Merge pull request #215 from pepijndevos/pv/exptang

add truncate for explicit tangent

Author: oxinabox

src/tangent.jl

@@ -84,7 +84,7 @@ abstract type AbstractTangentSpace; end
     struct ExplicitTangent{P}
 
 A fully explicit coordinate representation of the tangent space,
-represented by a vector of `2^(N-1)` partials.
+represented by a vector of `2^N-1` partials.
 """
 struct ExplicitTangent{P <: Tuple} <: AbstractTangentSpace
     partials::P
@@ -242,6 +242,14 @@ function truncate(tb::TangentBundle, order::Val)
     _TangentBundle(order, tb.primal, truncate(tb.tangent, order))
 end
 
+function truncate(tb::ExplicitTangent, order::Val{N}) where {N}
+    ExplicitTangent(tb.partials[1:2^N-1])
+end
+
+function truncate(et::ExplicitTangent, order::Val{1})
+    TaylorTangent(et.partials[1:1])
+end
+
 const UniformBundle{N, B, U} = TangentBundle{N, B, UniformTangent{U}}
 UniformBundle{N, B, U}(primal::B, partial::U) where {N,B,U} = _TangentBundle(Val{N}(), primal, UniformTangent{U}(partial))
 UniformBundle{N, B, U}(primal::B) where {N,B,U} = _TangentBundle(Val{N}(), primal, UniformTangent{U}(U.instance))

test/tangent.jl

@@ -2,6 +2,7 @@ module tagent
 using Diffractor
 using Diffractor: AbstractZeroBundle, ZeroBundle, DNEBundle
 using Diffractor: TaylorBundle, TaylorTangentIndex, CompositeBundle
+using Diffractor: ExplicitTangent, TaylorTangent, truncate
 using ChainRulesCore
 using Test
 
@@ -44,4 +45,12 @@ end
     @test f'(23.5) == Tangent{Foo152}(; x=1.0)
 end
 
+@testset "truncate" begin
+    tt = TaylorTangent((1.0,2.0,3.0,4.0,5.0,6.0,7.0))
+    @test truncate(tt, Val(2)) == TaylorTangent((1.0,2.0))
+    et = ExplicitTangent((1.0,2.0,3.0,4.0,5.0,6.0,7.0))
+    @test truncate(et, Val(2)) == ExplicitTangent((1.0,2.0,3.0))
+    @test truncate(et, Val(1)) == TaylorTangent((1.0,))
+end
+
 end  # module