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 difference of two values, with operands `[left, right]`.
///
/// The derivative with respect to the left operand is one and with
/// respect to the right operand minus one, so `backward` hands the
/// incoming gradient onward and negated respectively.
///
/// This is the one operation kept although its composition is
/// bit-exact: IEEE 754 defines `a - b` as `a + (-b)`, so `Add` of
/// `Neg` would reproduce these bits everywhere (short of NaN
/// sign-propagation corners). Keeping the variant is a practical
/// decision, on record: the spec prints one `Sub` line per
/// subtraction instead of a `Neg`/`Add` pair, the unfused oracle pays
/// one pass and one activation-sized buffer less per site, and the
/// node preserves the one-pass `a - b` intent for lower tiers instead
/// of leaving them an idiom to re-fuse.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) struct Sub;

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

    /// Returns the read set of the derivative rule below.
    /// It reads no payloads: the cotangents are the gradient and its negation.
    pub(crate) fn reads(&self) -> Reads {
        Reads::NOTHING
    }

    /// 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, "subtraction requires operands of equal shapes");
        left.clone()
    }
}

impl Sub {
    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 Sub {
    fn backward(&self, _operands: &[&Rule], _output: &Rule, gradient: &Rule) -> Cotangents<Rule> {
        smallvec![Some(gradient.clone()), Some(-gradient.clone())]
    }
}