Qubit Codec

qubit-codec gives Rust codec and adapter authors one set of contracts for single-value codecs, owned convenience APIs, and caller-buffered streaming. Format crates keep their binary, text, or character-set rules local while this crate supplies checked capacity handling, explicit lifecycle semantics, and policy-ready conversion loops.

Installation

[dependencies]
qubit-codec = "0.13"

The default feature set is empty. Enable io only for the qubit-io buffered bridges:

[dependencies]
qubit-codec = { version = "0.13", features = ["io"] }

The minimum supported Rust version is 1.94.

Quick Start

Start by choosing the smallest public contract that matches the format:

For example, a crate that owns a fixed-width big-endian u16 codec implements Codec once. It can then expose an owned operation without duplicating bounds or lifecycle handling:

use core::{convert::Infallible, num::NonZeroUsize};
use qubit_codec::{Codec, CodecValueEncoder, DecodeFailure, ValueEncoder};

#[derive(Default)]
struct U16BeCodec;

impl Codec for U16BeCodec {
    type Value = u16;
    type Unit = u8;
    type DecodeError = Infallible;
    type EncodeError = Infallible;

    const MIN_UNITS_PER_VALUE: usize = 2;
    const MAX_ENCODE_UNITS_PER_VALUE: usize = 2;
    const MAX_DECODE_UNITS_PER_VALUE: usize = 2;

    unsafe fn decode(
        &mut self,
        input: &[u8],
        input_index: usize,
    ) -> Result<(u16, NonZeroUsize), DecodeFailure<Infallible>> {
        debug_assert!(input_index + 2 <= input.len());
        let value = u16::from_be_bytes([
            input[input_index],
            input[input_index + 1],
        ]);
        Ok((value, NonZeroUsize::new(2).expect("two is non-zero")))
    }

    unsafe fn encode(
        &mut self,
        value: &u16,
        output: &mut [u8],
        output_index: usize,
    ) -> Result<usize, Infallible> {
        debug_assert!(output_index + 2 <= output.len());
        output[output_index..output_index + 2]
            .copy_from_slice(&value.to_be_bytes());
        Ok(2)
    }
}

let mut encoder = CodecValueEncoder::new(U16BeCodec);
let bytes = encoder.encode(&0x1234).expect("encoding is infallible");
assert_eq!(vec![0x12, 0x34], bytes);

The user guide continues this exact scenario with owned decode, caller-buffered streaming, incomplete input, lifecycle output, and policy hooks.

Why This Project Exists

A format crate often begins with one value-to-unit operation and later grows owned helpers, streaming adapters, malformed-input policy, and I/O integration. If every layer reimplements indices, capacity, EOF, and reset/finish behavior, the same format acquires incompatible contracts. qubit-codec keeps those mechanics shared while leaving domain rules in the format crate that owns them.

What It Provides

Boundaries and Guarantees

• The crate does not implement concrete binary formats, character sets, Base64, hex, percent encoding, or std::io extensions.
• Codec::decode separates an incomplete visible prefix from invalid domain input through DecodeFailure; an incomplete prefix is not an EOF decision.
• Codec::encode_len must equal the units written by the following successful Codec::encode in the same state, including intentional zero-output buffering.
• A Transcoder follows reset -> transcode/transcode_eof -> finish. NeedInput leaves its tail with the caller; NeedOutput requires more destination capacity; Complete means all visible input was consumed.
• TranscodeDecodeHooks::handle_incomplete_decode lets decode engines choose Reject, Skip, or Emit for an incomplete tail only after EOF is known.
• Capacity bounds must cover every reachable transient state. Owned adapters may allocate; streaming APIs use caller-provided buffers.

Learn More

• User guide: implement a codec, expose adapters, drive a stream, and diagnose failures.
• 中文用户手册
• API documentation
• Lifecycle-aware decode example
• 中文 README

Testing

# Run tests with the default feature set
cargo test

# Run tests with all declared features
cargo test --all-features

# Project CI checks
./ci-check.sh

# Check code coverage
./coverage.sh

License

Copyright (c) 2025 - 2026. Haixing Hu. All rights reserved.

Licensed under the Apache License, Version 2.0. See LICENSE for the full license text.

Contributing

Contributions are welcome. Please follow the Rust API guidelines, keep public API documentation and tests current, and run ./align-ci.sh to format code and ./ci-check.sh to satisfy CI requirements before submitting a pull request.

Author

Haixing Hu - Qubit Co. Ltd.

Repository: https://github.com/qubit-ltd/rs-codec