emkornfield commented on code in PR #48345: URL: https://github.com/apache/arrow/pull/48345#discussion_r2723663239
########## cpp/src/arrow/util/alp/alp_sampler.cc: ########## @@ -0,0 +1,150 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you 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 +// +// http://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 "arrow/util/alp/alp_sampler.h" + +#include <cmath> + +#include "arrow/util/alp/alp.h" +#include "arrow/util/alp/alp_constants.h" +#include "arrow/util/logging.h" +#include "arrow/util/ubsan.h" + +namespace arrow { +namespace util { +namespace alp { + +// ---------------------------------------------------------------------- +// AlpSampler implementation + +template <typename T> +AlpSampler<T>::AlpSampler() + : sample_vector_size_(AlpConstants::kSamplerVectorSize), + rowgroup_size_(AlpConstants::kSamplerRowgroupSize), + samples_per_vector_(AlpConstants::kSamplerSamplesPerVector), + sample_vectors_per_rowgroup_(AlpConstants::kSamplerSampleVectorsPerRowgroup), + rowgroup_sample_jump_((rowgroup_size_ / sample_vectors_per_rowgroup_) / + sample_vector_size_) {} + +template <typename T> +void AlpSampler<T>::AddSample(arrow::util::span<const T> input) { + for (uint64_t i = 0; i < input.size(); i += sample_vector_size_) { + const uint64_t elements = std::min(input.size() - i, sample_vector_size_); + AddSampleVector({input.data() + i, elements}); + } +} + +template <typename T> +void AlpSampler<T>::AddSampleVector(arrow::util::span<const T> input) { + const bool must_skip_current_vector = + MustSkipSamplingFromCurrentVector(vectors_count_, vectors_sampled_count_, + input.size()); + + vectors_count_ += 1; + total_values_count_ += input.size(); + if (must_skip_current_vector) { + return; + } + + const AlpSamplingParameters sampling_params = GetAlpSamplingParameters(input.size()); + + // Slice: take first num_lookup_value elements. + std::vector<T> current_vector_values( + input.begin(), + input.begin() + std::min<size_t>(sampling_params.num_lookup_value, input.size())); + + // Stride: take every num_sampled_increments-th element. + std::vector<T> current_vector_sample; + for (size_t i = 0; i < current_vector_values.size(); + i += sampling_params.num_sampled_increments) { + current_vector_sample.push_back(current_vector_values[i]); + } + sample_stored_ += current_vector_sample.size(); + + complete_vectors_sampled_.push_back(std::move(current_vector_values)); + rowgroup_sample_.push_back(std::move(current_vector_sample)); + vectors_sampled_count_++; +} + +template <typename T> +typename AlpSampler<T>::AlpSamplerResult AlpSampler<T>::Finalize() { + ARROW_LOG(DEBUG) << "AlpSampler finalized: vectorsSampled=" << vectors_sampled_count_ + << "/" << vectors_count_ << " total" + << ", valuesSampled=" << sample_stored_ << "/" << total_values_count_ + << " total"; + + AlpSamplerResult result; + result.alp_preset = AlpCompression<T>::CreateEncodingPreset(rowgroup_sample_); + + ARROW_LOG(DEBUG) << "AlpSampler preset: " << result.alp_preset.combinations.size() + << " exponent/factor combinations" + << ", estimatedSize=" << result.alp_preset.best_compressed_size + << " bytes"; + + return result; +} + +template <typename T> +typename AlpSampler<T>::AlpSamplingParameters AlpSampler<T>::GetAlpSamplingParameters( + uint64_t num_current_vector_values) { + const uint64_t num_lookup_values = + std::min(num_current_vector_values, + static_cast<uint64_t>(AlpConstants::kAlpVectorSize)); + // Sample equidistant values within a vector; jump a fixed number of values. + const uint64_t num_sampled_increments = + std::max(uint64_t{1}, static_cast<uint64_t>(std::ceil( + static_cast<double>(num_lookup_values) / + samples_per_vector_))); + const uint64_t num_sampled_values = + std::ceil(static_cast<double>(num_lookup_values) / num_sampled_increments); + + ARROW_CHECK(num_sampled_values < AlpConstants::kAlpVectorSize) << "alp_sample_too_large"; Review Comment: ARROW_CHECK will abort the program if it fails. Can you add a comment on Safey on why this would never fail? If this isn't peformance critical, it is better to return `Result<T>` to propagate errors. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
