BIND 10 #2148: extendable LabelSequence

[BIND 10 Development]

#2148: extendable LabelSequence
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            Reporter:  jinmei        |                        Owner:
                Type:  task          |  UnAssigned
            Priority:  medium        |                       Status:  new
           Component:  libdns++      |                    Milestone:
           Sensitive:  0             |  Sprint-20120731
         Sub-Project:  DNS           |                     Keywords:
Estimated Difficulty:  0             |              Defect Severity:  N/A
         Total Hours:  0             |  Feature Depending on Ticket:
                                     |  scalable inmemory
                                     |          Add Hours to Ticket:  0
                                     |                    Internal?:  0
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 While working on other tickets, I've come up with an idea of
 possible extension mentioned in #2093.  It's a two-fold extension
 to `LabelSequence`:

 - Allow constructing the object with an external buffer:
 {{{#!cpp
    LabelSequence(const LabelSequence& src, uint8_t
 buf[MAX_SERIALIZED_LENGTH]);
    // MAX_SERIALIZED_LENGTH is the constant introduced in trac2091a.  It
    // should have a sufficient space to internally hold the name and
    // offset data.
 }}}
   this constructor makes a "deep copy" of the "src" (LabelSequence),
   copying name and offset data to `buf` (name data would start
   at buf[0], and offsets would start at buf[Name::MAX_WIRE]).
 - Introduce another method "extend" to `LabelSequence`
 {{{
     void extend(const LabelSequence labels, uint8_t
 buf[MAX_SERIALIZED_LENGTH]);
 }}}
   This works only for `LabelSequence` constructed with the external
   buffer.  This method checks the condition by comparing the passed
   buf and internal pointers, and if it's okay, it appends the data of
   `labels` to `buf` and adjusts `last_label_` as long as the total
   length allows the extension.

 With these extensions, `getAbsoluteLabelSequence` of #2093 will be
 easily implemented by starting with the new construct and repeated
 calls to `extend`.

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Ticket URL: <http://bind10.isc.org/ticket/2148>
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