Rules for sortslices, unique (#546)

* unique, take 1

* add shortcut, and tests

* sortslices, too

* fixup

* Apply 3 suggestions

Co-authored-by: Lyndon White <oxinabox@ucc.asn.au>

* comment

Co-authored-by: Lyndon White <oxinabox@ucc.asn.au>

Co-authored-by: Lyndon White <oxinabox@ucc.asn.au>

Author: mcabbott

src/rulesets/Base/sort.jl

@@ -1,3 +1,7 @@
+#####
+##### `sort`
+#####
+
 function rrule(::typeof(partialsort), xs::AbstractVector, k::Union{Integer,OrdinalRange}; kwargs...)
     inds = partialsortperm(xs, k; kwargs...)
     ys = xs[inds]
@@ -33,3 +37,55 @@ function rrule(::typeof(sort), xs::AbstractVector; kwargs...)
     end
     return ys, sort_pullback
 end
+
+#####
+##### `sortslices`
+#####
+
+function rrule(::typeof(sortslices), x::AbstractArray; dims::Integer, kw...)
+    p = sortperm(collect(eachslice(x; dims=dims)); kw...)
+    inds = ntuple(d -> d == dims ? p : (:), ndims(x))
+    function sortslices_pullback(dy)
+        # No actual need to zero this, and if you didn't, then you could widen eltype
+        # Also, you could use similar(dy) here not x, same size?
+        dx = _zerolike_writeat(x, unthunk(dy), (), inds...)
+        return (NoTangent(), ProjectTo(x)(dx))
+    end
+    return x[inds...], sortslices_pullback
+end
+
+#####
+##### `unique`
+#####
+
+function rrule(::typeof(unique), x::AbstractArray{<:Number}; dims=:)
+    axes_x = axes(x)
+    y = unique(x; dims=dims)  # accepts only dims=: or dims::Integer
+    function unique_pullback(dy_raw)
+        dy = unthunk(dy_raw)
+        if length(x) == length(y)
+            # Short-circuit for the case of all unique, since `mask` is fairly expensive:
+            dx = reshape(dy, axes_x)
+            return (NoTangent(), ProjectTo(x)(dx))
+        end
+
+        if dims isa Colon
+            xs, ys = vec(x), y
+        else
+            xs, ys = collect(eachslice(x; dims=dims)), collect(eachslice(y; dims=dims))
+        end
+        mask = isequal.(permutedims(ys), xs)  # unique([0.0, -0.0, NaN, NaN])
+        mask .= (mask .== cumsum(mask, dims=1) .== true)  # this implements  findfirst(mask; dims=1)
+        keep = map(I -> I[1], findall(mask))
+        if dims isa Colon
+            # The function `_zerolike_writeat` allows second derivatives.
+            # Should perhaps eventually be shared with `getindex`.
+            dx = reshape(_zerolike_writeat(vec(x), vec(dy), (), keep), axes_x)
+        else
+            inds = ntuple(d -> d==dims ? keep : (:), length(axes_x))
+            dx = _zerolike_writeat(x, dy, (), inds...)
+        end
+        return (NoTangent(), ProjectTo(x)(dx))
+    end
+    return y, unique_pullback
+end

test/rulesets/Base/sort.jl

@@ -12,4 +12,33 @@
 
         test_rrule(partialsort, a, 4, fkwargs=(;rev=true))
     end
+
+    @testset "sortslices" begin
+        test_rrule(sortslices, rand(3,4); fkwargs=(; dims=2))
+        test_rrule(sortslices, rand(5,4); fkwargs=(; dims=1, rev=true, by=last))
+        test_rrule(sortslices, rand(3,4,5); fkwargs=(; dims=3, by=sum), check_inferred=false)
+
+        @test_throws Exception sortslices(Diagonal(1:3), dims=1)
+    end
+
+    @testset "unique" begin
+        # Trivial case, all unique:
+        test_rrule(unique, rand(5))
+        test_rrule(unique, rand(3,4))
+        test_rrule(unique, rand(3,4); fkwargs=(; dims=2))
+
+        # Not all unique:
+        @test rrule(unique, [1,1,2,3])[1] == [1,2,3]
+        @test rrule(unique, [1,1,2,3])[2]([10,20,30]) == (NoTangent(), [10, 0, 20, 30])
+
+        @test rrule(unique, [1 2; 1 4])[1] == [1,2,4]
+        @test rrule(unique, [1 2; 1 4])[2]([10,20,30]) == (NoTangent(), [10 20; 0 30])
+
+        @test rrule(unique, [1 2 1 2; 1 2 1 4], dims=2)[1] == [1 2 2; 1 2 4]
+        @test rrule(unique, [1 2 1 2; 1 2 1 4], dims=2)[2]([10 20 30; 40 50 60])[2] == [10 20 0 30; 40 50 0 60]
+
+        @test rrule(unique, Diagonal([1,2,3]))[1] == [1,0,2,3]
+        @test rrule(unique, Diagonal([1,2,3]))[2]([10 20 30 40])[2] == [10.0 0.0 0.0; 0.0 30.0 0.0; 0.0 0.0 40.0]
+        @test rrule(unique, Diagonal([1,2,3]))[2]([10 20 30 40])[2] isa Diagonal
+    end
 end