c++ - 在 protobuf 中使用 int32 的原因

Question

在 gpb proto2 ( https://developers.google.com/protocol-buffers/docs/proto#scalar ) 标量类型的描述中，它说:

• int32

  Uses variable-length encoding. Inefficient for encoding negative numbers – if your field is likely to have negative values, use sint32 instead.

• sint32

  Uses variable-length encoding. Signed int value. These more efficiently encode negative numbers than regular int32s.

对于正值，sint32 是否与 int32 一样有效？

换句话说，有没有理由使用int32？

如果使用什么语言很重要，我只对 C++ 感兴趣。

Answer 1

https://developers.google.com/protocol-buffers/docs/encoding#signed-integers

带符号的 varint 是通过在正值和负值之间交替进行编码的。例如，

   value     int32    zigzag    sint32
          (binary)            (binary)
       0  00000000         0  00000000
      -1  11111111         1  00000001
          11111111
          11111111
          11111111
          00001111
       1  00000001         2  00000010
      -2  11111110         3  00000011
          11111111
          11111111
          11111111
          00001111
...
      63  00111111       126  01111110
     -64  11000000       127  01111111
          11111111
          11111111
          11111111
          00001111
      64  01000000       128  10000000
                              00000001
...

平均而言，将正数编码为 sint 比编码为 int 需要多一位。

(展开并运行以下代码片段以进行现场演示。)

function encode_varint(number) {
  if (!number) return [0];
  var bytes = [];
  while (number) {
    var byte = number & 0x7F;
    number >>>= 7;
    if (number) byte |= 0x80;
    bytes.push(byte);
  }
  return bytes;
}
function format_bytes(bytes) {
  var output = '';
  for (var i = 0; i < bytes.length; i++) {
    if (i) output += ' ';
    output += bytes[i].toString(2).padStart(8, '0');
  }
  return output;
}
var valueElem = document.getElementById('value');
var int32Elem = document.getElementById('int32');
var sint32Elem = document.getElementById('sint32');
function update() {
  var value = parseInt(valueElem.value);
  var int32 = encode_varint(value);
  var sint32 = encode_varint(value << 1 ^ -(value < 0));
  int32Elem.value = format_bytes(int32);
  sint32Elem.value = format_bytes(sint32);
}
valueElem.addEventListener('change', update);
update();

#varint {
  display: grid;
  grid-template-columns: max-content auto;
  grid-row-gap: 1ex;
  grid-column-gap: 1ch;
}
#varint label {
  text-align: right;
}
#varint input {
  font-family: monospace;
}

<form id='varint' onsubmit='return false'>
  <label for='value'>value</label>
  <input id='value' type='number' value='0'>
  <label for='int32'>int32</label>
  <input id='int32' type='text' readonly>
  <label for='sint32'>sint32</label>
  <input id='sint32' type='text' readonly>
</form>