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

//! `Select` — the stateless 2:1 mux `bool × f64 × f64 -> f64`: forwards
//! `then` when `cond` is true, `otherwise` when it is false.
//!
//! The missing RTL standard cell (#281): the generic consumer of the Bool
//! plane back into F64. Until it existed, conditional values required
//! arithmetic contortions; with it, `count_true` composes as
//! `CumSum(Select(cond, 1.0, 0.0))` — no Counter primitive needed.
//!
//! Three inputs (`cond: Bool`, `then: F64`, `otherwise: F64`, each
//! `Firing::Any`), one `f64` output, no params, stateless. Emits `None`
//! until **all three** legs are warm — the mux reads both data inputs, as
//! [`Gt`](crate::Gt) requires both of its legs.

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

/// Stateless 2:1 mux: `out = if cond { then } else { otherwise }`, decided
/// fresh each fired cycle. Emits `None` until all three legs are warm (C8).
pub struct Select {
    out: [Cell; 1],
}

impl Select {
    /// Build a `Select` node.
    pub fn new() -> Self {
        Self { out: [Cell::from_f64(0.0)] }
    }

    /// The param-generic recipe for a blueprint primitive: paramless, builds
    /// through `Select::new`.
    pub fn builder() -> PrimitiveBuilder {
        PrimitiveBuilder::new(
            "Select",
            NodeSchema {
                inputs: vec![
                    PortSpec { kind: ScalarKind::Bool, firing: Firing::Any, name: "cond".into() },
                    PortSpec { kind: ScalarKind::F64, firing: Firing::Any, name: "then".into() },
                    PortSpec { kind: ScalarKind::F64, firing: Firing::Any, name: "otherwise".into() },
                ],
                output: vec![FieldSpec { name: "value".into(), kind: ScalarKind::F64 }],
                params: vec![],
                doc: "chooses between two inputs by a boolean selector",
            },
            |_| Box::new(Select::new()),
        )
    }
}

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

impl Node for Select {
    fn lookbacks(&self) -> Vec<usize> {
        vec![1, 1, 1]
    }

    fn eval(&mut self, ctx: Ctx<'_>) -> Option<&[Cell]> {
        let cond = ctx.bool_in(0);
        let then = ctx.f64_in(1);
        let otherwise = ctx.f64_in(2);
        if cond.is_empty() || then.is_empty() || otherwise.is_empty() {
            return None; // all three legs required (C8 warm-up gate)
        }
        let c = cond.get(0).unwrap_or(false);
        self.out[0] = Cell::from_f64(if c { then[0] } else { otherwise[0] });
        Some(&self.out)
    }

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

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

    fn mux_inputs() -> Vec<AnyColumn> {
        vec![
            AnyColumn::with_capacity(ScalarKind::Bool, 1),
            AnyColumn::with_capacity(ScalarKind::F64, 1),
            AnyColumn::with_capacity(ScalarKind::F64, 1),
        ]
    }

    #[test]
    fn select_muxes_per_cycle_by_cond() {
        // PROPERTY: out = if cond { then } else { otherwise }, re-decided each
        // cycle (stateless) — the 2:1 mux, both branches exercised plus a flip.
        let mut node = Select::new();
        let mut inputs = mux_inputs();
        let cases = [((true, 1.0, 9.0), 1.0), ((false, 1.0, 9.0), 9.0), ((true, 5.0, 7.0), 5.0)];
        for ((c, t, o), want) in cases {
            inputs[0].push(Scalar::bool(c)).unwrap();
            inputs[1].push(Scalar::f64(t)).unwrap();
            inputs[2].push(Scalar::f64(o)).unwrap();
            let got = node.eval(Ctx::new(&inputs, Timestamp(0)));
            assert_eq!(got, Some([Cell::from_f64(want)].as_slice()));
        }
    }

    #[test]
    fn select_is_none_until_all_three_legs_warm() {
        // The mux reads BOTH data inputs: a cold otherwise-leg gates the output
        // even while cond is true and then is present.
        let mut node = Select::new();
        let mut inputs = mux_inputs();
        inputs[0].push(Scalar::bool(true)).unwrap();
        inputs[1].push(Scalar::f64(1.0)).unwrap();
        assert_eq!(node.eval(Ctx::new(&inputs, Timestamp(0))), None);
        inputs[2].push(Scalar::f64(9.0)).unwrap();
        assert_eq!(
            node.eval(Ctx::new(&inputs, Timestamp(0))),
            Some([Cell::from_f64(1.0)].as_slice())
        );
    }

    #[test]
    fn input_slots_are_named_cond_then_otherwise() {
        let names: Vec<String> =
            Select::builder().schema().inputs.iter().map(|p| p.name.clone()).collect();
        assert_eq!(names, ["cond", "then", "otherwise"]);
    }
}