# Sum Combinators (`Add` + `LinComb`) — Implementation Plan

> **Parent spec:** `docs/specs/0008-sum-combinators.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement` skill to
> run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Ship two new `aura-std` leaf nodes — `Add` (two-input f64 sum) and
`LinComb { weights }` (N-input weighted sum) — so the north-star "combine
signals" move (C10) is expressible from shipped blocks.

**Architecture:** Two additive leaf nodes, one file each, mirroring the existing
`sub.rs` / `sma.rs` pattern (struct + `Node` impl + co-located hand-driven
`#[cfg(test)]` tests). `Add` mirrors `sub.rs` modulo the `+` operator; `LinComb`
carries a `Vec<f64>` weight param (assert-non-empty at construction, like
`Sma::new`) and builds a variadic input schema from `weights.len()`. Both
withhold output until every input is present (no implicit cold-leg `0.0`). Each
node is module-declared and re-exported in `lib.rs`. No `aura-core` change, no
new dependency.

**Tech Stack:** `aura-core` `Node`/`Ctx`/`NodeSchema`/`Scalar` contract;
`crates/aura-std/`.

---

**Files this plan creates or modifies:**

- Create: `crates/aura-std/src/add.rs` — `Add` leaf node + tests.
- Create: `crates/aura-std/src/lincomb.rs` — `LinComb` leaf node + tests.
- Modify: `crates/aura-std/src/lib.rs:18-25` — module declarations + `pub use`
  exports for both nodes (alphabetical order).
- Test: `crates/aura-std/src/add.rs` (`#[cfg(test)] mod tests`) — sum-once-both-present.
- Test: `crates/aura-std/src/lincomb.rs` (`#[cfg(test)] mod tests`) — weighted
  sum, unit-weights-equal-Add identity, three-input warm-up, empty-weights panic.

Mirror templates (read-only, do not edit): `crates/aura-std/src/sub.rs:1-70`,
`crates/aura-std/src/sma.rs:16-19` (the `assert!` precedent). The `aura_core`
import set (`Ctx, FieldSpec, Firing, InputSpec, Node, NodeSchema, Scalar,
ScalarKind`, plus test-only `AnyColumn, Timestamp`) is re-exported from the
`aura_core` crate root (`crates/aura-core/src/lib.rs:38-43`). `crates/aura-std/
Cargo.toml` already depends on `aura-core` — no manifest change.

---

### Task 1: `Add` — two-input f64 sum

**Files:**
- Create: `crates/aura-std/src/add.rs`
- Modify: `crates/aura-std/src/lib.rs:18,22`
- Test: `crates/aura-std/src/add.rs`

- [ ] **Step 1: Write the failing test + declare the module**

Create `crates/aura-std/src/add.rs` with the top-level import set and the test
module only (no `Add` struct yet — that is what makes the test fail):

```rust
//! `Add` — two-input f64 sum (input 0 plus input 1), the companion to `Sub`.
//! Combines two signal streams into one — the most basic combinator for the
//! north-star "combine one signal with another" research move (C10).

use aura_core::{Ctx, FieldSpec, Firing, InputSpec, Node, NodeSchema, Scalar, ScalarKind};

#[cfg(test)]
mod tests {
    use super::*;
    use aura_core::{AnyColumn, Timestamp};

    #[test]
    fn add_is_sum_once_both_inputs_present() {
        let mut add = Add::new();
        let mut inputs = vec![
            AnyColumn::with_capacity(ScalarKind::F64, 1),
            AnyColumn::with_capacity(ScalarKind::F64, 1),
        ];

        // only input 0 present -> None
        inputs[0].push(Scalar::F64(10.0)).unwrap();
        assert_eq!(add.eval(Ctx::new(&inputs, Timestamp(0))), None);

        // both present -> a + b
        inputs[1].push(Scalar::F64(4.0)).unwrap();
        assert_eq!(add.eval(Ctx::new(&inputs, Timestamp(0))), Some([Scalar::F64(14.0)].as_slice()));
    }
}
```

Then declare the module in `crates/aura-std/src/lib.rs` — insert `mod add;` as
the first line of the `mod` block (before `mod exposure;` at line 18):

```rust
mod add;
mod exposure;
```

- [ ] **Step 2: Run test to verify it fails**

Run: `cargo test -p aura-std add_is_sum_once_both_inputs_present`
Expected: FAIL — compile error `E0433`/`E0422` "failed to resolve" / "cannot
find function, struct, or type `Add` in this scope" (the test references
`Add::new()`, which does not exist yet).

- [ ] **Step 3: Write the `Add` node + export it**

Insert the struct and impls into `crates/aura-std/src/add.rs` between the
top-level `use` line and the `#[cfg(test)]` line:

```rust
/// Two-input f64 sum: input 0 plus input 1. Emits `None` until both inputs
/// have a value.
pub struct Add {
    out: [Scalar; 1],
}

impl Add {
    /// Build an `Add` node.
    pub fn new() -> Self {
        Self { out: [Scalar::F64(0.0)] }
    }
}

impl Default for Add {
    fn default() -> Self {
        Self::new()
    }
}

impl Node for Add {
    fn schema(&self) -> NodeSchema {
        NodeSchema {
            inputs: vec![
                InputSpec { kind: ScalarKind::F64, lookback: 1, firing: Firing::Any },
                InputSpec { kind: ScalarKind::F64, lookback: 1, firing: Firing::Any },
            ],
            output: vec![FieldSpec { name: "value", kind: ScalarKind::F64 }],
        }
    }

    fn eval(&mut self, ctx: Ctx<'_>) -> Option<&[Scalar]> {
        let a = ctx.f64_in(0);
        let b = ctx.f64_in(1);
        if a.is_empty() || b.is_empty() {
            return None;
        }
        self.out[0] = Scalar::F64(a[0] + b[0]);
        Some(&self.out)
    }
}
```

Then export it from `crates/aura-std/src/lib.rs` — insert `pub use add::Add;`
as the first line of the `pub use` block (before `pub use exposure::Exposure;`
at line 22):

```rust
pub use add::Add;
pub use exposure::Exposure;
```

- [ ] **Step 4: Run test to verify it passes**

Run: `cargo test -p aura-std add_is_sum_once_both_inputs_present`
Expected: PASS — `test add::tests::add_is_sum_once_both_inputs_present ... ok`.

---

### Task 2: `LinComb` — N-input weighted sum

**Files:**
- Create: `crates/aura-std/src/lincomb.rs`
- Modify: `crates/aura-std/src/lib.rs:19,23`
- Test: `crates/aura-std/src/lincomb.rs`

- [ ] **Step 1: Write the failing tests + declare the module**

Create `crates/aura-std/src/lincomb.rs` with the top-level import set and the
test module only (no `LinComb` struct yet):

```rust
//! `LinComb` — weighted sum of `N` f64 inputs (`Σ weights[i] · input[i]`), the
//! general combinator for the north-star "combine signals with weights" move
//! (C10). `LinComb([1.0, 1.0])` is `Add`; `LinComb([1.0, -1.0])` is `Sub`. The
//! weights are the node's tunable parameters (C8/C12) and fix its arity.

use aura_core::{Ctx, FieldSpec, Firing, InputSpec, Node, NodeSchema, Scalar, ScalarKind};

#[cfg(test)]
mod tests {
    use super::*;
    use aura_core::{AnyColumn, Timestamp};

    #[test]
    fn lincomb_weighted_sum_once_all_present() {
        let mut lc = LinComb::new(vec![0.5, 2.0]);
        let mut inputs = vec![
            AnyColumn::with_capacity(ScalarKind::F64, 1),
            AnyColumn::with_capacity(ScalarKind::F64, 1),
        ];

        // only input 0 present -> None
        inputs[0].push(Scalar::F64(10.0)).unwrap();
        assert_eq!(lc.eval(Ctx::new(&inputs, Timestamp(0))), None);

        // both present -> 0.5*10 + 2.0*3 = 11.0
        inputs[1].push(Scalar::F64(3.0)).unwrap();
        assert_eq!(lc.eval(Ctx::new(&inputs, Timestamp(0))), Some([Scalar::F64(11.0)].as_slice()));
    }

    #[test]
    fn lincomb_unit_weights_equal_add() {
        let mut lc = LinComb::new(vec![1.0, 1.0]);
        let mut inputs = vec![
            AnyColumn::with_capacity(ScalarKind::F64, 1),
            AnyColumn::with_capacity(ScalarKind::F64, 1),
        ];
        inputs[0].push(Scalar::F64(7.0)).unwrap();
        inputs[1].push(Scalar::F64(5.0)).unwrap();
        // unit weights reproduce Add: 7 + 5
        assert_eq!(lc.eval(Ctx::new(&inputs, Timestamp(0))), Some([Scalar::F64(12.0)].as_slice()));
    }

    #[test]
    fn lincomb_three_inputs_warm_up() {
        let mut lc = LinComb::new(vec![1.0, 1.0, 1.0]);
        let mut inputs = vec![
            AnyColumn::with_capacity(ScalarKind::F64, 1),
            AnyColumn::with_capacity(ScalarKind::F64, 1),
            AnyColumn::with_capacity(ScalarKind::F64, 1),
        ];
        inputs[0].push(Scalar::F64(1.0)).unwrap();
        inputs[1].push(Scalar::F64(2.0)).unwrap();
        // third leg still cold -> None (withheld until every leg is present)
        assert_eq!(lc.eval(Ctx::new(&inputs, Timestamp(0))), None);

        inputs[2].push(Scalar::F64(3.0)).unwrap();
        // all warm -> 1 + 2 + 3
        assert_eq!(lc.eval(Ctx::new(&inputs, Timestamp(0))), Some([Scalar::F64(6.0)].as_slice()));
    }

    #[test]
    #[should_panic(expected = "LinComb needs at least one weight")]
    fn lincomb_empty_weights_panics() {
        let _ = LinComb::new(vec![]);
    }
}
```

Then declare the module in `crates/aura-std/src/lib.rs` — insert `mod lincomb;`
between `mod exposure;` and `mod sim_broker;`:

```rust
mod exposure;
mod lincomb;
mod sim_broker;
```

- [ ] **Step 2: Run tests to verify they fail**

Run: `cargo test -p aura-std lincomb`
Expected: FAIL — compile error `E0433`/`E0422` "cannot find function, struct,
or type `LinComb` in this scope" (the tests reference `LinComb::new`, which does
not exist yet).

- [ ] **Step 3: Write the `LinComb` node + export it**

Insert the struct and impls into `crates/aura-std/src/lincomb.rs` between the
top-level `use` line and the `#[cfg(test)]` line:

```rust
/// Weighted sum of `N` f64 inputs: `Σ weights[i] · input[i]`. The `weights`
/// are the node's tunable parameters and fix its arity (`weights.len()` inputs,
/// in slot order). Emits `None` until *all* inputs have a value.
pub struct LinComb {
    weights: Vec<f64>,
    out: [Scalar; 1],
}

impl LinComb {
    /// Build a `LinComb` with one weight per input (at least one required).
    ///
    /// # Panics
    /// Panics if `weights` is empty.
    pub fn new(weights: Vec<f64>) -> Self {
        assert!(!weights.is_empty(), "LinComb needs at least one weight");
        Self { weights, out: [Scalar::F64(0.0)] }
    }
}

impl Node for LinComb {
    fn schema(&self) -> NodeSchema {
        NodeSchema {
            inputs: self
                .weights
                .iter()
                .map(|_| InputSpec { kind: ScalarKind::F64, lookback: 1, firing: Firing::Any })
                .collect(),
            output: vec![FieldSpec { name: "value", kind: ScalarKind::F64 }],
        }
    }

    fn eval(&mut self, ctx: Ctx<'_>) -> Option<&[Scalar]> {
        let mut acc = 0.0;
        for (i, &w) in self.weights.iter().enumerate() {
            let w_in = ctx.f64_in(i);
            if w_in.is_empty() {
                return None; // not yet warmed up — withhold until every leg is present
            }
            acc += w * w_in[0];
        }
        self.out[0] = Scalar::F64(acc);
        Some(&self.out)
    }
}
```

Then export it from `crates/aura-std/src/lib.rs` — insert `pub use
lincomb::LinComb;` between `pub use exposure::Exposure;` and `pub use
sim_broker::SimBroker;`:

```rust
pub use exposure::Exposure;
pub use lincomb::LinComb;
pub use sim_broker::SimBroker;
```

- [ ] **Step 4: Run tests to verify they pass**

Run: `cargo test -p aura-std lincomb`
Expected: PASS — all four `lincomb::tests::*` tests `... ok`
(`lincomb_weighted_sum_once_all_present`, `lincomb_unit_weights_equal_add`,
`lincomb_three_inputs_warm_up`, `lincomb_empty_weights_panics`).

---

### Task 3: Crate-wide gates

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

- [ ] **Step 1: Full test suite**

Run: `cargo test -p aura-std`
Expected: PASS — all existing `aura-std` tests plus the five new ones
(`add::tests::add_is_sum_once_both_inputs_present` and the four
`lincomb::tests::*`); `0 failed`.

- [ ] **Step 2: Clippy, warnings denied**

Run: `cargo clippy -p aura-std --all-targets -- -D warnings`
Expected: PASS — no warnings. (`Add` has a `Default` impl so
`clippy::new_without_default` does not fire; `LinComb::new` takes an argument so
the lint does not apply.)

- [ ] **Step 3: Doc build, warnings denied**

Run: `RUSTDOCFLAGS="-D warnings" cargo doc -p aura-std --no-deps`
Expected: PASS — clean; the new intra-doc references (`Sub`, `Add`) resolve.
