Create crc.md

docs/Computer Networks/crc.md

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+# Cyclic Redundancy Check
+
+CRC or Cyclic Redundancy Check is a method of detecting accidental changes/errors in the communication channel. 
+
+```java
+
+import java.util.Arrays; 
+class Program { 
+
+
+	static String Xor(String a, String b) 
+	{ 
+
+
+		String result = ""; 
+		int n = b.length(); 
+
+		for (int i = 1; i < n; i++) { 
+			if (a.charAt(i) == b.charAt(i)) 
+				result += "0"; 
+			else
+				result += "1"; 
+		} 
+		return result; 
+	} 
+	static String Mod2Div(String dividend, String divisor) 
+	{ 
+
+		int pick = divisor.length(); 
+
+
+		String tmp = dividend.substring(0, pick); 
+
+		int n = dividend.length(); 
+
+		while (pick < n) { 
+			if (tmp.charAt(0) == '1') 
+
+				tmp = Xor(divisor, tmp) 
+					+ dividend.charAt(pick); 
+			else
+
+
+				tmp = Xor(new String(new char[pick]) 
+							.replace("\0", "0"), 
+						tmp) 
+					+ dividend.charAt(pick); 
+
+
+			pick += 1; 
+		} 
+
+
+		if (tmp.charAt(0) == '1') 
+			tmp = Xor(divisor, tmp); 
+		else
+			tmp = Xor(new String(new char[pick]) 
+						.replace("\0", "0"), 
+					tmp); 
+
+		return tmp; 
+	} 
+
+
+	static void EncodeData(String data, String key) 
+	{ 
+		int l_key = key.length(); 
+
+		String appended_data 
+			= (data 
+			+ new String(new char[l_key - 1]) 
+					.replace("\0", "0")); 
+
+		String remainder = Mod2Div(appended_data, key); 
+
+
+		String codeword = data + remainder; 
+		System.out.println("Remainder : " + remainder); 
+		System.out.println( 
+			"Encoded Data (Data + Remainder) :" + codeword 
+			+ "\n"); 
+	} 
+	static void Receiver(String data, String key) 
+	{ 
+		String currxor 
+			= Mod2Div(data.substring(0, key.length()), key); 
+		int curr = key.length(); 
+		while (curr != data.length()) { 
+			if (currxor.length() != key.length()) { 
+				currxor += data.charAt(curr++); 
+			} 
+			else { 
+				currxor = Mod2Div(currxor, key); 
+			} 
+		} 
+		if (currxor.length() == key.length()) { 
+			currxor = Mod2Div(currxor, key); 
+		} 
+		if (currxor.contains("1")) { 
+			System.out.println( 
+				"there is some error in data"); 
+		} 
+		else { 
+			System.out.println("correct message received"); 
+		} 
+	} 
+
+	public static void main(String[] args) 
+	{ 
+		String data = "100100"; 
+		String key = "1101"; 
+		System.out.println("\nSender side..."); 
+		EncodeData(data, key); 
+
+		System.out.println("Receiver side..."); 
+		Receiver(data+Mod2Div(data+new String(new char[key.length() - 1]) 
+							.replace("\0", "0"),key),key); 
+	} 
+} 
+
+```