main.go

package main

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

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

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

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

// Message takes incoming JSON payload for writing heart rate
type Message struct {
	BPM int
}

var mutex = &sync.Mutex{}

var bcServer chan []Block

func main() {
	err := godotenv.Load()
	if err != nil {
		log.Fatal(err)
	}
	bcServer = make(chan []Block)

	/* go func() {
		t := time.Now()
		genesisBlock := Block{0, t.String(), 0, "", ""}
		spew.Dump(genesisBlock)
		Blockchain = append(Blockchain, genesisBlock)
	}() */
	// create genesis block
	t := time.Now()
	genesisBlock := Block{0, t.String(), 0, "", ""}
	spew.Dump(genesisBlock)
	Blockchain = append(Blockchain, genesisBlock)

	//httpPort := os.Getenv("PORT")

	//log.Fatal(run())

	// start TCP and serve TCP server
	server, err := net.Listen("tcp", ":"+os.Getenv("ADDR"))
	if err != nil {
		log.Fatal(err)
	}
	defer server.Close()

	for {
		conn, err := server.Accept()
		if err != nil {
			log.Fatal(err)
		}
		go handleConn(conn)
	}

}

// calculate the new hash value
func calHash(block Block) string {
	record := string(block.Index) + block.Timestamp + string(block.BPM) + block.PrevHash
	h := sha256.New()
	h.Write([]byte(record))
	hashed := h.Sum(nil)
	return hex.EncodeToString(hashed)
}

// generate new block
func generateBlock(oldBlock Block, BPM int) (Block, error) {

	var newBlock Block

	t := time.Now()

	newBlock.Index = oldBlock.Index + 1
	newBlock.Timestamp = t.String()
	newBlock.BPM = BPM
	newBlock.PrevHash = oldBlock.Hash
	newBlock.Hash = calHash(newBlock)

	return newBlock, nil
}
func isBlockValid(newBlock, oldBlock Block) bool {
	if oldBlock.Index+1 != newBlock.Index {
		return false
	}

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

	return true
}

// overwrite old chain with the new one that has the new block(s).
func replaceChain(newBlocks []Block) {
	mutex.Lock()
	if len(newBlocks) > len(Blockchain) {
		Blockchain = newBlocks
	}
	mutex.Unlock()
}

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

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

	return nil
}

// generate MUXRouter
func makeMuxRouter() http.Handler {
	muxRouter := mux.NewRouter()
	muxRouter.HandleFunc("/", handleGetBlockchain).Methods("GET")
	muxRouter.HandleFunc("/", handleWriteBlock).Methods("POST")
	return muxRouter
}

// Read
func handleGetBlockchain(w http.ResponseWriter, r *http.Request) {
	bytes, err := json.MarshalIndent(Blockchain, "", "  ")
	if err != nil {
		http.Error(w, err.Error(), http.StatusInternalServerError)
		return
	}
	io.WriteString(w, string(bytes))
}

// Write
func handleWriteBlock(w http.ResponseWriter, r *http.Request) {
	var m Message

	decoder := json.NewDecoder(r.Body)
	if err := decoder.Decode(&m); err != nil {
		respondWithJSON(w, r, http.StatusBadRequest, r.Body)
		return
	}
	defer r.Body.Close()

	newBlock, err := generateBlock(Blockchain[len(Blockchain)-1], m.BPM)
	if err != nil {
		respondWithJSON(w, r, http.StatusInternalServerError, m)
		return
	}
	if isBlockValid(newBlock, Blockchain[len(Blockchain)-1]) {
		newBlockchain := append(Blockchain, newBlock)
		replaceChain(newBlockchain)
		spew.Dump(Blockchain)
	}

	respondWithJSON(w, r, http.StatusCreated, newBlock)

}

// Post method to add new blocks
func respondWithJSON(w http.ResponseWriter, r *http.Request, code int, payload interface{}) {
	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)
}

// connection handler, Brodcast b/w connections
func handleConn(conn net.Conn) {
	defer conn.Close()
	io.WriteString(conn, "Enter a new BPM:")

	scanner := bufio.NewScanner(conn)

	// take in BPM from stdin and add it to blockchain after conducting necessary validation
	go func() {
		for scanner.Scan() {
			bpm, err := strconv.Atoi(scanner.Text())
			if err != nil {
				log.Printf("%v not a number: %v", scanner.Text(), err)
				continue
			}
			newBlock, err := generateBlock(Blockchain[len(Blockchain)-1], bpm)
			if err != nil {
				log.Println(err)
				continue
			}
			if isBlockValid(newBlock, Blockchain[len(Blockchain)-1]) {
				newBlockchain := append(Blockchain, newBlock)
				replaceChain(newBlockchain)
			}

			bcServer <- Blockchain
			io.WriteString(conn, "\nEnter a new BPM:")
		}
	}()

	go func() {
		for {
			time.Sleep(30 * time.Second)
			output, err := json.Marshal(Blockchain)
			if err != nil {
				log.Fatal(err)
			}
			io.WriteString(conn, string(output))
		}
	}()

	for _ = range bcServer {
		spew.Dump(Blockchain)
	}
}