实现 Protocol Buffer 中的 varint 编码

June 1, 2024 · Technology

Protobuf Encoding

Encode

Start with the integer you want to encode.
Break it into 7-bit chunks. These chunks are extracted from the least significant bits of the number.
Set the most significant bit (MSB) of each chunk to 1 if there are more chunks to follow, and to 0 for the last chunk.
Combine these chunks into bytes and output them in order.

300 in binary: 100101100
 
100101100 (original)
00101100 (first 7 bits: 44 in decimal)
00000010 (remaining bits: 2 in decimal)
 
 
00101100  ->  10101100 (since there are more chunks, MSB is 1)
00000010  ->  00000010 (last chunk, MSB is 0)
 
First byte:  10101100 (172 in decimal)
Second byte: 00000010 (2 in decimal)
 
0xAC 0x02

Decode

Read bytes one by one.
For each byte, strip the MSB and collect the remaining 7 bits.
If the MSB is 1, continue to the next byte. If the MSB is 0, this is the last byte.
Combine all 7-bit groups into the original number.

 
10010110 00000001        // Original inputs.
 0010110  0000001        // Drop continuation bits.
 0000001  0010110        // Convert to big-endian.
   00000010010110        // Concatenate.
 128 + 16 + 4 + 2 = 150  // Interpret as an unsigned 64-bit integer.

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