//
// Copyright 2026 Aarav Ravindra Kharade
//
// 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
//
//     http://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 CSystem

// ═══════════════════════════════════════════════════════════════════
//  ArkOS Network Service
// ═══════════════════════════════════════════════════════════════════
//  Background service that manages network connectivity for the OS.
//  Handles interface monitoring, DHCP client triggering, and DNS
//  resolver configuration.
//
//  Lifecycle:
//    1. Enumerate all non-loopback interfaces via /sys/class/net/
//    2. Bring up interfaces via ip(8) / ifconfig(8) if available
//    3. Trigger DHCP on wired interfaces via udhcpc (from busybox)
//    4. Monitor link state changes and re-acquire addresses as needed
//    5. Configure /etc/resolv.conf with received nameservers
//
//  In QEMU user-mode networking, the virtual NIC gets an IP via
//  QEMU's built-in DHCP server (typically 10.0.2.15).
// ═══════════════════════════════════════════════════════════════════

/// Manages network interface lifecycle and connectivity.
public class NetworkService {

    /// Singleton instance (started by ServiceManager or arkrt main).
    public static let shared = NetworkService()

    /// Polling interval for link state monitoring (seconds).
    private let pollInterval: UInt32 = 5

    /// Whether the service is currently running its monitor loop.
    private var isRunning = false

    private init() {}

    // ── Interface Bring-Up ────────────────────────────────────────

    /// Attempts to bring up all non-loopback network interfaces and
    /// acquire IP addresses via DHCP.
    public func configureInterfaces() {
        let interfaces = NetworkManager.getInterfaceNames()

        if interfaces.isEmpty {
            LogManager.log("No network interfaces found", level: .warn, component: "ark.service.network")
            return
        }

        LogManager.log("Found \(interfaces.count) interface(s): \(interfaces)", component: "ark.service.network")

        for iface in interfaces {
            bringUpInterface(iface)
        }
    }

    /// Brings up a single interface and starts DHCP if it's wired.
    private func bringUpInterface(_ name: String) {
        LogManager.log("Bringing up interface '\(name)'...", component: "ark.service.network")

        // Try to bring the interface up using ip(8) first, fall back to ifconfig
        let ipResult = runCommand("/sbin/ip link set \(name) up")
        if ipResult != 0 {
            let _ = runCommand("/sbin/ifconfig \(name) up")
        }

        // Check if the interface has carrier (cable plugged / associated)
        let carrierPath = "/sys/class/net/\(name)/carrier"
        if let c = readFileContents(carrierPath), trimWhitespace(c) == "1" {
            LogManager.log("'\(name)' has carrier — requesting DHCP lease...", component: "ark.service.network")
            requestDHCP(interface: name)
        } else {
            LogManager.log("'\(name)' has no carrier — will retry on link change", component: "ark.service.network")
        }
    }

    // ── DHCP Client ───────────────────────────────────────────────

    /// Runs udhcpc (busybox DHCP client) to acquire an IP address.
    /// Uses a short timeout to avoid blocking the boot process.
    private func requestDHCP(interface name: String) {
        // udhcpc flags:
        //   -i <interface>  — target interface
        //   -n              — exit if lease not obtained
        //   -q              — quit after obtaining lease
        //   -t 3            — try 3 times
        //   -T 2            — 2 second timeout between tries
        let result = runCommand("/sbin/udhcpc -i \(name) -n -q -t 3 -T 2 2>/dev/null")
        if result == 0 {
            LogManager.log("DHCP lease acquired on '\(name)'", component: "ark.service.network")
            configureDNS()
        } else {
            LogManager.log("DHCP failed on '\(name)' — will retry", level: .warn, component: "ark.service.network")
        }
    }

    // ── DNS Configuration ─────────────────────────────────────────

    /// Writes a basic resolv.conf with common public DNS servers.
    /// In production, this would be populated by DHCP lease data.
    private func configureDNS() {
        let resolv = """
        # Generated by ArkOS NetworkService
        nameserver 8.8.8.8
        nameserver 8.8.4.4
        nameserver 1.1.1.1
        """

        if let fp = fopen("/etc/resolv.conf", "w") {
            fputs(resolv, fp)
            fclose(fp)
            LogManager.log("DNS resolver configured", component: "ark.service.network")
        }
    }

    // ── Link State Monitor ────────────────────────────────────────

    /// Starts the background link monitoring loop. Checks for carrier
    /// changes and re-runs DHCP when a cable is plugged in.
    public func startMonitoring() {
        guard !isRunning else { return }
        isRunning = true

        LogManager.log("Starting link state monitor (poll every \(pollInterval)s)", component: "ark.service.network")

        ResourceController.executeOnCorePool { [self] in
            var previousState: [String: Bool] = [:]

            while isRunning {
                let interfaces = NetworkManager.getAllInterfaces()

                for iface in interfaces {
                    let wasUp = previousState[iface.name] ?? false
                    let isNowUp = iface.hasCarrier

                    // Detect cable plug-in events
                    if isNowUp && !wasUp {
                        LogManager.log("Carrier detected on '\(iface.name)' — acquiring address...", component: "ark.service.network")
                        requestDHCP(interface: iface.name)
                    }

                    // Detect cable unplug events
                    if !isNowUp && wasUp {
                        LogManager.log("Carrier lost on '\(iface.name)'", level: .warn, component: "ark.service.network")
                    }

                    previousState[iface.name] = isNowUp
                }

                sleep(pollInterval)
            }
        }
    }

    /// Stops the link monitoring loop.
    public func stopMonitoring() {
        isRunning = false
        LogManager.log("Link monitor stopped", component: "ark.service.network")
    }

    // ── Utility ───────────────────────────────────────────────────

    /// Execute a shell command and return its exit code.
    @discardableResult
    private func runCommand(_ cmd: String) -> Int32 {
        let result = system(cmd)
        return result
    }
}