From 1e979a5fc1ba1c8e21c7d06920e4a659161ea897 Mon Sep 17 00:00:00 2001
From: js <None>
Date: Mon, 3 Feb 2020 15:19:45 +0100
Subject: [PATCH] add doc to checksum

---
 src/protocolP2P/ProtocolP2PDatagram.java | 25 +++++++++++++++++++++++-
 src/tools/BytesArrayTools.java           |  4 ++--
 2 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/src/protocolP2P/ProtocolP2PDatagram.java b/src/protocolP2P/ProtocolP2PDatagram.java
index 079c0fd..cf2a02d 100644
--- a/src/protocolP2P/ProtocolP2PDatagram.java
+++ b/src/protocolP2P/ProtocolP2PDatagram.java
@@ -243,15 +243,33 @@ public class ProtocolP2PDatagram {
 		this.hostR = hostR;
 	}
 
+	/** Used to compute Checksum of a specific datagram
+	* @param datagram full datagram
+	* @return checksum
+	* @throws SizeError
+	*/
+
 	private int computeCheckSum(byte [] datagram) throws SizeError {
+		/*
+		* The checksum field is the 16 bit one’s complement of the one’s complement sum of all 16-bit words
+		* in the header and text. If a segment contains an odd number of header and text octets to be checksummed,
+		* the last octet is padded on the right with zeros to form a 16-bit word for checksum purposes.
+		* The pad is not transmitted as part of the segment. While computing the checksum, the checksum field
+		* itself is replaced with zeros.
+		*/
 		int checksum = 0;
-		for (int i=CHECKSUM_POSITION+2; i<CHECKSUM_POSITION+2+4+Payload.getPayloadSize(datagram); ++i){
+		for (int i=CHECKSUM_POSITION+2; i<CHECKSUM_POSITION+2+4+Payload.getPayloadSize(datagram); ++i) {
 			checksum += BytesArrayTools.readInt16Bits(datagram,i);
 			checksum &= 0xffff;
 		}
 		return checksum ^ 0xffff;
 	}
 
+	/** Used to set checksum into datagram
+	* @param datagram full datagram
+	* @throws InternalError
+	*/
+
 	private void setCheckSum(byte [] datagram) throws InternalError {
 		try {
 			int checksum = computeCheckSum(datagram);
@@ -261,6 +279,11 @@ public class ProtocolP2PDatagram {
 		}
 	}
 
+	/** Used to check if the checksum is correct
+	* @param datagram full datagram
+	* @throws TransmissionError
+	*/
+
 	private void checkCheckSum(byte [] datagram) throws TransmissionError {
 		try {
 			int checksum = BytesArrayTools.readInt16Bits(datagram,CHECKSUM_POSITION);
diff --git a/src/tools/BytesArrayTools.java b/src/tools/BytesArrayTools.java
index 3642bd8..7e21038 100644
--- a/src/tools/BytesArrayTools.java
+++ b/src/tools/BytesArrayTools.java
@@ -63,10 +63,10 @@ public class BytesArrayTools {
 		return size;
 	}
 
-	/** Read int from a bytearray
+	/** Read int from a bytearray of 16 bits
 	 * @param array	the array to read
 	 * @param start	where to begin reading
-	 * @return value read as int
+	 * @return value read as 16 bits int
 	 */
 	public static int readInt16Bits(byte[] array, int start) {
 		int size = 0;