我认为这是字节的sizeinfo,让我解释一下我是如何得出这个结论的.
首先我的上下文：我通过Adobe AIR(客户端)和System.data.sqlite(C#服务器端)操作sqlite数据库
使用System.data.sqlite,如果我通过Adobe AIR读取一个填充了BLOB的sqlite数据库,我必须得到AIR开头附加的那些字节,然后我的二进制数据都很好.完善！
使用Adobe AIR,如果我尝试读取一个由System.data.sqlite填充BLOB的sqlite数据库,数据已损坏我收到错误！显然是因为我没有AIR研究过的缺少的字节.
当然,我试图通过复制我在第一种情况下删除的一系列3个字节来添加这些字节,但之后它部分返回数据,而在图像的情况下,最后一行像素全部变为灰色,在某些图像中我有或多或少的灰色线条.因为数据对应于相同~4k大小的一系列图像,并且我从其中一个添加了3个字节,我得到了这个结果.
而Air也有时会抛出这个错误：
Error: Error #2030: End of file was
encountered.
显然这些字节给出了大小的信息,但我真的不知道它是怎么做的！？！
我尝试添加一个长度为4k的bytearray,它往往会增加3个字节,但我尝试添加4M,最多可达5个字节.
我发现这个问题How do you convert 3 bytes into a 24 bit number in C#?,我认为这可能是如何存储大小信息.
但我仍然没有得到它……
因为在我的Adobe AIR项目中,我必须将我的air.ByteArray对象作为参数传递给sqlite Blob字段中的所有内容; AIR会将所有内容序列化为AMF,这是一种紧凑的二进制动作消息格式.
Page 11 Secion 3.14 ByteArray类型
http://opensource.adobe.com/wiki/download/attachments/1114283/amf3_spec_05_05_08.pdf
该文件规定：
AMF 3 serializes this type using a
variable length encoding 29-bit
integer for the byte-length prefix
followed by the raw bytes of the
ByteArray.
但不是全部,我搜索了一个AMF .NET开源项目,并创建了FluorineFX.查看代码,我发现每个AMF二进制文件都以字节TypeCode为前缀,ByteArray为12：
/// <summary> /// AMF ByteArray data type. /// </summary> public const byte ByteArray = 12;
进一步搜索并再次在FluorineFX源中找到AMFReader.ReadAMF3ByteArray()和AMFWriter.WriteByteArray()
这有助于我快速构建我需要的东西：
private static byte[] RemoveAMF3ByteArrayPrefixBytes(byte[] ar)
{
var ms = new MemoryStream(ar);
var br = new BinaryReader(ms);
// if first byte is AMF TypeCode for ByteArray
if (br.Read() != 12)
return ar;
int handle = ReadAMF3IntegerData(br);
bool inline = ((handle & 1) != 0);
handle = handle >> 1;
if (inline)
{
int length = handle;
byte[] buffer = br.ReadBytes(length);
return buffer;
}
return ar;
}
private static byte[] AddAMF3ByteArrayPrefixBytes(byte[] ar)
{
var ms = new MemoryStream();
var bw = new BinaryWriter(ms);
bw.Write((byte)12); // AMF TypeCode for ByteArray
var handle = (int)ar.Length;
handle = handle << 1;
handle = handle | 1;
WriteAMF3IntegerData(bw,handle);
bw.Write(ar);
return ms.ToArray();
}
/// <summary>
/// Handle decoding of the variable-length representation which gives seven bits of value per serialized byte by using the high-order bit
/// of each byte as a continuation flag.
/// </summary>
/// <returns></returns>
private static int ReadAMF3IntegerData(BinaryReader br)
{
int acc = br.ReadByte();
if(acc < 128)
return acc;
else
{
acc = (acc & 0x7f) << 7;
int tmp = br.ReadByte();
if(tmp < 128)
acc = acc | tmp;
else
{
acc = (acc | tmp & 0x7f) << 7;
tmp = br.ReadByte();
if(tmp < 128)
acc = acc | tmp;
else
{
acc = (acc | tmp & 0x7f) << 8;
tmp = br.ReadByte();
acc = acc | tmp;
}
}
}
//To sign extend a value from some number of bits to a greater number of bits just copy the sign bit into all the additional bits in the new format.
//convert/sign extend the 29bit two's complement number to 32 bit
int mask = 1 << 28; // mask
int r = -(acc & mask) | acc;
return r;
//The following variation is not portable,but on architectures that employ an
//arithmetic right-shift,maintaining the sign,it should be fast.
//s = 32 - 29;
//r = (x << s) >> s;
}
private static void WriteAMF3IntegerData(BinaryWriter bw,int value)
{
//Sign contraction - the high order bit of the resulting value must match every bit removed from the number
//Clear 3 bits
value &= 0x1fffffff;
if (value < 0x80)
bw.Write((byte)value);
else
if (value < 0x4000)
{
bw.Write((byte)(value >> 7 & 0x7f | 0x80));
bw.Write((byte)(value & 0x7f));
}
else
if (value < 0x200000)
{
bw.Write((byte)(value >> 14 & 0x7f | 0x80));
bw.Write((byte)(value >> 7 & 0x7f | 0x80));
bw.Write((byte)(value & 0x7f));
}
else
{
bw.Write((byte)(value >> 22 & 0x7f | 0x80));
bw.Write((byte)(value >> 15 & 0x7f | 0x80));
bw.Write((byte)(value >> 8 & 0x7f | 0x80));
bw.Write((byte)(value & 0xff));
}
}
我希望这会有助于其他人.
