From ea830e85bdb562c024ff5e2b0b0b8ddf6acf347d Mon Sep 17 00:00:00 2001
From: "Claudio Maggioni (maggicl)" <maggicl@kolabnow.ch>
Date: Thu, 7 May 2020 17:08:12 +0200
Subject: [PATCH] hw2: completed but untested

---
 hw2/src/GBNTReceiver.java | 65 +++++++++++++++++++++++++++++++++++++--
 hw2/src/GBNTSender.java   |  9 ++++--
 2 files changed, 69 insertions(+), 5 deletions(-)

diff --git a/hw2/src/GBNTReceiver.java b/hw2/src/GBNTReceiver.java
index bf3cf83..363f781 100644
--- a/hw2/src/GBNTReceiver.java
+++ b/hw2/src/GBNTReceiver.java
@@ -2,13 +2,72 @@ import transport.Receiver;
 import transport.TimeoutAction;
 
 public class GBNTReceiver extends Receiver implements TimeoutAction {
+
+    private char sequence;
+    boolean firstPacket = true;
+    private static final int RECEIVER_TIMEOUT_MS = 1000;
+    private State state = State.SETUP;
+
+    enum State {
+        SETUP,
+        RECEIVING
+    }
+
+    private static byte[] generateACK(char sequence) {
+        byte[] dest = new byte[2];
+        dest[0] = (byte) ((sequence >> 8) & 0xFF);
+        dest[1] = (byte) (sequence & 0xFF);
+        return dest;
+    }
+
+    private static char readBigEndianChar(byte[] src, int offset) {
+        return (char) (src[offset] << 8 + src[offset + 1]);
+    }
+
+    private static char incrementAndRollover(char c) {
+        return (char) ((c + 1) % 65536);
+    }
+
     @Override
     public void timeoutAction() {
-
+        try {
+            disconnect();
+        } catch (java.lang.InterruptedException ex) {
+            System.out.println("Thread interrupted.  Exiting directly.");
+            System.exit(0);
+        }
     }
 
     @Override
-    protected void unreliableReceive(byte[] bytes, int i, int i1) {
-
+    protected void unreliableReceive(byte[] bytes, int offset, int length) {
+        switch(this.state) {
+            case SETUP:
+                if (length != 2) {
+                    if (firstPacket) return;
+                    else {
+                        state = State.RECEIVING;
+                        this.unreliableReceive(bytes, offset, length);
+                    }
+                }
+                sequence = readBigEndianChar(bytes, 0);
+                unreliableSend(bytes, offset, 2);
+                firstPacket = false;
+                break;
+            case RECEIVING:
+                if (length == 2) { // if packet is a close packet
+                    if (incrementAndRollover(this.sequence) != readBigEndianChar(bytes, offset)) return; // ignore if not synchronized
+                    this.unreliableSend(bytes, offset, 2);
+                    timeoutAction();
+                }
+                if (length < 3) return;
+                char seq = readBigEndianChar(bytes, 0);
+                if (seq != incrementAndRollover(sequence)) return; //drop the packet
+                else {
+                    this.unreliableReceive(bytes, offset + 2, length);
+                    sequence = incrementAndRollover(sequence);
+                    this.unreliableSend(generateACK(sequence), 0, 2);
+                }
+                break;
+        }
     }
 }
diff --git a/hw2/src/GBNTSender.java b/hw2/src/GBNTSender.java
index 06fa398..612c26c 100644
--- a/hw2/src/GBNTSender.java
+++ b/hw2/src/GBNTSender.java
@@ -13,6 +13,10 @@ public class GBNTSender extends transport.Sender implements TimeoutAction {
         return (char) (src[offset] << 8 + src[offset + 1]);
     }
 
+    private static char addAndRollover(char c, char i) {
+        return (char) ((c + i) % 65536);
+    }
+
     private static char incrementAndRollover(char c) {
         return (char) ((c + 1) % 65536);
     }
@@ -57,7 +61,7 @@ public class GBNTSender extends transport.Sender implements TimeoutAction {
     private static final char W = 10;
     private char base = (char) (new Random().nextInt() % 65536);
     private char waitingACK = 0;
-    private int timeoutMs = 1000;
+    private int timeoutMs = 500;
     private double rttEWMA = 0;
     private double devRttEWMA = 0;
 
@@ -107,7 +111,7 @@ public class GBNTSender extends transport.Sender implements TimeoutAction {
     protected int reliableSend(byte[] bytes, int offset, int length) {
         if (state != State.SENDING) throw new IllegalStateException();
 
-        Packet p = new Packet(base, bytes, offset, length);
+        Packet p = new Packet(addAndRollover(base, waitingACK), bytes, offset, length);
         waitingACK++;
         packets.add(p);
         p.send();
@@ -142,6 +146,7 @@ public class GBNTSender extends transport.Sender implements TimeoutAction {
                 byte[] expected = createSetupPacket();
                 if (bytes[offset] == expected[0] && bytes[1] == expected[offset + 1]) {
                     state = State.SENDING;
+                    base = incrementAndRollover(base);
                     resumeSender();
                 } else {
                     sendSetup();