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package dns
// Dedup removes identical RRs from rrs. It preserves the original ordering.
// The lowest TTL of any duplicates is used in the remaining one. Dedup modifies
// rrs.
// m is used to store the RRs temporary. If it is nil a new map will be allocated.
func Dedup(rrs []RR, m map[string]RR) []RR {
if m == nil {
m = make(map[string]RR)
}
// Save the keys, so we don't have to call normalizedString twice.
keys := make([]*string, 0, len(rrs))
for _, r := range rrs {
key := normalizedString(r)
keys = append(keys, &key)
if _, ok := m[key]; ok {
// Shortest TTL wins.
if m[key].Header().Ttl > r.Header().Ttl {
m[key].Header().Ttl = r.Header().Ttl
}
continue
}
m[key] = r
}
// If the length of the result map equals the amount of RRs we got,
// it means they were all different. We can then just return the original rrset.
if len(m) == len(rrs) {
return rrs
}
j := 0
for i, r := range rrs {
// If keys[i] lives in the map, we should copy and remove it.
if _, ok := m[*keys[i]]; ok {
delete(m, *keys[i])
rrs[j] = r
j++
}
if len(m) == 0 {
break
}
}
return rrs[:j]
}
// normalizedString returns a normalized string from r. The TTL
// is removed and the domain name is lowercased. We go from this:
// DomainName<TAB>TTL<TAB>CLASS<TAB>TYPE<TAB>RDATA to:
// lowercasename<TAB>CLASS<TAB>TYPE...
func normalizedString(r RR) string {
// A string Go DNS makes has: domainname<TAB>TTL<TAB>...
b := []byte(r.String())
// find the first non-escaped tab, then another, so we capture where the TTL lives.
esc := false
ttlStart, ttlEnd := 0, 0
for i := 0; i < len(b) && ttlEnd == 0; i++ {
switch {
case b[i] == '\\':
esc = !esc
case b[i] == '\t' && !esc:
if ttlStart == 0 {
ttlStart = i
continue
}
if ttlEnd == 0 {
ttlEnd = i
}
case b[i] >= 'A' && b[i] <= 'Z' && !esc:
b[i] += 32
default:
esc = false
}
}
// remove TTL.
copy(b[ttlStart:], b[ttlEnd:])
cut := ttlEnd - ttlStart
return string(b[:len(b)-cut])
}
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