fix set_property alloc

src/states.jl

@@ -203,11 +203,37 @@ vstate_type(S::Type, X::XState) = vstate_type(zero(S), X)
 
 ## ---------- a simple copying setter-like functionality 
 
-function set_property(x::S, s::Symbol, val) where {S}
-   nt = _x(x) 
-   return S((; nt..., s => val,))
+# this seems allocating, unclear why hence a generated implementation below 
+
+@generated function set_property(x::TX, s::Symbol, val) where {TX <: XState} 
+   SYMS, TT = _symstt(TX)
+   code = "$(nameof(TX))("
+   for i = 1:length(SYMS)
+      sym = SYMS[i]
+      code *= "$sym = (:$sym == s ? val : x.$sym)::(typeof(x.$sym)), "
+   end 
+   code *= ")"
+   quote
+      $(Meta.parse(code))      
+   end
 end
 
+@generated function set_property(x::TX, ::Val{sym1}, val1) where {TX <: XState, sym1}
+   SYMS, TT = _symstt(TX)
+   code = "$(nameof(TX))("
+   for (sym, T) in zip(SYMS, TT.types)
+      if sym == sym1 
+         code *= "$sym = val1, " 
+      else 
+         code *= "$sym = x.$sym, " 
+      end 
+   end
+   code *= ")"
+   quote
+      $(Meta.parse(code))      
+   end
+end 
+
 
 ## ---------- explicit real/complex conversion 
 # this feels a bit like a hack but might be unavoidable; 

test/test_atomsbase.jl

@@ -100,8 +100,8 @@ DP.set_positions!(aos1, X)
 DP.set_position!(aos1, 1, 𝐫1)
 @test position(aos1, 1) == 𝐫1
 
-@allocated DP.set_position(x, 𝐫1)
-@allocated DP.set_positions!(aos1, X)
+@allocated DecoratedParticles.set_position(x, 𝐫1)
+@test (@allocated DP.set_positions!(aos1, X)) == 0
 
 ## 
 
@@ -112,8 +112,7 @@ DP.set_positions!(soa, X)
 DP.set_position!(soa, 1, 𝐫1)
 @test position(soa, 1) == 𝐫1
 
-# much nicer behaved for now! no allocation
-@allocated DP.set_positions!(soa, X)
+@test (@allocated DP.set_positions!(soa, X)) == 0 
 
 ## 
 

test/test_states.jl

@@ -88,3 +88,32 @@ let Ys = Ys, Xs = Xs, a = a
    println(@test nalloc == 0) 
 end
 
+## 
+
+x = PState(a = 1.0, b = 2, c = true)
+y = DP.set_property(x, :a, 2.3)
+z = DP.set_property(x, Val(:a), 2.3)
+@test y.a == 2.3 && y.b == x.b && y.c == x.c
+@test z.a == 2.3 && z.b == x.b && z.c == x.c
+@test y == z
+
+function transform_states!(Xs, sym)
+   for i = 1:length(Xs)
+      xi = Xs[i] 
+      Xs[i] = DecoratedParticles.set_property(xi, sym, rand())
+   end
+   return nothing 
+end
+
+# test that there are no allocations 
+Xs = [ PState(a = 1.0, b = 2, c = true)  for _ = 1:1000 ]
+transform_states!(Xs, :a)
+@test (@allocated transform_states!(Xs, :a)) == 0
+transform_states!(Xs, Val{:a}())
+@test (@allocated transform_states!(Xs, Val{:a}())) == 0 
+
+# the two codes are essentially equivalent!!
+# using BenchmarkTools
+# @btime transform_states!($Xs, $(:a))
+# @btime transform_states!($Xs, Val{:a}())
+