Replace NEKit dependency with reduced subset

GlassVPN/zhuhaow-NEKit/IPStack/Packet/PacketProtocolParser.swift

@@ -0,0 +1,72 @@
+import Foundation
+
+protocol TransportProtocolParserProtocol {
+    var packetData: Data! { get set }
+
+    var offset: Int { get set }
+
+    var bytesLength: Int { get }
+
+    var payload: Data! { get set }
+
+    func buildSegment(_ pseudoHeaderChecksum: UInt32)
+}
+
+/// Parser to process UDP packet and build packet.
+class UDPProtocolParser: TransportProtocolParserProtocol {
+    /// The source port.
+    var sourcePort: Port!
+
+    /// The destination port.
+    var destinationPort: Port!
+
+    /// The data containing the UDP segment.
+    var packetData: Data!
+
+    /// The offset of the UDP segment in the `packetData`.
+    var offset: Int = 0
+
+    /// The payload to be encapsulated.
+    var payload: Data!
+
+    /// The length of the UDP segment.
+    var bytesLength: Int {
+        return payload.count + 8
+    }
+
+    init() {}
+
+    init?(packetData: Data, offset: Int) {
+        guard packetData.count >= offset + 8 else {
+            return nil
+        }
+
+        self.packetData = packetData
+        self.offset = offset
+
+        sourcePort = Port(bytesInNetworkOrder: (packetData as NSData).bytes.advanced(by: offset))
+        destinationPort = Port(bytesInNetworkOrder: (packetData as NSData).bytes.advanced(by: offset + 2))
+
+        payload = packetData.subdata(in: offset+8..<packetData.count)
+    }
+
+    func buildSegment(_ pseudoHeaderChecksum: UInt32) {
+        sourcePort.withUnsafeBufferPointer {
+            self.packetData.replaceSubrange(offset..<offset+2, with: $0)
+        }
+        destinationPort.withUnsafeBufferPointer {
+            self.packetData.replaceSubrange(offset+2..<offset+4, with: $0)
+        }
+        var length = NSSwapHostShortToBig(UInt16(bytesLength))
+        withUnsafeBytes(of: &length) {
+            packetData.replaceSubrange(offset+4..<offset+6, with: $0)
+        }
+        packetData.replaceSubrange(offset+8..<offset+8+payload.count, with: payload)
+
+        packetData.resetBytes(in: offset+6..<offset+8)
+        var checksum = Checksum.computeChecksum(packetData, from: offset, to: nil, withPseudoHeaderChecksum: pseudoHeaderChecksum)
+        withUnsafeBytes(of: &checksum) {
+            packetData.replaceSubrange(offset+6..<offset+8, with: $0)
+        }
+    }
+}