Type promotion between MixedDestabilizer, Stabilizer and others, also…

… used in ⊗ (#354) --------- Co-authored-by: Stefan Krastanov <[email protected]>
QuantumSavory · Sep 27, 2024 · 8173b45 · 8173b45 · Krastanov · Sep 27, 2024
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diff --git a/src/QuantumClifford.jl b/src/QuantumClifford.jl
@@ -989,6 +989,16 @@ function _apply!(stab::AbstractStabilizer, p::PauliOperator, indices; phases::Va
     stab
 end
 
+##############################
+# Conversion and promotion
+##############################
+
+Base.promote_rule(::Type{<:Destabilizer{T}}   , ::Type{<:MixedDestabilizer{T}}) where {T<:Tableau} = MixedDestabilizer{T}
+Base.promote_rule(::Type{<:MixedStabilizer{T}}, ::Type{<:MixedDestabilizer{T}}) where {T<:Tableau} = MixedDestabilizer{T}
+Base.promote_rule(::Type{<:Stabilizer{T}}     , ::Type{<:S}                   ) where {T<:Tableau, S<:Union{MixedStabilizer{T}, Destabilizer{T}, MixedDestabilizer{T}}} = S
+
+Base.convert(::Type{<:MixedDestabilizer{T}}, x::Union{Destabilizer{T}, MixedStabilizer{T}, Stabilizer{T}}) where {T <: Tableau} = MixedDestabilizer(x)
+
 ##############################
 # Helpers for binary codes
 ##############################

diff --git a/src/linalg.jl b/src/linalg.jl
@@ -161,8 +161,10 @@ julia> tensor(s, s)
 See also [`tensor_pow`](@ref)."""
 function tensor end
 
-function tensor(ops::AbstractStabilizer...) # TODO optimize this by doing conversion to common type to enable preallocation
-    foldl(⊗, ops[2:end], init=ops[1])
+function tensor(ops::AbstractStabilizer...) # TODO optimize by pre-allocating one large tableau instead of the current quadratic fold
+    ct = promote_type(map(typeof, ops)...)
+    conv_ops = map(x -> convert(ct, x), ops)
+    return foldl(⊗, conv_ops)
 end
 
 """Repeated tensor product of an operators or a tableau.

diff --git a/test/test_stabs.jl b/test/test_stabs.jl
@@ -55,6 +55,12 @@
             stabs = [s[1:i] for s in [random_stabilizer(n) for n in [32,16,16,64,63,65,129,128,127]] for i in rand(1:10)];
             mdstabs = MixedDestabilizer.(stabs);
             @test canonicalize!(⊗(stabs...)) == canonicalize!(stabilizerview(⊗(mdstabs...)))
+            md = MixedDestabilizer(random_destabilizer(n))
+            s = random_stabilizer(n)
+            mds = md⊗s
+            @test mixed_destab_looks_good(mds)
+            estab = stabilizerview(md)⊗s
+            @test canonicalize!(copy(stabilizerview(mds))) == canonicalize!(estab)
         end
     end