Check if position is already occupied (#758)

* Update grid_single.jl

* Update grid_single.jl

* improve error message

* Change tense in error message to past

* Update src/spaces/grid_single.jl

Co-authored-by: Martin Smit <57063134+jacobusmmsmit@users.noreply.github.com>

* add test

---------

Co-authored-by: Martin Smit <57063134+jacobusmmsmit@users.noreply.github.com>

Author: Tortar

src/spaces/grid_single.jl

@@ -37,9 +37,12 @@ function GridSpaceSingle(d::NTuple{D,Int}; periodic = true, metric = :chebyshev)
 end
 # Implementation of space API
 function add_agent_to_space!(a::A, model::ABM{<:GridSpaceSingle,A}) where {A<:AbstractAgent}
-    model.space.stored_ids[a.pos...] = a.id
+    pos = a.pos
+    !isempty(pos, model) && error("Cannot add agent $(a) to occupied position $(pos)")
+    model.space.stored_ids[pos...] = a.id
     return a
 end
+
 function remove_agent_from_space!(a::A, model::ABM{<:GridSpaceSingle,A}) where {A<:AbstractAgent}
     model.space.stored_ids[a.pos...] = 0
     return a

test/grid_space_tests.jl

@@ -37,6 +37,9 @@ using StableRNGs
         agent = model[3]
         move_agent_single!(agent, model)
         @test agent.pos == posx
+        if SpaceType == GridSpaceSingle
+            @test_throws ErrorException add_agent!(posx, model)
+        end
     end
 
     @testset "positions + empty" begin