func formatNsnUsingPatternWithCarrier()

in phonenumbers.go [1849:1920]
65 lines of code
12 McCabe index (conditional complexity)

func formatNsnUsingPatternWithCarrier(
	nationalNumber string,
	formattingPattern *NumberFormat,
	numberFormat PhoneNumberFormat,
	carrierCode string) string {

	numberFormatRule := formattingPattern.GetFormat()
	m := regexFor(formattingPattern.GetPattern())

	formattedNationalNumber := ""
	if numberFormat == NATIONAL &&
		len(carrierCode) > 0 &&
		len(formattingPattern.GetDomesticCarrierCodeFormattingRule()) > 0 {
		// Replace the $CC in the formatting rule with the desired carrier code.
		carrierCodeFormattingRule := formattingPattern.GetDomesticCarrierCodeFormattingRule()
		i := 1
		carrierCodeFormattingRule =
			CC_PATTERN.ReplaceAllStringFunc(carrierCodeFormattingRule,
				func(s string) string {
					if i > 0 {
						i -= 1
						return carrierCode
					}
					return s
				})
		// Now replace the $FG in the formatting rule with the first group
		// and the carrier code combined in the appropriate way.
		i = 1
		numberFormatRule = FIRST_GROUP_PATTERN.ReplaceAllStringFunc(
			numberFormatRule,
			func(s string) string {
				if i > 0 {
					i -= 1
					return carrierCodeFormattingRule
				}
				return s
			})
		formattedNationalNumber = m.ReplaceAllString(numberFormatRule, nationalNumber)
	} else {
		// Use the national prefix formatting rule instead.
		nationalPrefixFormattingRule :=
			formattingPattern.GetNationalPrefixFormattingRule()
		if numberFormat == NATIONAL &&
			len(nationalPrefixFormattingRule) > 0 {
			i := 1
			fgp := FIRST_GROUP_PATTERN.ReplaceAllStringFunc(numberFormatRule,
				func(s string) string {
					if i > 0 {
						i -= 1
						return nationalPrefixFormattingRule
					}
					return s
				})
			formattedNationalNumber = m.ReplaceAllString(nationalNumber, fgp)
		} else {
			formattedNationalNumber = m.ReplaceAllString(
				nationalNumber,
				numberFormatRule,
			)
		}
	}
	if numberFormat == RFC3966 {
		// Strip any leading punctuation.
		inds := SEPARATOR_PATTERN.FindStringIndex(formattedNationalNumber)
		if len(inds) > 0 && inds[0] == 0 {
			formattedNationalNumber = formattedNationalNumber[inds[1]:]
		}
		allStr := NOT_SEPARATOR_PATTERN.FindAllString(formattedNationalNumber, -1)
		formattedNationalNumber = strings.Join(allStr, "-")
	}
	return formattedNationalNumber
}