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