big_random function: fixed description and improved implementation

README.md

@@ -2,7 +2,6 @@
   <img alt="BigInt" src="logo.png">
 </p>
 <h3 align="center">Arbitrary-sized integer class for C++</h3>
-
 ---
 
 [![Release version][release-shield]][release-link]
@@ -200,14 +199,14 @@
 
 * #### Random
   * #### `big_random`
-    Get a random `BigInt`, that either has a random number of digits (up to
+    Get a random positive `BigInt`, that either has a random number of digits (up to
     1000), or a specific number of digits.
 
     ```c++
-    // get a random BigInt that has a random number of digits (up to 1000):
+    // get a random postive BigInt that has a random number of digits (up to 1000):
     big1 = big_random();
 
-    // get a random BigInt that has 12345 digits:
+    // get a random positive BigInt that has 12345 digits:
     big1 = big_random(12345);
     ```
 

include/functions/random.hpp

@@ -20,29 +20,42 @@ const size_t MAX_RANDOM_LENGTH = 1000;
 /*
     big_random (num_digits)
     -----------------------
-    Returns a random BigInt with a specific number of digits.
+    Returns a random positive BigInt with a specific number of digits.
 */
 
 BigInt big_random(size_t num_digits = 0) {
-    std::random_device rand_generator;      // true random number generator
-
-    if (num_digits == 0)    // the number of digits were not specified
+    /**
+     * This generators needed to be created only once,
+     * because properly seeded std::mt19937 can generate
+     * 2^19937 - 1 potential states which is more than
+     * enough for most usecases 
+     */
+
+    // seed sequence, seeded with true random generator
+    // used for better entropy
+    static std::seed_seq sseq{std::random_device{}()};
+    // generator itself
+    static std::mt19937 gen{sseq};                      
+    // different purpose distributions(cheap to create and use)
+    static std::uniform_int_distribution<> digit_distr(0, 9);
+    static std::uniform_int_distribution<> first_digit_distr(0, 9);
+    static std::uniform_int_distribution<> length_distribution(1, MAX_RANDOM_LENGTH);
+    // number to generate
+    static BigInt big_rand;
+
+    // first digit
+    big_rand.value = std::to_string(first_digit_distr(gen));
+
+    if (num_digits == 0)
+        // the number of digits were not specified
         // use a random number for it:
-        num_digits = 1 + rand_generator() % MAX_RANDOM_LENGTH;
-
-    BigInt big_rand;
-    big_rand.value = "";    // clear value to append digits
-
-    // ensure that the first digit is non-zero
-    big_rand.value += std::to_string(1 + rand_generator() % 9);
+        num_digits = length_distribution(gen);
 
+    // insert other digits
     while (big_rand.value.size() < num_digits)
-        big_rand.value += std::to_string(rand_generator());
-    if (big_rand.value.size() != num_digits)
-        big_rand.value.erase(num_digits);   // erase extra digits
+        big_rand.value += std::to_string(digit_distr(gen));
 
     return big_rand;
 }
 
-
 #endif  // BIG_INT_RANDOM_FUNCTIONS_HPP