main.go

package main

import (
	"crypto/sha256"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"log"
	"net/http"
	"os"
	"strconv"
	"strings"
	"sync"
	"time"

	"github.com/davecgh/go-spew/spew"
	"github.com/joho/godotenv"
)

const difficulty = 1

// Block represents each 'item' in the blockchain
type Block struct {
	Index      int
	Timestamp  string
	Data       int
	Hash       string
	PrevHash   string
	Difficulty int
	Nonce      string
}

// Blockchain is a series of validated Blocks
var Blockchain []Block

// Message takes incoming JSON payload for writing data
type Message struct {
	Data int
}

var mutex = &sync.Mutex{}

func main() {
	// Load .env file, with fallback to default port and difficulty
	if err := godotenv.Load(); err != nil {
		log.Println("No .env file found, using default configurations")
	}

	go initializeBlockchain()
	log.Fatal(run())
}

func initializeBlockchain() {
	t := time.Now()
	genesisBlock := Block{
		Index:      0,
		Timestamp:  t.String(),
		Data:       0,
		Hash:       calculateHash(Block{}),
		PrevHash:   "",
		Difficulty: getDifficulty(),
	}

	spew.Dump(genesisBlock)

	mutex.Lock()
	Blockchain = append(Blockchain, genesisBlock)
	mutex.Unlock()
}

// web server
func run() error {
	mux := makeMuxRouter()
	httpPort := os.Getenv("PORT")
	log.Println("HTTP Server Listening on port :", httpPort)
	s := &http.Server{
		Addr:           ":" + httpPort,
		Handler:        mux,
		ReadTimeout:    10 * time.Second,
		WriteTimeout:   10 * time.Second,
		MaxHeaderBytes: 1 << 20,
	}

	if err := s.ListenAndServe(); err != nil {
		return err
	}

	return nil
}

func respondWithJSON(w http.ResponseWriter, r *http.Request, code int, payload interface{}) {
	w.Header().Set("Content-Type", "application/json")
	response, err := json.MarshalIndent(payload, "", "  ")
	if err != nil {
		w.WriteHeader(http.StatusInternalServerError)
		w.Write([]byte("HTTP 500: Internal Server Error"))
		return
	}
	w.WriteHeader(code)
	w.Write(response)
}

// make sure block is valid by checking index, and comparing the hash of the previous block
func isBlockValid(newBlock, oldBlock Block) bool {
	if oldBlock.Index+1 != newBlock.Index {
		return false
	}

	if oldBlock.Hash != newBlock.PrevHash {
		return false
	}

	if calculateHash(newBlock) != newBlock.Hash {
		return false
	}

	return true
}

// SHA256 hasing
func calculateHash(block Block) string {
	record := strconv.Itoa(block.Index) + block.Timestamp + strconv.Itoa(block.Data) + block.PrevHash + block.Nonce
	h := sha256.New()
	h.Write([]byte(record))
	hashed := h.Sum(nil)
	return hex.EncodeToString(hashed)
}

// create a new block using previous block's hash
func generateBlock(oldBlock Block, Data int) Block {
	var newBlock Block

	t := time.Now()

	newBlock.Index = oldBlock.Index + 1
	newBlock.Timestamp = t.String()
	newBlock.Data = Data
	newBlock.PrevHash = oldBlock.Hash
	newBlock.Difficulty = difficulty

	for i := 0; ; i++ {
		hex := fmt.Sprintf("%x", i)
		newBlock.Nonce = hex
		if !isHashValid(calculateHash(newBlock), newBlock.Difficulty) {
			fmt.Println(calculateHash(newBlock), " do more work!")
			time.Sleep(time.Second)
			continue
		} else {
			fmt.Println(calculateHash(newBlock), " work done!")
			newBlock.Hash = calculateHash(newBlock)
			break
		}

	}
	return newBlock
}

func isHashValid(hash string, difficulty int) bool {
	prefix := strings.Repeat("0", difficulty)
	return strings.HasPrefix(hash, prefix)
}