SQLite文件格式初步分析之varint

varint为变长整数。长度为1到9个字节。最大可表示64位整数。

使用varint表示整数的原因：大多数情况下，整数数值比较小，如果使用64位整数保存的话，会浪费空间。

Varint格式如下(源于源码util.c)：

A = 0xxxxxxx 7位数据，一个标志位(这里就是高位的0)

B = 1xxxxxxx 7位数据，一个标志位(这里就是高位的1)

C = xxxxxxxx 8位数据

7 bits - A

14 bits - BA

21 bits - BBA

28 bits - BBBA

35 bits - BBBBA

42 bits - BBBBBA

49 bits - BBBBBBA

56 bits - BBBBBBBA

64 bits - BBBBBBBBC

对于前8个字节，以A为结尾，通过判断字节是否为A来来确定是否到达varint尾部。

对于9个字节组成的varint来说，如果前8个字节都为B，则说明此varint为9个字节长。如果为8个B，则说明下一个字节就是varint的尾部。SQLite的作者设计果然精巧。

以下是源码分析:

/*
** 从p[0]开始的内存处读取64位变长整数。
** 返回varint的字节数。varint值保存在*v中。
*/
u8 sqlite3GetVarint(const unsigned char *p,u64 *v){
  u32 a,b,s;

  a = *p;
  /* a: p0 (unmasked) */
  //varint为一个字节时
  if (!(a&0x80)) //如果a<=127=0x7F=2222211b
  {
	//值在0到127之间
    *v = a; //直接取值 之后返回
    return 1;
  }

  p++;
  b = *p;
  /* b: p1 (unmasked) */
  //varint为两个字节时
  if (!(b&0x80))//判断第二个字节是否为A。如果是，则此varint占用两个字节 格式为BA
  {
    a &= 0x7f; //a为第一个字节 取低7位
    a = a<<7; //a左移7位 为B的7位腾出空间
    a |= b; //A的低7位+B的低7位
    *v = a;
    return 2;
  }

  /* Verify that constants are precomputed correctly */
  assert( SLOT_2_0 == ((0x7f<<14) | (0x7f)) );
  assert( SLOT_4_2_0 == ((0xfU<<28) | (0x7f<<14) | (0x7f)) );

  p++;
  a = a<<14;
  a |= *p; //此后a为BXA或BXB
  /* a: p0<<14 | p2 (unmasked) */
  if (!(a&0x80)) //判断第三个字节是否为A。如果是，则此varint占用三个字节，格式为BBA。
  {
    a &= SLOT_2_0; //SLOT_2_0=222221100000002222211 清空X位
    b &= 0x7f;
    b = b<<7;
    a |= b; //填充X位 合成BBA
    *v = a;
    return 3;
  }

  //</略过分析>
  /* CSE1 from below */
  a &= SLOT_2_0;
  p++;
  b = b<<14;
  b |= *p;
  /* b: p1<<14 | p3 (unmasked) */
  if (!(b&0x80))
  {
    b &= SLOT_2_0;
    /* moved CSE1 up */
    /* a &= (0x7f<<14)|(0x7f); */
    a = a<<7;
    a |= b;
    *v = a;
    return 4;
  }

  /* a: p0<<14 | p2 (masked) */
  /* b: p1<<14 | p3 (unmasked) */
  /* 1:save off p0<<21 | p1<<14 | p2<<7 | p3 (masked) */
  /* moved CSE1 up */
  /* a &= (0x7f<<14)|(0x7f); */
  b &= SLOT_2_0;
  s = a;
  /* s: p0<<14 | p2 (masked) */

  p++;
  a = a<<14;
  a |= *p;
  /* a: p0<<28 | p2<<14 | p4 (unmasked) */
  if (!(a&0x80))
  {
    /* we can skip these cause they were (effectively) done above
    ** while calculating s */
    /* a &= (0x7f<<28)|(0x7f<<14)|(0x7f); */
    /* b &= (0x7f<<14)|(0x7f); */
    b = b<<7;
    a |= b;
    s = s>>18;
    *v = ((u64)s)<<32 | a;
    return 5;
  }

  /* 2:save off p0<<21 | p1<<14 | p2<<7 | p3 (masked) */
  s = s<<7;
  s |= b;
  /* s: p0<<21 | p1<<14 | p2<<7 | p3 (masked) */

  p++;
  b = b<<14;
  b |= *p;
  /* b: p1<<28 | p3<<14 | p5 (unmasked) */
  if (!(b&0x80))
  {
    /* we can skip this cause it was (effectively) done above in calc'ing s */
    /* b &= (0x7f<<28)|(0x7f<<14)|(0x7f); */
    a &= SLOT_2_0;
    a = a<<7;
    a |= b;
    s = s>>18;
    *v = ((u64)s)<<32 | a;
    return 6;
  }

  p++;
  a = a<<14;
  a |= *p;
  /* a: p2<<28 | p4<<14 | p6 (unmasked) */
  if (!(a&0x80))
  {
    a &= SLOT_4_2_0;
    b &= SLOT_2_0;
    b = b<<7;
    a |= b;
    s = s>>11;
    *v = ((u64)s)<<32 | a;
    return 7;
  }

  /* CSE2 from below */
  a &= SLOT_2_0;
  p++;
  b = b<<14;
  b |= *p;
  /* b: p3<<28 | p5<<14 | p7 (unmasked) */
  if (!(b&0x80))
  {
    b &= SLOT_4_2_0;
    /* moved CSE2 up */
    /* a &= (0x7f<<14)|(0x7f); */
    a = a<<7;
    a |= b;
    s = s>>4;
    *v = ((u64)s)<<32 | a;
    return 8;
  }
  //</略过分析>
  
  //前8个字节都为B，所以此varint有9个字节，格式为BBBBBBBBC。
  p++;
  a = a<<15;
  a |= *p;
  /* a: p4<<29 | p6<<15 | p8 (unmasked) */

  /* moved CSE2 up */
  /* a &= (0x7f<<29)|(0x7f<<15)|(0xff); */
  b &= SLOT_2_0;
  b = b<<8;
  a |= b;

  s = s<<4;
  b = p[-4];
  b &= 0x7f;
  b = b>>3;
  s |= b;

  *v = ((u64)s)<<32 | a;

  return 9;
}

（编辑：李大同）

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