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refactor: make the vjp's for min max abs more consistent #11

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merged 1 commit into from
Jan 13, 2025
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refactor: make the vjp's for min max abs more consistent #11

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47 changes: 11 additions & 36 deletions Sources/Differentiation/DerivativesOfNativeFunctions.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -6,55 +6,30 @@ import _Differentiation
/// https://github.com/apple/swift/blob/main/stdlib/public/core/Algorithm.swift#L18
@inlinable
@derivative(of: min(_:_:))
public func _vjpMin<T: Comparable & Differentiable>(
_ lhs: T,
_ rhs: T
) -> (value: T, pullback: (T.TangentVector) -> (T.TangentVector, T.TangentVector)) {
func pullback(_ tangentVector: T.TangentVector) -> (T.TangentVector, T.TangentVector) {
if lhs <= rhs {
return (tangentVector, .zero)
}
else {
return (.zero, tangentVector)
}
}
return (value: min(lhs, rhs), pullback: pullback)
public func _vjpMin<T: Comparable & Differentiable>(_ x: T, _ y: T)
-> (value: T, pullback: (T.TangentVector) -> (T.TangentVector, T.TangentVector))
{
y < x ? (value: y, pullback: { v in (.zero, v) }) : (value: x, pullback: { v in (v, .zero) })
}

/// For max(): "Returns: The greater of `x` and `y`. If `x` is equal to `y`, returns `y`."
/// https://github.com/apple/swift/blob/main/stdlib/public/core/Algorithm.swift#L52
@inlinable
@derivative(of: max(_:_:))
public func _vjpMax<T: Comparable & Differentiable>(
_ lhs: T,
_ rhs: T
) -> (value: T, pullback: (T.TangentVector) -> (T.TangentVector, T.TangentVector)) {
func pullback(_ tangentVector: T.TangentVector) -> (T.TangentVector, T.TangentVector) {
if lhs < rhs {
return (.zero, tangentVector)
}
else {
return (tangentVector, .zero)
}
}
return (value: max(lhs, rhs), pullback: pullback)
public func _vjpMax<T: Comparable & Differentiable>(_ x: T, _ y: T)
-> (value: T, pullback: (T.TangentVector) -> (T.TangentVector, T.TangentVector))
{
y >= x ? (value: y, pullback: { v in (.zero, v) }) : (value: x, pullback: { v in (v, .zero) })
}

/// To differentiate ``abs``
/// https://github.com/swiftlang/swift/blob/main/stdlib/public/core/Integers.swift#L346
@inlinable
@derivative(of: abs(_:))
public func _vjpAbs<T: Comparable & SignedNumeric & Differentiable>(_ value: T)
public func _vjpAbs<T: SignedNumeric & Comparable & Differentiable>(_ x: T)
-> (value: T, pullback: (T.TangentVector) -> T.TangentVector)
{
func pullback(_ tangentVector: T.TangentVector) -> T.TangentVector {
if value < 0 {
return .zero - tangentVector
}
else {
return tangentVector
}
}
return (value: abs(value), pullback: pullback)
x < 0 ? (value: -x, pullback: { v in .zero - v }) : (value: x, pullback: { v in v })
}

#endif
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