[gtk: 1/2] gsk: Fix angle normalization

[Matthias Clasen <matthiasc src gnome org>]

commit 1f0438e7fe7a6580116d95a922948ee63c7547b4
Author: Alexander Larsson <alexl redhat com>
Date:   Tue Feb 11 10:00:34 2020 +0100

    gsk: Fix angle normalization
    
    I was getting assertions that normalize_angle() failed the
    result < 260 check. Doing some research on this it turns out
    to be a precision issue. If the incomming angle is very slightly
    below zero, then adding 360 to it may end up with exactly 360.
    
    I simplified the code a bit to avoid division and rounding, because in
    practice most angles will be "just outside" the 0-360 degree anyway.
    And i also added a workaround for the "result is 360" case by just
    setting it to 0.

 gsk/gsktransform.c | 22 +++++++++++++++-------
 1 file changed, 15 insertions(+), 7 deletions(-)
---
diff --git a/gsk/gsktransform.c b/gsk/gsktransform.c
index b474ec4114..c8e0d321b7 100644
--- a/gsk/gsktransform.c
+++ b/gsk/gsktransform.c
@@ -821,20 +821,28 @@ static const GskTransformClass GSK_ROTATE_TRANSFORM_CLASS =
 static inline float
 normalize_angle (float angle)
 {
-  float f;
 
   if (angle >= 0 && angle < 360)
     return angle;
 
-  f = angle - (360 * ((int)(angle / 360.0)));
+  while (angle >= 360)
+    angle -= 360;
+  while (angle < 0)
+    angle += 360;
 
-  if (f < 0)
-    f = 360 + f;
+  /* Due to precision issues we may end up with a result that is just
+   * past the allowed range when rounded. For example, something like
+   * -epsilon + 360 when rounded to a float may end up with 360.
+   * So, we handle these cases by returning the exact value 0.
+   */
+
+  if (angle >= 360)
+    angle = 0;
 
-  g_assert (f < 360.0);
-  g_assert (f >= 0.0);
+  g_assert (angle < 360.0);
+  g_assert (angle >= 0.0);
 
-  return f;
+  return angle;
 }
 
 /**