# HG changeset patch
# User Rik <rik@octave.org>
# Date 1629218812 25200
# Node ID e3e0193963ea74aee98eb6c904541e0eff0601e3
# Parent  ac5e1b64f8c98fd391a73e8b9a0e671722a68dd9
Improve performance of filter along dimensions > 1 (bug #60928).

Replacing code pattern using while loop to calculate offset
with division and multiplication leads to 100X improvement
in filtering along second dimension.

* filter.cc (filter (b, a, x, si, dim)): Replace while loop to
calculate offset with division and multiplication.

diff -r ac5e1b64f8c9 -r e3e0193963ea libinterp/corefcn/filter.cc
--- a/libinterp/corefcn/filter.cc	Tue Aug 17 09:12:20 2021 -0700
+++ b/libinterp/corefcn/filter.cc	Tue Aug 17 09:46:52 2021 -0700
@@ -120,14 +120,9 @@
         x_offset = num * x_len;
       else
         {
-          octave_idx_type x_offset2 = 0;
           x_offset = num;
-          while (x_offset >= x_stride)
-            {
-              x_offset -= x_stride;
-              x_offset2++;
-            }
-          x_offset += x_offset2 * x_stride * x_len;
+          octave_idx_type n_strides = num / x_stride;
+          x_offset += n_strides * x_stride * (x_len - 1);
         }
       octave_idx_type si_offset = num * si_len;