KTLN-856: Implement a blockchain in Kotlin (#1038)

Author: harry9656

kotlin-blockchain/README.md

@@ -0,0 +1 @@
+## Relevant Articles

kotlin-blockchain/pom.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+    <artifactId>kotlin-blockchain</artifactId>
+    <name>kotlin-blockchain</name>
+    <packaging>jar</packaging>
+
+    <parent>
+        <groupId>com.baeldung</groupId>
+        <artifactId>kotlin-modules</artifactId>
+        <version>1.0.0-SNAPSHOT</version>
+    </parent>
+
+</project>
+

kotlin-blockchain/src/main/kotlin/com/baeldung/chain/Block.kt

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+package com.baeldung.chain
+
+import java.security.MessageDigest
+
+data class Block(
+    val previousHash: String,
+    val data: String,
+    val timestamp: Long = System.currentTimeMillis(),
+    val nonce: Int = 0
+) {
+
+    val hash: String by lazy { calculateHash() }
+
+    fun calculateHash(): String {
+        val input = "$previousHash$data$timestamp$nonce"
+        return input.sha256()
+    }
+}
+
+fun mineBlock(previousHash: String, data: String, acceptingRegexp: Regex): Block {
+    var finalBlock = Block(previousHash, data)
+    while (!finalBlock.hash.matches(acceptingRegexp)) {
+        finalBlock = finalBlock.copy(nonce = finalBlock.nonce + 1)
+    }
+    return finalBlock
+}
+
+fun String.sha256(): String {
+    val bytes = this.toByteArray()
+    val md = MessageDigest.getInstance("SHA-256")
+    val digest = md.digest(bytes)
+    return digest.fold("") { str, it -> str + "%02x".format(it) }
+}

kotlin-blockchain/src/main/kotlin/com/baeldung/chain/Blockchain.kt

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+package com.baeldung.chain
+
+class Blockchain(difficulty: Int = 5) {
+    val chain = mutableListOf<Block>()
+    val acceptanceRegex = "^[0]{$difficulty}.+".toRegex();
+
+init {
+    val genesisBlock = mineBlock("0", "Genesis Block", acceptanceRegex)
+    chain.add(genesisBlock)
+}
+
+    fun addBlock(block: Block) {
+        if (isValidBlock(block)) {
+            chain.add(block)
+        } else {
+            throw IllegalArgumentException("Invalid block")
+        }
+    }
+
+    private fun isValidBlock(block: Block): Boolean {
+        val lastBlock = chain.last()
+        return block.previousHash == lastBlock.hash &&
+                block.hash.matches(acceptanceRegex)
+    }
+
+    fun isValid(): Boolean {
+        if (chain.size < 2) return true
+
+        chain.zipWithNext().forEach { (prev, current) ->
+            if (current.previousHash != prev.hash) {
+                return false
+            }
+        }
+        return true
+    }
+}

kotlin-blockchain/src/main/kotlin/com/baeldung/chain/main.kt

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+package com.baeldung.chain
+
+fun main() {
+    val blockchain = Blockchain(3)
+
+    val block1 = mineBlock(blockchain.chain.last().hash, "Block 1 Data", blockchain.acceptanceRegex)
+    blockchain.addBlock(block1)
+
+    val block2 = mineBlock(blockchain.chain.last().hash, "Block 2 Data", blockchain.acceptanceRegex)
+    blockchain.addBlock(block2)
+
+    val block3 = mineBlock(blockchain.chain.last().hash, "Block 3 Data", blockchain.acceptanceRegex)
+    blockchain.addBlock(block3)
+
+    println("Blockchain valid? ${blockchain.isValid()}")
+
+    blockchain.chain.forEach {
+        println("Block Data: ${it.data}, Hash: ${it.hash}")
+    }
+}

pom.xml

@@ -528,6 +528,7 @@
                 <module>kotlin-rsocket</module>
                 <module>kotlin-self-executable-jar</module>
                 <module>kotlin-spark</module>
+                <module>kotlin-blockchain</module>
                 <module>kotlin-testing</module>
                 <module>kotlin-mockito</module>
                 <!-- <module>kotlin-tornadofx</module> --> <!-- not compatible with Java 9+ -->
@@ -610,6 +611,7 @@
                 <module>kotlin-rsocket</module>
                 <module>kotlin-self-executable-jar</module>
                 <module>kotlin-spark</module>
+                <module>kotlin-blockchain</module>
                 <module>kotlin-testing</module>
                 <module>kotlin-mockito</module>
                 <!-- <module>kotlin-tornadofx</module> --> <!-- not compatible with Java 9+ -->