Aura/crates/aura-std/src/lincomb.rs

//! `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 construction parameters that configure the node and fix its
//! arity (`weights.len()` inputs); they are also the combination's tunable
//! params, declared in the schema (cycle 0015) as `weights[0..N]` — N flat
//! indexed `F64` knobs that `Blueprint::param_space` aggregates (C8/C12/C19).

use aura_core::{
    Ctx, FieldSpec, Firing, Node, NodeSchema, ParamSpec, PortSpec, PrimitiveBuilder, Scalar,
    ScalarKind,
};

/// Weighted sum of `N` f64 inputs: `Σ weights[i] · input[i]`. The `weights` are
/// construction parameters that configure the node and fix its arity
/// (`weights.len()` inputs, in slot order). Emits `None` until *all* inputs
/// have a value.
///
/// # Firing and warm-up
///
/// Every input is [`Firing::Any`](aura_core::Firing::Any) — a *mode-A as-of
/// join*: the node fires on every cycle in which any leg is fresh (once all
/// legs have produced a value), pairing each fresh leg with the held value of
/// the others. Until every leg has a value it emits `None` (no cold-leg-as-
/// `0.0`). With heterogeneous sources sharing a timestamp (same `ts` from two
/// sources = two distinct cycles, C4), a fired node emits one row per *cycle*,
/// so a recorded combined stream may carry more than one row per timestamp.
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)] }
    }

    /// The param-generic recipe for a blueprint primitive. The `arity` is topology
    /// (fixed per blueprint, C19), taken as a builder arg; only the weight *values*
    /// are injected, slot by slot, through `LinComb::new` (the single sizing gate).
    pub fn builder(arity: usize) -> PrimitiveBuilder {
        let inputs = (0..arity)
            .map(|_| PortSpec { kind: ScalarKind::F64, firing: Firing::Any })
            .collect();
        let params = (0..arity)
            .map(|i| ParamSpec { name: format!("weights[{i}]"), kind: ScalarKind::F64 })
            .collect();
        PrimitiveBuilder::new(
            "LinComb",
            NodeSchema { inputs, output: vec![FieldSpec { name: "value", kind: ScalarKind::F64 }], params },
            |p| Box::new(LinComb::new(
                p.iter().map(|s| s.as_f64().expect("weight slot is F64")).collect(),
            )),
        )
    }
}

impl Node for LinComb {
    fn lookbacks(&self) -> Vec<usize> {
        vec![1; self.weights.len()]
    }

    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)
    }

    fn label(&self) -> String {
        "LinComb".to_string()
    }
}

#[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![]);
    }
}