import Foundation
import Darwin

/// Best-effort reverse DNS (IP -> PTR hostname). Blocking — call it off the main
/// thread. Returns nil when there's no PTR record (we don't want a numeric echo).
public enum ReverseDNS {

    public static func resolve(_ ip: String) -> String? {
        var storage = sockaddr_storage()
        var length: socklen_t = 0

        if ip.contains(":") {
            var sa = sockaddr_in6()
            sa.sin6_family = sa_family_t(AF_INET6)
            sa.sin6_len = UInt8(MemoryLayout<sockaddr_in6>.size)
            guard inet_pton(AF_INET6, ip, &sa.sin6_addr) == 1 else { return nil }
            withUnsafeBytes(of: &sa) { src in
                withUnsafeMutableBytes(of: &storage) { dst in
                    dst.copyMemory(from: UnsafeRawBufferPointer(rebasing: src[0..<MemoryLayout<sockaddr_in6>.size]))
                }
            }
            length = socklen_t(MemoryLayout<sockaddr_in6>.size)
        } else {
            var sa = sockaddr_in()
            sa.sin_family = sa_family_t(AF_INET)
            sa.sin_len = UInt8(MemoryLayout<sockaddr_in>.size)
            guard inet_pton(AF_INET, ip, &sa.sin_addr) == 1 else { return nil }
            withUnsafeBytes(of: &sa) { src in
                withUnsafeMutableBytes(of: &storage) { dst in
                    dst.copyMemory(from: UnsafeRawBufferPointer(rebasing: src[0..<MemoryLayout<sockaddr_in>.size]))
                }
            }
            length = socklen_t(MemoryLayout<sockaddr_in>.size)
        }

        var host = [CChar](repeating: 0, count: Int(NI_MAXHOST))
        let result = withUnsafePointer(to: &storage) { ptr in
            ptr.withMemoryRebound(to: sockaddr.self, capacity: 1) { sa in
                getnameinfo(sa, length, &host, socklen_t(host.count), nil, 0, NI_NAMEREQD)
            }
        }
        guard result == 0 else { return nil }
        let name = String(cString: host)
        return name.isEmpty ? nil : name
    }
}