Add copy method for SystemStructure

`SystemStructure` is mutable and mutated by various structure operations.
It helps debugging to be able to start from a clean copy every time,
so add an appropriate method to `Base.copy`.

Author: Keno

src/bipartite_graph.jl

@@ -59,8 +59,8 @@ end
 Base.size(m::Matching) = Base.size(m.match)
 Base.getindex(m::Matching, i::Integer) = m.match[i]
 Base.iterate(m::Matching, state...) = iterate(m.match, state...)
-function Base.copy(m::Matching)
-    Matching(copy(m.match), m.inv_match === nothing ? nothing : copy(m.inv_match))
+function Base.copy(m::Matching{U}) where {U}
+    Matching{U}(copy(m.match), m.inv_match === nothing ? nothing : copy(m.inv_match))
 end
 function Base.setindex!(m::Matching{U}, v::Union{Integer, U}, i::Integer) where {U}
     if m.inv_match !== nothing

src/systems/systemstructure.jl

@@ -37,12 +37,18 @@ struct DiffGraph <: Graphs.AbstractGraph{Int}
     primal_to_diff::Vector{Union{Int, Nothing}}
     diff_to_primal::Union{Nothing, Vector{Union{Int, Nothing}}}
 end
+
 DiffGraph(primal_to_diff::Vector{Union{Int, Nothing}}) = DiffGraph(primal_to_diff, nothing)
 function DiffGraph(n::Integer, with_badj::Bool = false)
     DiffGraph(Union{Int, Nothing}[nothing for _ in 1:n],
               with_badj ? Union{Int, Nothing}[nothing for _ in 1:n] : nothing)
 end
 
+function Base.copy(dg::DiffGraph)
+    DiffGraph(copy(dg.primal_to_diff),
+              dg.diff_to_primal === nothing ? nothing : copy(dg.diff_to_primal))
+end
+
 @noinline function require_complete(dg::DiffGraph)
     dg.diff_to_primal === nothing &&
         error("Not complete. Run `complete` first.")
@@ -145,6 +151,13 @@ Base.@kwdef mutable struct SystemStructure
     solvable_graph::Union{BipartiteGraph{Int, Nothing}, Nothing}
     only_discrete::Bool
 end
+
+function Base.copy(structure::SystemStructure)
+    SystemStructure(copy(structure.var_to_diff), copy(structure.eq_to_diff),
+                    copy(structure.graph), copy(structure.solvable_graph),
+                    structure.only_discrete)
+end
+
 is_only_discrete(s::SystemStructure) = s.only_discrete
 isdervar(s::SystemStructure, i) = invview(s.var_to_diff)[i] !== nothing
 function isalgvar(s::SystemStructure, i)