implement PRBS generator.

Fixes root-project#8199

math/mathcore/CMakeLists.txt

@@ -94,6 +94,7 @@ set(HEADERS
   TRandom1.h
   TRandom2.h
   TRandom3.h
+  TRandomBinary.h
   TRandomGen.h
   TStatistic.h
   VectorizedTMath.h
@@ -186,6 +187,7 @@ ROOT_STANDARD_LIBRARY_PACKAGE(MathCore
     src/TRandom1.cxx
     src/TRandom2.cxx
     src/TRandom3.cxx
+    src/TRandomBinary.cxx
     src/TRandomGen.cxx
     src/TStatistic.cxx
     src/UnBinData.cxx

math/mathcore/inc/LinkDef2.h

@@ -32,6 +32,7 @@
 #pragma link C++ class TRandom1+;
 #pragma link C++ class TRandom2+;
 #pragma link C++ class TRandom3-;
+#pragma link C++ class TRandomBinary+;
 
 #pragma link C++ class ROOT::Math::TRandomEngine+;
 #pragma link C++ class ROOT::Math::LCGEngine+;

math/mathcore/inc/TRandomBinary.h

@@ -0,0 +1,150 @@
+// @(#)root/mathcore:$Id$
+// Author: Fernando Hueso-González   04/08/2021
+
+#ifndef ROOT_TRandomBinary
+#define ROOT_TRandomBinary
+
+//////////////////////////////////////////////////////////////////////////
+//                                                                      //
+// TRandomBinary                                                        //
+//                                                                      //
+// Pseudo Random Binary Sequence generator class (periodicity = 2**n-1) //
+//                                                                      //
+//////////////////////////////////////////////////////////////////////////
+
+#include <array>
+#include <bitset>
+#include <vector>
+#include <set>
+#include <cmath>
+#include "Rtypes.h"
+#include "TError.h"
+
+class TRandomBinary  {
+public:
+   /**
+    * @brief Generate the next pseudo-random bit using the current state of a linear feedback shift register (LFSR) and update it
+    * @tparam k the length of the LFSR, usually also the order of the monic polynomial PRBS-k (last exponent)
+    * @tparam nTaps the number of taps
+    * @param lfsr the current value of the LFSR. Passed by reference, it will be updated with the next value
+    * @param taps the taps that will be XOR-ed to calculate the new bit. They are the exponents of the monic polynomial. Ordering is unimportant. Note that an exponent E in the polynom maps to bit index E-1 in the LFSR.
+    * @param left if true, the direction of the register shift is to the left <<, the newBit is set on lfsr at bit position 0 (right). If false, shift is to the right and the newBit is stored at bit position (k-1)
+    * @return the new random bit
+    * @throw an exception is thrown if taps are out of the range [1, k]
+    * @see https://en.wikipedia.org/wiki/Monic_polynomial
+    * @see https://en.wikipedia.org/wiki/Linear-feedback_shift_register
+    * @see https://en.wikipedia.org/wiki/Pseudorandom_binary_sequence
+    */
+   template <size_t k, size_t nTaps>
+   static bool
+   NextLFSR(std::bitset<k>& lfsr, const std::array<uint16_t, nTaps> taps, const bool left = true)
+   {
+      static_assert(k <= 32, "For the moment, only supported until k == 32.");
+      static_assert(k > 0, "Non-zero degree is needed for the LFSR.");
+      static_assert(nTaps > 0, "At least one tap is needed for the LFSR.");
+      static_assert(nTaps <= k, "Cannot use more taps than polynomial order");
+
+      // First, calculate the XOR (^) of all selected bits (marked by the taps)
+      bool newBit = lfsr[taps.at(0) - 1]; // the exponent E of the polynomial correspond to index E - 1 in the bitset
+      for(uint16_t j = 1; j < nTaps ; ++j)
+      {
+         newBit ^= lfsr[taps.at(j) - 1];
+      }
+
+      //Apply the shift to the register in the right direction, and overwrite the empty one with newBit
+      if(left)
+      {
+         lfsr <<= 1;
+         lfsr[0] = newBit;
+      }
+      else
+      {
+         lfsr >>= 1;
+         lfsr[k-1] = newBit;
+      }
+
+      return newBit;
+   }
+
+   /**
+    * @brief Generation of a sequence of pseudo-random bits using a linear feedback shift register (LFSR), until a register value is repeated (or maxPeriod is reached)
+    * @tparam k the length of the LFSR, usually also the order of the monic polynomial PRBS-k (last exponent)
+    * @tparam nTaps the number of taps
+    * @param start the start value (seed) of the LFSR
+    * @param taps the taps that will be XOR-ed to calculate the new bit. They are the exponents of the monic polynomial. Ordering is unimportant. Note that an exponent E in the polynom maps to bit index E-1 in the LFSR.
+    * @param left if true, the direction of the register shift is to the left <<, the newBit is set on lfsr at bit position 0 (right). If false, shift is to the right and the newBit is stored at bit position (k-1)
+    * @param wrapping if true, allow repetition of values in the LFSRhistory, until maxPeriod is reached or the repeated value == start. Enabling this option saves memory as no history is kept
+    * @param oppositeBit if true, use the high/low bit of the LFSR to store output (for left=true/false, respectively) instead of the newBit returned by ::NextLFSR
+    * @return the array of pseudo random bits, or an empty array if input was incorrect
+    * @see https://en.wikipedia.org/wiki/Monic_polynomial
+    * @see https://en.wikipedia.org/wiki/Linear-feedback_shift_register
+    * @see https://en.wikipedia.org/wiki/Pseudorandom_binary_sequence
+    */
+   template <size_t k, size_t nTaps>
+   static std::vector<bool>
+   GenerateSequence(const std::bitset<k> start, const std::array<uint16_t, nTaps> taps, const bool left = true, const bool wrapping = false, const bool oppositeBit = false)
+   {
+      std::vector<bool> result; // Store result here
+
+      //Sanity-checks
+      static_assert(k <= 32, "For the moment, only supported until k == 32.");
+      static_assert(k > 0, "Non-zero degree is needed for the LFSR.");
+      static_assert(nTaps >= 2, "At least two taps are needed for a proper sequence");
+      static_assert(nTaps <= k, "Cannot use more taps than polynomial order");
+      for(auto tap : taps) {
+         if(tap > k || tap == 0) {
+            Error("TRandomBinary", "Tap %u is out of range [1,%lu]", tap, k);
+            return result;
+         }
+      }
+      if(start.none()) {
+         Error("TRandomBinary", "A non-zero start value is needed");
+         return result;
+      }
+
+      // Calculate maximum period and pre-allocate space in result
+      const uint32_t maxPeriod = pow(2,k) - 1;
+      result.reserve(maxPeriod);
+
+      std::set<uint32_t> lfsrHistory; // a placeholder to store the history of all different values of the LFSR
+      std::bitset<k> lfsr(start); // a variable storing the current value of the LFSR
+      uint32_t i = 0; // a loop counter
+      if(oppositeBit) // if oppositeBit enabled, first value is already started with the seed
+         result.emplace_back(left ? lfsr[k-1] : lfsr[0]);
+
+      //Loop now until maxPeriod or a lfsr value is repeated. If wrapping enabled, allow repeated values if not equal to seed
+      do {
+         bool newBit = NextLFSR(lfsr, taps, left);
+
+         if(!oppositeBit)
+            result.emplace_back(newBit);
+         else
+            result.emplace_back(left ? lfsr[k-1] : lfsr[0]);
+
+         ++i;
+
+         if(!wrapping) // If wrapping not allowed, break the loop once a repeated value is encountered
+         {
+            if(lfsrHistory.count(lfsr.to_ulong()))
+               break;
+
+            lfsrHistory.insert(lfsr.to_ulong()); // Add to the history
+         }
+      }
+      while(lfsr != start && i < maxPeriod);
+
+      if(oppositeBit)
+         result.pop_back();// remove last element, as we already pushed the one from the seed above the while loop
+
+      result.shrink_to_fit();//only some special taps will lead to the maxPeriod, others will stop earlier
+
+      return result;
+   }
+
+   TRandomBinary() = default;
+   virtual ~TRandomBinary() = default;
+
+   ClassDef(TRandomBinary,0)  //Pseudo Random Binary Sequence (periodicity = 2**n - 1)
+};
+
+#endif

math/mathcore/src/TRandomBinary.cxx

@@ -0,0 +1,37 @@
+// @(#)root/mathcore:$Id$
+// Author: Fernando Hueso-González   04/08/2021
+
+/**
+
+\class TRandomBinary
+
+@ingroup Random
+
+This class contains a generator of Pseudo Random Binary Sequences (<a
+href="https://en.wikipedia.org/wiki/Pseudorandom_binary_sequence">PRBS</a>).
+It should NOT be used for general-purpose random number generation or any
+statistical study, see ::TRandom2 class instead.
+
+The goal is to generate binary bit sequences with the same algorithm as the ones usually implemented
+in electronic chips, so that the theoretically expected ones can be compared with the acquired sequences.
+
+The main ingredients of a PRBS generator are a monic polynomial of maximum degree \f$n\f$, with coefficients
+either 0 or 1, and a <a href="https://www.nayuki.io/page/galois-linear-feedback-shift-register">Galois</a>
+linear-feedback shift register with a non-zero seed. When the monic polynomial exponents are chosen appropriately,
+the period of the resulting bit sequence (0s and 1s) yields \f$2^n - 1\f$.
+
+Other implementations can be found here:
+
+- https://gist.github.com/mattbierner/d6d989bf26a7e54e7135
+- https://root.cern/doc/master/civetweb_8c_source.html#l06030
+- https://cryptography.fandom.com/wiki/Linear_feedback_shift_register
+- https://www3.advantest.com/documents/11348/33b24c8a-c8cb-40b8-a2a7-37515ba4abc8
+- https://www.reddit.com/r/askscience/comments/63a10q/for_prbs3_with_clock_input_on_each_gate_how_can/
+- https://es.mathworks.com/help/serdes/ref/prbs.html
+- https://metacpan.org/pod/Math::PRBS
+
+*/
+
+#include "TRandomBinary.h"
+
+ClassImp(TRandomBinary);

tutorials/math/PRBS.C

@@ -0,0 +1,46 @@
+/// \file
+/// \ingroup tutorial_math
+/// \notebook -nodraw
+/// Tutorial illustrating the use of TRandomBinary::GenerateSequence
+/// can be run with:
+///
+/// ~~~{.cpp}
+/// root > .x PRBS.C
+/// root > .x PRBS.C+ with ACLIC
+/// ~~~
+///
+/// \macro_output
+/// \macro_code
+///
+/// \author Fernando Hueso-González
+
+#include <TRandomBinary.h>
+#include <iostream>
+
+void PRBS ()
+{
+   printf("\nTRandomBinary::GenerateSequence PRBS3, PRBS4 and PRBS5 tests\n");
+   printf("==========================\n");
+
+   //PRBS3
+   std::array<uint16_t, 2> taps3 = {2, 3}; // Exponents of the monic polynomial
+   auto prbs3 = TRandomBinary::GenerateSequence(std::bitset<3>().flip(), taps3);// Start value all high
+
+   //PRBS4
+   std::array<uint16_t, 2> taps4 = {3, 4}; // Exponents of the monic polynomial
+   auto prbs4 = TRandomBinary::GenerateSequence(std::bitset<4>().flip(), taps4);// Start value all high
+
+   //PRBS7
+   std::array<uint16_t, 2> taps5 = {5, 3}; // Exponents of the monic polynomial
+   auto prbs5 = TRandomBinary::GenerateSequence(std::bitset<5>().flip(), taps5);// Start value all high
+
+   for(auto prbs : {prbs3, prbs4, prbs5})
+   {
+      std::cout << "PRBS period " << prbs.size() << ":\t";
+      for(auto p : prbs)
+      {
+         std::cout << p << " ";
+      }
+      std::cout << std::endl;
+   }
+}