// Package gtin validates and normalizes GS1 trade item numbers (GTIN-8/12/13/14).
// Only globally-unique GS1 codes are accepted: store-internal / variable-weight /
// coupon codes (which are not globally unique) are rejected on purpose.
package gtin

import (
	"errors"
	"strings"
)

// Validation errors.
var (
	ErrEmpty      = errors.New("条码不能为空")
	ErrFormat     = errors.New("条码必须为 8/12/13/14 位数字")
	ErrCheck      = errors.New("条码校验位不正确")
	ErrRestricted = errors.New("店内码/变量重量码/优惠券码等非全球唯一码，不予收录")
)

// Normalize trims and validates a GTIN, returning the cleaned digit string.
// It enforces length, the GS1 mod-10 check digit, and rejects restricted
// (non-globally-unique) number ranges.
func Normalize(raw string) (string, error) {
	s := strings.TrimSpace(raw)
	if s == "" {
		return "", ErrEmpty
	}
	for _, c := range s {
		if c < '0' || c > '9' {
			return "", ErrFormat
		}
	}
	switch len(s) {
	case 8, 12, 13, 14:
	default:
		return "", ErrFormat
	}
	if !validCheckDigit(s) {
		return "", ErrCheck
	}
	if restricted(s) {
		return "", ErrRestricted
	}
	return s, nil
}

// InferType returns the conventional GTIN type label for a normalized code.
func InferType(s string) string {
	switch len(s) {
	case 8:
		return "EAN8"
	case 12:
		return "UPC"
	case 14:
		return "GTIN14"
	default:
		return "EAN13"
	}
}

// validCheckDigit verifies the trailing GS1 mod-10 check digit. The digit
// immediately left of the check digit carries weight 3, then weights alternate.
func validCheckDigit(s string) bool {
	n := len(s)
	sum := 0
	for i := 0; i < n-1; i++ {
		d := int(s[i] - '0')
		if (n-1-i)%2 == 1 {
			sum += d * 3
		} else {
			sum += d
		}
	}
	check := (10 - (sum % 10)) % 10
	return check == int(s[n-1]-'0')
}

// restricted reports whether a (length/check-digit valid) code falls in a
// number range reserved for non-globally-unique use.
func restricted(s string) bool {
	switch len(s) {
	case 13:
		p2 := s[:2]
		switch {
		case s[0] == '2': // 20-29 restricted distribution / in-store
			return true
		case p2 == "02": // 020-029 variable-measure within a store
			return true
		case p2 == "04": // 040-049 restricted circulation within a company
			return true
		case p2 == "05": // 050-059 coupons
			return true
		case p2 == "98" || p2 == "99": // 980-989/99 coupons & refund receipts
			return true
		}
	case 12: // UPC-A: leading number-system digit
		switch s[0] {
		case '2': // in-store / random weight
			return true
		case '4': // unrestricted in-store use
			return true
		case '5': // coupons
			return true
		}
	case 8: // EAN-8: 0/2 prefixes reserved for in-store use
		if s[0] == '0' || s[0] == '2' {
			return true
		}
	}
	return false
}