Module aim 

One reading of pointer intent for the whole system: is the pointer passing over something, or aiming at it?

Passing over a control is not hovering it. Only aim counts: the pointer slowing and settling on a target, or decelerating so that it will stop inside one. Every surface that reacts to the pointer before a click (a tab pill opening its chips, a maximized window’s bar revealed at the top edge, a desk widget brought by reaching for its edge, a banner that must not expire under the pointer) reads intent from this one Trajectory, so they agree about what a hand movement means and are tuned in one place.

• settled: slow at the newest sample (SETTLE_SPEED), or silent for SETTLE since it (a moving pointer reports far more often, so silence means it stopped);
• settling into a target: settled inside it, or decelerating so that, at constant deceleration, it stops inside it;
• heading for a target: inside the convex hull of a recent position (the apex) and the target, so a pointer moving away or sideways past it is not;
• in transit: fast and not braking, its speed still above a share of its recent peak;
• aiming at a target (Trajectory::aims_at): on it and settled or settling there. This is what “the pointer is on this control” means everywhere: an aimed-at control never moves, and something that pauses for the person (a banner’s timer) pauses only while aimed at.

Pure: every method takes the instant it reasons about, so slow and fast pointer paths are tested against a synthetic timeline.

Structs

Trajectory

A pointer’s recent path, newest last.

Constants

APEX_SAMPLES

How many samples back the apex of a heading cone sits, so a single jittery sample cannot point the cone the wrong way.

MAX_SAMPLES

Samples kept: enough to span a push to the screen edge (160 ms) at any sane report rate.

SETTLE

A moving pointer reports at least every 16 ms; one silent this long has stopped where it last was.

SETTLE_SPEED

At or below this speed (px/ms, 50 px/s) a pointer is settling where it is.

Functions

in_aim_hull

Whether p lies in the convex hull of apex and target: inside the target, or inside a triangle from the apex to one of its edges.