Allow chain id to be assigned with chainid!()

Adds two function for assigning the chainid, chainid!(chain, new_id) and
chainid!(residue, new_id). For GitHub issue BioJulia#22.

src/model.jl

@@ -46,6 +46,7 @@ export
     resnames,
     chain,
     chainid,
+    chainid!,
     resids,
     residues,
     model,
@@ -127,7 +128,7 @@ A residue (amino acid) or other molecule - either a `Residue` or a
 abstract type AbstractResidue <: StructuralElement end
 
 "A residue (amino acid) or other molecule."
-struct Residue <: AbstractResidue
+mutable struct Residue <: AbstractResidue
     name::String
     number::Int
     ins_code::Char
@@ -147,7 +148,7 @@ struct DisorderedResidue <: AbstractResidue
 end
 
 "A chain (molecule) from a macromolecular structure."
-struct Chain <: StructuralElement
+mutable struct Chain <: StructuralElement
     id::String # mmCIF files can have multi-character chain IDs
     res_list::Vector{String}
     residues::Dict{String, AbstractResidue}
@@ -816,6 +817,48 @@ Get the chain ID of an `AbstractAtom`, `AbstractResidue` or `Chain` as a
 chainid(el::Union{AbstractResidue, AbstractAtom}) = chainid(chain(el))
 chainid(ch::Chain) = ch.id
 
+"""
+    chainid!(ch, id)
+
+Set the chain ID of an `Chain` to a new `String`.
+"""
+chainid!(ch::Chain, id::String) = ch.id = id
+
+"""
+    chainid!(res, id)
+
+Set the chain ID of an `AbstractResidue` to a new `String`.
+
+If a chain with this ID already exists, it will be removed from its current
+chain and added to that chain. If a chain with this ID does not exist, a new
+chain will be added to the model and this residue will be added to it. If
+moving this residue from a chain to a new chain renders the old chain without
+residues, the old chain will be removed from the `Model`.
+"""
+function chainid!(res::AbstractResidue, id::String)
+    current_chain = res.chain
+    model_chains = current_chain.model.chains
+
+    # find the currently-assigned resid, which may not have been created from the resid function
+    current_resid = first(filter(el -> el.second == res, current_chain.residues)).first
+
+    if id in keys(model_chains)
+        model_chains[id].residues[resid(res)] = res
+    else
+        model_chains[id] = Chain(id, [], Dict(current_resid => res), current_chain.model)
+    end
+    res.chain = model_chains[id]
+
+    # remove the residue from its current chain
+    delete!(current_chain.residues, current_resid)
+    if isempty(current_chain.residues)
+        delete!(model_chains, current_chain.id)
+    end
+
+    fixlists!(structure(res))
+end
+
+
 """
     resids(ch)
 
@@ -1522,7 +1565,7 @@ fullresname(res::Residue) = res.name
 function fixlists!(struc::ProteinStructure)
     for mod in struc
         for ch in mod
-            append!(ch.res_list, resid.(sort(collect(values(residues(ch))))))
+            ch.res_list = resid.(sort(collect(values(residues(ch)))))
             for res in ch
                 if isa(res, Residue)
                     fixlists!(res)
@@ -1537,7 +1580,7 @@ function fixlists!(struc::ProteinStructure)
 end
 
 function fixlists!(res::Residue)
-    append!(res.atom_list, fullatomname.(sort(collect(values(atoms(res))))))
+    res.atom_list = fullatomname.(sort(collect(values(atoms(res)))))
 end
 
 fullatomname(at::Atom) = at.name

test/runtests.jl

@@ -415,6 +415,33 @@ end
     @test chainid(dis_res) == "A"
     @test chainid(ch) == "A"
 
+    # test modifying the chain ids
+    chainid!(ch, "C")
+    @test chainid(at) == "C"
+    @test chainid(dis_at) == "C"
+    @test chainid(res) == "C"
+    @test chainid(dis_res) == "C"
+    @test chainid(ch) == "C"
+
+    @test chainids(mod) == ["B", "C"]
+    chainid!(ch, "A")
+
+    # move one of the residues to a new chain and StructuralElements below it
+    # should identify on the new chain
+    chainid!(res, "C")
+    @test chainid(res) == "C"
+    @test chainid(at) == "C"
+    @test chainid(dis_at) == "C"
+    @test chainid(dis_res) == "A"
+    @test chainid(ch) == "A"
+
+    @test chainids(mod) == ["A", "B", "C"]
+
+    # Emptying a chain of residues by moving its residues deletes the chain
+    chainid!(res, "A")
+
+    @test chainids(mod) == ["A", "B"]
+
     @test resids(ch) == ["10", "H_20A"]
 
     @test isa(residues(ch), Dict{String, AbstractResidue})