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diff --git a/third_party/jpeg-xl/lib/jxl/enc_cluster.h b/third_party/jpeg-xl/lib/jxl/enc_cluster.h
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+++ b/third_party/jpeg-xl/lib/jxl/enc_cluster.h
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+// Copyright (c) the JPEG XL Project Authors. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Functions for clustering similar histograms together.
+
+#ifndef LIB_JXL_ENC_CLUSTER_H_
+#define LIB_JXL_ENC_CLUSTER_H_
+
+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+
+#include <vector>
+
+#include "lib/jxl/ans_params.h"
+#include "lib/jxl/enc_ans.h"
+
+namespace jxl {
+
+struct Histogram {
+  Histogram() {
+    total_count_ = 0;
+    entropy_ = 0.0;
+  }
+  void Clear() {
+    data_.clear();
+    total_count_ = 0;
+  }
+  void Add(size_t symbol) {
+    if (data_.size() <= symbol) {
+      data_.resize(DivCeil(symbol + 1, kRounding) * kRounding);
+    }
+    ++data_[symbol];
+    ++total_count_;
+  }
+  void AddHistogram(const Histogram& other) {
+    if (other.data_.size() > data_.size()) {
+      data_.resize(other.data_.size());
+    }
+    for (size_t i = 0; i < other.data_.size(); ++i) {
+      data_[i] += other.data_[i];
+    }
+    total_count_ += other.total_count_;
+  }
+  float PopulationCost() const {
+    return ANSPopulationCost(data_.data(), data_.size());
+  }
+  float ShannonEntropy() const;
+
+  std::vector<ANSHistBin> data_;
+  size_t total_count_;
+  mutable float entropy_;  // WARNING: not kept up-to-date.
+  static constexpr size_t kRounding = 8;
+};
+
+void ClusterHistograms(HistogramParams params, const std::vector<Histogram>& in,
+                       size_t max_histograms, std::vector<Histogram>* out,
+                       std::vector<uint32_t>* histogram_symbols);
+}  // namespace jxl
+
+#endif  // LIB_JXL_ENC_CLUSTER_H_