// Copyright 2024 Google LLC
// SPDX-License-Identifier: Apache-2.0
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     https://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#include "compression/stats.h"

#include <stdio.h>

#include <algorithm>  // std::min
#include <string>

#include "hwy/base.h"  // HWY_ASSERT

namespace gcpp {

void Stats::Assimilate(const Stats& other) {
  const int64_t total_n = n_ + other.n_;
  if (total_n == 0) return;  // Nothing to do; prevents div by zero.

  min_ = std::min(min_, other.min_);
  max_ = std::max(max_, other.max_);

  product_ *= other.product_;

  const double product_n = n_ * other.n_;
  const double n2 = n_ * n_;
  const double other_n2 = other.n_ * other.n_;
  const int64_t total_n2 = total_n * total_n;
  const double total_n3 = static_cast<double>(total_n2) * total_n;
  // Precompute reciprocal for speed - used at least twice.
  const double inv_total_n = 1.0 / total_n;
  const double inv_total_n2 = 1.0 / total_n2;

  const double delta = other.m1_ - m1_;
  const double delta2 = delta * delta;
  const double delta3 = delta * delta2;
  const double delta4 = delta2 * delta2;

  m1_ = (n_ * m1_ + other.n_ * other.m1_) * inv_total_n;

  const double new_m2 = m2_ + other.m2_ + delta2 * product_n * inv_total_n;

  const double new_m3 =
      m3_ + other.m3_ + delta3 * product_n * (n_ - other.n_) * inv_total_n2 +
      3.0 * delta * (n_ * other.m2_ - other.n_ * m2_) * inv_total_n;

  m4_ += other.m4_ +
         delta4 * product_n * (n2 - product_n + other_n2) / total_n3 +
         6.0 * delta2 * (n2 * other.m2_ + other_n2 * m2_) * inv_total_n2 +
         4.0 * delta * (n_ * other.m3_ - other.n_ * m3_) * inv_total_n;

  m2_ = new_m2;
  m3_ = new_m3;
  n_ = total_n;
}

std::string Stats::ToString(int exclude) const {
  if (Count() == 0) return std::string("(none)");

  char buf[300];
  int pos = 0;
  int ret;  // snprintf - bytes written or negative for error.

  if ((exclude & kNoCount) == 0) {
    ret = snprintf(buf + pos, sizeof(buf) - pos, "Count=%9zu ",
                   static_cast<size_t>(Count()));
    HWY_ASSERT(ret > 0);
    pos += ret;
  }

  if ((exclude & kNoMeanSD) == 0) {
    const float sd = StandardDeviation();
    if (sd > 100) {
      ret = snprintf(buf + pos, sizeof(buf) - pos, "Mean=%8.2E SD=%7.1E ",
                     Mean(), sd);
    } else {
      ret = snprintf(buf + pos, sizeof(buf) - pos, "Mean=%8.6f SD=%7.5f ",
                     Mean(), sd);
    }
    HWY_ASSERT(ret > 0);
    pos += ret;
  }

  if ((exclude & kNoMinMax) == 0) {
    ret = snprintf(buf + pos, sizeof(buf) - pos, "Min=%8.5e Max=%8.5e ", Min(),
                   Max());
    HWY_ASSERT(ret > 0);
    pos += ret;
  }

  if ((exclude & kNoSkewKurt) == 0) {
    ret = snprintf(buf + pos, sizeof(buf) - pos, "Skew=%5.2f Kurt=%7.2f ",
                   Skewness(), Kurtosis());
    HWY_ASSERT(ret > 0);
    pos += ret;
  }

  if ((exclude & kNoGeomean) == 0) {
    ret = snprintf(buf + pos, sizeof(buf) - pos, "GeoMean=%9.6f ",
                   GeometricMean());
    HWY_ASSERT(ret > 0);
    pos += ret;
  }

  HWY_ASSERT(pos < static_cast<int>(sizeof(buf)));
  return buf;
}

}  // namespace gcpp