Implement std::student_t_distribution

libcudacxx/include/cuda/std/__random/student_t_distribution.h

@@ -0,0 +1,186 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------====//
+
+#ifndef _CUDA_STD___STUDENT_T_DISTRIBUTION_H
+#define _CUDA_STD___STUDENT_T_DISTRIBUTION_H
+
+#include <cuda/std/detail/__config>
+
+#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
+#  pragma GCC system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
+#  pragma clang system_header
+#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
+#  pragma system_header
+#endif // no system header
+
+#include <cuda/std/__cmath/roots.h>
+#include <cuda/std/__limits/numeric_limits.h>
+#include <cuda/std/__random/gamma_distribution.h>
+#include <cuda/std/__random/is_valid.h>
+#include <cuda/std/__random/normal_distribution.h>
+
+#if !_CCCL_COMPILER(NVRTC)
+#  include <iosfwd>
+#endif // !_CCCL_COMPILER(NVRTC)
+
+#include <cuda/std/__cccl/prologue.h>
+
+_CCCL_BEGIN_NAMESPACE_CUDA_STD
+
+template <class _RealType = double>
+class student_t_distribution
+{
+  static_assert(__libcpp_random_is_valid_realtype<_RealType>, "RealType must be a supported floating-point type");
+
+public:
+  // types
+  using result_type = _RealType;
+
+  class param_type
+  {
+  private:
+    result_type __n_ = result_type{1};
+
+  public:
+    using distribution_type = student_t_distribution;
+
+    _CCCL_API constexpr explicit param_type(result_type __n = result_type{1}) noexcept
+        : __n_{__n}
+    {}
+
+    [[nodiscard]] _CCCL_API constexpr result_type n() const noexcept
+    {
+      return __n_;
+    }
+
+    [[nodiscard]] _CCCL_API friend constexpr bool operator==(const param_type& __x, const param_type& __y) noexcept
+    {
+      return __x.__n_ == __y.__n_;
+    }
+#if _CCCL_STD_VER <= 2017
+    [[nodiscard]] _CCCL_API friend constexpr bool operator!=(const param_type& __x, const param_type& __y) noexcept
+    {
+      return !(__x == __y);
+    }
+#endif // _CCCL_STD_VER <= 2017
+  };
+
+private:
+  param_type __p_{};
+  normal_distribution<result_type> __nd_{};
+
+public:
+  // constructor and reset functions
+  constexpr student_t_distribution() noexcept = default;
+
+  _CCCL_API constexpr explicit student_t_distribution(result_type __n) noexcept
+      : __p_{param_type{__n}}
+  {}
+  _CCCL_API constexpr explicit student_t_distribution(const param_type& __p) noexcept
+      : __p_{__p}
+  {}
+  _CCCL_API constexpr void reset() noexcept
+  {
+    __nd_.reset();
+  }
+
+  // generating functions
+  template <class _URng>
+  [[nodiscard]] _CCCL_API result_type operator()(_URng& __g)
+  {
+    return (*this)(__g, __p_);
+  }
+  template <class _URng>
+  [[nodiscard]] _CCCL_API result_type operator()(_URng& __g, const param_type& __p)
+  {
+    static_assert(__cccl_random_is_valid_urng<_URng>, "URng must meet the UniformRandomBitGenerator requirements");
+    gamma_distribution<result_type> __gd{__p.n() * result_type{.5}, result_type{2}};
+    return __nd_(__g) * cuda::std::sqrt(__p.n() / __gd(__g));
+  }
+
+  // property functions
+  [[nodiscard]] _CCCL_API constexpr result_type n() const noexcept
+  {
+    return __p_.n();
+  }
+
+  [[nodiscard]] _CCCL_API constexpr param_type param() const noexcept
+  {
+    return __p_;
+  }
+  _CCCL_API constexpr void param(const param_type& __p) noexcept
+  {
+    __p_ = __p;
+  }
+
+  [[nodiscard]] _CCCL_API static constexpr result_type min() noexcept
+  {
+    return -numeric_limits<result_type>::infinity();
+  }
+  [[nodiscard]] _CCCL_API static constexpr result_type max() noexcept
+  {
+    return numeric_limits<result_type>::infinity();
+  }
+
+  [[nodiscard]] _CCCL_API friend constexpr bool
+  operator==(const student_t_distribution& __x, const student_t_distribution& __y) noexcept
+  {
+    return __x.__p_ == __y.__p_;
+  }
+#if _CCCL_STD_VER <= 2017
+  [[nodiscard]] _CCCL_API friend constexpr bool
+  operator!=(const student_t_distribution& __x, const student_t_distribution& __y) noexcept
+  {
+    return !(__x == __y);
+  }
+#endif // _CCCL_STD_VER <= 2017
+
+#if !_CCCL_COMPILER(NVRTC)
+  template <class _CharT, class _Traits>
+  friend ::std::basic_ostream<_CharT, _Traits>&
+  operator<<(::std::basic_ostream<_CharT, _Traits>& __os, const student_t_distribution& __x)
+  {
+    using _Ostream = ::std::basic_ostream<_CharT, _Traits>;
+    auto __flags   = __os.flags();
+    __os.flags(_Ostream::dec | _Ostream::left | _Ostream::scientific);
+    _CharT __sp      = __os.widen(' ');
+    _CharT __fill    = __os.fill(__sp);
+    auto __precision = __os.precision(numeric_limits<result_type>::max_digits10);
+    __os << __x.n();
+    __os.precision(__precision);
+    __os.fill(__fill);
+    __os.flags(__flags);
+    return __os;
+  }
+
+  template <class _CharT, class _Traits>
+  friend ::std::basic_istream<_CharT, _Traits>&
+  operator>>(::std::basic_istream<_CharT, _Traits>& __is, student_t_distribution& __x)
+  {
+    using _Istream = ::std::basic_istream<_CharT, _Traits>;
+    auto __flags   = __is.flags();
+    __is.flags(_Istream::skipws);
+    result_type __n;
+    __is >> __n;
+    if (!__is.fail())
+    {
+      __x.param(param_type{__n});
+    }
+    __is.flags(__flags);
+    return __is;
+  }
+#endif // !_CCCL_COMPILER(NVRTC)
+};
+
+_CCCL_END_NAMESPACE_CUDA_STD
+
+#include <cuda/std/__cccl/epilogue.h>
+
+#endif // _CUDA_STD___STUDENT_T_DISTRIBUTION_H

libcudacxx/include/cuda/std/__random_

@@ -30,6 +30,7 @@
 #include <cuda/std/__random/normal_distribution.h>
 #include <cuda/std/__random/philox_engine.h>
 #include <cuda/std/__random/seed_seq.h>
+#include <cuda/std/__random/student_t_distribution.h>
 #include <cuda/std/__random/uniform_int_distribution.h>
 #include <cuda/std/__random/uniform_real_distribution.h>
 #include <cuda/std/__random/weibull_distribution.h>

libcudacxx/test/libcudacxx/std/random/distribution/student_t.pass.cpp

@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+// SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES.
+//
+//===----------------------------------------------------------------------===//
+//
+// REQUIRES: long_tests
+
+// <random>
+
+// template<class RealType = double>
+// class student_t_distribution
+
+#include <cuda/std/__random_>
+#include <cuda/std/cassert>
+#include <cuda/std/cmath>
+
+#include "random_utilities/stats_functions.h"
+#include "random_utilities/test_distribution.h"
+#include "test_macros.h"
+
+template <class T>
+struct student_t_cdf
+{
+  using P = typename cuda::std::student_t_distribution<T>::param_type;
+
+  __host__ __device__ double operator()(double x, const P& p) const
+  {
+    // CDF of Student's t-distribution: F(x) = 0.5 + 0.5 * sgn(x) * I_{t²/(n+t²)}(0.5, n/2)
+    // where I is the regularized incomplete beta function and t = x
+    double t2       = x * x;
+    double n        = p.n();
+    double ratio    = t2 / (n + t2);
+    double beta_val = incomplete_beta(0.5, n / 2.0, ratio);
+
+    if (x >= 0)
+    {
+      return 0.5 + 0.5 * beta_val;
+    }
+    else
+    {
+      return 0.5 - 0.5 * beta_val;
+    }
+  }
+};
+
+template <class T>
+__host__ __device__ void test()
+{
+  [[maybe_unused]] const bool test_constexpr = false;
+  using D                                    = cuda::std::student_t_distribution<T>;
+  using P                                    = typename D::param_type;
+  using G                                    = cuda::std::philox4x64;
+  cuda::std::array<P, 5> params              = {P(1), P(2), P(5), P(10), P(30)};
+  test_distribution<D, true, G, test_constexpr>(params, student_t_cdf<T>{});
+}
+
+int main(int, char**)
+{
+  test<double>();
+  test<float>();
+  return 0;
+}