Rename ProjectionStyle's -> AdjointStyles and improve docs

Author: gaurav-arya

docs/src/api.md

@@ -21,6 +21,10 @@ AbstractFFTs.plan_brfft
 AbstractFFTs.plan_irfft
 AbstractFFTs.fftdims
 Base.adjoint
+AbstractFFTs.FFTAdjointStyle
+AbstractFFTs.RFFTAdjointStyle
+AbstractFFTs.BRFFTAdjointStyle
+AbstractFFTs.UnitaryAdjointStyle
 AbstractFFTs.fftshift
 AbstractFFTs.fftshift!
 AbstractFFTs.ifftshift

docs/src/implementations.md

@@ -32,10 +32,9 @@ To define a new FFT implementation in your own module, you should
 
 * You can also define similar methods of `plan_rfft` and `plan_brfft` for real-input FFTs.
 
-* To enable automatic computation of adjoint plans via [`Base.adjoint`](@ref) (used in rules for reverse-mode differentiation), define the trait `AbstractFFTs.ProjectionStyle(::MyPlan)`, which can return:
-    * `AbstractFFTs.NoProjectionStyle()`,
-    * `AbstractFFTs.RealProjectionStyle()`, for plans that halve one of the output's dimensions analogously to [`rfft`](@ref),
-    * `AbstractFFTs.RealInverseProjectionStyle(d::Int)`, for plans that expect an input with a halved dimension analogously to [`irfft`](@ref), where `d` is the original length of the dimension.
+* We offer an experimental `AdjointStyle` trait to enable automatic computation of adjoint plans via [`Base.adjoint`](@ref),
+(which `AbstractFFTs` uses to implement reverse-mode differentiation rules). To support adjoints in a new plan, define the trait `AbstractFFTs.AdjointStyle(::MyPlan)`. This should return a subtype of `AS <: AbstractFFTs.AdjointStyle` supporting `AbstractFFTs.adjoint_mul(::AdjointPlan, ::AbstractArray, ::AS)`. `AbstractFFTs` pre-implements [`AbstractFFTs.FFTAdjointStyle`](@ref), [`AbstractFFTs.RFFTAdjointStyle`](@ref),
+[`AbstractFFTs.BRFFTAdjointStyle`](@ref), and [`AbstractFFTs.UnitaryAdjointStyle`](@ref).
 
 The normalization convention for your FFT should be that it computes ``y_k = \sum_j x_j \exp(-2\pi i j k/n)`` for a transform of
 length ``n``, and the "backwards" (unnormalized inverse) transform computes the same thing but with ``\exp(+2\pi i jk/n)``.

src/definitions.jl

@@ -583,35 +583,57 @@ plan_brfft
 
 ##############################################################################
 
-abstract type ProjectionStyle end
+abstract type AdjointStyle end
 
 """
-    NoProjectionStyle()
+    FFTAdjointStyle()
 
-Projection style for complex to complex discrete Fourier transform
+Projection style for complex to complex discrete Fourier transforms. 
+    
+Since the Fourier transform is unitary up to a scaling, the adjoint simply applies 
+the transform's inverse with an appropriate scaling.
 """
-struct NoProjectionStyle <: ProjectionStyle end
+struct FFTAdjointStyle <: AdjointStyle end
 
 """
-    RealProjectionStyle()
+    RFFTAdjointStyle()
 
-Projection style for complex to real discrete Fourier transform
+Projection style for real to complex discrete Fourier transforms, for plans that 
+halve one of the output's dimensions analogously to [`rfft`](@ref).
+    
+Since the Fourier transform is unitary up to a scaling, the adjoint applies the transform's 
+inverse, but with additional logic to handle the fact that the output is projected 
+to exploit its conjugate symmetry (see [`rfft`](@ref)). 
 """
-struct RealProjectionStyle <: ProjectionStyle end 
+struct RFFTAdjointStyle <: AdjointStyle end 
 
 """
-    RealInverseProjectionStyle()
+    BRFFTAdjointStyle(d::Dim)
 
-Projection style for inverse of complex to real discrete Fourier transform
+Projection style for complex to real discrete Fourier transforms, for plans that 
+expect an input with a halved dimension analogously to [`irfft`](@ref), where `d` 
+is the original length of the dimension.
+    
+Since the Fourier transform is unitary up to a scaling, the adjoint applies the transform's 
+inverse, but with additional logic to handle the fact that the input is projected 
+to exploit its conjugate symmetry (see [`irfft`](@ref)). 
 """
-struct RealInverseProjectionStyle <: ProjectionStyle
+struct BRFFTAdjointStyle <: AdjointStyle
     dim::Int
 end
 
-output_size(p::Plan) = _output_size(p, ProjectionStyle(p))
-_output_size(p::Plan, ::NoProjectionStyle) = size(p)
-_output_size(p::Plan, ::RealProjectionStyle) = rfft_output_size(size(p), fftdims(p))
-_output_size(p::Plan, s::RealInverseProjectionStyle) = brfft_output_size(size(p), s.dim, fftdims(p))
+"""
+    UnitaryAdjointStyle()
+
+Projection style for unitary transforms, whose adjoint equals their inverse.
+"""
+struct UnitaryAdjointStyle <: AdjointStyle end
+
+output_size(p::Plan) = _output_size(p, AdjointStyle(p))
+_output_size(p::Plan, ::FFTAdjointStyle) = size(p)
+_output_size(p::Plan, ::RFFTAdjointStyle) = rfft_output_size(size(p), fftdims(p))
+_output_size(p::Plan, s::BRFFTAdjointStyle) = brfft_output_size(size(p), s.dim, fftdims(p))
+_output_size(p::Plan, ::UnitaryAdjointStyle) = size(p)
 
 struct AdjointPlan{T,P<:Plan} <: Plan{T}
     p::P
@@ -638,15 +660,15 @@ Base.adjoint(p::ScaledPlan) = ScaledPlan(p.p', p.scale)
 size(p::AdjointPlan) = output_size(p.p)
 output_size(p::AdjointPlan) = size(p.p)
 
-Base.:*(p::AdjointPlan, x::AbstractArray) = _mul(p, x, ProjectionStyle(p.p))
+Base.:*(p::AdjointPlan, x::AbstractArray) = adjoint_mul(p, x, AdjointStyle(p.p))
 
-function _mul(p::AdjointPlan{T}, x::AbstractArray, ::NoProjectionStyle) where {T}
+function adjoint_mul(p::AdjointPlan{T}, x::AbstractArray, ::FFTAdjointStyle) where {T}
     dims = fftdims(p.p)
     N = normalization(T, size(p.p), dims)
     return (p.p \ x) / N
 end
 
-function _mul(p::AdjointPlan{T}, x::AbstractArray, ::RealProjectionStyle) where {T<:Real}
+function adjoint_mul(p::AdjointPlan{T}, x::AbstractArray, ::RFFTAdjointStyle) where {T<:Real}
     dims = fftdims(p.p)
     N = normalization(T, size(p.p), dims)
     halfdim = first(dims)
@@ -659,7 +681,7 @@ function _mul(p::AdjointPlan{T}, x::AbstractArray, ::RealProjectionStyle) where
     return p.p \ (x ./ convert(typeof(x), scale))
 end
 
-function _mul(p::AdjointPlan{T}, x::AbstractArray, ::RealInverseProjectionStyle) where {T}
+function adjoint_mul(p::AdjointPlan{T}, x::AbstractArray, ::BRFFTAdjointStyle) where {T}
     dims = fftdims(p.p)
     N = normalization(real(T), output_size(p.p), dims)
     halfdim = first(dims)
@@ -672,6 +694,8 @@ function _mul(p::AdjointPlan{T}, x::AbstractArray, ::RealInverseProjectionStyle)
     return (convert(typeof(x), scale) ./ N) .* (p.p \ x)
 end
 
+adjoint_mul(p::AdjointPlan, x::AbstractArray, ::UnitaryAdjointStyle) = p.p \ x
+
 # Analogously to ScaledPlan, define both plan_inv (for no caching) and inv (caches inner plan only).
 plan_inv(p::AdjointPlan) = adjoint(plan_inv(p.p)) 
 inv(p::AdjointPlan) = adjoint(inv(p.p))

test/testplans.jl

@@ -21,8 +21,8 @@ Base.ndims(::TestPlan{T,N}) where {T,N} = N
 Base.size(p::InverseTestPlan) = p.sz
 Base.ndims(::InverseTestPlan{T,N}) where {T,N} = N
 
-AbstractFFTs.ProjectionStyle(::TestPlan) = AbstractFFTs.NoProjectionStyle()
-AbstractFFTs.ProjectionStyle(::InverseTestPlan) = AbstractFFTs.NoProjectionStyle()
+AbstractFFTs.AdjointStyle(::TestPlan) = AbstractFFTs.FFTAdjointStyle()
+AbstractFFTs.AdjointStyle(::InverseTestPlan) = AbstractFFTs.FFTAdjointStyle()
 
 function AbstractFFTs.plan_fft(x::AbstractArray{T}, region; kwargs...) where {T}
     return TestPlan{T}(region, size(x))
@@ -110,8 +110,8 @@ mutable struct InverseTestRPlan{T,N,G} <: Plan{Complex{T}}
     end
 end
 
-AbstractFFTs.ProjectionStyle(::TestRPlan) = AbstractFFTs.RealProjectionStyle()
-AbstractFFTs.ProjectionStyle(p::InverseTestRPlan) = AbstractFFTs.RealInverseProjectionStyle(p.d)
+AbstractFFTs.AdjointStyle(::TestRPlan) = AbstractFFTs.RFFTAdjointStyle()
+AbstractFFTs.AdjointStyle(p::InverseTestRPlan) = AbstractFFTs.BRFFTAdjointStyle(p.d)
 
 function AbstractFFTs.plan_rfft(x::AbstractArray{T}, region; kwargs...) where {T<:Real}
     return TestRPlan{T}(region, size(x))
@@ -241,7 +241,7 @@ end
 
 Base.size(p::InplaceTestPlan) = size(p.plan)
 Base.ndims(p::InplaceTestPlan) = ndims(p.plan)
-AbstractFFTs.ProjectionStyle(p::InplaceTestPlan) = AbstractFFTs.ProjectionStyle(p.plan)
+AbstractFFTs.AdjointStyle(p::InplaceTestPlan) = AbstractFFTs.AdjointStyle(p.plan)
 
 function AbstractFFTs.plan_fft!(x::AbstractArray, region; kwargs...)
     return InplaceTestPlan(plan_fft(x, region; kwargs...))