--- /dev/null
+package dns
+
+// Holds a bunch of helper functions for dealing with labels.
+
+// SplitDomainName splits a name string into it's labels.
+// www.miek.nl. returns []string{"www", "miek", "nl"}
+// .www.miek.nl. returns []string{"", "www", "miek", "nl"},
+// The root label (.) returns nil. Note that using
+// strings.Split(s) will work in most cases, but does not handle
+// escaped dots (\.) for instance.
+// s must be a syntactically valid domain name, see IsDomainName.
+func SplitDomainName(s string) (labels []string) {
+ if len(s) == 0 {
+ return nil
+ }
+ fqdnEnd := 0 // offset of the final '.' or the length of the name
+ idx := Split(s)
+ begin := 0
+ if IsFqdn(s) {
+ fqdnEnd = len(s) - 1
+ } else {
+ fqdnEnd = len(s)
+ }
+
+ switch len(idx) {
+ case 0:
+ return nil
+ case 1:
+ // no-op
+ default:
+ end := 0
+ for i := 1; i < len(idx); i++ {
+ end = idx[i]
+ labels = append(labels, s[begin:end-1])
+ begin = end
+ }
+ }
+
+ return append(labels, s[begin:fqdnEnd])
+}
+
+// CompareDomainName compares the names s1 and s2 and
+// returns how many labels they have in common starting from the *right*.
+// The comparison stops at the first inequality. The names are downcased
+// before the comparison.
+//
+// www.miek.nl. and miek.nl. have two labels in common: miek and nl
+// www.miek.nl. and www.bla.nl. have one label in common: nl
+//
+// s1 and s2 must be syntactically valid domain names.
+func CompareDomainName(s1, s2 string) (n int) {
+ // the first check: root label
+ if s1 == "." || s2 == "." {
+ return 0
+ }
+
+ l1 := Split(s1)
+ l2 := Split(s2)
+
+ j1 := len(l1) - 1 // end
+ i1 := len(l1) - 2 // start
+ j2 := len(l2) - 1
+ i2 := len(l2) - 2
+ // the second check can be done here: last/only label
+ // before we fall through into the for-loop below
+ if equal(s1[l1[j1]:], s2[l2[j2]:]) {
+ n++
+ } else {
+ return
+ }
+ for {
+ if i1 < 0 || i2 < 0 {
+ break
+ }
+ if equal(s1[l1[i1]:l1[j1]], s2[l2[i2]:l2[j2]]) {
+ n++
+ } else {
+ break
+ }
+ j1--
+ i1--
+ j2--
+ i2--
+ }
+ return
+}
+
+// CountLabel counts the the number of labels in the string s.
+// s must be a syntactically valid domain name.
+func CountLabel(s string) (labels int) {
+ if s == "." {
+ return
+ }
+ off := 0
+ end := false
+ for {
+ off, end = NextLabel(s, off)
+ labels++
+ if end {
+ return
+ }
+ }
+}
+
+// Split splits a name s into its label indexes.
+// www.miek.nl. returns []int{0, 4, 9}, www.miek.nl also returns []int{0, 4, 9}.
+// The root name (.) returns nil. Also see SplitDomainName.
+// s must be a syntactically valid domain name.
+func Split(s string) []int {
+ if s == "." {
+ return nil
+ }
+ idx := make([]int, 1, 3)
+ off := 0
+ end := false
+
+ for {
+ off, end = NextLabel(s, off)
+ if end {
+ return idx
+ }
+ idx = append(idx, off)
+ }
+}
+
+// NextLabel returns the index of the start of the next label in the
+// string s starting at offset.
+// The bool end is true when the end of the string has been reached.
+// Also see PrevLabel.
+func NextLabel(s string, offset int) (i int, end bool) {
+ quote := false
+ for i = offset; i < len(s)-1; i++ {
+ switch s[i] {
+ case '\\':
+ quote = !quote
+ default:
+ quote = false
+ case '.':
+ if quote {
+ quote = !quote
+ continue
+ }
+ return i + 1, false
+ }
+ }
+ return i + 1, true
+}
+
+// PrevLabel returns the index of the label when starting from the right and
+// jumping n labels to the left.
+// The bool start is true when the start of the string has been overshot.
+// Also see NextLabel.
+func PrevLabel(s string, n int) (i int, start bool) {
+ if n == 0 {
+ return len(s), false
+ }
+ lab := Split(s)
+ if lab == nil {
+ return 0, true
+ }
+ if n > len(lab) {
+ return 0, true
+ }
+ return lab[len(lab)-n], false
+}
+
+// equal compares a and b while ignoring case. It returns true when equal otherwise false.
+func equal(a, b string) bool {
+ // might be lifted into API function.
+ la := len(a)
+ lb := len(b)
+ if la != lb {
+ return false
+ }
+
+ for i := la - 1; i >= 0; i-- {
+ ai := a[i]
+ bi := b[i]
+ if ai >= 'A' && ai <= 'Z' {
+ ai |= 'a' - 'A'
+ }
+ if bi >= 'A' && bi <= 'Z' {
+ bi |= 'a' - 'A'
+ }
+ if ai != bi {
+ return false
+ }
+ }
+ return true
+}
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