NexusVideoPlayer/H264.m

//
//  H264.m
//  Dropcam
//
//  Created by Loren Kirkby on 12/18/09.
//  Copyright 2009 Dropcam. All rights reserved.
//

#import "H264.h"

uint8_t get_h264_nalu_type(NSData *packet) 
{
  if ([packet length] < 1)
    return -1;
  
  uint8_t header[1] = {0};
  [packet getBytes:header length:sizeof(header)];
  
  uint8_t nal_unit_type = (header[0] & 0x1F);
  return nal_unit_type;
}

BOOL isStartPacket(NSData *packet) 
{
  if ([packet length] < 1)
    return NO;
  
  uint8_t header[1] = {0};
  [packet getBytes:header length:sizeof(header)];
  
  uint8_t nal_unit_type = (header[0] & 0x1F);
  return nal_unit_type == 5 || nal_unit_type == 7 || nal_unit_type == 8;
}


static BOOL readBit(NSData *data, int bitpos)
{
  uint8_t byte = ((uint8_t*)data.bytes)[bitpos / 8];
  int shift = 7 - (bitpos % 8);
  uint8_t mask = byte & (1 << shift);
  return mask >> shift;
}

static uint32_t readExpGolomb(NSData *data, int *bitpos)
{
  // Section 9.1
  int leadingZeroBits = -1;
  for (int b = 0; !b; leadingZeroBits++) {
    b = readBit(data, (*bitpos)++);
  }
  
  uint32_t suffix = 0;
  for (int i = 1; i <= leadingZeroBits; i++) {
    suffix |= readBit(data, (*bitpos)++) << (leadingZeroBits - i);
  }
  
  uint32_t pow2 = (1 << leadingZeroBits);
  return pow2 - 1 + suffix;
}

H264SliceType getH264SliceType(NSData *packet)
{
  if ([packet length] < 2)
    return -1;
  
  // Section 7.2
  uint8_t naluType = get_h264_nalu_type(packet);
  if (naluType != H264_NAL_SLICE_TYPE && naluType != H264_NAL_IDR_TYPE) {
    return -1;
  }
  
  // Section 7.3
  int bitpos = 8;
  uint32_t first_mb = readExpGolomb(packet, &bitpos); (void)first_mb;
  uint32_t slice_type = readExpGolomb(packet, &bitpos);
  return slice_type;
}