Introduce Catch's own RNG based on the PCG family of RNGs

In the future, we will also want to introduce our own
`uniform_int_distribution` and `uniform_real_distribution` to get
repeatable test runs across different platforms.

include/internal/catch_random_number_generator.cpp

@@ -7,23 +7,67 @@
 
 #include "catch_random_number_generator.h"
 #include "catch_context.h"
+#include "catch_run_context.h"
 #include "catch_interfaces_config.h"
 
 namespace Catch {
 
-    std::mt19937& rng() {
-        static std::mt19937 s_rng;
-        return s_rng;
+namespace {
+
+#if defined(_MSC_VER)
+#pragma warning(push)
+#pragma warning(disable:4146) // we negate uint32 during the rotate
+#endif
+        // Safe rotr implementation thanks to John Regehr
+        uint32_t rotate_right(uint32_t val, uint32_t count) {
+            const uint32_t mask = 31;
+            count &= mask;
+            return (val >> count) | (val << (-count & mask));
+        }
+
+#if defined(_MSC_VER)
+#pragma warning(pop)
+#endif
+
+}
+
+
+    SimplePcg32::SimplePcg32(result_type seed_) {
+        seed(seed_);
     }
 
-    void seedRng( IConfig const& config ) {
-        if( config.rngSeed() != 0 ) {
-            std::srand( config.rngSeed() );
-            rng().seed( config.rngSeed() );
+
+    void SimplePcg32::seed(result_type seed_) {
+        m_state = 0;
+        (*this)();
+        m_state += seed_;
+        (*this)();
+    }
+
+    void SimplePcg32::discard(uint64_t skip) {
+        // We could implement this to run in O(log n) steps, but this
+        // should suffice for our use case.
+        for (uint64_t s = 0; s < skip; ++s) {
+            static_cast<void>((*this)());
         }
     }
 
-    unsigned int rngSeed() {
-        return getCurrentContext().getConfig()->rngSeed();
+    SimplePcg32::result_type SimplePcg32::operator()() {
+        // prepare the output value
+        const uint32_t xorshifted = static_cast<uint32_t>(((m_state >> 18u) ^ m_state) >> 27u);
+        const auto output = rotate_right(xorshifted, m_state >> 59u);
+
+        // advance state
+        m_state = m_state * 6364136223846793005ULL + s_inc;
+
+        return output;
+    }
+
+    bool operator==(SimplePcg32 const& lhs, SimplePcg32 const& rhs) {
+        return lhs.m_state == rhs.m_state;
+    }
+
+    bool operator!=(SimplePcg32 const& lhs, SimplePcg32 const& rhs) {
+        return lhs.m_state != rhs.m_state;
     }
 }