main.go

package main

import (
	"crypto/sha256"
	"encoding/hex"
	"fmt"
	log "github.com/sirupsen/logrus"
	"github.com/urfave/cli/v2"
	"io"
	"os"
)

func main() {
	app := &cli.App{
		Usage: "a cli to calculate a hash chain but tediously",
		Flags: []cli.Flag{},
		Commands: []*cli.Command{
			{
				Name:    "add",
				Aliases: []string{"a"},
				Usage:   "returns a new hash based on an (optional) previous hash and the specified files",
				Action: func(c *cli.Context) error {
					if c.NArg() < 1 {
						return fmt.Errorf("must specify at least one file")
					}
					hash, err := add(c.String("previous"), c.Args().Slice())
					if err != nil {
						return err
					}
					fmt.Printf("%x\n", hash)
					return nil
				},
				Flags: []cli.Flag{
					&cli.StringFlag{
						Name:        "previous",
						Aliases:     []string{"p"},
						Usage:       "previous hash for hash chain",
						DefaultText: "",
					},
				},
			},
		},
	}
	err := app.Run(os.Args)
	if err != nil {
		log.Fatal(err)
	}
}

func add (previous string, files []string) ([]byte, error) {

	var err error
	var hashes []byte

	if previous != "" {
		hashes, err = hex.DecodeString(previous)
		if err != nil {
			return nil, err
		}
	}

	for _, f := range files {
		hashes, err = appendFileHash(hashes, f)
		if err != nil {
			return nil, err
		}
		log.Infof("file: %s", f)
	}

	hash := sha256.New()
	hash.Write(hashes)
	return hash.Sum(nil), nil
}

func appendFileHash(hash []byte, path string) ([]byte, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	h := sha256.New()
	if _, err := io.Copy(h, f); err != nil {
		return nil, err
	}

	return h.Sum(hash), nil
}