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 // SPDX-License-Identifier: MIT
 // Copyright (c) 2018-2023, Advanced Micro Devices, Inc. All rights reserved.
  
 #pragma once
  
 #include "ck/utility/math_v2.hpp"
  
 namespace ck {
  
 // Assume
 //  1) XDesc is known at compile-time
 //  2) MeanVarDesc is known at compile-time
 //  3) XBuffer is static buffer
 //  4) MeanBuffer is static buffer
 //  5) VarBuffer is static buffer
 template <typename T, typename XThreadDesc_M_K, typename MeanVarThreadDesc_M>
 struct ThreadwiseWelford
 {
     static constexpr auto x_thread_desc_m_k      = XThreadDesc_M_K{};
     static constexpr auto mean_var_thread_desc_m = MeanVarThreadDesc_M{};
  
     static constexpr auto thread_x_length_m        = x_thread_desc_m_k.GetLength(Number<0>{});
     static constexpr auto thread_x_length_k        = x_thread_desc_m_k.GetLength(Number<1>{});
     static constexpr auto thread_mean_var_length_m = mean_var_thread_desc_m.GetLength(Number<0>{});
  
     static_assert(thread_x_length_m == thread_mean_var_length_m,
                   "lengths of source and mean/var buffer must match!");
  
     __device__ constexpr ThreadwiseWelford() : cur_count_(0), max_count_(0) {}
  
     __device__ inline void Update(T& mean, T& var, T x)
     {
         using ck::math::isnan;
  
         if(isnan(x))
         {
             mean = x;
             var  = x;
         }
         else
         {
             T delta = x - mean;
             mean += delta / cur_count_;
             T delta2 = x - mean;
             var += delta * delta2;
         }
     }
  
     template <typename XBufferType, typename MeanBufferType, typename VarBufferType>
     __device__ void
     Run(const XBufferType& x_buf_m_k, MeanBufferType& mean_buf_m, VarBufferType& var_buf_m)
     {
         // FIXME - Better naming for var_buf_m
  
         static_for<0, thread_x_length_k, 1>{}([&](auto iK) {
             if(cur_count_ < max_count_)
             {
                 ++cur_count_;
  
                 static_for<0, thread_x_length_m, 1>{}([&](auto iM) {
                     constexpr index_t out_offset =
                         mean_var_thread_desc_m.CalculateOffset(make_tuple(iM));
  
                     constexpr auto in_offset =
                         x_thread_desc_m_k.CalculateOffset(make_tuple(iM, iK));
                     Update(mean_buf_m(Number<out_offset>{}),
                            var_buf_m(Number<out_offset>{}),
                            x_buf_m_k[Number<in_offset>{}]);
                 });
             }
         });
     };
  
     int cur_count_;
     int max_count_;
 };
  
 template <typename T,
           typename SrcMeanVarCountThreadDesc_M_K,
           typename DstMeanVarThreadDesc_M,
           bool GetActualVariance = false>
 struct ThreadwiseWelfordMerge
 {
     static constexpr auto src_thread_desc_m_k = SrcMeanVarCountThreadDesc_M_K{};
     static constexpr auto dst_thread_desc_m   = DstMeanVarThreadDesc_M{};
  
     static constexpr auto src_length_m = src_thread_desc_m_k.GetLength(Number<0>{});
     static constexpr auto src_length_k = src_thread_desc_m_k.GetLength(Number<1>{});
     static constexpr auto dst_length_m = dst_thread_desc_m.GetLength(Number<0>{});
  
     static_assert(src_length_m == dst_length_m, "lengths of source and dst buffer must match!");
  
     __device__ static void
     Merge(T& mean_a, T& var_a, int32_t& count_a, T mean_b, T var_b, int32_t count_b)
     {
         int count            = count_a + count_b;
         T count_b_over_count = count == 0 ? type_convert<T>(0) : type_convert<T>(count_b) / count;
         T delta              = mean_b - mean_a;
         mean_a += delta * count_b_over_count;
         var_a += var_b + delta * delta * count_a * count_b_over_count;
         count_a = count;
     }
  
     template <typename SrcMeanBufferType,
               typename SrcVarBufferType,
               typename SrcCountBufferType,
               typename DstMeanBufferType,
               typename DstVarBufferType,
               typename DstCountBufferType>
     __device__ static void Run(const SrcMeanBufferType& src_mean_buf,
                                const SrcVarBufferType& src_var_buf,
                                const SrcCountBufferType& src_count_buf,
                                DstMeanBufferType& dst_mean_buf,
                                DstVarBufferType& dst_var_buf,
                                DstCountBufferType& dst_count_buf)
     {
         static_for<0, src_length_m, 1>{}([&](auto iM) {
             static_for<0, src_length_k, 1>{}([&](auto iK) {
                 constexpr auto src_offset = src_thread_desc_m_k.CalculateOffset(make_tuple(iM, iK));
  
                 Merge(dst_mean_buf(iM),
                       dst_var_buf(iM),
                       dst_count_buf(iM),
                       src_mean_buf[Number<src_offset>{}],
                       src_var_buf[Number<src_offset>{}],
                       src_count_buf[Number<src_offset>{}]);
             });
  
             if constexpr(GetActualVariance)
             {
                 dst_var_buf(iM) = dst_var_buf[iM] / dst_count_buf[iM];
             };
         });
     };
 };
  
 } // namespace ck