VLAArchiveInput.h

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//# VLAArchiveInput.h: An abstract class for reading VLA archive records
//# Copyright (C) 1999,2000
//# Associated Universities, Inc. Washington DC, USA.
//#
//# This library is free software; you can redistribute it and/or modify it
//# under the terms of the GNU Library General Public License as published by
//# the Free Software Foundation; either version 2 of the License, or (at your
//# option) any later version.
//#
//# This library is distributed in the hope that it will be useful, but WITHOUT
//# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
//# FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Library General Public
//# License for more details.
//#
//# You should have received a copy of the GNU Library General Public License
//# along with this library; if not, write to the Free Software Foundation,
//# Inc., 675 Massachusetts Ave, Cambridge, MA 02139, USA.
//#
//# Correspondence concerning AIPS++ should be addressed as follows:
//#        Internet email: aips2-request@nrao.edu.
//#        Postal address: AIPS++ Project Office
//#                        National Radio Astronomy Observatory
//#                        520 Edgemont Road
//#                        Charlottesville, VA 22903-2475 USA
//#
//# $Id$

#ifndef NRAO_VLAARCHIVEINPUT_H
#define NRAO_VLAARCHIVEINPUT_H

#include <casa/aips.h>
#include <casa/IO/ByteSinkSource.h>
#include <casa/IO/ConversionIO.h>
#include <casa/IO/MemoryIO.h>
#include <casa/OS/ModcompDataConversion.h>

#include <casa/namespace.h>
namespace casa { //# NAMESPACE CASA - BEGIN
class ByteSource;
} //# NAMESPACE CASA - END


// <summary>This class reads VLA archive records from a Tape</summary>

// <reviewed reviewer="" date="" tests="" demos="">

// <prerequisite>
// <ol>
//   <li> The IO Module
// </ol>
// </prerequisite>
//
// <etymology>
// This class is designed to reads VLA archive records from a Tape
// </etymology>
//
// <synopsis> 

// This class is designed to read VLA archive data.  The data may be read from
// a disk, tape drive or any other data source supported by the IO module.  A
// call to the operator++() function assembles the next reconstructed VLA
// archive data record from the input. A reference to this data can be
// obtained using the logicalRecord function.
//
// Refer to the "VLA Archive Data Format", VLA Computer Memorandum 186
// by G.C. Hunt, K.P. Sowinski, and T.J. Bottomly; June 1993.
// (This is also available as AIPS++ note 159)
//
// The VLA archive records are always a multiple of 2048 bytes.  The
// record sizes were designed for use with magnetic tapes, so there is
// a maximum physical record size of 13*2048=26624 bytes.
//
// The low level class (blockio), that actually does the I/O, allows
// for a record (hereinafter chunk) size and for a input block size of
// a multiple of the chunk size.  The low level read operation tests
// for the number of bytes actually read from the device.
//
// The helper classes VlaDiskInput, VlaTapeInput, and VlaStdInput are
// designed to deal with the low level input from the devices in an
// analogous fashion to the ones used for FITS input.
//
// Since a read may be issued for an arbitrary number of bytes from a
// disk, the chunk multiple is arbitrary and may be used to tune the
// speed of operation.  There is an obvious trade-off between the
// block size created in the blockio class and the number of read
// operations.
//
// The story is quite different for tape input.  A read request for at
// least the maximum physical record size must be made to avoid loss of
// data.  Since a single tape record will be read with a single read
// operation, there is no point is having it any larger.  The chunk
// multiple must be exactly 13 so that the block size is 26624.
//
// The reconstitution algorithm is as follows:
//
// 1. Read a 2048 chunk from the input.
//
// The first two 16-bit integers should contain the values 1 and n,
// where n is the number of "physical records" in the current "logical  
// record."  (If the first value is not 1, then the chunk is rejected
// and a new one read until the first 16-bit value is 1.)  These two
// values are not part of the reconstituted "logical record."
//
// 2. The next 32-bit integer contains the length of the "logical
// record" in 16-bit words.  The buffer is resized so that it can
// contain the whole reconstituted "logical record."
//
// 3. The remainder of the chunk is copied to the buffer.
//
// 4. Successive chunks are read from the input.
//
// The chunks are copied into the buffer until the "logical record"
// is complete.  For "logical records" longer than 26620 byte, this is
// not the whole story.  Each "physical record" contains maximum of 13
// chunks.  When the first "physical record" of 13 chunks has been read,
// the next chunk will be the first of the next "physical record."  The
// first two 16-bit integers will now be 2 and n, to indicate that this
// is the second "physical record" of the sequence.  These 4 bytes are
// decoded and the rest of this chunk is copied to the buffer.  And so
// on...
//
// An end-of-file condition on the input will cause record processing
// to be declared complete.  
// </synopsis> 
//
// <example>
// To open and read a VLA archive data file
// <code>
//    VLAArchiveInput *in;
//    Block <Char> *buff;
//    String fileName = " ";
//    String fileType = "tape";
//
//    if (fileType == String("tape")) {
//        in = new VLAArchiveInput(fileName.chars(), VLAArchiveInput::Tape);
//    } else {
//        in = new VLAArchiveInput(fileName.chars(), VLAArchiveInput::Disk);
//    }
//
//    uInt record = 0;
//    for (buff=&(in->next()); in->atEnd()==False; buff=&(in->next()), record++) {
//        cout << "Record" << record << endl;
//        // process record pointed to by buff
//    }
// </code>
//
// </example>
//
// <motivation>
// </motivation>
//
// <todo asof="">
// <ol>
//   <li> Bulletproofing - check for realistic buffer size (<1e6)
//   <li> Bulletproofing - check newn and newm on each read
//   <li> What happens after a single end-of-file on a tape drive?
//   <li> Add record skipping
//   <li> Should it work with stdin?  This is in place but not debugged.
// </ol>
// </todo>

class VLAArchiveInput
{
public: 
  // The destructor is virtual to ensure that the destructor in a derived
  // class is actually used. 
  virtual ~VLAArchiveInput();

  // This returns a reconstructed VLA logical record from the input
  // stream. This ByteSource has the data stored in memory, and hence is
  // seekable. Data read from this ByteSource will have the ModComp numeric
  // conversions applied.
  ByteSource& logicalRecord();
  const ByteSource& logicalRecord() const;

  // Reads the next logical record from specified IO source. Returns False if
  // there was a problem assembling the next record ie., it returns the value
  // of the hasData() member function.
  virtual Bool read() = 0;

  // Returns if true if the current record contains data. The current record
  // could be empty for a number of reasons including:
  // <ul> 
  // <li> You attempted to read beyond the end of the file.
  // <li> The physical record sequence numbers were not the expected ones
  // <li> An I/O Error occured while trying to read from the file.
  // <li> The beginning of a logical record could not be found. This is after
  //      searching 5MB of data.
  // </ul>
  Bool hasData() const;

protected: 
  //# the default constructor initialises the itsRecord data member
  VLAArchiveInput();
  
  //# All reads will be in multiples of this blocksize.
  static const uInt BlockSize;
  //# The size in bytes of the physical record sequence numbers
  static const uInt HeaderSize;
  //# A Physical Record can never be bigger than this many blocks.
  static const uInt MaxBlocksPerPhysicalRecord;

  //# These objects contains the current logical record. The memory IO object
  //# is used to put the data. It is taken out using the ByteSinkSource which
  //# will apply any numeric conversions.
  MemoryIO itsMemIO;

private:
  ModcompDataConversion itsModComp;
  ConversionIO itsCtrIO;

protected: 
  ByteSinkSource itsRecord;
};
#endif