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diff --git a/extern/draco/draco/src/draco/compression/bit_coders/adaptive_rans_bit_coding_shared.h b/extern/draco/draco/src/draco/compression/bit_coders/adaptive_rans_bit_coding_shared.h
new file mode 100644
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+++ b/extern/draco/draco/src/draco/compression/bit_coders/adaptive_rans_bit_coding_shared.h
@@ -0,0 +1,43 @@
+// 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 provides shared functions for adaptive rANS bit coding.
+#ifndef DRACO_COMPRESSION_BIT_CODERS_ADAPTIVE_RANS_BIT_CODING_SHARED_H_
+#define DRACO_COMPRESSION_BIT_CODERS_ADAPTIVE_RANS_BIT_CODING_SHARED_H_
+
+#include "draco/core/macros.h"
+
+namespace draco {
+
+// Clamp the probability p to a uint8_t in the range [1,255].
+inline uint8_t clamp_probability(double p) {
+  DRACO_DCHECK_LE(p, 1.0);
+  DRACO_DCHECK_LE(0.0, p);
+  uint32_t p_int = static_cast<uint32_t>((p * 256) + 0.5);
+  p_int -= (p_int == 256);
+  p_int += (p_int == 0);
+  return static_cast<uint8_t>(p_int);
+}
+
+// Update the probability according to new incoming bit.
+inline double update_probability(double old_p, bool bit) {
+  static constexpr double w = 128.0;
+  static constexpr double w0 = (w - 1.0) / w;
+  static constexpr double w1 = 1.0 / w;
+  return old_p * w0 + (!bit) * w1;
+}
+
+}  // namespace draco
+
+#endif  // DRACO_COMPRESSION_BIT_CODERS_ADAPTIVE_RANS_BIT_CODING_SHARED_H_