# Core Streaming Substrate — Implementation Plan

> **Parent spec:** `docs/specs/0001-core-streaming-substrate.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement` skill to run
> this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Deliver the `aura-core` streaming substrate — the four scalar base
types, the bounded-lookback financial-indexed `Column`/`Window`, the type-erased
`AnyColumn` edge, and the `KindMismatch` guard — fully built, tested, and lint-clean.

**Architecture:** Four new private modules in `aura-core` (`scalar`, `column`,
`any`, `error`), each with its own inline `#[cfg(test)] mod tests`, re-exported
from `lib.rs`. No dependencies added. Interior-mutability-free: a sim is
single-threaded (C1), so columns are owned `&mut` on push, borrowed `&` on read.

**Tech Stack:** Rust (edition 2024, `aura-core` crate), `cargo build/test/clippy
--workspace`.

**Module ordering (by dependency):** `scalar` (standalone) → `column`
(generic, standalone) → `error` (uses `scalar::ScalarKind`) → `any` (uses all
three) → final workspace gate. Each task adds its file, its `mod`+`pub use` line
in `lib.rs`, and its tests, so the crate compiles green after every task.

**Greenfield note:** every type here is new; the tests pin each module's contract
and run green on first build. There is no pre-existing behaviour to RED against —
the must-fail guard (`wrong_kind_push_is_rejected`, Task 4) is the one test whose
*correct behaviour is rejection*.

**Files this plan creates or modifies:**

- Create: `crates/aura-core/src/scalar.rs` — `Timestamp`, `Scalar`, `ScalarKind`, `kind()`, four `From` impls.
- Create: `crates/aura-core/src/column.rs` — `Column<T>` ring + zero-copy `Window<'_, T>` with `Index`.
- Create: `crates/aura-core/src/error.rs` — `KindMismatch { expected, got }`.
- Create: `crates/aura-core/src/any.rs` — `AnyColumn` type-erased edge.
- Modify: `crates/aura-core/src/lib.rs:1-19` — module declarations + re-exports (doc prose updated).
- Test: inline `#[cfg(test)] mod tests` in each of the four module files.

---

## Task 1: The scalar set (`scalar.rs`)

**Files:**
- Create: `crates/aura-core/src/scalar.rs`
- Modify: `crates/aura-core/src/lib.rs`
- Test: inline in `crates/aura-core/src/scalar.rs`

- [ ] **Step 1: Create `crates/aura-core/src/scalar.rs` with the scalar set**

```rust
//! The closed four-type scalar set streamed on aura's hot path (C7):
//! `i64`, `f64`, `bool`, and `Timestamp` (epoch-ns UTC). The carrier `Scalar`
//! is a `Copy` POD enum — no type-erased payloads, no heap, no reference counting.

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

/// The kind tag of a scalar / column, used for the edge-time type check (C7).
#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum ScalarKind {
    I64,
    F64,
    Bool,
    Timestamp,
}

/// The four scalar base types, type-erased into one `Copy` carrier (C7).
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum Scalar {
    I64(i64),
    F64(f64),
    Bool(bool),
    Ts(Timestamp),
}

impl Scalar {
    /// The kind tag of this scalar.
    pub fn kind(self) -> ScalarKind {
        match self {
            Scalar::I64(_) => ScalarKind::I64,
            Scalar::F64(_) => ScalarKind::F64,
            Scalar::Bool(_) => ScalarKind::Bool,
            Scalar::Ts(_) => ScalarKind::Timestamp,
        }
    }
}

impl From<i64> for Scalar {
    fn from(v: i64) -> Self {
        Scalar::I64(v)
    }
}
impl From<f64> for Scalar {
    fn from(v: f64) -> Self {
        Scalar::F64(v)
    }
}
impl From<bool> for Scalar {
    fn from(v: bool) -> Self {
        Scalar::Bool(v)
    }
}
impl From<Timestamp> for Scalar {
    fn from(v: Timestamp) -> Self {
        Scalar::Ts(v)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn kind_matches_variant() {
        assert_eq!(Scalar::I64(1).kind(), ScalarKind::I64);
        assert_eq!(Scalar::F64(1.0).kind(), ScalarKind::F64);
        assert_eq!(Scalar::Bool(true).kind(), ScalarKind::Bool);
        assert_eq!(Scalar::Ts(Timestamp(1)).kind(), ScalarKind::Timestamp);
    }

    #[test]
    fn from_widenings() {
        assert_eq!(Scalar::from(7i64), Scalar::I64(7));
        assert_eq!(Scalar::from(2.5f64), Scalar::F64(2.5));
        assert_eq!(Scalar::from(true), Scalar::Bool(true));
        assert_eq!(Scalar::from(Timestamp(9)), Scalar::Ts(Timestamp(9)));
    }

    #[test]
    fn timestamp_orders_and_defaults() {
        assert!(Timestamp(1) < Timestamp(2));
        assert_eq!(Timestamp::default(), Timestamp(0));
    }

    #[test]
    fn scalar_is_copy() {
        let s = Scalar::F64(1.0);
        let a = s;
        let b = s; // Copy: `s` still usable after move-by-value
        assert_eq!(a, b);
        assert_eq!(s, a);
    }
}
```

- [ ] **Step 2: Rewrite `lib.rs` doc and declare the `scalar` module only**

Each task adds only its own `mod` + `pub use` lines so the crate compiles green
after every task (the `mod` lines accumulate; by Task 4 all four are present).
Replace the entire current body of `crates/aura-core/src/lib.rs` (19 lines,
doc-only) with:

```rust
//! `aura-core` — the shared contract every aura component links against.
//!
//! This crate is the boundary forced by the cdylib hot-reload model: both the
//! engine and every hot-reloadable node cdylib depend on it, so it stays small
//! and stable.
//!
//! Delivered so far (cycle 0001 — the streaming substrate):
//!
//! - the four scalar base types streamed as SoA: [`Scalar`] / [`ScalarKind`] /
//!   [`Timestamp`] (C7);
//! - [`Column`] — a fixed-capacity, bounded-lookback ring with financial
//!   indexing (index 0 = newest) and a `run_count` freshness primitive (C5/C8),
//!   read zero-copy through a [`Window`];
//! - [`AnyColumn`] — the type-erased edge over the four column kinds, with an
//!   edge-time kind check ([`KindMismatch`]) (C7).
//!
//! Still to come (subsequent cycles): the `Node` trait and its `schema`/`eval`,
//! the evaluation context `Ctx`, the firing policies (A: fire-on-any-fresh +
//! hold; B: all-fresh barrier), and the deterministic sim loop.

mod scalar;

pub use scalar::{Scalar, ScalarKind, Timestamp};
```

The intra-doc links to `Column`/`Window`/`AnyColumn`/`KindMismatch` resolve once
Tasks 2–4 re-export them; they are not checked by `cargo build`/`test`/`clippy`
(only `cargo doc`), so they break no gate in the interim.

- [ ] **Step 3: Build and test the scalar module**

Run: `cargo test -p aura-core scalar::`
Expected: the build succeeds and the four `scalar::tests::*` tests report `ok`;
0 failed.

---

## Task 2: The column and its window (`column.rs`)

**Files:**
- Create: `crates/aura-core/src/column.rs`
- Test: inline in `crates/aura-core/src/column.rs`

- [ ] **Step 1: Create `crates/aura-core/src/column.rs`**

```rust
//! `Column<T>` — a fixed-capacity, bounded-lookback SoA ring with financial
//! indexing (index 0 = newest), and its zero-copy read view `Window` (C5/C8).

/// A fixed-capacity bounded-lookback column. Pre-sized at construction
/// (C8: no realloc in the hot loop); index 0 = newest; carries a monotonic
/// `run_count` of total pushes — the freshness primitive (C5).
pub struct Column<T> {
    buf: Box<[T]>,
    head: usize,    // next write slot
    len: usize,     // valid elements, saturates at capacity
    run_count: u64, // monotonic total pushes (C5)
}

impl<T: Copy + Default> Column<T> {
    /// Create a column sized to `lookback` (must be >= 1). The engine sizes this
    /// once at wiring; it is never reallocated (C8).
    pub fn with_capacity(lookback: usize) -> Self {
        assert!(lookback >= 1, "Column lookback must be >= 1");
        Column {
            buf: vec![T::default(); lookback].into_boxed_slice(),
            head: 0,
            len: 0,
            run_count: 0,
        }
    }

    /// Append the newest value. O(1), allocation-free. On overflow the oldest
    /// value is overwritten (true ring). Bumps `run_count`.
    pub fn push(&mut self, v: T) {
        let cap = self.buf.len();
        self.buf[self.head] = v;
        self.head = (self.head + 1) % cap;
        if self.len < cap {
            self.len += 1;
        }
        self.run_count += 1;
    }

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

    pub fn len(&self) -> usize {
        self.len
    }

    pub fn is_empty(&self) -> bool {
        self.len == 0
    }

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

    /// The monotonic push counter — the freshness primitive (C5). Never moved
    /// by a read.
    pub fn run_count(&self) -> u64 {
        self.run_count
    }

    /// A zero-copy read view handed to a reading node (C8).
    pub fn window(&self) -> Window<'_, T> {
        Window { col: self }
    }
}

/// Read-only, zero-copy view into a [`Column`] — what a reading node sees (C8).
/// Carries no storage, only a borrow.
pub struct Window<'a, T> {
    col: &'a Column<T>,
}

impl<'a, T: Copy + Default> Window<'a, T> {
    /// Financial index, 0 = newest; `None` if cold / out of range.
    pub fn get(&self, k: usize) -> Option<T> {
        self.col.get(k)
    }

    pub fn len(&self) -> usize {
        self.col.len()
    }

    pub fn is_empty(&self) -> bool {
        self.col.is_empty()
    }

    pub fn run_count(&self) -> u64 {
        self.col.run_count()
    }
}

/// `window[k]` — newest-at-0 indexed access; panics on out-of-range like a
/// slice. Nodes that may be cold use [`Window::get`] instead.
impl<'a, T: Copy + Default> core::ops::Index<usize> for Window<'a, T> {
    type Output = T;

    fn index(&self, k: usize) -> &T {
        let col = self.col;
        assert!(k < col.len, "Window index {k} out of range (len {})", col.len);
        let cap = col.buf.len();
        &col.buf[(col.head + cap - 1 - k) % cap]
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn newest_at_zero_oldest_at_len_minus_one() {
        let mut c = Column::<i64>::with_capacity(4);
        c.push(10);
        c.push(20);
        c.push(30);
        assert_eq!(c.len(), 3);
        assert_eq!(c.get(0), Some(30)); // newest
        assert_eq!(c.get(1), Some(20));
        assert_eq!(c.get(2), Some(10)); // oldest
        assert_eq!(c.get(3), None); // beyond len
    }

    #[test]
    fn wraparound_keeps_newest_capacity_values() {
        let mut c = Column::<i64>::with_capacity(3);
        for v in [1, 2, 3, 4, 5] {
            c.push(v);
        }
        assert_eq!(c.len(), 3); // saturated
        assert_eq!(c.capacity(), 3);
        assert_eq!(c.get(0), Some(5)); // newest
        assert_eq!(c.get(1), Some(4));
        assert_eq!(c.get(2), Some(3)); // oldest retained
        assert_eq!(c.get(3), None); // 1 and 2 were dropped
    }

    #[test]
    fn run_count_counts_pushes_not_len() {
        let mut c = Column::<i64>::with_capacity(2);
        assert_eq!(c.run_count(), 0);
        for v in [1, 2, 3, 4, 5] {
            c.push(v);
        }
        assert_eq!(c.run_count(), 5); // counts pushes, past the ring wrap
        assert_eq!(c.len(), 2); // while len stays at capacity
        let _ = c.get(0);
        assert_eq!(c.run_count(), 5); // a read never moves it
    }

    #[test]
    fn window_is_a_view_matching_the_column() {
        let mut c = Column::<f64>::with_capacity(4);
        c.push(1.0);
        c.push(2.0);
        let cap_before = c.capacity();
        let w = c.window();
        assert_eq!(w.len(), 2);
        assert_eq!(w.get(0), Some(2.0));
        assert_eq!(w[1], 1.0); // Index, newest-at-0
        assert_eq!(w.run_count(), 2);
        assert_eq!(c.capacity(), cap_before); // taking a window did not realloc
    }

    #[test]
    fn empty_column_reads_none() {
        let c = Column::<bool>::with_capacity(2);
        assert!(c.is_empty());
        assert_eq!(c.get(0), None);
        assert_eq!(c.run_count(), 0);
    }
}
```

- [ ] **Step 2: Declare and re-export the `column` module in `lib.rs`**

Add to `crates/aura-core/src/lib.rs`, after the `mod scalar;` line and after the
`pub use scalar::...;` line respectively:

```rust
mod column;
```
```rust
pub use column::{Column, Window};
```

- [ ] **Step 3: Test the column module**

Run: `cargo test -p aura-core column::`
Expected: the five `column::tests::*` tests report `ok`; 0 failed.

---

## Task 3: The guard error (`error.rs`)

**Files:**
- Create: `crates/aura-core/src/error.rs`
- Test: inline in `crates/aura-core/src/error.rs`

- [ ] **Step 1: Create `crates/aura-core/src/error.rs`**

```rust
//! `KindMismatch` — the wiring-time guard error for a wrong-kind edge push (C7).

use crate::scalar::ScalarKind;

/// Returned by [`crate::AnyColumn::push`] when a `Scalar`'s kind does not match
/// the column's kind; the column is left untouched.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct KindMismatch {
    /// The kind the column holds.
    pub expected: ScalarKind,
    /// The kind of the value that was offered.
    pub got: ScalarKind,
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn kind_mismatch_is_a_plain_value() {
        let e = KindMismatch { expected: ScalarKind::I64, got: ScalarKind::F64 };
        assert_eq!(e, KindMismatch { expected: ScalarKind::I64, got: ScalarKind::F64 });
        assert_ne!(e, KindMismatch { expected: ScalarKind::F64, got: ScalarKind::I64 });
    }
}
```

- [ ] **Step 2: Declare and re-export the `error` module in `lib.rs`**

Add to `crates/aura-core/src/lib.rs`:

```rust
mod error;
```
```rust
pub use error::KindMismatch;
```

- [ ] **Step 3: Test the error module**

Run: `cargo test -p aura-core error::`
Expected: `error::tests::kind_mismatch_is_a_plain_value` reports `ok`.

---

## Task 4: The type-erased edge (`any.rs`)

**Files:**
- Create: `crates/aura-core/src/any.rs`
- Test: inline in `crates/aura-core/src/any.rs` (incl. the must-fail guard)

- [ ] **Step 1: Create `crates/aura-core/src/any.rs`**

```rust
//! `AnyColumn` — the type-erased edge: the four concrete `Column<T>` behind one
//! enum. The kind check is paid at the edge (wiring time); the hot loop grabs
//! the concrete column once (`as_*_mut`) and pushes monomorphically (C7).

use crate::column::Column;
use crate::error::KindMismatch;
use crate::scalar::{Scalar, ScalarKind, Timestamp};

/// A type-erased edge over the four scalar columns (C7).
pub enum AnyColumn {
    I64(Column<i64>),
    F64(Column<f64>),
    Bool(Column<bool>),
    Ts(Column<Timestamp>),
}

impl AnyColumn {
    /// Create a type-erased column of `kind`, sized to `lookback` (C8).
    pub fn with_capacity(kind: ScalarKind, lookback: usize) -> Self {
        match kind {
            ScalarKind::I64 => AnyColumn::I64(Column::with_capacity(lookback)),
            ScalarKind::F64 => AnyColumn::F64(Column::with_capacity(lookback)),
            ScalarKind::Bool => AnyColumn::Bool(Column::with_capacity(lookback)),
            ScalarKind::Timestamp => AnyColumn::Ts(Column::with_capacity(lookback)),
        }
    }

    pub fn kind(&self) -> ScalarKind {
        match self {
            AnyColumn::I64(_) => ScalarKind::I64,
            AnyColumn::F64(_) => ScalarKind::F64,
            AnyColumn::Bool(_) => ScalarKind::Bool,
            AnyColumn::Ts(_) => ScalarKind::Timestamp,
        }
    }

    pub fn len(&self) -> usize {
        match self {
            AnyColumn::I64(c) => c.len(),
            AnyColumn::F64(c) => c.len(),
            AnyColumn::Bool(c) => c.len(),
            AnyColumn::Ts(c) => c.len(),
        }
    }

    pub fn is_empty(&self) -> bool {
        self.len() == 0
    }

    pub fn run_count(&self) -> u64 {
        match self {
            AnyColumn::I64(c) => c.run_count(),
            AnyColumn::F64(c) => c.run_count(),
            AnyColumn::Bool(c) => c.run_count(),
            AnyColumn::Ts(c) => c.run_count(),
        }
    }

    /// Edge-time typed push. `Err(KindMismatch)` if `v`'s kind != this column's
    /// kind; the column is left untouched (C7 guard). The hot path bypasses this
    /// by taking the concrete `Column<T>` once via `as_*_mut`.
    pub fn push(&mut self, v: Scalar) -> Result<(), KindMismatch> {
        match (self, v) {
            (AnyColumn::I64(c), Scalar::I64(x)) => {
                c.push(x);
                Ok(())
            }
            (AnyColumn::F64(c), Scalar::F64(x)) => {
                c.push(x);
                Ok(())
            }
            (AnyColumn::Bool(c), Scalar::Bool(x)) => {
                c.push(x);
                Ok(())
            }
            (AnyColumn::Ts(c), Scalar::Ts(x)) => {
                c.push(x);
                Ok(())
            }
            (this, v) => Err(KindMismatch { expected: this.kind(), got: v.kind() }),
        }
    }

    /// Read one type-erased value (0 = newest). `None` if cold / out of range.
    pub fn get(&self, k: usize) -> Option<Scalar> {
        match self {
            AnyColumn::I64(c) => c.get(k).map(Scalar::I64),
            AnyColumn::F64(c) => c.get(k).map(Scalar::F64),
            AnyColumn::Bool(c) => c.get(k).map(Scalar::Bool),
            AnyColumn::Ts(c) => c.get(k).map(Scalar::Ts),
        }
    }

    /// Concrete-column accessor for the monomorphic hot path; `Some` only for
    /// the matching kind.
    pub fn as_i64_mut(&mut self) -> Option<&mut Column<i64>> {
        match self {
            AnyColumn::I64(c) => Some(c),
            _ => None,
        }
    }

    pub fn as_f64_mut(&mut self) -> Option<&mut Column<f64>> {
        match self {
            AnyColumn::F64(c) => Some(c),
            _ => None,
        }
    }

    pub fn as_bool_mut(&mut self) -> Option<&mut Column<bool>> {
        match self {
            AnyColumn::Bool(c) => Some(c),
            _ => None,
        }
    }

    pub fn as_ts_mut(&mut self) -> Option<&mut Column<Timestamp>> {
        match self {
            AnyColumn::Ts(c) => Some(c),
            _ => None,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn same_kind_push_round_trips() {
        let mut e = AnyColumn::with_capacity(ScalarKind::F64, 4);
        e.push(Scalar::F64(1.5)).unwrap();
        e.push(Scalar::F64(2.5)).unwrap();
        assert_eq!(e.kind(), ScalarKind::F64);
        assert_eq!(e.len(), 2);
        assert_eq!(e.run_count(), 2);
        assert_eq!(e.get(0), Some(Scalar::F64(2.5))); // newest
        assert_eq!(e.get(1), Some(Scalar::F64(1.5)));
        assert_eq!(e.get(2), None);
    }

    #[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(); // 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
    }

    #[test]
    fn concrete_accessor_matches_only_its_kind() {
        let mut e = AnyColumn::with_capacity(ScalarKind::F64, 2);
        assert!(e.as_f64_mut().is_some());
        assert!(e.as_i64_mut().is_none());
        assert!(e.as_bool_mut().is_none());
        assert!(e.as_ts_mut().is_none());
        // hot-path push through the concrete column, monomorphic:
        e.as_f64_mut().unwrap().push(3.0);
        assert_eq!(e.get(0), Some(Scalar::F64(3.0)));
        assert_eq!(e.run_count(), 1);
    }

    #[test]
    fn timestamp_edge_round_trips() {
        let mut e = AnyColumn::with_capacity(ScalarKind::Timestamp, 2);
        e.push(Scalar::Ts(Timestamp(1_700_000_000_000_000_000))).unwrap();
        assert_eq!(e.get(0), Some(Scalar::Ts(Timestamp(1_700_000_000_000_000_000))));
    }
}
```

- [ ] **Step 2: Declare and re-export the `any` module in `lib.rs`**

Add to `crates/aura-core/src/lib.rs` (this completes the four modules):

```rust
mod any;
```
```rust
pub use any::AnyColumn;
```

- [ ] **Step 3: Test the any module**

Run: `cargo test -p aura-core any::`
Expected: the four `any::tests::*` tests report `ok`, including
`wrong_kind_push_is_rejected`.

---

## Task 5: Workspace gate (build + full test + lint)

**Files:** none (verification only).

- [ ] **Step 1: Full workspace build**

Run: `cargo build --workspace`
Expected: `Finished` with 0 errors, 0 warnings.

- [ ] **Step 2: Full workspace test with a count assertion**

Run: `cargo test --workspace`
Expected: all tests pass; the `aura-core` unit-test binary reports
`test result: ok. 14 passed; 0 failed` (4 scalar + 5 column + 1 error + 4 any).
If the count is not 14, a test was lost or misnamed — investigate before
proceeding.

- [ ] **Step 3: Lint clean under `-D warnings`**

Run: `cargo clippy --workspace --all-targets -- -D warnings`
Expected: `Finished` with no clippy diagnostics (exit 0).

- [ ] **Step 4: Surface-purity check — no `RefCell`/`Rc`/`dyn Any` leaked**

Run: `! grep -rnE 'RefCell|Rc<|dyn Any' crates/aura-core/src`
Expected: exit 0 (no matches). The substrate must carry none of these on its
surface (C1/C7). A match is a regression to fix before the cycle closes.