//===----------------------------------------------------------------------===//
// Copyright © 2025-2026 Apple Inc. and the Containerization project authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//   https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//===----------------------------------------------------------------------===//

import ContainerizationError
import ContainerizationExtras

/// DNS configuration for a container. The values will be used to
/// construct /etc/resolv.conf for a given container.
public struct DNS: Sendable {
    /// The set of default nameservers to use if none are provided
    /// in the constructor.
    public static let defaultNameservers = ["1.1.1.1"]

    /// The nameservers a container should use.
    public var nameservers: [String]
    /// The DNS domain to use.
    public var domain: String?
    /// The DNS search domains to use.
    public var searchDomains: [String]
    /// The DNS options to use.
    public var options: [String]

    public init(
        nameservers: [String] = defaultNameservers,
        domain: String? = nil,
        searchDomains: [String] = [],
        options: [String] = []
    ) {
        self.nameservers = nameservers
        self.domain = domain
        self.searchDomains = searchDomains
        self.options = options
    }

    /// Validates the DNS configuration.
    ///
    /// Ensures that all nameserver entries are valid IPv4 or IPv6 addresses.
    /// Arbitrary hostnames are not permitted as nameservers.
    ///
    /// - Throws: ``ContainerizationError`` with code `.invalidArgument` if
    ///   any nameserver is not a valid IP address.
    public func validate() throws {
        for nameserver in nameservers {
            let isValidIPv4 = (try? IPv4Address(nameserver)) != nil
            let isValidIPv6 = (try? IPv6Address(nameserver)) != nil
            if !isValidIPv4 && !isValidIPv6 {
                throw ContainerizationError(
                    .invalidArgument,
                    message: "nameserver '\(nameserver)' is not a valid IPv4 or IPv6 address"
                )
            }
        }
    }
}

extension DNS {
    public var resolvConf: String {
        var text = ""

        if !nameservers.isEmpty {
            text += nameservers.map { "nameserver \($0)" }.joined(separator: "\n") + "\n"
        }

        if let domain {
            text += "domain \(domain)\n"
        }

        if !searchDomains.isEmpty {
            text += "search \(searchDomains.joined(separator: " "))\n"
        }

        if !options.isEmpty {
            text += "options \(options.joined(separator: " "))\n"
        }

        return text
    }
}