Tests for compressed StateSpace printing; fix D for 0-state systems

Move the D-matrix println back outside `if nstates(sys) > 0` so pure
feedthrough systems (state dimension 0) still show D. Add tests covering
the new compressed format for large A/B/C/D, the zero-D shortcut, and
the mixed case (large A with small B/C/D). Update the four existing
print test strings to drop the trailing blank line that the previous
`println(..., "\n")` produced.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: baggepinnen

lib/ControlSystemsBase/src/types/StateSpace.jl

@@ -573,9 +573,9 @@ function Base.show(io::IO, sys::AbstractStateSpace)
             println(io, "B = $(typeof(B))$(size(B))")
         length(C) < 200 ? println(io, "C = \n", _string_mat_with_headers(C)) :
             println(io, "C = $(typeof(C))$(size(C))")
-        length(D) < 200 ? println(io, "D = \n", _string_mat_with_headers(D)) :
-            println(io, "D = ", iszero(D) ? "  0" : "$(typeof(D))$(size(D))")
     end
+    length(D) < 200 ? println(io, "D = \n", _string_mat_with_headers(D)) :
+        println(io, "D = ", iszero(D) ? "  0" : "$(typeof(D))$(size(D))")
     # Print sample time
     if isdiscrete(sys)
         println(io, "Sample Time: ", sys.Ts, " (seconds)")

lib/ControlSystemsBase/test/test_statespace.jl

@@ -217,10 +217,24 @@
 
         # Printing
         if SS <: StateSpace
-            @test sprint(show, C_222) == "StateSpace{Continuous, Int64}\nA = \n -5  -3\n  2  -9\nB = \n 1  0\n 0  2\nC = \n 1  0\n 0  1\nD = \n 0  0\n 0  0\n\nContinuous-time state-space model"
-            @test sprint(show, C_022) == "StateSpace{Continuous, Float64}\nD = \n 4.0  0.0\n 0.0  4.0\n\nContinuous-time state-space model"
-            @test sprint(show, D_022) == "StateSpace{Discrete{Float64}, Float64}\nD = \n 4.0  0.0\n 0.0  4.0\n\nSample Time: 0.005 (seconds)\nDiscrete-time state-space model"
-            @test sprint(show, D_222) == "StateSpace{Discrete{Float64}, Float64}\nA = \n  0.2  -0.8\n -0.8   0.07\nB = \n 1.0  0.0\n 0.0  2.0\nC = \n 1.0  0.0\n 0.0  1.0\nD = \n 0.0  0.0\n 0.0  0.0\n\nSample Time: 0.005 (seconds)\nDiscrete-time state-space model"
+            @test sprint(show, C_222) == "StateSpace{Continuous, Int64}\nA = \n -5  -3\n  2  -9\nB = \n 1  0\n 0  2\nC = \n 1  0\n 0  1\nD = \n 0  0\n 0  0\nContinuous-time state-space model"
+            @test sprint(show, C_022) == "StateSpace{Continuous, Float64}\nD = \n 4.0  0.0\n 0.0  4.0\nContinuous-time state-space model"
+            @test sprint(show, D_022) == "StateSpace{Discrete{Float64}, Float64}\nD = \n 4.0  0.0\n 0.0  4.0\nSample Time: 0.005 (seconds)\nDiscrete-time state-space model"
+            @test sprint(show, D_222) == "StateSpace{Discrete{Float64}, Float64}\nA = \n  0.2  -0.8\n -0.8   0.07\nB = \n 1.0  0.0\n 0.0  2.0\nC = \n 1.0  0.0\n 0.0  1.0\nD = \n 0.0  0.0\n 0.0  0.0\nSample Time: 0.005 (seconds)\nDiscrete-time state-space model"
+
+            # Large systems use a compressed format
+            G_large_zeroD = ss(zeros(15,15), zeros(15,15), zeros(15,15), zeros(15,15))
+            @test sprint(show, G_large_zeroD) == "StateSpace{Continuous, Float64}\nA = Matrix{Float64}(15, 15)\nB = Matrix{Float64}(15, 15)\nC = Matrix{Float64}(15, 15)\nD =   0\nContinuous-time state-space model"
+
+            G_large_nonzeroD = ss(zeros(15,15), zeros(15,15), zeros(15,15), ones(15,15))
+            @test sprint(show, G_large_nonzeroD) == "StateSpace{Continuous, Float64}\nA = Matrix{Float64}(15, 15)\nB = Matrix{Float64}(15, 15)\nC = Matrix{Float64}(15, 15)\nD = Matrix{Float64}(15, 15)\nContinuous-time state-space model"
+
+            G_large_A = ss(zeros(15,15), zeros(15,1), zeros(1,15), 0)
+            out_large_A = sprint(show, G_large_A)
+            @test occursin("A = Matrix{Float64}(15, 15)", out_large_A)
+            @test occursin("B = \n", out_large_A)
+            @test occursin("C = \n", out_large_A)
+            @test occursin("D = \n", out_large_A)
         end
 
         G = ssrand(1,2,3)