topos 0.13.1

An autodiff compiler stack in Rust.
Documentation 
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use smallvec::smallvec;

use crate::{Element, Recordable, Shape, Tensor};

use super::{Cotangents, Operation, Reads, binary};

/// The product of two values, with operands `[left, right]`.
///
/// The derivative with respect to each operand is the other operand, so
/// `backward` scales the incoming gradient by the opposite side's value.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct Mul;

impl Mul {
    /// Returns the arity: two operands.
    pub(crate) fn arity(&self) -> usize {
        2
    }

    /// Returns the read set of the derivative rule below.
    /// It reads both operand payloads: each side's cotangent scales by the other.
    pub(crate) fn reads(&self) -> Reads {
        Reads {
            operands: [true, true],
            output: false,
        }
    }

    /// Infers the shape of the result, which both operands must share.
    pub(crate) fn infer_shape(&self, operands: &[Shape]) -> Shape {
        let (left, right) = binary(operands);
        assert_eq!(
            left, right,
            "multiplication requires operands of equal shapes"
        );
        left.clone()
    }
}

impl Mul {
    pub(crate) fn forward<E: Element>(&self, operands: &[&Tensor<E>]) -> Tensor<E> {
        let (&left, &right) = binary(operands);
        left.clone() * right.clone()
    }
}

impl<Rule: Recordable> Operation<Rule> for Mul {
    fn backward(&self, operands: &[&Rule], _output: &Rule, gradient: &Rule) -> Cotangents<Rule> {
        let (&left, &right) = binary(operands);
        smallvec![
            Some(gradient.clone() * right.clone()),
            Some(gradient.clone() * left.clone()),
        ]
    }
}