[gegl/samplers] lohalo: Knuth's floored division by power of 2 implemented with right shift (assumes two's complemen

[Nicolas Robidoux <nrobidoux src gnome org>]

commit 48052c3c8bf6a6bca6de403919b1ca7fe574ea7a
Author: Nicolas Robidoux <nicolas robidoux gmail com>
Date:   Tue Jun 28 15:46:14 2011 -0400

    lohalo: Knuth's floored division by power of 2 implemented with right shift (assumes two's complement arch.)

 gegl/buffer/gegl-sampler-lohalo.c |   10 +++++++++-
 1 files changed, 9 insertions(+), 1 deletions(-)
---
diff --git a/gegl/buffer/gegl-sampler-lohalo.c b/gegl/buffer/gegl-sampler-lohalo.c
index 3fa6c79..5d11d0d 100644
--- a/gegl/buffer/gegl-sampler-lohalo.c
+++ b/gegl/buffer/gegl-sampler-lohalo.c
@@ -187,8 +187,16 @@
  * Special case of of Knuth's floored division, that is:
  *
  * FLOORED_DIVISION(a,b) (((a) - ((a)<0 ? (b)-1 : 0)) / (b))
+ *
+ * When b is 2, this gives, for example,
+ *
+ * FLOORED_DIVISION_BY_2(a) (((a) - ((a)>=0 ? 0 : 1)) / 2)
+ *
+ * On a two's complement machine, this is even simpler:
+ *
+ * FLOORED_DIVISION_BY_2(a) ( (a)>>1 )
  */
-#define LOHALO_FLOORED_DIVISION_BY_2(a) (((a) - ((a)>=0 ? 0 : 1)) / 2)
+#define LOHALO_FLOORED_DIVISION_BY_2(a) ( (a)>>1 )
 
 /*
  * General convention: Looking at the image as a (linear algebra)