Prevent Difficulty Jumps

blockchain.js

@@ -37,14 +37,16 @@ class Blockchain {
 
     for (let i=1; i<chain.length; i++) {
       const { timestamp, lastHash, hash, nonce, difficulty, data } = chain[i];
-
       const actualLastHash = chain[i-1].hash;
+      const lastDifficulty = chain[i-1].difficulty;
 
       if (lastHash !== actualLastHash) return false;
 
       const validatedHash = cryptoHash(timestamp, lastHash, data, nonce, difficulty);
 
       if (hash !== validatedHash) return false;
+
+      if (Math.abs(lastDifficulty - difficulty) > 1) return false;
     }
 
     return true;

blockchain.test.js

@@ -1,5 +1,6 @@
 const Blockchain = require('./blockchain');
 const Block = require('./block');
+const cryptoHash = require('./crypto-hash');
 
 describe('Blockchain', () => {
   let blockchain, newChain, originalChain;
@@ -58,6 +59,27 @@ describe('Blockchain', () => {
         });
       });
 
+      describe('and the chain contains a block with a jumped difficulty', () => {
+        it('returns false', () => {
+          const lastBlock = blockchain.chain[blockchain.chain.length-1];
+          const lastHash = lastBlock.hash;
+          const timestamp = Date.now();
+          const nonce = 0;
+          const data = [];
+          const difficulty = lastBlock.difficulty - 3;
+
+          const hash = cryptoHash(timestamp, lastHash, difficulty, nonce, data);
+
+          const badBlock = new Block({
+            timestamp, lastHash, hash, nonce, difficulty, data
+          });
+
+          blockchain.chain.push(badBlock);
+
+          expect(Blockchain.isValidChain(blockchain.chain)).toBe(false);
+        });
+      });
+
       describe('and the chain does not contain any invalid blocks', () => {
         it('returns true', () => {
           expect(Blockchain.isValidChain(blockchain.chain)).toBe(true);