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filter: avoid division-by-zero when delta_time is zero in custom filter

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delta_time can be zero when:
- the fallback acceleration function is used for multiple movement types
  (for example, pointer motion and wheel scrolling simultaneously)
- two different methods produce the same movement at the same time
  (for example, button-scrolling and wheel-scrolling)

Reusing the last delta_time is a graceful fallback even if there are
duplicate events or event-ordering bugs.

Signed-off-by: Yinon Burgansky <yinonburgansky@gmail.com>
Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1336>
This commit is contained in:
Yinon Burgansky 2025-10-22 19:27:15 +03:00 • committed by Marge Bot
parent 94b7836456
commit cc7dfccd22
1 changed files with 18 additions and 4 deletions
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22
src/filter-custom.c
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@ -34,6 +34,7 @@
struct custom_accel_function {
uint64_t last_time;
uint64_t last_delta_time;
double step;
size_t npoints;
double points[];
@ -58,6 +59,7 @@ create_custom_accel_function(double step, const double *points, size_t npoints)
struct custom_accel_function *cf =
zalloc(sizeof(*cf) + npoints * sizeof(*points));
cf->last_time = 0;
cf->last_delta_time = FIRST_MOTION_TIME_INTERVAL;
cf->step = step;
cf->npoints = npoints;
memcpy(cf->points, points, sizeof(*points) * npoints);
@ -110,13 +112,25 @@ custom_accel_function_calculate_speed(struct custom_accel_function *cf,
/* calculate speed based on time passed since last event */
double distance = hypot(unaccelerated->x, unaccelerated->y);
/* delta_time can be zero when:
* - the fallback acceleration function is used for multiple movement types
* (for example, pointer motion and wheel scrolling simultaneously)
* - two different methods produce the same movement at the same time
* (for example, button-scrolling and wheel-scrolling)
*
* Reusing the last delta_time is a graceful fallback even if there are
* duplicate events or event-ordering bugs.
*/
uint64_t delta_time =
(time > cf->last_time) ? time - cf->last_time : cf->last_delta_time;
/* handle first event in a motion */
if (time - cf->last_time > MOTION_TIMEOUT)
cf->last_time = time - FIRST_MOTION_TIME_INTERVAL;
if (delta_time > MOTION_TIMEOUT)
delta_time = FIRST_MOTION_TIME_INTERVAL;
double dt = us2ms_f(time - cf->last_time);
double speed = distance / dt; /* speed is in device-units per ms */
/* speed is in device-units per ms */
double speed = distance / us2ms_f(delta_time);
cf->last_time = time;
cf->last_delta_time = delta_time;
return speed;
}