Fix uniform random key generation (#630)

Fixes #532

Author: sleeepyjack

include/cuco/utility/key_generator.cuh

@@ -20,7 +20,8 @@
 #include <cuco/detail/pair/helpers.cuh>
 #include <cuco/detail/utility/strong_type.cuh>
 
-#include <cuda/functional>
+#include <cuda/std/cmath>
+#include <cuda/std/functional>  // TODO include <cuda/std/algorithm> instead once available
 #include <cuda/std/limits>
 #include <cuda/std/span>
 #include <thrust/device_vector.h>
@@ -80,6 +81,7 @@ struct gaussian : public cuco::detail::strong_type<double> {
 }  // namespace distribution
 
 namespace detail {
+
 /**
  * @brief Generate uniform functor
  *
@@ -94,29 +96,37 @@ struct generate_uniform_fn {
    *
    * @param num Number of elements to generate
    * @param dist Random number distribution
+   * @param seed Random seed
    */
-  __host__ __device__ constexpr generate_uniform_fn(std::size_t num, Dist dist)
-    : num_{num}, dist_{dist}
+  __host__ __device__ constexpr generate_uniform_fn(std::size_t num, Dist dist, std::size_t seed)
+    : num_{num}, dist_{dist}, seed_{seed}
   {
   }
 
   /**
    * @brief Generates a random number of type `T` based on the given `seed`
    *
-   * @param seed Random number generator seed
+   * @param idx Index of the output element
    *
    * @return A resulting random number
    */
-  __host__ __device__ constexpr T operator()(std::size_t seed) const noexcept
+  __host__ __device__ constexpr T operator()(std::size_t idx) const noexcept
   {
     RNG rng;
-    thrust::uniform_int_distribution<T> uniform_dist{1, static_cast<T>(num_ / dist_.value)};
-    rng.seed(seed);
+    // Improved seeding using a linear congruential generator
+    rng.seed(seed_ + idx * 1664525ull + 1013904223ull);
+    // Calculate number of unique keys
+    auto num_unique_keys = cuda::std::max<size_t>(
+      1ull,
+      static_cast<size_t>(
+        cuda::std::ceil(static_cast<double>(num_) / static_cast<double>(dist_.value))));
+    thrust::uniform_int_distribution<T> uniform_dist{0, static_cast<T>(num_unique_keys - 1)};
     return uniform_dist(rng);
   }
 
-  std::size_t num_;  ///< Number of elements to generate
-  Dist dist_;        ///< Random number distribution
+  std::size_t num_;   ///< Number of elements to generate
+  Dist dist_;         ///< Random number distribution
+  std::size_t seed_;  ///< Random seed
 };
 
 /**
@@ -270,18 +280,17 @@ class key_generator {
     using value_type = typename std::iterator_traits<OutputIt>::value_type;
 
     if constexpr (std::is_same_v<Dist, distribution::unique>) {
-      thrust::sequence(exec_policy, out_begin, out_end, 0);
+      thrust::sequence(exec_policy, out_begin, out_end, value_type{0});
       thrust::shuffle(exec_policy, out_begin, out_end, this->rng_);
     } else if constexpr (std::is_same_v<Dist, distribution::uniform>) {
       size_t num_keys = thrust::distance(out_begin, out_end);
-
-      thrust::counting_iterator<size_t> seeds(this->rng_());
+      size_t seed     = this->rng_();
 
       thrust::transform(exec_policy,
-                        seeds,
-                        seeds + num_keys,
+                        thrust::make_counting_iterator<size_t>(0),
+                        thrust::make_counting_iterator<size_t>(num_keys),
                         out_begin,
-                        detail::generate_uniform_fn<value_type, Dist, RNG>{num_keys, dist});
+                        detail::generate_uniform_fn<value_type, Dist, RNG>{num_keys, dist, seed});
     } else if constexpr (std::is_same_v<Dist, distribution::gaussian>) {
       size_t num_keys = thrust::distance(out_begin, out_end);