From 16ae765e1c9c2ce68125ba1d6b1ed18c5b8b3cf2 Mon Sep 17 00:00:00 2001
From: Jacob Patterson-Stein <jacobpstein@gmail.com>
Date: Fri, 11 Oct 2024 21:34:56 -0400
Subject: [PATCH] Adding ACORN algo rng

An initial attempt at implementing a simple ACORN random number generator
---
 src/acorn.cpp | 60 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 60 insertions(+)
 create mode 100644 src/acorn.cpp

diff --git a/src/acorn.cpp b/src/acorn.cpp
new file mode 100644
index 0000000..dea0494
--- /dev/null
+++ b/src/acorn.cpp
@@ -0,0 +1,60 @@
+#include <Rcpp.h>
+using namespace Rcpp;
+
+//' Additive Congruential Random Number (ACORN) Pseudorandom Number Generator
+ //'
+ //' The Additive Congruential Random Number (ACORN) pseudorandom number generator was developed in 1989 by R.S.Wikramaratna.
+ //' The ACORN algorithm is based on additive congruential methods, which use modular arithmetic combined with addition to evolve a sequence of random numbers.
+ //' The state is maintained in a vector {x} of length {k}, initialized with a seed. At each iteration, the state is updated through additive congruence.
+ //'
+ //' The generator operates as follows:
+ //' 1. Select an {n} value of numbers to generate.
+ //' 2. Set a {seed} for the algorithm.
+ //' 3. Choose a {k} value to set the order of the algorithm and determine how many additive steps occur.
+ //' 4. The result is derived from the last value in the state vector.
+ //'
+ //' @param seed Initial seed for the generator. This should be an integer.
+ //' @param n The number of random numbers to generate.
+ //' @param k The order of the ACORN algorithm, determining how many additive steps occur.
+ //'
+ //' @return A numeric vector of pseudorandom values generated by the ACORN algorithm.
+ //'
+ //' @references Wikramaratna, R.S. (1989). ACORN — A new method for generating sequences of uniformly distributed Pseudo-random Numbers. Journal of Computational Physics. 83. 16-31.
+ //'
+ //' @examples
+ //' # Example usage of the acorn_rng function
+ //' acorn_rng(10, seed = 123, k = 5)
+
+ //'
+ // [[Rcpp::export]]
+
+NumericVector acorn_rng(int n, int seed, int k) {
+  NumericVector result(n);
+
+  // Initialize state variables
+  std::vector<long long> x(k, seed);  // Seed replicated across the vector
+
+  // Generate random numbers
+  for (int i = 0; i < n; ++i) {
+    // Update the state vector
+    for (int j = k - 1; j > 0; --j) {
+      x[j] = x[j] + x[j - 1];  // Additive step
+    }
+
+    // Modulo to ensure we stay within limits
+    x[0] = x[0] + 1;  // Keep evolving the seed
+    long long random_value = x[k - 1];  // Use the last element as the random number
+
+    // Store the random number in the result vector
+    result[i] = random_value % 1000000000;  // Modulus to limit large numbers
+  }
+
+  return result;
+}
+
+
+
+
+/*** R
+acorn_rng(10, seed = 123, k = 5)
+*/