Struct Decoder 

pub struct Decoder { /* private fields */ }

A reusable FIX message decoder.

Owns a SmallVec buffer that is allocated once (at startup or first use) and reused across every decode call — zero allocation per message on the hot path.

Stores (tag, value_start, value_end) byte offsets rather than slices, eliminating all unsafe lifetime transmutes while preserving zero-allocation and zero-copy semantics.

Example

let mut decoder = Decoder::new();

loop {
    let msg = decoder.decode(buf)?;  // zero allocation after first call
    process(msg);
    // msg dropped here — decoder buffer ready for next call
}

Implementations

impl Decoder

pub fn new() -> Self

Create a new decoder with a default inline capacity of 32 fields.

pub fn with_capacity(capacity: usize) -> Self

Create a new decoder pre-allocated for capacity fields. Use this when messages consistently exceed 32 fields (e.g. MarketData).

pub fn decode<'a>(&'a mut self, buf: &'a [u8]) -> Result<Message<'a>, FixError>

Decode a raw FIX byte buffer into a Message.

Clears and reuses the internal offset buffer — zero allocation per call after the first. The returned Message<'a> borrows both from self (the offset slice) and from buf (the raw bytes). Drop Message before calling decode again.

The sorted tag index used by Message::find is built lazily on the first find() call and cached for the message lifetime. If find() is never called, no sort ever happens.

Errors

• FixError::IncompleteMessage — the buffer contains a partial field (no = or no SOH delimiter found); buffer more bytes before retrying.
• FixError::InvalidTag — a tag contained non-digit bytes or overflowed u32.

Trait Implementations

impl Default for Decoder

fn default() -> Self

Returns the “default value” for a type.