/* behavior.js — deterministic ghost motion so all viewers see the same movement.
 * Motion is a pure function of (server spawn record, wall-clock time): no per-client
 * randomness, so phones stay in sync without streaming positions.
 */
import * as THREE from 'three';

function hashNoise(seed, k) {
  // cheap deterministic pseudo-noise in [-1, 1]
  const x = Math.sin(seed * 127.1 + k * 311.7) * 43758.5453;
  return (x - Math.floor(x)) * 2 - 1;
}

export function ghostTransform(rec, nowMs, out) {
  const t = (nowMs - rec.spawnedAt) / 1000;
  const base = new THREE.Vector3(...rec.pos);
  const b = rec.behavior || { type: 'static' };

  if (b.type === 'wander') {
    const s = b.seed || 1;
    const R = b.radius ?? 0.6;
    const v = b.speed ?? 0.15;
    // smooth pseudo-random orbit-drift: two incommensurate sines per axis
    const ph = t * v;
    out.position.set(
      base.x + R * 0.9 * Math.sin(ph * 1.0 + hashNoise(s, 1) * 6.28) * 0.7
             + R * 0.4 * Math.sin(ph * 2.3 + hashNoise(s, 2) * 6.28) * 0.3,
      base.y + 0.06 * Math.sin(t * 1.7 + hashNoise(s, 3) * 6.28),
      base.z + R * 0.9 * Math.cos(ph * 0.8 + hashNoise(s, 4) * 6.28) * 0.7
             + R * 0.4 * Math.cos(ph * 1.9 + hashNoise(s, 5) * 6.28) * 0.3
    );
    // face travel direction (finite difference)
    const eps = 0.05;
    const ahead = (t2) => new THREE.Vector3(
      base.x + R * 0.9 * Math.sin(t2 * v + hashNoise(s, 1) * 6.28) * 0.7,
      0,
      base.z + R * 0.9 * Math.cos(t2 * v * 0.8 + hashNoise(s, 4) * 6.28) * 0.7);
    const dir = ahead(t + eps).sub(ahead(t));
    out.rotationY = Math.atan2(dir.x, dir.z);
  } else {
    const amp = b.bobAmp ?? 0.06;
    const hz = b.bobHz ?? 0.4;
    out.position.set(base.x, base.y + amp * Math.sin(t * hz * Math.PI * 2), base.z);
    out.rotationY = 0.25 * Math.sin(t * 0.3); // slow idle sway
  }

  // crossfade opacity: fade in on spawn, fade out approaching `until`
  const fade = (rec.crossfade || 3) * 1000;
  const inA = Math.min(1, (nowMs - rec.spawnedAt) / fade);
  const outA = Math.min(1, Math.max(0, (rec.until - nowMs) / fade));
  out.opacity = Math.min(inA, outA);
  return out;
}