/*! \page Stars Stars    
  \tableofcontents
  Design document for the implementation of star catalog support in KStars.
  \section StarsClassInheritance Class Inheritance
  + StarComponent - Manages all stars
      - struct starName - Holds a star's names (defined within StarComponent)
  + DeepStarComponent - Manages unnamed stars (instances managed by StarComponent)
  + StarBlock - A block of many (usually 100) stars
  + StarBlockList - A list of StarBlocks; machinery to load data dynamically from catalog files
  + StarBlockFactory - Maintains a cache of recently used StarBlocks
  + BinFileHelper - A generic helper class used to deal with binary data files (See ../binfilehelper.h)
  + starData - holds 32 bits of data describing a star (See ../skyobjects/stardata.h)
  + deepStarData - holds 16 bits of data describing a (faint) star (See ../skyobjects/deepstardata.h)

  \section Categorization Categorization of Stars
  Stars could be either named or unnamed. Named stars and unnamed stars
  are treated differently in KStars. Stars having names are handled by
  the methods defined in class StarComponent. Unnamed stars are handled
  by the methods defined in class DeepStarComponent, which are invoked
  via the methods of StarComponent.

  A second categorization is "static" and "dynamic" stars. The
  StarObjects for "static" stars are always kept in memory. They are
  loaded into memory when KStars initializes. "Dynamic" stars are loaded
  into memory from the disk when required. When the sky map is slewed to
  a new region, or zoomed to a greater depth, dynamic stars are loaded
  "on-demand" into memory from the disk and drawn. Named stars are
  always static. Unnamed stars could be either static or
  dynamic. Naturally, StarComponent deals only with static stars whereas
  DeepStarComponent deals with both static and dynamic stars.

  A third categorization is "shallow" and "deep" stars. Shallow stars
  are the brighter stars, which have enough catalog data to require a
  32-byte data structure (starData) to hold them. Deep stars are the
  fainter ones, data for which can be fit into a 16-byte structure
  (deepStarData). All named stars and all static stars are shallow. All
  deep stars are dynamic and unnamed. StarComponent manages only shallow
  stars. DeepStarComponent manages both shallow and deep stars.
  
  Stars are indexed using \subpage HTMIndexing "multi-level Hierarchical Triangular Mesh" to speed
  up drawing and searching for stars in the skymap.
  
  Stars are stored in a custom \ref BinaryFormat Binary Format to speed up reading and loading of data into memory.

  \section Catalogs Catalogs
  As of this writing, KStars supports the following catalogs:
  1. Default named stars catalog ("namedstars.dat" in the data directory)
     [Together with default unnamed stars catalog, makes up all stars to mag 8]
       + Named
       + Static
       + Shallow
       + Managed by StarComponent's methods
       + Names of stars stored in "starnames.dat" in the data directory,
         in order of appearance in namedstars.dat

  2. Default unnamed stars catalog ("unnamedstars.dat" in the data directory)
     [Together with default named stars catalog, makes up all stars to mag 8]
       + Unnamed
       + Static
       + Shallow
       + Managed by DeepStarComponent

  3. Tycho-2 catalog (Downloadable add-on)
     Prerequisite: Default catalogs (namedstars.dat, unnamedstars.dat and starnames.dat)
       + Unnamed
       + Dynamic
       + Shallow
       + Managed by DeepStarComponent
       + Reside on a level-3 \subpage HTMIndexing HTMesh
       + Down to mag ~12

  4. USNO NOMAD (part) 100 million star catalog (Downloadable add-on)
     Prerequisite: Tycho-2 catalog
       + Unnamed
       + Dynamic
       + Deep
       + Managed by DeepStarComponent
       + Reside on a level-6 \subpage HTMIndexing HTMesh
       + Down to mag ~16

  \section NamedStars Named Stars
  Named Stars are dealt with by StarComponent. Their case is very simple
  because they are all static stars. The star names are stored in a
  separate file (starnames.dat), in which the names are maintained in
  the same order in which stars appear in the named star catalog
  (namedstars.dat). Both files are opened while reading (See
  StarComponent::loadStaticData()) and data is read off from the two
  files sequentially, side-by-side.

  Each record in starnames.dat is 40 bytes long, containing a long name
  (32 bytes) and a Bayer designation (8 bytes).

  Each record in namedstars.dat is 32 bytes long, and holds a direct
  memory dump of struct starData, made on a little-endian system. (On
  big endian systems, byte order inversion is done at runtime)

  Named stars are read off the disk at initialization time by
  StarComponent::loadStaticStars() and are drawn by the loops in
  StarComponent::draw(). All of them reside in the memory throughout
  KStars' life.

  \section UnnamedStars Unnamed Stars
  Much of the machinery listed out above go into maintenance of unnamed
  stars.

  Unnamed stars are dealt with by DeepStarComponent.

  StarComponent has a QVector of DeepStarComponent objects, which are
  created for each unnamed-star catalog loaded. (See
  StarComponent::loadDeepStarCatalogs().) StarComponent takes
  responsibility to call methods of the DeepStarComponents when
  required. For instance, StarComponent::draw() calls
  DeepStarComponent::draw() on each catalog, after drawing the named
  stars. (StarComponent, on the other hand, is managed by
  SkyMapComposite)

  \subsection MemoryMap Memory map
  Memory map of unnamed stars:
  + StarComponent
    + One DeepStarComponent for every catalog
      + One StarBlockList for each trixel       + StarBlockFactory churns out StarBlocks
        + Several StarBlocks are attached / detached dynamically to a StarBlockList
          + Several StarObjects (usu. 100) in a StarBlock
  \subsection Loading Loading unnamed stars
  As mentioned earlier, unnamed stars may be both static and dynamic,
  and could be both deep and shallow.
  \subsubsection Creation Creation of unnamed StarObjects
  TODO: This is outdated. Stars are now created differently.

  IMPORTANT: Unnamed stars, for fast loading, are not constructed!!!
             They are allocated in memory (using malloc) and data is
             copied directly (using memcpy) onto them.
             This can lead to very strange segfaults with misleading
             backtraces if not understood carefully.

  For each record read off the data file, a StarObject instance
  (plainStarTemplate) is loaded with the data using
  StarObject::init(...) and then memcpy()'d and held in a StarBlock
  \subsubsection LoadingStatic Loading unnamed, static stars
  The method DeepStarComponent::loadStaticStars() (invoked from
  StarComponent) loads all static stars in an unnamed-star catalog.

  The star data files are organized trixel by trixel. The method enters
  each trixel, creates a StarBlock of variable size, enough to hold all
  the unnamed stars in the current trixel, and then reads star-by-star
  into memory.

  Static stars always occupy the first StarBlock in a StarBlockList.
  \subsubsection LoadingDynamic Loading unnamed, dynamic stars
  Dynamic stars are loaded on-demand (during the draw() call) in
  StarBlockList::fillToMag().

  NOTE: This can be a problem for objectNearest(). objectNearest() calls
  work correctly only when the sky map is centred on the required
  trixel(s). For extending this when required, one must call
  StarBlockList::fillToMag() from objectNearest()

  fillToMag() is invoked by DeepStarComponent::draw(), for every trixel
  present on-screen, with the current magnitude limit (calculated from
  the zoom level) as an argument. fillToMag()'s job is to ensure that
  the StarBlockList is filled with stars up to that magnitude by loading
  more stars from disk if required.

  An important point to note is that StarBlockList maintains stars
  sorted by magnitude - brightest first - to match the data
  file. StarBlocks at the "tail end" of the StarBlockList are the
  faintest. fillToMag() quickly checks the magnitude of the last star in
  the last StarBlock of the StarBlockList and starts filling into the
  last StarBlock.

  Each StarBlockList maintains the current read offset in the data file,
  so that it can quickly seek to the point where it last loaded and
  continue loading fainter stars. When stars are removed from a
  StarBlockList, the read offset is decremented.

  StarBlocks holding dynamic stars are always of fixed size
  (default=100). There could be half-filled StarBlocks. (We save
  allocation overhead by recycling StarBlocks using
  StarBlockFactory. More about this later.) StarBlockList::fillToMag()
  is aware of this fact and continues to fill half-filled StarBlocks
  when required.

  \subsection Recycling Recycling of StarBlocks
  StarBlockFactory maintains a Least-Recently-Used cache of "dynamic"
  StarBlocks (which hold dynamic stars).

  When StarBlockList::fillToMag() asks for new star blocks,
  StarBlockFactory sees if there are unused (trixel not-on-screen)
  StarBlocks lying around. If there are none, it allocates a new
  StarBlock and hands it over to StarBlockList. But if there are unused
  StarBlocks lying around, StarBlockFactory recycles them.

  Imagine a situation where the map is panned. A trixel that was
  on-screen till now might no longer displayed on the screen, but a new
  trixel might come into focus. In this case, StarBlockFactory detaches
  the StarBlocks attached to the StarBlockList of the old trixel and
  gives them to the StarBlockList of the new trixel. The new data read
  off the disk is then StarObject::init()'ed into the StarBlock.

  Each draw cycle in KStars has a drawID that is incremented upon every
  draw. This makes the drawID almost unique, since it has a very large
  period. StarBlockFactory has a local copy of this drawID that is
  maintained in sync with KStars' drawID. When stars are drawn in
  StarComponent::draw(), every dynamic StarBlock that's drawn is
  "touched" by calling StarBlockFactory::mark*().

  When drawing a trixel, markFirst() is called on the first StarBlock in
  every StarBlockList and markNext() is called on subsequent ones. The
  freshest StarBlocks are maintained at the beginning of the linked list
  inside StarBlockFactory and equally-fresh StarBlocks are ordered in
  the same order as they appear in StarBlockList. Thus, StarBlockFactory
  can safely pluck out the last StarBlock in the linked list and recycle
  it (provided it was last touched in an older draw) without affecting
  the ordering of stars in StarBlockList (i.e. we ensure that we don't
  pluck out a StarBlock from in-between a StarBlockList. Only the last
  StarBlock of a StarBlockList can be released at any time). Thus, the
  ordering of StarBlocks in the linked-list of StarBlockFactory, is:
   + First, by drawID
   + Then, by catalog (i.e. unnamedstars.dat / Tycho-2 / USNO-NOMAD-1e8)
   + Then, by trixel (only those trixels that are on-screen are considered)
   + Finally, by magnitude

  When StarBlockFactory wants to re-use a StarBlock, it first verifies
  that the drawID is outdated. Then, it calls
  StarBlockList::releaseBlock() on the parent StarBlockList and thus
  orphans the StarBlock. The orphaned StarBlock is handed over to the
  new StarBlockList which adopts it.

  Thus, KStars initially allocates new StarBlocks until a saturation
  point is reached, where there are enough StarBlocks to keep
  recycling. Sagittarius has the maximum number of stars, and setting
  the focus on Sagittarius will usually lead to saturation (where KStars
  consumes 200-odd MB of memory with USNO-NOMAD-1e8), and this memory is
  not freed till KStars exits. A possible extension would be to free
  StarBlocks lazily after their drawID's get really outdated, so that
  the memory surge is temporary.

  \subsection Files Star Catalog Files
  Star catalogs are stored in the format described in
  ../data/README.binfileformat

  Star catalog files were created by first taking ASCII data and putting
  them into MySQL database tables and then dumped in a sorted manner
  into the catalog files. The tools used are in ../data/tools. The ASCII
  originals from public domain sources are available with Akarsh Simha
  and Jason Harris, or with the US Naval Observatory itself.

  Stars in the catalog files are first ordered by trixel (the "Index"
  parameter in the language of README.binfileformat) and then ordered by
  magnitude (brightest first) within each trixel.

  \subsection ProperMotion Proper Motion
  All star data in KStars has first order proper motion rates (called
  "dRA" and "dDec" or "pmRA" and "pmDec"), which are used to correct for
  proper motion.

  This leads to a problem in the case of dynamic stars, as a star may
  cross over from one trixel to another trixel due to proper motion, in
  which case it might not be drawn despite its corrected coordinates
  being on-screen.

  To solve this, stars are duplicated in all trixels that they will pass
  through in a span of +/-10,000 years from the epoch. StarObject has
  some commented code that can be used to track the position of a
  star. [Maybe this should be turned into a feature in future]
  
  TODO: Describe details like the 25 arcminute "safety" margin.

  \section HDCatalog Henry-Draper Catalog Numbers
  StarObject allows for a Henry-Draper catalog number to be associated
  with a star. KStars allows for both static and dynamic stars to have
  an associated catalog number. This is currently used for the
  Henry-Draper number, although it could be extended to other catalogs.

  Both static and dynamic stars from Tycho-2 appear in the HD
  catalog. To find dynamic stars by HD number, a downloadable add-on
  "Henry Draper Index" is available. This file contains a list of 4-byte
  offsets in sequential order of HD number. These offsets tell KStars
  where to find the star in the Tycho-2 catalog binary data file. See
  StarComponent::findByHDIndex() for details. See
  ../data/README.indexfileformat for details about the format followed
  by the Henry-Draper index file.  
  
  \section BinaryFormat Binary Format
  
  Stars data are stored in binary format to speed up reading and loading data into memory. There are a number of tools in the data/tools directory that parses the original ASCII data files and convert them to a KStars compatible binary format.
  
  \subsection Header The Header
  \subsubsection Preamble Preamble
  The preamble consists of the following:

 * A 124 byte-long string containing human-readable information about
   the file

 * A 2 byte-long endinanness indicator field, preinitialized to 'KS'
   (0x4B53) on the host machine

 The purpose of the endianness indicator is to make the file portable
 across machines of different endianness. A machine on which the
 endianness indicator field is read as 'SK' instead of 'KS' is of
 different endinanness from the machine on which the file was
 generated, and hence byteswapping must be performed.

 * A 1 byte-long unsigned integer for the version of the binary format structure.
 
 \subsubsection FieldDescriptor Field Descriptor
 The field descriptor describes the various fields used in the latter
 half of the data region. The field descriptor consists of the
 following:

 * A 2 byte-long unsigned integer, indicating the number of fields per
   record

 * A set of entries describing the fields, amounting to the number
   previously mentioned, each consisting of:

     - A 10 byte-long character array, providing a human-readable name
       for the field

     - A 1 byte-long integer carrying the size of the field in bytes

     - A 1 byte-long unsigned integer carrying the data type (see
       Appendix A)

     - A 4 byte-long integer storing a scale factor to divide the
       contents of the associated field by, to interpret it
       correctly. This is used for storing fixed-point fields. (Eg: A
       fixed-point number with 2 decimal places would have a scale
       factor of 100)

  The set of entries is read directly into dataElement structure and byte swapping is performed if needed.

  \subsubsection IndexTable Index Table
   The format permits an index on one parameter. The parameter is to be
 represented by 4-bytes (eg: A running index from 0 to 4,294,967,295). The
 index table consists of the following:

 * A 4 byte unsigned integer indicating the number of index entries

 * Index table entries, each consisting of:

     - A 4 byte-long unsigned integer parameter, which is the index
     
     - A 4 byte-long unsigned integer, storing the offset of the first
       record with this index parameter in the file

     - A 4 byte-long unsigned integer, storing the number of records
       under this index parameter.

  \subsection TheData The Data

  \subsubsection ExpansionFields Expansion Fields
   Certain specific information about the contents of the field may be
 added here. The contents could be specific to each application, and
 could be empty. For KStars star data files, the entries in this
 region are:

   + Faint magnitude limit * 100, stored as a 16-bit integer

   + HTM Level for this file, stored as a 8-bit integer. The
     convention followed is that "N0000" would belong to a HTMesh of
     level 3.

   + Maximum Stars per Trixel, stored as a 2-byte unsigned integer.
   
   \subsubsection DataRecords Data Records
   This region starts at the offset pointed out in the first entry of
 the index table. The data consists of a series of records, the
 details of whose fields (assumed to be placed continuously, without
 any gaps) are as described in the field descriptor in the header.

 NOTE: KStars _overrides_ the above defined convention for performance
 gains and ease of programming in the star names file, where records
 are stored as single blocks of the "starName" structure defined in
 kstars/kstars/skycomponents/starcomponent.h
 
 \section HDIndex Henry Draper Indexing
The index file used to store the Henry Draper catalog indexes for
KStars is just a list of 4-byte long offsets (to be treated as 32-bit
integers ) describing the position of the Henry Draper stars' records
in order, in the binary data files.

If the offset is negative, it indicates that the star is in the
shallow star file and not in the Tycho-2 deep star add on. The actual
offset in this case is just the absolute value of the offset.

The first four bytes in the index file correspond to the offset of the
star HD000001 in the data files, and so on.

\section Credits Credits
  Most of the memory layout and machinery outline described above was
  the brainchild of James Bowlin and it was implemented in code by
  Akarsh Simha. Jason Harris helped with the catalog data and also
  mentored Akarsh along with James. Some of the parsing tools in the
  kstars/data/tools directory have code due to James.
  
\author Akarsh Simha
*/
