ipv4AddressString: String? = nil
                var ipv6AddressString: String? = nil
                while (resultPointer != nil) {
                    let result = resultPointer!.pointee
                    resultPointer = result.ai_next
                    if (result.ai_family == AF_INET && result.ai_addrlen == INET_ADDRSTRLEN) {
                        if (inet_ntop(result.ai_family, result.ai_addr, ipv4Buffer, result.ai_addrlen) != nil) {
                            ipv4AddressString = String(cString: ipv4Buffer)
                            // If we found an IPv4 address, we can stop
                            break
                        }
                    } else if (result.ai_family == AF_INET6 && result.ai_addrlen == INET6_ADDRSTRLEN) {
                        if (ipv6AddressString != nil) {
                            // If we already have an IPv6 address, we can skip this one
                            continue
                        }
                        if (inet_ntop(result.ai_family, result.ai_addr, ipv6Buffer, result.ai_addrlen) != nil) {
                            ipv6AddressString = String(cString: ipv6Buffer)
                        }
                    }
                }
                ipv4Buffer.deallocate()
                ipv6Buffer.deallocate()
                // We prefer an IPv4 address over an IPv6 address
                if let ipv4AddressString = ipv4AddressString, let ipv4Address = IPv4Address(ipv4AddressString) {
                    return Endpoint(host: NWEndpoint.Host.ipv4(ipv4Address), port: endpoint.port)
                } else if let ipv6AddressString = ipv6AddressString, let ipv6Address = IPv6Address(ipv6AddressString) {
                    return Endpoint(host: NWEndpoint.Host.ipv6(ipv6Address), port: endpoint.port)
                } else {
                    return nil
                }
            } else {
                // getaddrinfo failed
                return nil
            }
        default:
            // The endpoint is already resolved
            return endpoint
        }
    }
}