anydifferentiable.swift

// RUN: %target-run-simple-swift
// REQUIRES: executable_test

import _Differentiation
import StdlibUnittest

var TypeErasureTests = TestSuite("DifferentiableTypeErasure")

struct Vector: Differentiable, Equatable {
  var x, y: Float
}
struct Generic<T: Differentiable & Equatable>: Differentiable, Equatable {
  var x: T
}

extension AnyDerivative {
  // This exists only to faciliate testing.
  func moved(along offset: TangentVector) -> Self {
    var result = self
    result.move(by: offset)
    return result
  }
}

TypeErasureTests.test("AnyDifferentiable operations") {
  do {
    var any = AnyDifferentiable(Vector(x: 1, y: 1))
    let tan = AnyDerivative(Vector.TangentVector(x: 1, y: 1))
    any.move(by: tan)
    expectEqual(Vector(x: 2, y: 2), any.base as? Vector)
  }

  do {
    var any = AnyDifferentiable(Generic<Float>(x: 1))
    let tan = AnyDerivative(Generic<Float>.TangentVector(x: 1))
    any.move(by: tan)
    expectEqual(Generic<Float>(x: 2), any.base as? Generic<Float>)
  }
}

TypeErasureTests.test("AnyDerivative operations") {
  do {
    var tan = AnyDerivative(Vector.TangentVector(x: 1, y: 1))
    tan += tan
    expectEqual(AnyDerivative(Vector.TangentVector(x: 2, y: 2)), tan)
    expectEqual(AnyDerivative(Vector.TangentVector(x: 4, y: 4)), tan + tan)
    expectEqual(AnyDerivative(Vector.TangentVector(x: 0, y: 0)), tan - tan)
    expectEqual(AnyDerivative(Vector.TangentVector(x: 4, y: 4)), tan.moved(along: tan))
    expectEqual(AnyDerivative(Vector.TangentVector(x: 2, y: 2)), tan)
  }

  do {
    var tan = AnyDerivative(Generic<Float>.TangentVector(x: 1))
    tan += tan
    expectEqual(AnyDerivative(Generic<Float>.TangentVector(x: 2)), tan)
    expectEqual(AnyDerivative(Generic<Float>.TangentVector(x: 4)), tan + tan)
    expectEqual(AnyDerivative(Generic<Float>.TangentVector(x: 0)), tan - tan)
    expectEqual(AnyDerivative(Generic<Float>.TangentVector(x: 4)), tan.moved(along: tan))
  }
}

TypeErasureTests.test("AnyDerivative.zero") {
  var zero = AnyDerivative.zero
  zero += zero
  zero -= zero
  expectEqual(zero, zero + zero)
  expectEqual(zero, zero - zero)
  expectEqual(zero, zero.moved(along: zero))

  var tan = AnyDerivative(Vector.TangentVector(x: 1, y: 1))
  expectEqual(zero, zero)
  expectEqual(AnyDerivative(Vector.TangentVector.zero), tan - tan)
  expectNotEqual(AnyDerivative(Vector.TangentVector.zero), zero)
  expectNotEqual(AnyDerivative.zero, tan - tan)
  tan += zero
  tan -= zero
  expectEqual(tan, tan + zero)
  expectEqual(tan, tan - zero)
  expectEqual(tan, tan.moved(along: zero))
  expectEqual(tan, zero.moved(along: tan))
  expectEqual(zero, zero)
  expectEqual(tan, tan)
}

TypeErasureTests.test("AnyDifferentiable casting") {
  let any = AnyDifferentiable(Vector(x: 1, y: 1))
  expectEqual(Vector(x: 1, y: 1), any.base as? Vector)

  let genericAny = AnyDifferentiable(Generic<Float>(x: 1))
  expectEqual(Generic<Float>(x: 1),
              genericAny.base as? Generic<Float>)
  expectEqual(nil, genericAny.base as? Generic<Double>)
}

TypeErasureTests.test("AnyDifferentiable reflection") {
  let originalVector = Vector(x: 1, y: 1)
  let vector = AnyDifferentiable(originalVector)
  let mirror = Mirror(reflecting: vector)
  let children = Array(mirror.children)
  expectEqual(2, children.count)
  expectEqual(["x", "y"], children.map(\.label))
  expectEqual([originalVector.x, originalVector.y], children.map { $0.value as! Float })
}

TypeErasureTests.test("AnyDerivative casting") {
  let tan = AnyDerivative(Vector.TangentVector(x: 1, y: 1))
  expectEqual(Vector.TangentVector(x: 1, y: 1), tan.base as? Vector.TangentVector)

  let genericTan = AnyDerivative(Generic<Float>.TangentVector(x: 1))
  expectEqual(Generic<Float>.TangentVector(x: 1),
              genericTan.base as? Generic<Float>.TangentVector)
  expectEqual(nil, genericTan.base as? Generic<Double>.TangentVector)

  let zero = AnyDerivative.zero
  expectEqual(nil, zero.base as? Float)
  expectEqual(nil, zero.base as? Vector.TangentVector)
  expectEqual(nil, zero.base as? Generic<Float>.TangentVector)
}

TypeErasureTests.test("AnyDerivative reflection") {
  let originalTan = Vector.TangentVector(x: 1, y: 1)
  let tan = AnyDerivative(originalTan)
  let mirror = Mirror(reflecting: tan)
  let children = Array(mirror.children)
  expectEqual(2, children.count)
  expectEqual(["x", "y"], children.map(\.label))
  expectEqual([originalTan.x, originalTan.y], children.map { $0.value as! Float })
}

TypeErasureTests.test("AnyDifferentiable differentiation") {
  // Test `AnyDifferentiable` initializer.
  do {
    let x: Float = 3
    let v = AnyDerivative(Float(2))
    let 𝛁x = pullback(at: x, of: { AnyDifferentiable($0) })(v)
    let expectedVJP: Float = 2
    expectEqual(expectedVJP, 𝛁x)
  }

  do {
    let x = Vector(x: 4, y: 5)
    let v = AnyDerivative(Vector.TangentVector(x: 2, y: 2))
    let 𝛁x = pullback(at: x, of: { AnyDifferentiable($0) })(v)
    let expectedVJP = Vector.TangentVector(x: 2, y: 2)
    expectEqual(expectedVJP, 𝛁x)
  }

  do {
    let x = Generic<Double>(x: 4)
    let v = AnyDerivative(Generic<Double>.TangentVector(x: 2))
    let 𝛁x = pullback(at: x, of: { AnyDifferentiable($0) })(v)
    let expectedVJP = Generic<Double>.TangentVector(x: 2)
    expectEqual(expectedVJP, 𝛁x)
  }
}

TypeErasureTests.test("AnyDerivative differentiation") {
  // Test `AnyDerivative` operations.
  func tripleSum(_ x: AnyDerivative, _ y: AnyDerivative) -> AnyDerivative {
    let sum = x + y
    return sum + sum + sum
  }

  do {
    let x = AnyDerivative(Float(4))
    let y = AnyDerivative(Float(-2))
    let v = AnyDerivative(Float(1))
    let expectedVJP: Float = 3

    let (𝛁x, 𝛁y) = pullback(at: x, y, of: tripleSum)(v)
    expectEqual(expectedVJP, 𝛁x.base as? Float)
    expectEqual(expectedVJP, 𝛁y.base as? Float)
  }

  do {
    let x = AnyDerivative(Vector.TangentVector(x: 4, y: 5))
    let y = AnyDerivative(Vector.TangentVector(x: -2, y: -1))
    let v = AnyDerivative(Vector.TangentVector(x: 1, y: 1))
    let expectedVJP = Vector.TangentVector(x: 3, y: 3)

    let (𝛁x, 𝛁y) = pullback(at: x, y, of: tripleSum)(v)
    expectEqual(expectedVJP, 𝛁x.base as? Vector.TangentVector)
    expectEqual(expectedVJP, 𝛁y.base as? Vector.TangentVector)
  }

  do {
    let x = AnyDerivative(Generic<Double>.TangentVector(x: 4))
    let y = AnyDerivative(Generic<Double>.TangentVector(x: -2))
    let v = AnyDerivative(Generic<Double>.TangentVector(x: 1))
    let expectedVJP = Generic<Double>.TangentVector(x: 3)

    let (𝛁x, 𝛁y) = pullback(at: x, y, of: tripleSum)(v)
    expectEqual(expectedVJP, 𝛁x.base as? Generic<Double>.TangentVector)
    expectEqual(expectedVJP, 𝛁y.base as? Generic<Double>.TangentVector)
  }

  // Test `AnyDerivative` initializer.
  func typeErased<T>(_ x: T) -> AnyDerivative
  where T: Differentiable, T.TangentVector == T {
    let any = AnyDerivative(x)
    return any + any
  }

  do {
    let x: Float = 3
    let v = AnyDerivative(Float(1))
    let 𝛁x = pullback(at: x, of: { x in typeErased(x) })(v)
    let expectedVJP: Float = 2
    expectEqual(expectedVJP, 𝛁x)
  }

  do {
    let x = Vector.TangentVector(x: 4, y: 5)
    let v = AnyDerivative(Vector.TangentVector(x: 1, y: 1))
    let 𝛁x = pullback(at: x, of: { x in typeErased(x) })(v)
    let expectedVJP = Vector.TangentVector(x: 2, y: 2)
    expectEqual(expectedVJP, 𝛁x)
  }

  do {
    let x = Generic<Double>.TangentVector(x: 4)
    let v = AnyDerivative(Generic<Double>.TangentVector(x: 1))
    let 𝛁x = pullback(at: x, of: { x in typeErased(x) })(v)
    let expectedVJP = Generic<Double>.TangentVector(x: 2)
    expectEqual(expectedVJP, 𝛁x)
  }
}

runAllTests()