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

//! `Latch` — a level-sensitive set/reset register that emits an `f64` exposure
//! magnitude (`1.0` latched / `0.0` flat), the C5 SR-register of the
//! session-breakout vocabulary.
//!
//! Two `bool` inputs (`set`, `reset`, `Firing::Any`); one `f64` output `value`;
//! no params. **Emits `f64` directly**, not `bool`: a held position *is*
//! exposure `+1`, flat *is* `0.0`, so the output feeds [`SimBroker`](crate::SimBroker)'s
//! `f64` `exposure` slot with no cast — this is the resolution of the `bool → f64`
//! seam (no separate `Exposure`/cast node).
//!
//! **State.** A persisted `held: f64`, initial `0.0`, mutated in `eval` and
//! carried across cycles in the struct — exactly how [`SimBroker`](crate::SimBroker)
//! holds `prev_price`/`prev_exposure` across cycles.
//!
//! **Logic — level SR latch, RESET-DOMINANT.** Per fired eval: if `reset` is on
//! → `held = 0.0`; else if `set` is on → `held = 1.0`; else `held` is unchanged
//! (sample-and-hold). Reset wins when both pulse (defensive default; in the
//! strategy bar3 ≠ bar5 so they never coincide).

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

/// Level-sensitive set/reset register emitting an `f64` exposure magnitude:
/// `held` → `1.0` on `set`, → `0.0` on `reset` (reset-dominant), held across
/// quiet cycles. Emits `None` only while **both** input legs are still cold
/// (nothing latched yet); a present-but-false leg is read as "no pulse".
pub struct Latch {
    held: f64, // persisted exposure magnitude: 1.0 = long, 0.0 = flat
    out: [Cell; 1],
}

impl Latch {
    /// Build a `Latch` node, initially flat (`held = 0.0`).
    pub fn new() -> Self {
        Self { held: 0.0, out: [Cell::from_f64(0.0)] }
    }

    /// The param-generic recipe for a blueprint primitive: paramless, builds
    /// through `Latch::new`.
    pub fn builder() -> PrimitiveBuilder {
        PrimitiveBuilder::new(
            "Latch",
            NodeSchema {
                inputs: vec![
                    PortSpec { kind: ScalarKind::Bool, firing: Firing::Any, name: "set".into() },
                    PortSpec { kind: ScalarKind::Bool, firing: Firing::Any, name: "reset".into() },
                ],
                output: vec![FieldSpec { name: "value".into(), kind: ScalarKind::F64 }],
                params: vec![],
            },
            |_| Box::new(Latch::new()),
        )
    }
}

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

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

    fn eval(&mut self, ctx: Ctx<'_>) -> Option<&[Cell]> {
        let set = ctx.bool_in(0);
        let reset = ctx.bool_in(1);
        if set.is_empty() && reset.is_empty() {
            return None; // cold: nothing latched yet (downstream reads flat 0.0)
        }
        // An absent/empty leg is read as "no pulse"; a present leg pulses when true.
        let set_on = set.get(0).unwrap_or(false);
        let reset_on = reset.get(0).unwrap_or(false);
        if reset_on {
            self.held = 0.0; // reset dominates (defensive default)
        } else if set_on {
            self.held = 1.0;
        } // else: sample-and-hold — `held` unchanged
        self.out[0] = Cell::from_f64(self.held);
        Some(&self.out)
    }

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

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

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

    #[test]
    fn latch_holds_set_until_reset_reset_dominant() {
        // The core property: a level SR latch that sample-and-HOLDS its f64
        // exposure magnitude across quiet bars, with RESET dominant when both
        // pulse. Both legs are present every cycle (as in the real graph: set =
        // `entry`, reset = `isBar5Close`, both fresh bool streams each bar).
        let mut node = Latch::new();
        let mut inputs = two_bool_inputs();

        // (set, reset) -> expected held
        let cases = [
            ((false, false), 0.0), // initial: flat
            ((true, false), 1.0),  // set latches long
            ((false, false), 1.0), // HOLD across a quiet bar (load-bearing sample-and-hold)
            ((false, true), 0.0),  // reset goes flat
            ((false, false), 0.0), // hold flat
            ((true, true), 0.0),   // reset-dominant: both on -> flat (pins the priority)
        ];

        for ((set, reset), want) in cases {
            inputs[0].push(Scalar::bool(set)).unwrap();
            inputs[1].push(Scalar::bool(reset)).unwrap();
            let got = node.eval(Ctx::new(&inputs, Timestamp(0)));
            assert_eq!(got, Some([Cell::from_f64(want)].as_slice()));
        }
    }

    #[test]
    fn latch_is_none_while_both_legs_cold() {
        // Before any input on either leg: nothing latched yet -> None (downstream
        // reads SimBroker's cold-leg-as-0.0, i.e. flat).
        let mut node = Latch::new();
        let inputs = two_bool_inputs();
        assert_eq!(node.eval(Ctx::new(&inputs, Timestamp(0))), None);
    }

    #[test]
    fn input_slots_are_named_set_reset() {
        let l = Latch::builder();
        let names: Vec<&str> = l.schema().inputs.iter().map(|p| p.name.as_str()).collect();
        assert_eq!(names, ["set", "reset"]);
    }
}