add non-allocating enumerate_paths!

Author: rafaqz

src/shortestpaths/bellman-ford.jl

@@ -116,13 +116,41 @@ to all other vertices. In addition, `enumerate_paths(state, v, d)` will return
 a vector representing the path from vertex `v` to vertex `d`.
 """
 function enumerate_paths(state::AbstractPathState, vs::AbstractVector{<:Integer})
-    parents = state.parents
-    T = eltype(parents)
+    T = eltype(state.parents)
+    all_paths = Vector{T}[Vector{eltype(state.parents)}() for _ in 1:length(vs)]
+    enumerate_paths!(all_paths, state, vs)
+end
+
+enumerate_paths(state::AbstractPathState, v::Integer) = enumerate_paths(state, v:v)[1]
+function enumerate_paths(state::AbstractPathState)
+    return enumerate_paths(state, 1:length(state.parents))
+end
+
+"""
+    enumerate_paths!(paths::AbstractVector{<:AbstractVector}, state, vs::AbstractVector{Int})
+
+In-place version of [`enumerate_paths`](@ref).
+
+`paths` must be a `Vector{Vectors{eltype(state.parents)}}`
 
+`enumerate_paths!` should be more efficient when used in a loop,
+as the same memory can be used for each iteration.
+"""
+function enumerate_paths!(
+    all_paths::AbstractVector{<:AbstractVector},
+    state::AbstractPathState, 
+    vs::AbstractVector{<:Integer}
+)
+    Base.require_one_based_indexing(all_paths)
+    Base.require_one_based_indexing(vs)
+    @assert length(all_paths) == length(vs)
+
+    parents = state.parents
+    T = eltype(state.parents)
     num_vs = length(vs)
-    all_paths = Vector{Vector{T}}(undef, num_vs)
+
     for i in 1:num_vs
-        all_paths[i] = Vector{T}()
+        empty!(all_paths[i])
         index = T(vs[i])
         if parents[index] != 0 || parents[index] == index
             while parents[index] != 0
@@ -134,9 +162,4 @@ function enumerate_paths(state::AbstractPathState, vs::AbstractVector{<:Integer}
         end
     end
     return all_paths
-end
-
-enumerate_paths(state::AbstractPathState, v::Integer) = enumerate_paths(state, [v])[1]
-function enumerate_paths(state::AbstractPathState)
-    return enumerate_paths(state, [1:length(state.parents);])
-end
+end