less push! in collect_with_eltype

Author: rafaqz

src/geometry_primitives.jl

@@ -74,15 +74,32 @@ collect_with_eltype(::Type{T}, vec::Vector{T}) where {T} = vec
 collect_with_eltype(::Type{T}, vec::AbstractVector{T}) where {T} = collect(vec)
 
 function collect_with_eltype(::Type{T}, iter) where {T}
-    # TODO we could be super smart about allocating the right length
-    # but its kinda annoying, since e.g. T == Triangle and first(iter) isa Quad
-    # will need double the length etc - but could all be figured out ;)
-    result = T[]
+    # We need to get `eltype` information from `iter`, it seems to be `Any`
+    # most of the time so the eltype checks here don't actually work
+    l = if Base.IteratorSize(iter) isa Union{Base.HasShape,Base.HasLength}
+        if Base.IteratorEltype(iter) isa Base.HasEltype && isconcretetype(eltype(iter))
+            # Work out the exact length 
+            length(convert_simplex(T, first(iter))) * length(iter)
+        else
+            # We know it is at least the length of iter,
+            # after that we will `push!` if we have to
+            length(iter)
+        end
+    else
+        0
+    end
+    n = 0
+    result = Vector{T}(undef, l)
     for element in iter
         # convert_simplex always returns a tuple,
         # so that e.g. convert(Triangle, quad) can return 2 elements
         for telement in convert_simplex(T, element)
-            push!(result, telement)
+            n += 1
+            if n > l
+                push!(result, telement)
+            else
+                result[n] = telement
+            end
         end
     end
     return result