common_video/h265/h265_inline.h

/*
 *  Copyright (c) 2023 The WebRTC 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 in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

// This header file includes the inline functions in H265 parser.

#ifndef COMMON_VIDEO_H265_H265_INLINE_H_
#define COMMON_VIDEO_H265_H265_INLINE_H_

#include <stdint.h>

#include "rtc_base/compile_assert_c.h"

extern const int8_t kWebRtcVideo_CountLeadingZeros32_Table[64];

static __inline int WebRtcVideo_CountLeadingZeros32_NotBuiltin(uint32_t n) {
  // Normalize n by rounding up to the nearest number that is a sequence of 0
  // bits followed by a sequence of 1 bits. This number has the same number of
  // leading zeros as the original n. There are exactly 33 such values.
  n |= n >> 1;
  n |= n >> 2;
  n |= n >> 4;
  n |= n >> 8;
  n |= n >> 16;

  // Multiply the modified n with a constant selected (by exhaustive search)
  // such that each of the 33 possible values of n give a product whose 6 most
  // significant bits are unique. Then look up the answer in the table.
  return kWebRtcVideo_CountLeadingZeros32_Table[(n * 0x8c0b2891) >> 26];
}

// Returns the number of leading zero bits in the argument.
static __inline int WebRtcVideo_CountLeadingZeros32(uint32_t n) {
#ifdef __GNUC__
  RTC_COMPILE_ASSERT(sizeof(unsigned int) == sizeof(uint32_t));
  return n == 0 ? 32 : __builtin_clz(n);
#else
  return WebRtcVideo_CountLeadingZeros32_NotBuiltin(n);
#endif
}
#endif  // COMMON_VIDEO_H265_H265_INLINE_H_