generate_payload.go

package main

import (
	"regexp"
	"strings"
	"fmt"
	"github.com/snksoft/crc"
)

const ID_PAYLOAD_FORMAT = "00"
const ID_POI_METHOD = "01"
const ID_MERCHANT_INFORMATION_BOT = "29"
const ID_TRANSACTION_CURRENCY = "53"
const ID_TRANSACTION_AMOUNT = "54"
const ID_COUNTRY_CODE = "58"
const ID_CRC = "63"

const PAYLOAD_FORMAT_EMV_QRCPS_MERCHANT_PRESENTED_MODE = "01"
const POI_METHOD_STATIC = "11"
const POI_METHOD_DYNAMIC = "12"
const MERCHANT_INFORMATION_TEMPLATE_ID_GUID = "00"
const BOT_ID_MERCHANT_PHONE_NUMBER = "01"
const BOT_ID_MERCHANT_TAX_ID = "02"
const BOT_ID_MERCHANT_EWALLET_ID = "03"
const GUID_PROMPTPAY = "A000000677010111"
const TRANSACTION_CURRENCY_THB = "764"
const COUNTRY_CODE_TH = "TH"

func f(id string, value string) string {
	return fmt.Sprintf("%s%02d%s", id, len(value), value)
}

func serialize (xs []string) string {
	return strings.Join(xs, "")
}

func sanitizeTarget (id string) string {
	regex := regexp.MustCompile(`[^0-9]`)
	return regex.ReplaceAllString(id, "")
}

func formatTarget (id string) string {
	numbers := sanitizeTarget(id)
	if len(numbers) >= 13 {
		return numbers
	}
	regex := regexp.MustCompile(`^0`)
	countryCoded := regex.ReplaceAllString(id, "66")
	return fmt.Sprintf("%013s", countryCoded)
}

func formatAmount (amount float32) string {
	return fmt.Sprintf("%.2f", amount)
}

func formatCrc (crcValue uint64) string {
	return fmt.Sprintf("%04X", crcValue)
}

func GeneratePayload (target string, amount float32) string {
	target = sanitizeTarget(target)
	var targetType string
	switch {
	case len(target) >= 15:
		targetType = BOT_ID_MERCHANT_EWALLET_ID
	case len(target) >= 13:
		targetType = BOT_ID_MERCHANT_TAX_ID
	default:
		targetType = BOT_ID_MERCHANT_PHONE_NUMBER
	}

	var data []string
	data = append(data, f(ID_PAYLOAD_FORMAT, PAYLOAD_FORMAT_EMV_QRCPS_MERCHANT_PRESENTED_MODE))
	if amount != 0 {
		data = append(data, f(ID_POI_METHOD, POI_METHOD_DYNAMIC))
	} else {
		data = append(data, f(ID_POI_METHOD, POI_METHOD_STATIC))
	}
	merchantInfo := serialize([] string { f(MERCHANT_INFORMATION_TEMPLATE_ID_GUID, GUID_PROMPTPAY),f(targetType, formatTarget(target)) })
	data = append(data, f(ID_MERCHANT_INFORMATION_BOT, merchantInfo))
	data = append(data, f(ID_COUNTRY_CODE, COUNTRY_CODE_TH))
	data = append(data, f(ID_TRANSACTION_CURRENCY, TRANSACTION_CURRENCY_THB))
	data = append(data, f(ID_PAYLOAD_FORMAT, PAYLOAD_FORMAT_EMV_QRCPS_MERCHANT_PRESENTED_MODE))
	if amount != 0 {
		data = append(data, f(ID_TRANSACTION_AMOUNT, formatAmount(amount)))
	}

	dataToCrc := fmt.Sprintf("%s%s%s", serialize(data), ID_CRC, "04")
	crcValue := crc.CalculateCRC(crc.CCITT, []byte(dataToCrc))
	data = append(data, f(ID_CRC, formatCrc(crcValue)))
	return serialize(data)
}