//! `Sub` — two-input f64 difference (input 0 minus input 1), e.g. a fast/slow
//! spread. The walking skeleton's second worked node: it gives the sim loop a
//! real fan-out + join to run (two SMAs joining into one node), exercising
//! multi-input `Ctx` access inside a running graph.

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

/// Two-input f64 difference: input 0 minus input 1. Emits `None` until both
/// inputs have a value.
#[derive(Default)]
pub struct Sub;

impl Sub {
    /// Build a `Sub` node.
    pub fn new() -> Self {
        Self
    }
}

impl Node for Sub {
    fn schema(&self) -> NodeSchema {
        NodeSchema {
            inputs: vec![
                InputSpec { kind: ScalarKind::F64, lookback: 1 },
                InputSpec { kind: ScalarKind::F64, lookback: 1 },
            ],
            output: 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;
        }
        Some(Scalar::F64(a[0] - b[0]))
    }
}

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

    #[test]
    fn sub_is_difference_once_both_inputs_present() {
        let mut sub = Sub::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!(sub.eval(Ctx::new(&inputs)), None);

        // both present -> a - b
        inputs[1].push(Scalar::F64(4.0)).unwrap();
        assert_eq!(sub.eval(Ctx::new(&inputs)), Some(Scalar::F64(6.0)));
    }
}