From 0a32b9087bd7fa58b4d545c9a699b783e41139db Mon Sep 17 00:00:00 2001
From: Ashish Gopal Hattimare
 <32576042+ashishgopalhattimare@users.noreply.github.com>
Date: Sun, 23 Jun 2019 19:55:29 +0530
Subject: [PATCH] Solution to Assignment 2

---
 src/blockchain/Part 2/Main.java | 114 ++++++++++++++++++++++++++++++++
 1 file changed, 114 insertions(+)
 create mode 100644 src/blockchain/Part 2/Main.java

diff --git a/src/blockchain/Part 2/Main.java b/src/blockchain/Part 2/Main.java
new file mode 100644
index 0000000..d69bb32
--- /dev/null
+++ b/src/blockchain/Part 2/Main.java	
@@ -0,0 +1,114 @@
+package jetbrains;
+
+import java.security.MessageDigest;
+import java.util.ArrayList;
+import java.util.Date;
+
+class Block
+{
+    public static final int DIFFICULTY = 5;
+
+    private long date;
+    private int id;
+    private long  nonce;
+    private String prev;
+    private String hash;
+
+    private Long duration;
+
+    public Block(int id, String prev) {
+        this.date = new Date().getTime();
+        this.prev = prev;
+        this.id = id;
+
+        StringBuilder sb = new StringBuilder("");
+        for(int i = 0; i < DIFFICULTY; i++) {
+            sb.append('0');
+        }
+        String starter = sb.toString();
+        Long start = System.nanoTime();
+
+        nonce = 0;
+        this.hash = generateHash(nonce + (id + prev + date));
+        while(!this.hash.startsWith(starter)) {
+            nonce++;
+            this.hash = generateHash(nonce + (id + prev + date));
+        }
+        this.duration = System.nanoTime() - start;
+    }
+
+    private String generateHash(String message) {
+        return Blockchain.applySha256(message);
+    }
+
+    public String getHash() {
+        return hash;
+    }
+
+    @Override
+    public String toString() {
+        return  "\nBlock:\nId: " + this.id +
+                "\nTimestamp: " + this.date +
+                "\nMagic number: " + nonce +
+                "\nHash of the previous block:\n" + this.prev +
+                "\nHash of the block:\n" + this.hash +
+                "\nBlock was generating for " + this.duration/1_000_000_000 + " seconds";
+    }
+}
+
+class Blockchain
+{
+    private static int id = 1;
+    private ArrayList<Block> chain;
+
+    public Blockchain()
+    {
+        chain = new ArrayList<>();
+        chain.add(new Block(id++, "0"));
+    }
+
+    /* Applies Sha256 to a string and returns a hash. */
+    public static String applySha256(String input){
+        try {
+            MessageDigest digest = MessageDigest.getInstance("SHA-256");
+            /* Applies sha256 to our input */
+            byte[] hash = digest.digest(input.getBytes("UTF-8"));
+            StringBuilder hexString = new StringBuilder();
+            for (byte elem: hash) {
+                String hex = Integer.toHexString(0xff & elem);
+                if(hex.length() == 1) hexString.append('0');
+                hexString.append(hex);
+            }
+            return hexString.toString();
+        }
+        catch(Exception e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    public void addBlock()
+    {
+        chain.add(new Block(id++, chain.get(chain.size() - 1).getHash()));
+    }
+
+    public void displayChain()
+    {
+        System.out.println("Enter how many zeros the hash must starts with: " + Block.DIFFICULTY);
+        for(Block block : chain) {
+            System.out.println(block);
+        }
+    }
+}
+
+public class Main {
+
+    public static void main(String[] args) {
+
+        Blockchain chain = new Blockchain();
+
+        for(int i = 0; i < 2; i++) {
+            chain.addBlock();
+        }
+        chain.displayChain();
+    }
+}