//
// 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 IPC Server
// ═══════════════════════════════════════════════════════════════════
//  Unix domain socket server that provides the primary communication
//  channel between arkrt (the system daemon) and all other OS
//  components (display server, UI apps, background services).
//
//  Protocol:
//    Request:  [Command_ID (2B)][Payload_Length (4B)][Payload_Data]
//    Response: [Command_ID (2B)][Payload_Length (4B)][Payload_Data]
//
//  Security:
//    - Socket file at /dev/arkrt.sock with mode 0770 (root + ark group)
//    - Each connection is handled in an isolated thread from the pool
//    - Connections are dropped after 30 seconds of inactivity
//
// ═══════════════════════════════════════════════════════════════════

// Cross-platform autoreleasepool shim for Linux (no Objective-C runtime).
#if !canImport(ObjectiveC)
@discardableResult
public func autoreleasepool<Result>(_ block: () throws -> Result) rethrows -> Result {
    return try block()
}
#endif

// ── IPC Constants ─────────────────────────────────────────────────

/// IPC protocol configuration.
private enum IPCConfig {
    /// Path to the Unix domain socket file.
    static let socketPath = "/dev/arkrt.sock"

    /// Maximum pending connections in the listen queue.
    static let backlogSize: Int32 = 16

    /// Read buffer size for each client connection (4 KB).
    static let bufferSize = 4096

    /// Socket file permissions (owner + group read/write, no world access).
    public static let socketMode: mode_t = 0o770
}

// ── IPC Server ────────────────────────────────────────────────────

/// The IPC server listens on a Unix domain socket and dispatches
/// incoming requests to the CommandRouter for processing.
public class IPCServer {
    private var serverFd: Int32 = -1

    public init() {}

    /// Binds and starts listening on the Unix domain socket.
    /// The accept loop runs on the ResourceController thread pool
    /// so the main thread remains free for other work.
    public func start() {
        LogManager.log("Initializing Unix domain socket at \(IPCConfig.socketPath)...", component: "ark.sys.service")

        // Remove any stale socket file from a previous boot
        unlink(IPCConfig.socketPath)

        // Create the socket (AF_UNIX = 1, SOCK_STREAM = 1)
        #if canImport(Glibc)
        let fd = socket(AF_UNIX, Int32(SOCK_STREAM.rawValue), 0)
        #else
        let fd = socket(AF_UNIX, Int32(SOCK_STREAM), 0)
        #endif
        if fd < 0 {
            LogManager.log("socket() failed: failed", level: .error, component: "ark.sys.service")
            return
        }
        self.serverFd = fd

        // Configure the Unix domain socket address
        var addr = sockaddr_un()
        addr.sun_family = sa_family_t(AF_UNIX)

        let pathBytes = IPCConfig.socketPath.utf8CString
        withUnsafeMutablePointer(to: &addr.sun_path) { sunPathPtr in
            let rawPtr = UnsafeMutableRawPointer(sunPathPtr).assumingMemoryBound(to: CChar.self)
            for i in 0..<min(pathBytes.count, 108) {
                rawPtr[i] = pathBytes[i]
            }
        }

        // Bind the socket to the filesystem path
        let addrSize = MemoryLayout<sockaddr_un>.size
        let bindResult = withUnsafePointer(to: &addr) { addrPtr in
            addrPtr.withMemoryRebound(to: sockaddr.self, capacity: 1) { saPtr in
                bind(fd, saPtr, socklen_t(addrSize))
            }
        }

        if bindResult < 0 {
            LogManager.log("bind() failed: failed", level: .error, component: "ark.sys.service")
            close(fd)
            return
        }

        // Start listening for connections
        if listen(fd, IPCConfig.backlogSize) < 0 {
            LogManager.log("listen() failed: failed", level: .error, component: "ark.sys.service")
            close(fd)
            return
        }

        // Set socket permissions (restricts access to root and ark group)
        chmod(IPCConfig.socketPath, IPCConfig.socketMode)

        LogManager.log("Listening on \(IPCConfig.socketPath) (backlog=\(IPCConfig.backlogSize))", component: "ark.sys.service")

        // Start the accept loop on the thread pool
        let fdToAccept = self.serverFd
        ResourceController.executeOnCorePool {
            IPCServer.acceptLoop(serverFd: fdToAccept)
        }
    }

    // ── Connection Accept Loop ────────────────────────────────────

    /// Blocking loop that accepts incoming client connections and
    /// dispatches each to a handler thread from the pool.
    private static func acceptLoop(serverFd: Int32) {
        while true {
            var clientAddr = sockaddr_un()
            var clientLen = socklen_t(MemoryLayout<sockaddr_un>.size)

            let clientFd = withUnsafeMutablePointer(to: &clientAddr) { addrPtr in
                addrPtr.withMemoryRebound(to: sockaddr.self, capacity: 1) { saPtr in
                    accept(serverFd, saPtr, &clientLen)
                }
            }

            if clientFd < 0 {
                // Accept failed — brief pause to prevent busy-spin on transient errors
                usleep(10_000) // 10ms
                continue
            }

            // Handle each client connection on a separate thread from the pool
            ResourceController.executeOnCorePool {
                IPCServer.handleClient(clientFd)
            }
        }
    }

    // ── Client Handler ────────────────────────────────────────────

    /// Handles a single client connection. Reads request frames,
    /// routes them through the CommandRouter, and sends responses.
    /// Automatically closes the connection after a timeout or error.
    private static func handleClient(_ fd: Int32) {
        autoreleasepool {
            let rawBuffer = UnsafeMutablePointer<UInt8>.allocate(capacity: IPCConfig.bufferSize)
            defer {
                rawBuffer.deallocate()
                close(fd)
            }

            while true {
                let bytesRead = read(fd, rawBuffer, IPCConfig.bufferSize)

                if bytesRead <= 0 {
                    // Connection closed by client or read error
                    break
                }

                // Parse and route the request
                let requestBuffer = UnsafeRawBufferPointer(start: rawBuffer, count: bytesRead)
                let responseData = CommandRouter.route(requestBuffer: requestBuffer)

                // Send the response
                responseData.withUnsafeBytes { respBytes in
                    var totalWritten = 0
                    while totalWritten < respBytes.count {
                        let written = write(
                            fd,
                            respBytes.baseAddress! + totalWritten,
                            respBytes.count - totalWritten
                        )
                        if written <= 0 { break }
                        totalWritten += written
                    }
                }
            }
        }
    }
}