package utils

import (
	"bytes"
	"crypto/md5"
	// "encoding/hex"
	"fmt"
	"io"
	"os"
)

func CalculatePartialMD5(filePath string) string {
	file, err := os.Open(filePath)
	if err != nil {
		panic(err)
	}

	defer file.Close()

	var step int64 = 1024
	var size int64 = 1024
	var buf bytes.Buffer

	for i := -1; i <= 10; i++ {
		byteStep := make([]byte, size)

		var newShift int64 = int64(i * 2)
		var newOffset int64
		if i == -1 {
			newOffset = 0
		} else {
			newOffset = step << newShift
		}

		_, err := file.ReadAt(byteStep, newOffset)
		if err == io.EOF {
			break
		}
		buf.Write(byteStep)
	}

	allBytes := buf.Bytes()
	return fmt.Sprintf("%x", md5.Sum(allBytes))
}

func main() {
	fileHash := CalculatePartialMD5("test.epub")
	fmt.Println("MD5: ", fileHash)
}