此文档提供简体中文版本。

LEB128

LEB128 or Little Endian Base 128 is a form of variable-length code compression used to store an arbitrarily large integer in a small number of bytes, unlike traditional 32-bit or 64-bit encoded integers that always occupy a fixed number of bytes.

LEB128 and VLQ (Variable-Length Quantity), used in MIDI and other protocols and formats, both compress any unsigned integer into not only the same number of bits, but exactly the same bits—the two formats differ only in exactly how those bits are arranged. In some context, LEB128 is referred to as a 7-bit Encoded Integer.

In its core, LEB128 splits the binary representation of a given integer into 7-bit chunks and uses the MSB of every byte to indicate whether another byte is required.

LEB128.Swift

Swift doesn’t have a system framework that deals with LEB128 encoding. LEB128.Swift is created to address this issue. It provides codecs for conversion between LEB128 encoded byte buffers and Swift native integer types: Int and UInt. It also has its own signed and unsigned integer types that acts as BigIntegers in other language to work with values larger than 64-bit.

Codec

LEB128Encoder and LEB128Decoder facilitate conversion between Swift integer types and LEB128 encoded byes. They are useful when working with file formats and data streams using LEB128 encoding.

• LEB128Encoder can transform any BinaryInteger conforming types to LEB128 encoded ContigousArray<UInt8>. Using ContigousArray<UInt8> ensures performance. When Foundation framework is available, additional extension methods that returns Data are also provided.
• LEB128Decoder can transform any Collection of UInt8s to platform-independent Int or UInt, depending on whether the original encoding is signed or unsigned. Because [UInt8] and Data conform to Collection, no extension methods are provided for more specialized collections. Be aware that the size of Int and UInt are limited, depending on the platform implementation, they are either 32-bit or 64-bit. Therefore, not all LEB128 encoded arbitrary-length integers can fit, when the size of encoded integer exceeds platform limitation, decoding methods return nil.

7-bit Encoded Integers

Signed7BitEncodedInteger, Unsigned7BitEncodedInteger and the base protocol SevenBitEncodedInteger store any integer value in a native LEB128 byte buffer. This allows storage and arithmetic operations on arbitrarily large values.

Swift Int and UInt values can be extracted from corresponding types. However, when the stored value exceeds platform limitation, properties returning Swift integers become unavailable and debugging prints raw bytes instead of human-readable decimals.

Signed7BitEncodedInteger and Unsigned7BitEncodedInteger have not yet supported all arithmetic operations. Conformance to BinaryInteger is scheduled on is in the works.