improve algebra and show implementation for (Abstract)SymmetryVectors

src/show.jl

@@ -540,9 +540,12 @@ end
 function Base.show(io :: IO, n :: SymmetryVector)
     print(io, "[")
     for (i, (mults_k, lgirs_k)) in enumerate(zip(n.multsv, n.lgirsv))
-        printstyled(io, 
-                    Crystalline.symvec2string(mults_k, label.(lgirs_k); braces=false);
-                    color=iseven(i) ? :normal : :light_blue)
+        str = if !iszero(mults_k)
+            Crystalline.symvec2string(mults_k, label.(lgirs_k); braces=false)
+        else # if there are no occupied irreps at the considered k-point print "0kᵢ"
+            "0" * klabel(first(lgirs_k)) * "ᵢ"
+        end
+        printstyled(io, str; color=iseven(i) ? :normal : :light_blue)
         i ≠ length(n.multsv) && print(io, ", ")
     end
     print(io, "]")

src/types_symmetryvectors.jl

@@ -211,9 +211,14 @@ Base.:-(n::AbstractSymmetryVector{D}, m::AbstractSymmetryVector{D}) where D = n
 function Base.:*(n::AbstractSymmetryVector{D}, k::Integer) where D
     SymmetryVector{D}(irreps(n), [ms .* k for ms in multiplicities(n)], occupation(n) * k)
 end
+Base.:*(k::Integer, n::AbstractSymmetryVector) = n * k
 function Base.zero(n::AbstractSymmetryVector{D}) where D
     SymmetryVector{D}(irreps(n), zero.(multiplicities(n)), 0)
 end
+# make sure `sum(::AbstractSymmetryVector)` is type-stable (necessary since the + operation
+# now may change the type of an `AbstractSymmetryVector` - so `n[1]` and `n[1]+n[2]` may
+# be of different types) and always returns a `SymmetryVector`
+Base.reduce_first(::typeof(+), n::AbstractSymmetryVector) = SymmetryVector(n)
 
 # ::: Utilities & misc :::
 dim(::AbstractSymmetryVector{D}) where D = D

test/runtests.jl

@@ -15,6 +15,9 @@ using Crystalline, Test
     include("kvecs.jl")
     include("wyckoff.jl")
 
+    # symmetry vectors
+    include("symmetryvectors.jl")
+
     # show, notation, and cached info
     include("show.jl")
     include("notation.jl")

test/symmetryvectors.jl

@@ -0,0 +1,25 @@
+using Test
+using Crystalline
+
+@testset "(Abstract)SymmetryVectors" begin
+    brs = calc_bandreps(221) # ::Collection{NewBandRep{3}}
+
+    # addition of symmetry vectors
+    @test brs[1] + brs[2] == SymmetryVector(brs[1]) + SymmetryVector(brs[2])
+    @test Vector(brs[1] + brs[2]) == Vector(brs[1]) + Vector(brs[2])
+
+    # multiplication of symmetry vectors
+    @test brs[1] + brs[1] == 2brs[1]
+    @test -brs[1] == 2brs[1] - 3brs[1]
+    @test zero(brs[1]) == 0*brs[1]
+    @test brs[3]*7 == 7*brs[3]
+
+    # type-stable summation
+    @test sum(brs[1:1]) isa SymmetryVector{3}
+    @test sum(brs[1:2]) isa SymmetryVector{3}
+
+    # printing SymmetryVector that have zero-contents
+    n = SymmetryVector(brs[1])
+    @test n*0 == zero(n)
+    @test string(n) == "[0Mᵢ, 0Xᵢ, 0Γᵢ, 0Rᵢ] (0 bands)"
+end # @testset "(Abstract)SymmetryVectors"