Fibonacci Linear Feedback Shift Register

git-svn-id: http://gnuradio.org/svn/gnuradio/trunk@9492 221aa14e-8319-0410-a670-987f0aec2ac5

1 files changed, 104 insertions, 0 deletions

diff --git a/gnuradio-core/src/lib/general/gri_lfsr.h b/gnuradio-core/src/lib/general/gri_lfsr.h
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+/* -*- c++ -*- */
+/*
+ * Copyright 2007 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_GRI_LFSR_H
+#define INCLUDED_GRI_LFSR_H
+
+#include <stdexcept>
+#include <stdint.h>
+
+/*!
+ * \brief Fibonacci Linear Feedback Shift Register using specified polynomial mask
+ * \ingroup math
+ *
+ * Generates a maximal length pseudo-random sequence of length 2^degree-1
+ * 
+ * Constructor: gri_lfsr(int mask, int seed, int reg_len);
+ *  
+ *      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
+ *
+ *      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.
+ *
+ *   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() - performs once cycle of the lfsr,
+ *               generating the new high order bit from the mask
+ *               and returning the low order bit which has been
+ *               shifted out of the register.
+ *
+ *
+ */
+
+
+class gri_lfsr
+{
+ private:
+  uint32_t d_shift_register;
+  uint32_t d_mask;
+  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:
+
+  gri_lfsr(uint32_t mask, uint32_t seed, uint32_t reg_len)
+    : d_shift_register(seed), d_mask(mask), 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 bit = 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 bit;
+  }
+
+  int mask() const { return d_mask; }
+};
+
+#endif /* INCLUDED_GRI_LFSR_H */