//! `Mul` — two-input f64 product (input 0 times input 1). The fundamental
//! multiplication primitive (e.g. squaring a return for a variance estimate:
//! `Mul(delta, delta)`). Emits `None` until both inputs have a value.

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

/// Two-input f64 product: input 0 times input 1. Emits `None` until both inputs
/// have a value.
pub struct Mul {
    out: [Cell; 1],
}

impl Mul {
    pub fn new() -> Self {
        Self { out: [Cell::from_f64(0.0)] }
    }
    pub fn builder() -> PrimitiveBuilder {
        PrimitiveBuilder::new(
            "Mul",
            NodeSchema {
                inputs: vec![
                    PortSpec { kind: ScalarKind::F64, firing: Firing::Any, name: "lhs".into() },
                    PortSpec { kind: ScalarKind::F64, firing: Firing::Any, name: "rhs".into() },
                ],
                output: vec![FieldSpec { name: "value".into(), kind: ScalarKind::F64 }],
                params: vec![],
            },
            |_| Box::new(Mul::new()),
        )
    }
}

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

impl Node for Mul {
    fn lookbacks(&self) -> Vec<usize> { vec![1, 1] }
    fn eval(&mut self, ctx: Ctx<'_>) -> Option<&[Cell]> {
        let a = ctx.f64_in(0);
        let b = ctx.f64_in(1);
        if a.is_empty() || b.is_empty() {
            return None;
        }
        self.out[0] = Cell::from_f64(a[0] * b[0]);
        Some(&self.out)
    }
    fn label(&self) -> String { "Mul".to_string() }
}

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

    #[test]
    fn mul_is_product_once_both_inputs_present() {
        let mut m = Mul::new();
        let mut inputs = vec![
            AnyColumn::with_capacity(ScalarKind::F64, 1),
            AnyColumn::with_capacity(ScalarKind::F64, 1),
        ];
        inputs[0].push(Scalar::f64(3.0)).unwrap();
        assert_eq!(m.eval(Ctx::new(&inputs, Timestamp(0))), None); // only one leg
        inputs[1].push(Scalar::f64(4.0)).unwrap();
        assert_eq!(m.eval(Ctx::new(&inputs, Timestamp(0))), Some([Cell::from_f64(12.0)].as_slice()));
    }
    #[test]
    fn input_slots_are_named_lhs_rhs() {
        let names: Vec<String> = Mul::builder().schema().inputs.iter().map(|p| p.name.clone()).collect();
        assert_eq!(names, ["lhs", "rhs"]);
    }
}
