NFC: style changes per review

shorter lines, more spaces, and comments (because I had already forgotten how this worked myself)

base/multidimensional.jl

@@ -487,13 +487,21 @@ module IteratorsMD
  return I, (I, n+1)
  end
 
- simd_outer_range(iter::CartesianPartition{CartesianIndex{2}}) = @inbounds CartesianIndices((iter[1][2]:iter[end][2],))
  @inline function simd_outer_range(iter::CartesianPartition)
+ # In general, the Cartesian Partition might start and stop in the middle of the outer
+ # dimensions — thus the outer range of a CartesianPartition is itself a
+ # CartesianPartition.
  t = tail(iter.parent.parent.indices)
  ci = CartesianIndices(t)
  li = LinearIndices(t)
  return @inbounds view(ci, li[tail(iter[1].I)...]:li[tail(iter[end].I)...])
  end
+ function simd_outer_range(iter::CartesianPartition{CartesianIndex{2}})
+ # But for two-dimensional Partitions the above is just a simple one-dimensional range
+ # over the second dimension; we don't need to worry about non-rectangular staggers in
+ # higher dimensions.
+ return @inbounds CartesianIndices((iter[1][2]:iter[end][2],))
+ end
  @inline function simd_inner_length(iter::CartesianPartition, I::CartesianIndex)
  inner = iter.parent.parent.indices[1]
  @inbounds fi = iter[1].I
@@ -503,8 +511,12 @@ module IteratorsMD
  return inner_end - inner_start + 1
  end
  @inline function simd_index(iter::CartesianPartition, Ilast::CartesianIndex, I1::Int)
+ # I1 is the 0-based distance from the first dimension's offest
+ offset = first(iter.parent.parent.indices[1]) # (this is 1 for 1-based arrays)
+ # In the first column we need to also add in the iter's starting point (branchlessly)
  f = @inbounds iter[1]
- CartesianIndex((I1+first(iter.parent.parent.indices[1])+(Ilast.I==tail(f.I))*(f[1]-1), Ilast.I...))
+ startoffset = (Ilast.I == tail(f.I))*(f[1] - 1)
+ CartesianIndex((I1 + offset + startoffset, Ilast.I...))
  end
 end # IteratorsMD