Source: compact.go in package golang.org/x/text/internal/language/compact

Package compact defines a compact representation of language tags. Common language tags (at least all for which locale information is defined in CLDR) are assigned a unique index. Each Tag is associated with such an ID for selecting language-related resources (such as translations) as well as one for selecting regional defaults (currency, number formatting, etc.) It may want to export this functionality at some point, but at this point this is only available for use within x/text.

package compact // import "golang.org/x/text/internal/language/compact"

import (
	"sort"
	"strings"

	"golang.org/x/text/internal/language"
)

ID is an integer identifying a single tag.

type ID uint16

func getCoreIndex(t language.Tag) (id ID, ok bool) {
	cci, ok := language.GetCompactCore(t)
	if !ok {
		return 0, false
	}
	i := sort.Search(len(coreTags), func(i int) bool {
		return cci <= coreTags[i]
	})
	if i == len(coreTags) || coreTags[i] != cci {
		return 0, false
	}
	return ID(i), true
}

Parent returns the ID of the parent or the root ID if id is already the root.

func (id ID) Parent() ID {
	return parents[id]
}

Tag converts id to an internal language Tag.

func (id ID) Tag() language.Tag {
	if int(id) >= len(coreTags) {
		return specialTags[int(id)-len(coreTags)]
	}
	return coreTags[id].Tag()
}

var specialTags []language.Tag

func init() {
	tags := strings.Split(specialTagsStr, " ")
	specialTags = make([]language.Tag, len(tags))
	for i, t := range tags {
		specialTags[i] = language.MustParse(t)
	}