analog: moving LFSR code into gr-analog.

1 files changed, 162 insertions, 0 deletions

diff --git a/gr-analog/include/analog/lfsr.h b/gr-analog/include/analog/lfsr.h
new file mode 100644
index 000000000..9612a9d43
--- /dev/null
+++ b/gr-analog/include/analog/lfsr.h
@@ -0,0 +1,162 @@
+/* -*- c++ -*- */
+/*
+ * Copyright 2008,2010,2012 Free Software Foundation, Inc.
+ *
+ * This file is part of GNU Radio
+ *
+ * GNU Radio is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3, or (at your option)
+ * any later version.
+ *
+ * GNU Radio 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with GNU Radio; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef INCLUDED_ANALOG_LFSR_H
+#define INCLUDED_ANALOG_LFSR_H
+
+#include <analog/api.h>
+#include <stdexcept>
+#include <stdint.h>
+
+namespace gr {
+  namespace analog {
+
+    /*!
+     * \brief Fibonacci Linear Feedback Shift Register using specified
+     * polynomial mask
+     * \ingroup misc
+     *
+     * Generates a maximal length pseudo-random sequence of length
+     * 2^degree-1
+     *
+     * Constructor: analog::lfsr(int mask, int seed, int reg_len);
+     *
+     * \param mask - polynomial coefficients representing the
+     *             locations of feedback taps from a shift register
+     *             which are xor'ed together to form the new high
+     *             order bit.
+     *
+     *             Some common masks might be:
+     *              x^4 + x^3 + x^0 = 0x19
+     *              x^5 + x^3 + x^0 = 0x29
+     *              x^6 + x^5 + x^0 = 0x61
+     *
+     * \param seed - the initialization vector placed into the
+     *             register durring initialization. Low order bit
+     *             corresponds to x^0 coefficient -- the first to be
+     *             shifted as output.
+     *
+     * \param reg_len - specifies the length of the feedback shift
+     *             register to be used. Durring each iteration, the
+     *             register is rightshifted one and the new bit is
+     *             placed in bit reg_len. reg_len should generally be
+     *             at least order(mask) + 1
+     *
+     *
+     * see http://en.wikipedia.org/wiki/Linear_feedback_shift_register
+     * for more explanation.
+     *
+     *  next_bit() - Standard LFSR operation
+     *
+     *      Perform one cycle of the LFSR.  The output bit is taken from
+     *      the shift register LSB.  The shift register MSB is assigned from
+     *      the modulo 2 sum of the masked shift register.
+     *
+     *  next_bit_scramble(unsigned char input) - Scramble an input stream
+     *
+     *      Perform one cycle of the LFSR.  The output bit is taken from
+     *      the shift register LSB.  The shift register MSB is assigned from
+     *      the modulo 2 sum of the masked shift register and the input LSB.
+     *
+     *  next_bit_descramble(unsigned char input) - Descramble an input stream
+     *
+     *      Perform one cycle of the LFSR.  The output bit is taken from
+     *      the modulo 2 sum of the masked shift register and the input LSB.
+     *      The shift register MSB is assigned from the LSB of the input.
+     *
+     * See http://en.wikipedia.org/wiki/Scrambler for operation of these
+     * last two functions (see multiplicative scrambler.)
+     */
+    class ANALOG_API lfsr
+    {
+    private:
+      uint32_t d_shift_register;
+      uint32_t d_mask;
+      uint32_t d_seed;
+      uint32_t d_shift_register_length;	// less than 32
+
+      static uint32_t
+	popCount(uint32_t x)
+      {
+	uint32_t r = x - ((x >> 1) & 033333333333)
+	  - ((x >> 2) & 011111111111);
+	return ((r + (r >> 3)) & 030707070707) % 63;
+      }
+
+    public:
+      lfsr(uint32_t mask, uint32_t seed, uint32_t reg_len)
+	: d_shift_register(seed),
+	d_mask(mask),
+	d_seed(seed),
+	d_shift_register_length(reg_len)
+	{
+	  if(reg_len > 31)
+	    throw std::invalid_argument("reg_len must be <= 31");
+	}
+
+      unsigned char next_bit()
+      {
+	unsigned char output = d_shift_register & 1;
+	unsigned char newbit = popCount( d_shift_register & d_mask )%2;
+	d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
+	return output;
+      }
+
+      unsigned char next_bit_scramble(unsigned char input)
+      {
+	unsigned char output = d_shift_register & 1;
+	unsigned char newbit = (popCount( d_shift_register & d_mask )%2)^(input & 1);
+	d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
+	return output;
+      }
+
+      unsigned char next_bit_descramble(unsigned char input)
+      {
+	unsigned char output = (popCount( d_shift_register & d_mask )%2)^(input & 1);
+	unsigned char newbit = input & 1;
+	d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));
+	return output;
+      }
+
+      /*!
+       * Reset shift register to initial seed value
+       */
+      void reset() { d_shift_register = d_seed; }
+
+      /*!
+       * Rotate the register through x number of bits
+       * where we are just throwing away the results to get queued up correctly
+       */
+      void pre_shift(int num)
+      {
+	for(int i=0; i<num; i++) {
+	  next_bit();
+	}
+      }
+
+      int mask() const { return d_mask; }
+    };
+
+  } /* namespace analog */
+} /* namespace gr */
+
+#endif /* INCLUDED_ANALOG_LFSR_H */