Lucene数据存储结构中的VInt（可变长度整型）
转载自：
http://www.blogjava.net/persister/archive/2010/02/02/311642.html
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A variable-length format for positive integers is defined where the high-order bit of each byte indicates whether more bytes remain to be read. The low-order seven bits are appended as increasingly more significant bits in the resulting integer value. Thus values from zero to 127 may be stored in a single byte, values from 128 to 16,383 may be stored in two bytes, and so on.
可变格式的整型定义：最高位表示是否还有字节要读取，低七位就是就是具体的有效位，添加到
结果数据中。比如00000001 最高位表示0，那么说明这个数就是一个字节表示，有效位是后面的七位0000001，值为1。10000010 00000001 第一个字节最高位为1，表示后面还有字节，第二位最高位0表示到此为止了，即就是两个字节，那么具体的值注意，是从最后一个字节的七位有效数放在最前面，依次放置，最后是第一个自己的七位有效位，所以这个数表示 0000001 0000010，换算成整数就是130
VInt Encoding Example
Value
First byte
Second byte
Third byte
0
00000000
?
?
1
00000001
?
?
2
00000010
?
?
...
?
?
?
127
01111111
?
?
128
10000000
00000001
?
129
10000001
00000001
?
130
10000010
00000001
?
...
?
?
?
16,383
11111111
01111111
?
16,384
10000000
10000000
00000001
16,385
10000001
10000000
00000001
...
?
?
?
?
Lucene源代码中进行存储和读取是这样的。OutputStream是负责写：
?1???/**?Writes?an?int?in?a?variable-length?format.??Writes?between?one?and?2????*?five?bytes.??Smaller?values?take?fewer?bytes.??Negative?numbers?are?not
?3????*?supported.
?4????*?@see?InputStream#readVInt()
?5????*/
?6???public?final?void?writeVInt(int?i)?throws?IOException?{
?7?????while?((i?&?~0x7F)?!=?0)?{
?8???????writeByte((byte)((i?&?0x7f)?|?0x80));
?9???????i?>>>=?7;
10?????}
11?????writeByte((byte)i);
12???}
InputStream负责读：
?1???/**?Reads?an?int?stored?in?variable-length?format.??Reads?between?one?and
?2????*?five?bytes.??Smaller?values?take?fewer?bytes.??Negative?numbers?are?not
?3????*?supported.
?4????*?@see?OutputStream#writeVInt(int)
?5????*/
?6???public?final?int?readVInt()?throws?IOException?{
?7?????byte?b?=?readByte();
?8?????int?i?=?b?&?0x7F;
?9?????for?(int?shift?=?7;?(b?&?0x80)?!=?0;?shift?+=?7)?{
10???????b?=?readByte();
11???????i?|=?(b?&?0x7F)?<<?shift;
12?????}
13?????return?i;
14???}
>>>表示无符号右移