Varint整数压缩算法
varint的整数压缩算法，应该很多地方都见到。 可以压缩int ，节省空间，适合存储。 thirft的CompactProtocol 其实就是应用varint来压缩整数
但持续的读写，解析应该还是慢很多的吧，所以有的库capnproto https://capnproto.org/ 就取消这个做法了，而是采用一种特殊 “packing”算法来压缩，说是直接压缩 0 字节。
确实如果很需要考虑带宽的时候可以考虑 LZ4 等压缩算法。 本来是希望在各程序内容的 IR 字节码采用 varint， 不知道会不会少用一些内存，缓存里面更好一些。 单看上去varint 有导致分支更多，估计不会有什么作用，反而得不偿失。
这个thrift 里面代码 https://github.com/apache/thrift/blob/master/lib/cpp/src/thrift/protocol/TCompactProtocol.tcc
/**
* Write an i32 as a varint. Results in 1-5 bytes on the wire.
*/
template <class Transport_>
uint32_t TCompactProtocolT<Transport_>::writeVarint32(uint32_t n) {
uint8_t buf[5];
uint32_t wsize = 0;
while (true) {
if ((n & ~0x7F) == 0) {
buf[wsize++] = (int8_t)n;
break;
} else {
buf[wsize++] = (int8_t)((n & 0x7F) | 0x80);
n >>= 7;
}
}
trans_->write(buf, wsize);
return wsize;
}
/**
* Read an i64 from the wire as a proper varint. The MSB of each byte is set
* if there is another byte to follow. This can read up to 10 bytes.
*/
template <class Transport_>
uint32_t TCompactProtocolT<Transport_>::readVarint64(int64_t& i64) {
uint32_t rsize = 0;
uint64_t val = 0;
int shift = 0;
uint8_t buf[10]; // 64 bits / (7 bits/byte) = 10 bytes.
uint32_t buf_size = sizeof(buf);
const uint8_t* borrowed = trans_->borrow(buf, &buf_size);
// Fast path.
if (borrowed != NULL) {
while (true) {
uint8_t byte = borrowed[rsize];
rsize++;
val |= (uint64_t)(byte & 0x7f) << shift;
shift += 7;
if (!(byte & 0x80)) {
i64 = val;
trans_->consume(rsize);
return rsize;
}
// Have to check for invalid data so we don't crash.
if (UNLIKELY(rsize == sizeof(buf))) {
throw TProtocolException(TProtocolException::INVALID_DATA, "Variable-length int over 10 bytes.");
}
}
}
// Slow path.
else {
while (true) {
uint8_t byte;
rsize += trans_->readAll(&byte, 1);
val |= (uint64_t)(byte & 0x7f) << shift;
shift += 7;
if (!(byte & 0x80)) {
i64 = val;
return rsize;
}
// Might as well check for invalid data on the slow path too.
if (UNLIKELY(rsize >= sizeof(buf))) {
throw TProtocolException(TProtocolException::INVALID_DATA, "Variable-length int over 10 bytes.");
}
}
}
}