简介：Varints are a crucial aspect of Protocol Buffers (Protobuf) encoding. This article demystifies the encoding process and provides a clear understanding of how varints work in practice.
Varints are a core component of Protocol Buffers (Protobuf) encoding, used to represent integer values in a compact and efficient manner. In this article, we will delve into the inner workings of varints and explore how they enable efficient serialization and deserialization of integer data in Protobuf.
What Are Varints?
Varints are a variable-length encoding scheme for integers. They use a minimum number of bytes to represent small integers, while reserving additional bytes for larger values. This allows varints to encode both small and large integers in a space-efficient manner.
Varints are encoded in little-endian format, where the least significant byte comes first. Each subsequent byte in the varint has its most significant bit (MSB) set to indicate that there are more bytes to follow. The MSB is unset in the last byte of the varint.
Encoding Process
The encoding process for varints begins by determining the number of bytes required to represent the integer. Smaller integers require fewer bytes, while larger integers require more bytes. The exact number of bytes needed depends on the size of the integer and the associated type (e.g., int32, int64, uint32, uint64, sint32, sint64, bool, enum).
Once the number of bytes is determined, the integer is broken down into individual bytes. The least significant byte is encoded first, followed by subsequent bytes in ascending order of significance. Each byte’s MSB is set to indicate that more bytes follow, except for the last byte where it is unset.
To illustrate the encoding process, let’s consider an example where we encode the integer value 300 using varints.
Example: Encoding 300 with Varints
Value: 300 (decimal)
Type: int32 (32-bit integer)
First, we determine the number of bytes required to represent 300 using the int32 type: one byte.
Next, we break down 300 into individual bytes: 0000 0010 1011 0000 (binary).
Finally, we encode the byte sequence with varints: 010 1100 000 0010 (little-endian binary).
The encoded varint represents the integer value 300 using a single byte.
Decoding Process
Decoding varints involves reversing the encoding process. To decode a varint, we read the bytes in reverse order starting from the least significant byte. We unset each MSB as we encounter it until we reach the last byte. The decoded value is then reconstructed by combining the individual bytes in their original order.
In our example, decoding the varint 010 1100 000 0010 yields the original value 300.
Applications in Practice
Varints are particularly useful in scenarios where space efficiency is crucial, such as when storing or transmitting large amounts of data. By using varints, Protobuf enables efficient serialization and deserialization of integer values, reducing storage requirements and improving data transmission efficiency. Varints are also beneficial for supporting both fixed-width and variable-width integer types within a single encoding scheme.
Conclusion
Varints are a fundamental aspect of Protocol Buffers encoding, enabling efficient representation of integer values in a compact and flexible manner. Understanding how varints work and their application in practice can help developers make informed decisions when using Protobuf for data serialization and inter-process communication.