feat(core): the node contract — Node, schema/eval, Ctx, and a worked SMA

Cycle 0002, the second walking-skeleton slice on top of the 0001 substrate:
the interface every node forever implements (C8), proven end-to-end by a real
node with no engine present.

- `Node` (aura-core) — `schema() -> NodeSchema` declares inputs (kind +
  lookback) and the single output kind; `eval(&mut self, Ctx) -> Option<Scalar>`
  computes one cycle's output (`None` = filter / not-yet-warmed-up). `&mut self`
  so a node may keep its own derived state.
- `Ctx<'a>` (aura-core) — a `Copy` borrow-wrapper handing `eval` zero-copy,
  financial-indexed `Window`s per input (`ctx.f64_in(i)[k]`, index 0 = newest).
  A kind mismatch panics ("engine bug") — wiring guarantees the kind, so a
  mismatch can only mean the wiring layer is broken; this keeps node-author code
  clean (`w[k]`, not `w?[k]`).
- `AnyColumn::as_f64/as_i64/as_bool/as_ts` (aura-core) — the read-side mirror of
  the existing `as_*_mut`, the mechanism `Ctx` uses to get a typed window from a
  type-erased edge. Closes the read-side gap the cycle-0001 audit recorded.
- `Sma` (aura-std) — the skeleton's first real block: a producer node computing
  the moving mean of one f64 input, emitting `None` until warmed up. Authored in
  a downstream crate (proving aura-core's contract is usable across the crate
  boundary) and driven by a hand-written test that mimics exactly what the sim
  loop will later generalize: push fresh input, eval, collect.

Deliberately deferred (spec 0002 "Out of scope", recorded as decisions): the
firing policies (C6 — InputSpec carries no firing field yet), the sim loop (C4)
and freshness gating (C5), schema-level tunable params (C12/C19), and the
no-output sink refinement (C8 consumer side).

Gates green: cargo build/test (20: 18 aura-core + 2 aura-std)/clippy -D warnings
all clean; surface-purity grep (no dyn-Any / Rc / RefCell) clean.

refs walking-skeleton

crates/aura-core/src/any.rs

@@ -120,6 +120,37 @@ impl AnyColumn {
             _ => None,
         }
     }
+
+    /// Read-side concrete-column accessor (the symmetric partner of
+    /// `as_*_mut`); `Some` only for the matching kind. `Ctx` uses these to hand
+    /// a typed window from a type-erased edge.
+    pub fn as_i64(&self) -> Option<&Column<i64>> {
+        match self {
+            AnyColumn::I64(c) => Some(c),
+            _ => None,
+        }
+    }
+
+    pub fn as_f64(&self) -> Option<&Column<f64>> {
+        match self {
+            AnyColumn::F64(c) => Some(c),
+            _ => None,
+        }
+    }
+
+    pub fn as_bool(&self) -> Option<&Column<bool>> {
+        match self {
+            AnyColumn::Bool(c) => Some(c),
+            _ => None,
+        }
+    }
+
+    pub fn as_ts(&self) -> Option<&Column<Timestamp>> {
+        match self {
+            AnyColumn::Ts(c) => Some(c),
+            _ => None,
+        }
+    }
 }
 
 #[cfg(test)]
@@ -161,6 +192,19 @@ mod tests {
         assert_eq!(e.run_count(), 1);
     }
 
+    #[test]
+    fn read_accessor_matches_only_its_kind() {
+        let f = AnyColumn::with_capacity(ScalarKind::F64, 2);
+        assert!(f.as_f64().is_some());
+        assert!(f.as_i64().is_none());
+        assert!(f.as_bool().is_none());
+        assert!(f.as_ts().is_none());
+
+        let i = AnyColumn::with_capacity(ScalarKind::I64, 2);
+        assert!(i.as_i64().is_some());
+        assert!(i.as_f64().is_none());
+    }
+
     #[test]
     fn timestamp_edge_round_trips() {
         let mut e = AnyColumn::with_capacity(ScalarKind::Timestamp, 2);

crates/aura-core/src/ctx.rs

@@ -0,0 +1,100 @@
+//! The evaluation context (C8): the read-side window access a node sees in
+//! `eval`. The engine sizes and types each input from the node's `schema` at
+//! wiring, so the typed accessors below treat a kind mismatch as an engine bug
+//! (panic), not a user-facing error.
+
+use crate::{AnyColumn, Timestamp, Window};
+
+/// Read-only, zero-copy view of a node's inputs for one `eval`, in schema
+/// order. `Copy` because it is just a borrow of the input slice.
+#[derive(Clone, Copy)]
+pub struct Ctx<'a> {
+    inputs: &'a [AnyColumn],
+}
+
+impl<'a> Ctx<'a> {
+    /// Wrap the per-input columns (in schema-declared order) for one `eval`.
+    pub fn new(inputs: &'a [AnyColumn]) -> Self {
+        Self { inputs }
+    }
+
+    /// Zero-copy `f64` window into input `i` (index 0 = newest). Panics if input
+    /// `i` is not an `f64` edge — a wiring bug, never reachable from a correctly
+    /// wired graph.
+    pub fn f64_in(&self, i: usize) -> Window<'a, f64> {
+        let inputs: &'a [AnyColumn] = self.inputs;
+        inputs[i]
+            .as_f64()
+            .expect("input kind mismatch (checked at wiring) — engine bug")
+            .window()
+    }
+
+    /// Zero-copy `i64` window into input `i` (index 0 = newest). See `f64_in`.
+    pub fn i64_in(&self, i: usize) -> Window<'a, i64> {
+        let inputs: &'a [AnyColumn] = self.inputs;
+        inputs[i]
+            .as_i64()
+            .expect("input kind mismatch (checked at wiring) — engine bug")
+            .window()
+    }
+
+    /// Zero-copy `bool` window into input `i` (index 0 = newest). See `f64_in`.
+    pub fn bool_in(&self, i: usize) -> Window<'a, bool> {
+        let inputs: &'a [AnyColumn] = self.inputs;
+        inputs[i]
+            .as_bool()
+            .expect("input kind mismatch (checked at wiring) — engine bug")
+            .window()
+    }
+
+    /// Zero-copy `timestamp` window into input `i` (index 0 = newest). See
+    /// `f64_in`.
+    pub fn ts_in(&self, i: usize) -> Window<'a, Timestamp> {
+        let inputs: &'a [AnyColumn] = self.inputs;
+        inputs[i]
+            .as_ts()
+            .expect("input kind mismatch (checked at wiring) — engine bug")
+            .window()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::{Scalar, ScalarKind};
+
+    #[test]
+    fn ctx_hands_financial_indexed_windows() {
+        let mut inputs = vec![AnyColumn::with_capacity(ScalarKind::F64, 4)];
+        for v in [10.0_f64, 20.0, 30.0] {
+            inputs[0].push(Scalar::F64(v)).unwrap();
+        }
+        let ctx = Ctx::new(&inputs);
+        let w = ctx.f64_in(0);
+        assert_eq!(w.len(), 3);
+        assert_eq!(w[0], 30.0); // newest
+        assert_eq!(w[2], 10.0); // oldest
+    }
+
+    #[test]
+    fn ctx_addresses_multiple_inputs() {
+        let mut inputs = vec![
+            AnyColumn::with_capacity(ScalarKind::F64, 2),
+            AnyColumn::with_capacity(ScalarKind::I64, 2),
+        ];
+        inputs[0].push(Scalar::F64(1.5)).unwrap();
+        inputs[1].push(Scalar::I64(42)).unwrap();
+        let ctx = Ctx::new(&inputs);
+        assert_eq!(ctx.f64_in(0)[0], 1.5);
+        assert_eq!(ctx.i64_in(1)[0], 42);
+    }
+
+    #[test]
+    #[should_panic(expected = "engine bug")]
+    fn ctx_panics_on_kind_mismatch() {
+        let mut inputs = vec![AnyColumn::with_capacity(ScalarKind::I64, 2)];
+        inputs[0].push(Scalar::I64(7)).unwrap();
+        let ctx = Ctx::new(&inputs);
+        let _ = ctx.f64_in(0); // wrong kind → panic
+    }
+}

crates/aura-core/src/lib.rs

@@ -14,16 +14,29 @@
 //! - [`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.
+//! Delivered in cycle 0002 — the node contract:
+//!
+//! - [`Node`] — the `schema`/`eval` contract every node implements (C8), with
+//!   [`NodeSchema`] / [`InputSpec`] declaring inputs (kind + lookback) and the
+//!   single output kind;
+//! - [`Ctx`] — the per-`eval` read-side: zero-copy, financial-indexed [`Window`]
+//!   access into each input (closing the cycle-0001 read-side gap on
+//!   [`AnyColumn`]).
+//!
+//! Still to come (subsequent cycles): the firing policies (A: fire-on-any-fresh +
+//! hold; B: all-fresh barrier), the deterministic sim loop, sources, and
+//! ingestion.
 
 mod any;
 mod column;
+mod ctx;
 mod error;
+mod node;
 mod scalar;
 
 pub use any::AnyColumn;
 pub use column::{Column, Window};
+pub use ctx::Ctx;
 pub use error::KindMismatch;
+pub use node::{InputSpec, Node, NodeSchema};
 pub use scalar::{Scalar, ScalarKind, Timestamp};

crates/aura-core/src/node.rs

@@ -0,0 +1,31 @@
+//! The node contract (C8): the interface every node implements. A node declares
+//! its inputs and output kind via `schema`, and computes one cycle's output via
+//! `eval`. Firing policy (C6) and tunable params (C12/C19) are deliberately not
+//! part of the schema yet — see spec 0002's "Out of scope".
+
+use crate::{Ctx, Scalar, ScalarKind};
+
+/// One declared input of a node: its scalar kind and the lookback depth the
+/// engine must pre-size for it (must be >= 1).
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct InputSpec {
+    pub kind: ScalarKind,
+    pub lookback: usize,
+}
+
+/// A node's declared interface: its inputs (in order) and its single output
+/// kind. Built once at wiring, never on the hot path — the `Vec` is fine here.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct NodeSchema {
+    pub inputs: Vec<InputSpec>,
+    pub output: ScalarKind,
+}
+
+/// The universal composable dataflow unit (C8): at most one output, a producer
+/// or transformer. `schema` declares the interface; `eval` computes one cycle's
+/// output (`None` = filter / not-yet-warmed-up). `&mut self` because a node may
+/// keep its own derived state.
+pub trait Node {
+    fn schema(&self) -> NodeSchema;
+    fn eval(&mut self, ctx: Ctx<'_>) -> Option<Scalar>;
+}

crates/aura-std/src/lib.rs

@@ -12,6 +12,8 @@
 //!   pulled in as cargo git dependencies;
 //! - project-local `nodes/` — experimental, project-specific blocks.
 //!
-//! Types are intentionally absent until the first spec needs them (the
-//! walking-skeleton milestone). This crate documents intent; it does not yet
-//! define API.
+//! The first block lands with the walking skeleton: [`Sma`], the simple moving
+//! average — a worked producer node proving the `aura-core` `Node` contract.
+
+mod sma;
+pub use sma::Sma;

crates/aura-std/src/sma.rs

@@ -0,0 +1,79 @@
+//! `Sma` — simple moving average over the last `length` values of one f64
+//! input. The walking skeleton's first worked node: it proves the `aura-core`
+//! `Node` contract is authorable from a downstream crate and evaluable with no
+//! engine present (the test drives it by hand, as the sim loop later will).
+
+use aura_core::{Ctx, InputSpec, Node, NodeSchema, Scalar, ScalarKind};
+
+/// Simple moving average over the last `length` values of one f64 input.
+pub struct Sma {
+    length: usize,
+}
+
+impl Sma {
+    /// Build an SMA of window `length` (must be >= 1).
+    pub fn new(length: usize) -> Self {
+        assert!(length >= 1, "SMA length must be >= 1");
+        Self { length }
+    }
+}
+
+impl Node for Sma {
+    fn schema(&self) -> NodeSchema {
+        NodeSchema {
+            inputs: vec![InputSpec { kind: ScalarKind::F64, lookback: self.length }],
+            output: ScalarKind::F64,
+        }
+    }
+
+    fn eval(&mut self, ctx: Ctx<'_>) -> Option<Scalar> {
+        let w = ctx.f64_in(0);
+        if w.len() < self.length {
+            return None; // not yet warmed up
+        }
+        let mut sum = 0.0;
+        for k in 0..self.length {
+            sum += w[k]; // index 0 = newest (financial indexing)
+        }
+        Some(Scalar::F64(sum / self.length as f64))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use aura_core::AnyColumn;
+
+    #[test]
+    fn sma_warms_up_then_tracks_the_window_mean() {
+        let mut sma = Sma::new(3);
+        let schema = sma.schema();
+
+        // size the input column from the schema, as the engine will at wiring
+        let mut inputs = vec![AnyColumn::with_capacity(
+            schema.inputs[0].kind,
+            schema.inputs[0].lookback,
+        )];
+
+        let feed = [1.0_f64, 2.0, 3.0, 4.0, 5.0];
+        // means of [1,2,3], [2,3,4], [3,4,5] once warmed up
+        let expect = [None, None, Some(2.0), Some(3.0), Some(4.0)];
+
+        for (v, want) in feed.iter().zip(expect) {
+            inputs[0].push(Scalar::F64(*v)).unwrap();
+            assert_eq!(sma.eval(Ctx::new(&inputs)), want.map(Scalar::F64));
+        }
+    }
+
+    #[test]
+    fn sma_length_one_is_identity() {
+        let mut sma = Sma::new(1);
+        let mut inputs = vec![AnyColumn::with_capacity(ScalarKind::F64, 1)];
+
+        inputs[0].push(Scalar::F64(7.0)).unwrap();
+        assert_eq!(sma.eval(Ctx::new(&inputs)), Some(Scalar::F64(7.0)));
+
+        inputs[0].push(Scalar::F64(9.0)).unwrap();
+        assert_eq!(sma.eval(Ctx::new(&inputs)), Some(Scalar::F64(9.0)));
+    }
+}