fable/val Public Public: anyone can read this; nobody can change it.

/**

 * Where this person is looking.

 *

 * The view angle is the browser's to move: the mouse arrives at frame rate,

 * and a snapshot is a description of a moment already gone. The client used

 * to take `snap.me.angle` on every snapshot, which dragged the camera back

 * to a stale angle thirty times a second — worst exactly when you turned

 * fastest, because that is when the stale angle is furthest away.

 *

 * So the local angle is the camera's, and the server's angle is taken only

 * when it is telling us something we did not say: there is no view yet, the

 * body under us changed, we respawned, or the angle that came back is one we

 * never sent. That last one is what the sent log is for — the server echoes

 * our own angle back, rounded, so an angle matching nothing we sent in the

 * last second came from the match (a round start recentring everybody at the

 * site) rather than from us, and is worth taking.

 *

 * Nothing here is sent anywhere: the angle still leaves through the input

 * timer exactly as it always did.

 */

/** How long a sent angle is remembered, in seconds. */

export const SENT_MEMORY = 1.2;

/** How close an echo must be to count as ours. The server rounds to 0.1. */

export const SENT_TOLERANCE = 0.08;

/** `corrected` counts the times the server's angle replaced ours: every one is a jump of the view. */

export const makeYaw = (angle = 0) => ({ angle, id: "", alive: false, ready: false, sent: [], corrected: 0 });

/** The short way round from one angle to another, in (-π, π]. */

export const shortWay = (from, to) => ((to - from + Math.PI * 3) % (Math.PI * 2)) - Math.PI;

/** A mouse delta, or an arrow key's share of a second. Ours, always taken. */

export function turn(yaw, delta) {

  yaw.angle += delta;

  return yaw.angle;

}

/** Point it somewhere outright: the top-down view's pointer aim. */

export function aim(yaw, angle) {

  yaw.angle = angle;

  return yaw.angle;

}

/** Remember an angle we told the server about, at a time in seconds. */

export function told(yaw, angle, at) {

  yaw.sent.push({ at, angle });

  while (yaw.sent.length && yaw.sent[0].at < at - SENT_MEMORY) yaw.sent.shift();

  return yaw;

}

/** Is this angle one we sent recently? An echo, not a correction. */

const ours = (yaw, angle, at) =>

  yaw.sent.some((s) => s.at >= at - SENT_MEMORY && Math.abs(shortWay(s.angle, angle)) <= SENT_TOLERANCE);

/**

 * A snapshot landed. Returns the angle to look along, having decided whether

 * the server's word replaces ours.

 *

 * `me` is the snapshot's own body: `{ id, angle, alive }`. `at` is now, in

 * seconds. An empty sent log is not evidence of anything — a client that has

 * not spoken yet keeps what it has.

 */

export function settle(yaw, me, at) {

  const seatChanged = !yaw.ready || me.id !== yaw.id;

  const respawned = me.alive && !yaw.alive;

  const corrected = yaw.sent.length > 0 && !ours(yaw, me.angle, at);

  if (seatChanged || respawned || corrected) {

    if (yaw.ready) yaw.corrected += 1;

    yaw.angle = me.angle;

    yaw.sent.length = 0;

  }

  yaw.id = me.id;

  yaw.alive = me.alive;

  yaw.ready = true;

  return yaw.angle;

}