reverse traversal of row entries if step(I) < 0 (JuliaLang#22544)

* reverse traversal of row entries if step(I) < 0

* add test

* Update sparse.jl

base/sparse/sparsematrix.jl

@@ -1851,6 +1851,9 @@ function getindex_cols(A::SparseMatrixCSC{Tv,Ti}, J::AbstractVector) where {Tv,T
  return SparseMatrixCSC(m, nJ, colptrS, rowvalS, nzvalS)
 end
 
+getindex_traverse_col(::AbstractUnitRange, lo::Int, hi::Int) = lo:hi
+getindex_traverse_col(I::StepRange, lo::Int, hi::Int) = step(I) > 0 ? (lo:1:hi) : (hi:-1:lo)
+
 function getindex(A::SparseMatrixCSC{Tv,Ti}, I::Range, J::AbstractVector) where {Tv,Ti<:Integer}
  # Ranges for indexing rows
  (m, n) = size(A)
@@ -1884,7 +1887,7 @@ function getindex(A::SparseMatrixCSC{Tv,Ti}, I::Range, J::AbstractVector) where
 
  @inbounds for j = 1:nJ
  col = J[j]
- for k = colptrA[col]:colptrA[col+1]-1
+ for k = getindex_traverse_col(I, colptrA[col], colptrA[col+1]-1)
  rowA = rowvalA[k]
  i = rangesearch(I, rowA)
  if i > 0

test/sparse/sparse.jl

@@ -1795,3 +1795,12 @@ end
  @test nnz(A) == n
  @test A == eye(n)
 end
+
+@testset "reverse search direction if step < 0 #21986" begin
+ srand(1234)
+ A = sprand(5, 5, 1/5)
+ A = max.(A, A')
+ A = spones(A)
+ B = A[5:-1:1, 5:-1:1]
+ @test issymmetric(B)
+end