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_context.cpp

@@ -7,6 +7,7 @@
  */
 #include "catch_context.h"
 #include "catch_common.h"
+#include "catch_random_number_generator.h"
 
 namespace Catch {
 
@@ -59,4 +60,11 @@ namespace Catch {
     IContext::~IContext() = default;
     IMutableContext::~IMutableContext() = default;
     Context::~Context() = default;
+
+
+    SimplePcg32& rng() {
+        static SimplePcg32 s_rng;
+        return s_rng;
+    }
+
 }

include/internal/catch_context.h

@@ -55,6 +55,9 @@ namespace Catch {
     }
 
     void cleanUpContext();
+
+    class SimplePcg32;
+    SimplePcg32& rng();
 }
 
 #endif // TWOBLUECUBES_CATCH_CONTEXT_H_INCLUDED

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;
     }
 }

include/internal/catch_random_number_generator.h

@@ -7,17 +7,52 @@
 #ifndef TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED
 #define TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED
 
-#include <algorithm>
-#include <random>
+#include <cstdint>
 
 namespace Catch {
 
-    struct IConfig;
+    // This is a simple implementation of C++11 Uniform Random Number
+    // Generator. It does not provide all operators, because Catch2
+    // does not use it, but it should behave as expected inside stdlib's
+    // distributions.
+    // The implementation is based on the PCG family (http://pcg-random.org)
+    class SimplePcg32 {
+        using state_type = std::uint64_t;
+    public:
+        using result_type = std::uint32_t;
+        static constexpr result_type min() {
+            return 0;
+        }
+        static constexpr result_type max() {
+            return static_cast<result_type>(-1);
+        }
 
-    std::mt19937& rng();
-    void seedRng( IConfig const& config );
-    unsigned int rngSeed();
+        // Provide some default initial state for the default constructor
+        SimplePcg32():SimplePcg32(0xed743cc4U) {}
 
-}
+        explicit SimplePcg32(result_type seed_);
+
+        void seed(result_type seed_);
+        void discard(uint64_t skip);
+
+        result_type operator()();
+
+    private:
+        friend bool operator==(SimplePcg32 const& lhs, SimplePcg32 const& rhs);
+        friend bool operator!=(SimplePcg32 const& lhs, SimplePcg32 const& rhs);
+
+        // In theory we also need operator<< and operator>>
+        // In practice we do not use them, so we will skip them for now
+
+
+        std::uint64_t m_state;
+        // This part of the state determines which "stream" of the numbers
+        // is chosen -- we take it as a constant for Catch2, so we only
+        // need to deal with seeding the main state.
+        // Picked by reading 8 bytes from `/dev/random` :-)
+        static const std::uint64_t s_inc = (0x13ed0cc53f939476ULL << 1ULL) | 1ULL;
+    };
+
+} // end namespace Catch
 
 #endif // TWOBLUECUBES_CATCH_RANDOM_NUMBER_GENERATOR_H_INCLUDED

include/internal/catch_run_context.cpp

@@ -506,4 +506,16 @@ namespace Catch {
         else
             CATCH_INTERNAL_ERROR("No result capture instance");
     }
+
+    void seedRng(IConfig const& config) {
+        if (config.rngSeed() != 0) {
+            std::srand(config.rngSeed());
+            rng().seed(config.rngSeed());
+        }
+    }
+
+    unsigned int rngSeed() {
+        return getCurrentContext().getConfig()->rngSeed();
+    }
+
 }

include/internal/catch_run_context.h

@@ -151,6 +151,8 @@ namespace Catch {
         bool m_includeSuccessfulResults;
     };
 
+    void seedRng(IConfig const& config);
+    unsigned int rngSeed();
 } // end namespace Catch
 
 #endif // TWOBLUECUBES_CATCH_RUNNER_IMPL_HPP_INCLUDED

include/internal/catch_test_case_registry_impl.cpp

@@ -11,6 +11,7 @@
 #include "catch_enforce.h"
 #include "catch_interfaces_registry_hub.h"
 #include "catch_random_number_generator.h"
+#include "catch_run_context.h"
 #include "catch_string_manip.h"
 #include "catch_test_case_info.h"