# Core Streaming Substrate — Design Spec

**Date:** 2026-06-03
**Status:** Draft — awaiting user spec review
**Authors:** orchestrator + Claude

## Goal

Deliver the foundation everything in aura streams through: the four scalar base
types, the bounded-lookback Structure-of-Arrays column with financial-style
indexing, the zero-copy read window a node sees, the type-erased edge carrier,
and the per-column run-count that the freshness model is built on. This is the
first cycle of the walking-skeleton milestone (ingest → one signal →
deterministic backtest → position table → sim-optimal broker → pip-equity); it
is the substrate the subsequent `Node`/`Ctx`/engine-loop cycles build on.

The reference is RustAst's `src/ast/rtl/series` and `streams` — a working
implementation of this substrate. aura keeps its *shape* (newest-at-0 indexing,
a per-series push counter for freshness, SoA composites) and **sharpens** it:
no boxed `Value`, no `VecDeque`, no `RefCell`/`Rc`, no per-event allocation.

This cycle implements only the data substrate. **Out of scope** (subsequent
cycles): the `Node` trait, the evaluation `Ctx`, firing policies A/B, the
deterministic sim loop / freshness-gated recompute, the ingestion boundary /
k-way merge, and the named OHLCV bundle. Those are named here only to fix this
cycle's boundary; this cycle delivers the primitives they will consume.

## Architecture

`aura-core` gains four small modules, no dependencies added:

- `scalar` — the closed four-type scalar set: the `Timestamp` newtype, the
  `Scalar` Copy-POD carrier, and the `ScalarKind` tag (C7).
- `column` — `Column<T>`: a fixed-capacity ring buffer, pre-sized to a lookback
  limit at construction, financial-indexed (index 0 = newest), carrying a
  monotonic `run_count` (C5/C8). `Window<'_, T>` is its zero-copy read view.
- `any` — `AnyColumn`: the type-erased edge carrier, an enum over the four
  `Column<T>` kinds. The kind check is paid at the edge (wiring time); the hot
  loop pushes into the concrete `Column<T>` monomorphically (C7).
- `error` — `KindMismatch`, the wiring-time guard error returned when a `Scalar`
  of the wrong kind is pushed into an `AnyColumn`.

The whole substrate is **interior-mutability-free**. A sim is single-threaded
and non-concurrent (C1), so the engine owns columns by `&mut` for the push side
and hands out `&Column` / `Window` (shared borrow) on the read side — RustAst's
`RefCell`-on-every-series disappears.

## Concrete code shapes

### North-star slice (the consumer this substrate exists to serve)

A future `aura-std` SMA node — *not built this cycle* — reads its input as an
indexed, newest-at-0 window. This is the minimal honest slice proving the
substrate's read API fits the node use case it exists for:

```rust
// Illustrative future consumer (NOT delivered this cycle): the read shape an
// indicator node wants. `length` is the node's pre-sized lookback (C8).
fn sma(window: &Window<'_, f64>, length: usize) -> Option<f64> {
    if window.len() < length {
        return None; // not yet warmed up (C8: None = not-yet-warmed-up)
    }
    let sum: f64 = (0..length).map(|k| window[k]).sum(); // [0] newest, [length-1] oldest
    Some(sum / length as f64)
}
```

The point of the cycle is that `Window` makes this code possible with no
lookback bookkeeping in the node: the engine pre-sized the buffer, the node only
reads by index.

### Delivered code — the scalar set (`scalar.rs`)

```rust
/// Canonical engine time: epoch-nanoseconds UTC. Newtype over i64 (C7).
/// `Default` (epoch 0) is required so `Column<Timestamp>` can pre-size its ring.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Timestamp(pub i64);

/// The four scalar base types, type-erased for an edge. Copy POD — no `dyn Any`,
/// no heap, no Rc (C7 forbids all three on the hot path).
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum Scalar {
    I64(i64),
    F64(f64),
    Bool(bool),
    Ts(Timestamp),
}

/// The kind tag of a scalar / column, used for the edge-time type check.
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum ScalarKind {
    I64,
    F64,
    Bool,
    Timestamp,
}

impl Scalar {
    pub fn kind(self) -> ScalarKind { /* match */ }
}

// Total, infallible widenings into the carrier:
impl From<i64> for Scalar { /* ... */ }
impl From<f64> for Scalar { /* ... */ }
impl From<bool> for Scalar { /* ... */ }
impl From<Timestamp> for Scalar { /* ... */ }
```

### Delivered code — the column and its window (`column.rs`)

```rust
/// A fixed-capacity, bounded-lookback SoA column. Pre-sized at construction
/// (C8: no realloc in the hot loop); financial-indexed (index 0 = newest);
/// carries a monotonic push counter for the freshness model (C5).
pub struct Column<T> {
    buf: Box<[T]>,   // capacity == lookback; never reallocated
    head: usize,     // ring write cursor (next slot)
    len: usize,      // valid elements, saturates at capacity
    run_count: u64,  // monotonic total pushes — RustAst's total_count (C5)
}

impl<T: Copy + Default> Column<T> {
    /// `lookback` is the window depth the engine sized for this edge (C8).
    pub fn with_capacity(lookback: usize) -> Self { /* Box<[T]> of len lookback */ }

    /// Append the newest value. O(1), no allocation. Bumps `run_count`.
    /// On overflow the oldest value is overwritten (true ring).
    pub fn push(&mut self, v: T) { /* buf[head]=v; head=(head+1)%cap; len=min(len+1,cap); run_count+=1 */ }

    /// Financial index: 0 = newest, `len()-1` = oldest. None if `k >= len`
    /// (C8: None == not-yet-warmed-up / out of range). Zero-copy.
    pub fn get(&self, k: usize) -> Option<T> { /* buf[(head - 1 - k) mod cap] */ }

    pub fn len(&self) -> usize { self.len }
    pub fn capacity(&self) -> usize { self.buf.len() }
    pub fn run_count(&self) -> u64 { self.run_count }

    /// A zero-copy read view handed to a reading node.
    pub fn window(&self) -> Window<'_, T> { Window { col: self } }
}

/// Read-only, zero-copy view into a `Column` — what a node sees (C8).
pub struct Window<'a, T> {
    col: &'a Column<T>,
}

impl<'a, T: Copy + Default> Window<'a, T> {
    pub fn get(&self, k: usize) -> Option<T> { self.col.get(k) }
    pub fn len(&self) -> usize { self.col.len() }
    pub fn run_count(&self) -> u64 { self.col.run_count() }
}

/// `window[k]` — newest-at-0 indexed access; panics on out-of-range, matching
/// slice `Index` semantics. Nodes that may be cold use `get` instead.
impl<'a, T: Copy + Default> core::ops::Index<usize> for Window<'a, T> {
    type Output = T;
    fn index(&self, k: usize) -> &T { /* &buf[(head-1-k) mod cap], bounds-checked */ }
}
```

### Delivered code — the type-erased edge (`any.rs`)

```rust
/// A type-erased edge: the four concrete columns behind one enum (C7). The kind
/// check happens here, at wiring time; the hot loop grabs the concrete column
/// once (`as_f64_mut`) and pushes monomorphically — no per-event dispatch.
pub enum AnyColumn {
    I64(Column<i64>),
    F64(Column<f64>),
    Bool(Column<bool>),
    Ts(Column<Timestamp>),
}

impl AnyColumn {
    pub fn with_capacity(kind: ScalarKind, lookback: usize) -> Self { /* match kind */ }

    pub fn kind(&self) -> ScalarKind { /* match self */ }
    pub fn len(&self) -> usize { /* match self */ }
    pub fn run_count(&self) -> u64 { /* match self */ }

    /// Edge-time typed push. Returns `KindMismatch` if `v.kind() != self.kind()`
    /// — the C7 type-erasure guard. The hot path avoids this by taking the
    /// concrete `Column<T>` once via `as_*_mut`.
    pub fn push(&mut self, v: Scalar) -> Result<(), KindMismatch> { /* match (self, v) */ }

    /// Read one type-erased value (0 = newest). None if cold / out of range.
    pub fn get(&self, k: usize) -> Option<Scalar> { /* match self -> Scalar::… */ }

    /// Concrete-column accessors for the monomorphic hot path.
    pub fn as_f64_mut(&mut self) -> Option<&mut Column<f64>> { /* … */ }
    pub fn as_i64_mut(&mut self) -> Option<&mut Column<i64>> { /* … */ }
    pub fn as_bool_mut(&mut self) -> Option<&mut Column<bool>> { /* … */ }
    pub fn as_ts_mut(&mut self) -> Option<&mut Column<Timestamp>> { /* … */ }
}
```

### Delivered code — the must-fail guard fixture (`error.rs` + a test)

The C7 type-erasure contract is load-bearing: a wrong-kind push must be rejected,
never silently coerced. The test whose *correct behaviour is rejection*:

```rust
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct KindMismatch {
    pub expected: ScalarKind,
    pub got: ScalarKind,
}

#[test]
fn wrong_kind_push_is_rejected() {
    let mut edge = AnyColumn::with_capacity(ScalarKind::I64, 8);
    let err = edge.push(Scalar::F64(1.5)).unwrap_err(); // pushing f64 into an i64 edge
    assert_eq!(err, KindMismatch { expected: ScalarKind::I64, got: ScalarKind::F64 });
    assert_eq!(edge.len(), 0);       // nothing was stored
    assert_eq!(edge.run_count(), 0); // and the counter did not move
}
```

### Implementation shape (secondary — before → after)

`aura-core/src/lib.rs` today is doc-comment-only (no `pub` items). After this
cycle it declares the four modules and re-exports the surface:

```rust
// before: lib.rs is documentation only, defines no types.

// after:
mod scalar;
mod column;
mod any;
mod error;

pub use scalar::{Scalar, ScalarKind, Timestamp};
pub use column::{Column, Window};
pub use any::AnyColumn;
pub use error::KindMismatch;
```

## Components

- **`Timestamp`** — `i64` epoch-ns newtype, fully ordered (the ingestion
  boundary will normalize source-native units into it in a later cycle; here it
  is just the type).
- **`Scalar` / `ScalarKind`** — the closed carrier + tag. `Scalar` is `Copy`;
  `From` impls give ergonomic widening; `kind()` projects to the tag.
- **`Column<T>`** — the ring. Owns a pre-sized `Box<[T]>`; `push` is O(1) and
  allocation-free; `get`/`Index` implement newest-at-0; `run_count` is the
  freshness primitive.
- **`Window<'_, T>`** — the borrow a reader gets; carries no storage, only a
  reference; `Index` for the warm path, `get` for the possibly-cold path.
- **`AnyColumn`** — the edge; type check at the boundary, concrete-column
  accessors for the hot loop.
- **`KindMismatch`** — the guard error.

## Data flow

1. The engine (later cycle) sizes an edge: `AnyColumn::with_capacity(kind,
   lookback)` — buffer allocated once, never again (C8).
2. A producer pushes its newest output for the cycle. Wiring-time / dynamic
   path: `edge.push(scalar)?` (kind-checked). Hot path: the engine took
   `edge.as_f64_mut()` once at sim start and calls `col.push(x)` monomorphically
   each cycle — no per-event dispatch, no allocation (C7).
3. `run_count` increments on every push; the freshness model (later cycle)
   compares an input's `run_count` against the value it last saw to decide
   fire-vs-hold (C5). This cycle delivers the counter, not the comparison loop.
4. A reading node receives `col.window()` and reads by index, newest at 0 (C8).

There is no merge, no clock, and no recompute logic in this cycle — those are the
engine's, downstream. The substrate is pure data structures + their invariants.

## Error handling

- **Wrong-kind push** → `Err(KindMismatch { expected, got })`; the column is
  left untouched (len and run_count unchanged). This is the only fallible
  operation in the substrate.
- **Cold / out-of-range read** → `get` returns `None` (C8: not-yet-warmed-up);
  `Index` panics like a slice (the warm-path ergonomic, used only when the node
  has already checked `len`).
- **No panics on the push/normal-read path**; no allocation after construction;
  no interior mutability to poison.

## Testing strategy

Unit tests live beside each module; all run under `cargo test --workspace`.

- **`Column` ring semantics:** push then `get(0)` is newest; `get(len-1)` is
  oldest; pushing more than capacity keeps exactly the newest `capacity` values
  and drops the oldest (wraparound correctness); `len` saturates at capacity;
  `get(k>=len) == None`.
- **`run_count` is the freshness primitive:** monotonic across pushes
  (including after the ring wraps — it counts *pushes*, not stored len); never
  moved by a read.
- **`Window` is a zero-copy view:** values read through the window equal the
  column's; taking a window does not change the column's capacity (no realloc);
  `Index` and `get` agree on the warm range.
- **`Scalar` / `ScalarKind`:** `kind()` is correct for each variant; `From`
  widenings round-trip through `get` on an `AnyColumn`.
- **`AnyColumn` type erasure:** same-kind push stores and is read back as the
  right-kind `Scalar`; `run_count`/`len` delegate; the concrete `as_*_mut`
  accessor returns `Some` only for the matching kind.
- **The must-fail guard** (`wrong_kind_push_is_rejected`, shown above): a
  wrong-kind push is rejected and leaves the column untouched.

## Acceptance criteria

Applying aura's feature-acceptance lens (CLAUDE.md):

1. **The intended audience naturally reaches for it.** The node author (LLM via
   builder APIs, later cycles) reads inputs as `window[k]` newest-at-0 — the
   north-star slice above is exactly the SMA an author writes, with zero
   lookback bookkeeping. The substrate is the thing every node, source, and sink
   stands on.
2. **It measurably improves correctness / removes redundancy.** Engine-sized,
   pre-allocated windows mean node code *cannot* mis-manage lookback (C8
   rationale); the closed `Scalar` set + `AnyColumn` removes RustAst's boxed
   `Value` and per-series `RefCell` entirely.
3. **It reintroduces no failure class the core constraints exist to kill.** No
   `dyn Any`, no per-event heap allocation, no interior mutability, fixed
   pre-sized buffers (C1/C7/C8 all upheld); the wrong-kind push is structurally
   rejected, not coerced.

Concretely, the cycle ships when:

- `cargo build --workspace`, `cargo test --workspace`, and
  `cargo clippy --workspace --all-targets -- -D warnings` are all green.
- Every test in the Testing-strategy list exists and passes, including the
  `wrong_kind_push_is_rejected` must-fail guard.
- `aura-core` exports `Scalar`, `ScalarKind`, `Timestamp`, `Column`, `Window`,
  `AnyColumn`, `KindMismatch` and nothing leaks `RefCell`/`Rc`/`dyn Any` on the
  substrate surface.