1 files changed, 54 insertions, 0 deletions
diff --git a/extern/draco/draco/src/draco/compression/entropy/rans_symbol_coding.h b/extern/draco/draco/src/draco/compression/entropy/rans_symbol_coding.h
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index 00000000000..0a68e29fe26
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+++ b/extern/draco/draco/src/draco/compression/entropy/rans_symbol_coding.h
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+// Copyright 2016 The Draco Authors.
+//
+// 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
+//
+//      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.
+//
+// File providing shared functionality for RAnsSymbolEncoder and
+// RAnsSymbolDecoder (see rans_symbol_encoder.h / rans_symbol_decoder.h).
+#ifndef DRACO_COMPRESSION_ENTROPY_RANS_SYMBOL_CODING_H_
+#define DRACO_COMPRESSION_ENTROPY_RANS_SYMBOL_CODING_H_
+
+#include "draco/compression/entropy/ans.h"
+
+namespace draco {
+
+// Computes the desired precision of the rANS method for the specified number of
+// unique symbols the input data (defined by their bit_length).
+constexpr int ComputeRAnsUnclampedPrecision(int symbols_bit_length) {
+  return (3 * symbols_bit_length) / 2;
+}
+
+// Computes the desired precision clamped to guarantee a valid functionality of
+// our rANS library (which is between 12 to 20 bits).
+constexpr int ComputeRAnsPrecisionFromUniqueSymbolsBitLength(
+    int symbols_bit_length) {
+  return ComputeRAnsUnclampedPrecision(symbols_bit_length) < 12
+             ? 12
+             : ComputeRAnsUnclampedPrecision(symbols_bit_length) > 20
+                   ? 20
+                   : ComputeRAnsUnclampedPrecision(symbols_bit_length);
+}
+
+// Compute approximate frequency table size needed for storing the provided
+// symbols.
+static inline int64_t ApproximateRAnsFrequencyTableBits(
+    int32_t max_value, int num_unique_symbols) {
+  // Approximate number of bits for storing zero frequency entries using the
+  // run length encoding (with max length of 64).
+  const int64_t table_zero_frequency_bits =
+      8 * (num_unique_symbols + (max_value - num_unique_symbols) / 64);
+  return 8 * num_unique_symbols + table_zero_frequency_bits;
+}
+
+}  // namespace draco
+
+#endif  // DRACO_COMPRESSION_ENTROPY_RANS_SYMBOL_CODING_H_