add Chebyshev transform

src/Transform/Transform.jl

@@ -17,3 +17,6 @@ export
 abstract type AbstractTransform end
 
 include("fourier_transform.jl")
+include("chebyshev_transform.jl")
+
+const truncate_modes = low_pass

src/Transform/chebyshev_transform.jl

@@ -0,0 +1,23 @@
+export ChebyshevTransform
+
+struct ChebyshevTransform{N, S}<:AbstractTransform
+    modes::NTuple{N, S} # N == ndims(x)
+end
+
+Base.ndims(::ChebyshevTransform{N}) where {N} = N
+
+function transform(t::ChebyshevTransform{N}, 𝐱::AbstractArray) where {N}
+    return FFTW.r2r(𝐱, FFTW.REDFT00, 1:N) # [size(x)..., in_chs, batch]
+end
+
+function low_pass(t::ChebyshevTransform, 𝐱̂::AbstractArray)
+    return view(𝐱̂, map(d->1:d, t.modes)..., :, :) # [ft.modes..., in_chs, batch]
+end
+
+function inverse(t::ChebyshevTransform{N}, 𝐱̂::AbstractArray) where {N}
+    return FFTW.r2r(
+        𝐱̂ ./ (prod(2 .* (size(𝐱̂)[1:N] .- 1))),
+        FFTW.REDFT00,
+        1:N,
+    ) # [size(x)..., in_chs, batch]
+end

test/Transform/Transform.jl

@@ -1 +1,4 @@
-@testset "Transform" begin include("fourier_transform.jl") end
+@testset "Transform" begin
+    include("fourier_transform.jl")
+    include("chebyshev_transform.jl")
+end

test/Transform/chebyshev_transform.jl

@@ -0,0 +1,11 @@
+@testset "Chebyshev transform" begin
+    ch = 6
+    batch = 7
+    𝐱 = rand(30, 40, 50, ch, batch)
+
+    t = ChebyshevTransform((3, 4, 5))
+
+    @test size(transform(t, 𝐱)) == (30, 40, 50, ch, batch)
+    @test size(truncate_modes(t, transform(t, 𝐱))) == (3, 4, 5, ch, batch)
+    @test size(inverse(t, truncate_modes(t, transform(t, 𝐱)))) == (3, 4, 5, ch, batch)
+end

test/Transform/fourier_transform.jl

@@ -1,9 +1,11 @@
-@testset "fourier transform" begin
-    𝐱 = rand(30, 40, 50, 6, 7) # where ch == 6 and batch == 7
+@testset "Fourier transform" begin
+    ch = 6
+    batch = 7
+    𝐱 = rand(30, 40, 50, ch, batch)
 
     ft = FourierTransform((3, 4, 5))
 
-    @test size(transform(ft, 𝐱)) == (30, 40, 50, 6, 7)
-    @test size(truncate_modes(ft, transform(ft, 𝐱))) == (3, 4, 5, 6, 7)
-    @test size(inverse(ft, truncate_modes(ft, transform(ft, 𝐱)))) == (3, 4, 5, 6, 7)
+    @test size(transform(ft, 𝐱)) == (30, 40, 50, ch, batch)
+    @test size(truncate_modes(ft, transform(ft, 𝐱))) == (3, 4, 5, ch, batch)
+    @test size(inverse(ft, truncate_modes(ft, transform(ft, 𝐱)))) == (3, 4, 5, ch, batch)
 end