Fix show for empty tensors (#329)

* avoid divide by zero for showing empty tensors

* add test

Author: lkdvos

src/tensors/blockiterator.jl

@@ -50,7 +50,7 @@ function show_blocks(io, mime::MIME"text/plain", iter; maytruncate::Bool = true)
         numlinesleft, numcols = get(io, :displaysize, displaysize(io))::Tuple{Int, Int}
         numlinesleft -= 2 # lines of headers have already been subtracted, but not the 2 spare lines for old and new prompts
         minlinesperblock = 7 # aim to have at least this many lines per printed block (= 5 lines for the actual matrix)
-        minnumberofblocks = min(3, length(iter)) # aim to show at least this many blocks
+        minnumberofblocks = clamp(length(iter), 1, 3) # aim to show at least this many blocks
         truncateblocks = sum(cb -> min(size(cb[2], 1) + 2, minlinesperblock), iter; init = 0) > numlinesleft
         maxnumlinesperblock = max(div(numlinesleft - 2 * truncateblocks, minnumberofblocks), minlinesperblock)
         # aim to show at least minnumberofblocks, but not if this means that there would be less than minlinesperblock

test/tensors/tensors.jl

@@ -629,8 +629,9 @@ end
     for V in (ℂ^2, Z2Space(0 => 2, 1 => 2), SU2Space(0 => 2, 1 => 2))
         t1 = ones(Float32, V ⊗ V, V)
         t2 = randn(ComplexF64, V ⊗ V ⊗ V)
+        t3 = randn(Float64, zero(V), zero(V))
         # test unlimited output
-        for t in (t1, t2, t1', t2')
+        for t in (t1, t2, t1', t2', t3)
             output = IOBuffer()
             summary(output, t)
             print(output, ":\n codomain: ")