Streaming and state

The terminal is streaming-native: it folds a feed of events into state one event at a time, in O(1) per event, and never recomputes over history. That is the whole moat — it is what lets the core sustain tens of thousands of frames per second regardless of how long a session has run.

The fold

AppState::fold applies a single event to one (SourceId, Symbol)'s state incrementally:

• Trade → update last, push into the bounded TapeRing, add to the Footprint (per-price buy/sell volume), advance the IndicatorSet, and append to a bounded price history.
• Ticker → update last.
• BookSnapshot / BookDelta → apply to the local L2 BookState (a BTreeMap-backed book; a zero-quantity level is a removal).
• Account / lifecycle events do not affect per-symbol market state.

Every buffer is bounded (the tape ring, the price history), so memory is bounded regardless of session length.

Determinism and the golden corpus

The fold is deterministic: the same recorded feed always produces the same frame. golden/ pins this byte-for-byte — a recorded feed (replay/basic.json) drives the terminal and must produce the exact frame view-models in expected/basic.json. Because the same command protocol crosses every binding, these fixtures are also the cross-language parity corpus: a binding replaying the feed must reproduce the same frame.

Regenerate the fixtures after an intentional schema change:

WICKRA_REGEN=1 cargo test -p terminal-core --test golden

Property and fuzz coverage

• tests/proptest_invariants.rs — arbitrary event streams keep the tape ring within its cap, track the last price and keep the book top-of-book ordered.
• fuzz/ — four cargo-fuzz targets (feed_event, state_fold, view_model, config_parse) drive arbitrary bytes through the parsing and fold paths; none may panic. The footprint accumulator saturates rather than overflowing on adversarial volumes.

Performance

Measured with cargo bench -p terminal-bench (see ../BENCHMARKS.md): folding one event ~333 ns, a full tick (poll + fold + build every panel) ~25 µs.