Add templates to RngEngine class (#690)

* Add templates to RngEngine class

* Use 64-bit mersenne twister rng engine

releasenotes/notes/rng-engine-8235ed6278c23765.yaml

@@ -0,0 +1,5 @@
+---
+upgrade:
+  - |
+    Change the RNG engine for simulators from 32-bit Mersenne twister to
+    64-bit Mersenne twister engine.

src/framework/rng.hpp

@@ -18,117 +18,85 @@
 #include <cstdint>
 #include <random>
 
-#include "framework/types.hpp"
-
 namespace AER {
 
-/***************************************************************************/ /**
-  *
-  * RngEngine Class
-  *
-  * Objects of this class are used to generate random numbers for backends.
-  *These
-  * are used to decide outcomes of measurements and resets, and for implementing
-  * noise.
-  *
-  ******************************************************************************/
+//============================================================================
+// RngEngine Class
+//
+// Objects of this class are used to generate random numbers for backends.
+// These are used to decide outcomes of measurements and resets, and for
+// implementing noise.
+//
+//============================================================================
 
 class RngEngine {
 public:
-  /**
-   * Generate a uniformly distributed pseudo random real in the half-open
-   * interval [a,b)
-   * @param a closed lower bound on interval
-   * @param b open upper bound on interval
-   * @return the generated double
-   */
-  double rand(double a, double b);
 
-  /**
-   * Generate a uniformly distributed pseudo random real in the half-open
-   * interval [0,b)
-   * @param b open upper bound on interval
-   * @return the generated double
-   */
-  inline double rand(double b) { return rand(0, b); };
+  //-----------------------------------------------------------------------
+  // Constructors
+  //-----------------------------------------------------------------------
 
-  /**
-   * Generate a uniformly distributed pseudo random real in the half-open
-   * interval [0,1)
-   * @return the generated double
-   */
-  inline double rand() { return rand(0, 1); };
+  // Default constructor initialize RNG engine with a random seed
+  RngEngine() { set_random_seed(); }
 
-  /**
-   * Generate a uniformly distributed pseudo random integer in the closed
-   * interval [a,b]
-   * @param a lower bound on interval
-   * @param b upper bound on interval
-   * @return the generated integer
-   */
-  int_t rand_int(int_t a, int_t b);
-  uint_t rand_int(uint_t a, uint_t b);
+  // Seeded constructor initialize RNG engine with a fixed seed
+  explicit RngEngine(size_t seed) { set_seed(seed); }
 
-  /**
-   * Generate a pseudo random integer from a a discrete distribution
-   * constructed from an input vector of probabilities for [0,..,n-1]
-   * where n is the lenght of the vector. If this vector is not normalized
-   * it will be scaled when it is converted to a discrete_distribution
-   * @param probs the vector of probabilities
-   * @return the generated integer
-   */
-  uint_t rand_int(const std::vector<double> &probs);
+  //-----------------------------------------------------------------------
+  // Set fixed or random seed for RNG
+  //-----------------------------------------------------------------------
 
-  /**
-   * Default constructor initialize RNG engine with a random seed
-   */
-  RngEngine() {
+  // Set random seed for the RNG engine
+  void set_random_seed() { 
     std::random_device rd;
-    rng.seed(rd());
-  };
-
-  /**
-   * Seeded constructor initialize RNG engine with a fixed seed
-   * @param seed integer to use as seed for mt19937 engine
-   */
-  explicit RngEngine(uint_t seed) { rng.seed(seed); };
-
+    set_seed(rd());
+  }
 
   // Set a fixed seed for the RNG engine
-  void set_seed(uint_t seed) { rng.seed(seed); };
+  void set_seed(size_t seed) { rng.seed(seed); }
   
-private:
-  std::mt19937 rng; // Mersenne twister rng engine
+  //-----------------------------------------------------------------------
+  // Sampling methods
+  //-----------------------------------------------------------------------
+
+  // Generate a uniformly distributed pseudo random real in the half-open
+  // interval [a,b)
+  double rand(double a, double b) {
+    return std::uniform_real_distribution<double>(a, b)(rng);
+  }
+
+  // Generate a uniformly distributed pseudo random real in the half-open
+  // interval [0,b)
+  double rand(double b) {
+    return rand(double(0), b);
   };
 
-/*******************************************************************************
- *
- * RngEngine Methods
- *
- ******************************************************************************/
+  // Generate a uniformly distributed pseudo random real in the half-open
+  // interval [0,1)
+  double rand() {return rand(0, 1); };
 
-double RngEngine::rand(double a, double b) {
-  double p = std::uniform_real_distribution<double>(a, b)(rng);
-  return p;
+  // Generate a uniformly distributed pseudo random integer in the closed
+  // interval [a,b]
+  template <typename Integer,
+            typename = std::enable_if_t<std::is_integral<Integer>::value>>
+  Integer rand_int(Integer a, Integer b) {
+    return std::uniform_int_distribution<Integer>(a, b)(rng);
   }
 
-// randomly distributed integers in [a,b]
-int_t RngEngine::rand_int(int_t a, int_t b) {
-  int_t n = std::uniform_int_distribution<int_t>(a, b)(rng);
-  return n;
+  // Generate a pseudo random integer from a a discrete distribution
+  // constructed from an input vector of probabilities for [0,..,n-1]
+  // where n is the lenght of the vector. If this vector is not normalized
+  // it will be scaled when it is converted to a discrete_distribution
+  template <typename Float,
+            typename = std::enable_if_t<std::is_floating_point<Float>::value>>
+  size_t rand_int(const std::vector<Float> &probs) {
+    return std::discrete_distribution<size_t>(probs.begin(), probs.end())(rng);
   }
 
-uint_t RngEngine::rand_int(uint_t a, uint_t b) {
-  int_t n = std::uniform_int_distribution<uint_t>(a, b)(rng);
-  return n;
-}
-
-// randomly distributed integers from vector
-uint_t RngEngine::rand_int(const std::vector<double> &probs) {
-  uint_t n = std::discrete_distribution<uint_t>(probs.begin(), probs.end())(rng);
-  return n;
-}
+private:
+  std::mt19937_64 rng; // Mersenne twister rng engine
+};
 
 //------------------------------------------------------------------------------
-} // End namespace QISKIT
+} // End namespace AER
 #endif