repack_bits_bb.h

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/* -*- c++ -*- */
/*
 * Copyright 2012 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * SPDX-License-Identifier: GPL-3.0-or-later
 *
 */
 
#ifndef INCLUDED_BLOCKS_REPACK_BITS_BB_H
#define INCLUDED_BLOCKS_REPACK_BITS_BB_H
 
#include <gnuradio/blocks/api.h>
#include <gnuradio/endianness.h>
#include <gnuradio/tagged_stream_block.h>
 
namespace gr {
namespace blocks {
 
/*!
 * \brief Repack \p k bits from the input stream onto \p l bits of the output stream.
 * \ingroup byte_operators_blk
 *
 * \details
 * No bits are lost here; any value for k and l (within [1, 8]) is allowed.
 * On every fresh input byte, it starts reading on the LSB, and starts copying
 * to the LSB as well.
 *
 * When supplying a tag name, this block operates on tagged streams.
 * In this case, it can happen that the input data or the output data
 * becomes unaligned when k * input length is not equal to l * output length.
 * In this case, the \p align_output parameter is used to decide which
 * data packet to align.
 *
 * Usually, \p align_output is false for unpacking (k=8, l < 8) and false for
 * reversing that.
 *
 * \section gr_blocks_repack_example Example
 *
 * Say you're tx'ing 8-PSK and therefore set k=8, l=3 on the transmit side
 * before the modulator. Now assume you're transmitting a single byte of data.
 * Your incoming tagged stream has length 1, the outgoing has length 3. However,
 * the third item is actually only carrying 2 bits of relevant data, the bits
 * do not align with the boundaries. So you set \p align_output = false,
 * because the output can be unaligned.
 *
 * Now say you're doing the inverse: packing those three items into full
 * bytes. How do you interpret those three bytes? Without this flag,
 * you'd have to assume there's 9 relevant bits in there, so you'd end up
 * with 2 bytes of output data. But in the packing case, you want the
 * \b output to be aligned; all output bits must be useful. By asserting this flag,
 * the packing algorithm tries to do this and in this case assumes that
 * since we have alignment after 8 bits, the 9th can be discarded.
 */
class BLOCKS_API repack_bits_bb : virtual public tagged_stream_block
{
public:
    typedef std::shared_ptr<repack_bits_bb> sptr;
 
    /*!
     * \param k Number of relevant bits on the input stream
     * \param l Number of relevant bits on the output stream
     * \param tsb_tag_key If not empty, this is the key for the length tag.
     * \param align_output If tsb_tag_key is given, this controls if the input
     *                     or the output is aligned.
     * \param endianness The endianness of the output data stream (LSB or MSB).
     */
    static sptr make(int k,
                     int l = 8,
                     const std::string& tsb_tag_key = "",
                     bool align_output = false,
                     endianness_t endianness = GR_LSB_FIRST);
    virtual void set_k_and_l(int k, int l) = 0; // callback function for bits per input
                                                // byte k and bits per output byte l.
};
 
} // namespace blocks
} // namespace gr
 
#endif /* INCLUDED_BLOCKS_REPACK_BITS_BB_H */