Enhanced documentation for NBLog.cpp

* Documentation: Workflow summary for histogram data processing:

* For more details on FIFO, please see system/media/audio_utils; doxygen

* TODO: add this documentation to doxygen once it is further developed

* 1) writing the data to a buffer

* onWork

* 1) Writing buffer period timestamp to the circular buffer

* onWork()

*     Called every period length (e.g., 4ms)

*     Calls LOG_HIST_TS

* LOG_HIST_TS

*     Determines readable buffer section via pointer arithmetic on reader

*     and writer pointers

*

* 2) reading the data from shared memory

* 2) Writing LOG_HIST_FLUSH event to console when audio is turned on or off

*    When this event is found when reading from the buffer, all histograms are

*    printed to the console

*    TODO: remove this: always write data to another data structure or the console

* FastMixer::onStateChange()

*     is called when audio is turned on/off

*     calls LOG_HIST_FLUSH()

* LOG_HIST_FLUSH()

*     calls logHistFlush

* NBLog::Writer::logHistFlush

*     records current timestamp to write it to the console

*     calls log(EVENT_HISTOGRAM_FLUSH)

*     From here, everything is the same as in 1), resulting in call to fifo write

*

* 3) reading the data from shared memory

* Thread::threadloop()

*     TODO: add description?

* NBLog::MergeThread::threadLoop()

*     calls NBLog::Merger::merge

* NBLog::Merger::merge

*     Merges snapshots sorted by timestamp

*     for each reader in vector of class NamedReader,

*     callsNamedReader::reader()->getSnapshot

*     TODO: check whether the rest of this function is relevant

*     calls mFifoReader->obtain to find readable data

*     sets snapshot.begin() and .end() iterators to boundaries of valid entries

*     moves the fifo reader index to after the last entry read

*     in this case, the buffer is in shared memory. in (3), the buffer is private

*     in this case, the buffer is in shared memory. in (4), the buffer is private

*

* 3) reading the data from private buffer

* 4) reading the data from private buffer

* MediaLogService::dump

*     calls NBLog::Reader::dump(int) on instance of subclass mergeReader

*     TODO: when was MediaLogService::dump called?

*     For each NBLog::Reader in vector NamedReaders (subclass mergeReader):

*     calls NBLog::Reader::dump(int)

* NBLog::Reader::dump(int)

*     calls getSnapshot on the current reader

*     calls dump(int, size_t, Snapshot)

*     (histogram entry) to NBLog::mHists

*     In the case of EVENT_HISTOGRAM_FLUSH, calls drawHistogram on each element in

*     the list and erases it

*     TODO: when do these events occur?

*     TODO: get rid of the FLUSH, instead add every HISTOGRAM_ENTRY_TS to two

*     circular buffers: one short-term and one long-term (can add even longer-term

*     structures in the future). When dump is called, print everything currently

*     in the buffer.

* NBLog::drawHistogram

*     input: timestamp array

*     buckets this to a histogram and prints