luhn.go

package luhn

import (
	"fmt"
)

type luhnNumber struct {
	baseNumber int64
	checkDigit int
}

// calculateCheckDigit calculates the check digit for the given number
func (l luhnNumber) calculateCheckDigit() (luhnNumber, error) {
	if l.baseNumber <= 0 {
		return l, fmt.Errorf("invalid input: number must be positive")
	}

	temp := l.baseNumber
	var total int
	multiplier := 0

	for temp > 0 {
		digit := int(temp % 10)
		temp /= 10

		if multiplier%2 == 0 {
			digit *= 2
			total += sumDoubledDigits(digit)
		} else {
			total += digit
		}
		multiplier++
	}

	checkDigit := (10 - (total % 10)) % 10
	l.checkDigit = checkDigit
	return l, nil
}

// FullNumber calculates the full number with the check digit
func FullNumber(number int64) (int64, error) {
	if number <= 0 {
		return 0, fmt.Errorf("invalid input: number must be positive")
	}

	ln := luhnNumber{
		baseNumber: number,
	}
	res, err := ln.calculateCheckDigit()
	if err != nil {
		return 0, fmt.Errorf("failed to generate check digit: %v", err)
	}

	// Append check digit arithmetically
	finalNumber := number*10 + int64(res.checkDigit)
	return finalNumber, nil
}

// CheckDigit calculates the check digit for the given number
func CheckDigit(number int64) (int, error) {
	if number <= 0 {
		return 0, fmt.Errorf("invalid input: number must be positive")
	}
	ln := luhnNumber{
		baseNumber: number,
	}
	res, err := ln.calculateCheckDigit()
	if err != nil {
		return 0, fmt.Errorf("failed to generate check digit: %v", err)
	}
	return res.checkDigit, nil
}

// IsValid checks if the number is valid according to the Luhn algorithm
func IsValid(number int64) bool {
	if number < 10 {
		return false
	}

	checkDigit := int(number % 10)
	baseNumber := number / 10

	ln := luhnNumber{
		baseNumber: baseNumber,
	}
	res, err := ln.calculateCheckDigit()
	if err != nil {
		return false
	}

	return res.checkDigit == checkDigit
}

// sumDoubledDigits sums the digits of a number after doubling
func sumDoubledDigits(digits int) int {
	if digits < 10 {
		return digits
	}

	total := 0
	for digits > 0 {
		total += digits % 10
		digits /= 10
	}
	return total
}