package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
	"strings"
)

func main() {

	fmt.Println("Welcome to Checkdigit Calculator")

	directory, err := os.Getwd()
	Error(err)

	fmt.Printf("What is the name of the file in this directory?: ")
	reader := bufio.NewReader(os.Stdin)
	input, err := reader.ReadString('\n')
	Error(err)

	input = strings.TrimSpace(input)
	file, err := os.Open(input)
	Error(err)
	var upcs []string
	scanner := bufio.NewScanner(file)
	for scanner.Scan() {
		upcs = append(upcs, scanner.Text())
	}
	err = file.Close()
	Error(err)
	if scanner.Err() != nil {
		fmt.Println(scanner.Err())
	}

	var gtins []string

	for _, upc := range upcs {

		upcLength := len(upc)

		// Check Digit Formula
		// Take all the digits in odd positions, add them and multiply by three
		// Take all the digits in even positions (except for the last one) and add to the number you got above
		// Divide that number by 10 and take the remainder
		// If the remainder is not 0, subtract it from 10
		//
		// 12/8/2023: New git updates from https://www.gs1.org/services/how-calculate-check-digit-manually

		oddNumber := 0
		evenNumber := 0
		var remainder int

		if upcLength < 8 {
			gtins = append(gtins, "Cannot process upcs less than 8 digits")
			continue
		}

		if upcLength > 17 {
			gtins = append(gtins, "Invalid barcode")
			continue
		}

		if upcLength%2 == 0 {
			for i, num := range upc {
				if i%2 != 0 {
					convertedOddNumber, err := strconv.Atoi(string(num))
					Error(err)
					convertedOddNumber = convertedOddNumber * 3
					oddNumber += convertedOddNumber
				} else {
					if upcLength-1 > i {
						convertedEvenNumber, err := strconv.Atoi(string(num))
						Error(err)
						evenNumber += convertedEvenNumber
					}
				}
			}
		} else {
			for i, num := range upc {
				if i%2 == 0 {
					convertedOddNumber, err := strconv.Atoi(string(num))
					Error(err)
					convertedOddNumber = convertedOddNumber * 3
					oddNumber += convertedOddNumber
				} else {
					if upcLength-1 > i {
						convertedEvenNumber, err := strconv.Atoi(string(num))
						Error(err)
						evenNumber += convertedEvenNumber
					}
				}
			}
		}

		remainder = (oddNumber + evenNumber) % 10
		if remainder != 0 {
			remainder = 10 - remainder
		}
		remainderString := strconv.Itoa(remainder)

		gtins = append(gtins, upc+remainderString)
	}

	if _, err := os.Stat(directory + "/answer.txt"); err == nil {
		e := os.Remove(directory + "/answer.txt")
		Error(e)
	}

	answerFile, err := os.Create(directory + "/answer.txt")
	Error(err)

	defer answerFile.Close()

	for _, gtin := range gtins {
		_, err := answerFile.WriteString(gtin + "\n")
		Error(err)
	}

	fmt.Println("The answer.txt file has been successfully generated with check digits.")
}