/* 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 === 'path') {
    pathTransform(b, t, out);
  } else if (b.type === 'wander') {
    const s = b.seed || 1;
    const R = b.radius ?? 60;      // cm
    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 + (b.vertical ?? 10) * Math.sin(t * v * 1.3 + hashNoise(s, 3) * 6.28)
             + 3 * Math.sin(t * 1.7 + hashNoise(s, 6) * 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 ?? 6;     // cm
    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; permanent residents (until == null) never fade out
  const fade = (rec.crossfade || 3) * 1000;
  const inA = Math.min(1, (nowMs - rec.spawnedAt) / fade);
  const outA = rec.until == null ? 1 : Math.min(1, Math.max(0, (rec.until - nowMs) / fade));
  out.opacity = Math.min(inA, outA);
  return out;
}

/* Walk a waypoint path at constant speed (cm/s), turning to face travel direction.
 * Deterministic: position is a pure function of elapsed time, so all viewers agree.
 * b: { points:[[x,y,z],...], mode:'loop'|'pingpong', speed, phase, seed } */
function pathTransform(b, t, out) {
  const raw = b.points || [];
  if (raw.length < 2) { out.position.set(...(raw[0] || [0, 0, 0])); out.rotationY = 0; return; }
  const pts = b.mode === 'loop' ? [...raw, raw[0]] : raw;

  // cumulative segment lengths
  const cum = [0];
  for (let i = 1; i < pts.length; i++) {
    const dx = pts[i][0] - pts[i - 1][0], dy = pts[i][1] - pts[i - 1][1], dz = pts[i][2] - pts[i - 1][2];
    cum.push(cum[i - 1] + Math.hypot(dx, dy, dz));
  }
  const total = cum[cum.length - 1] || 1;

  const wrap = (d) => {
    if (b.mode === 'pingpong') { const m = ((d % (2 * total)) + 2 * total) % (2 * total); return m < total ? m : 2 * total - m; }
    return ((d % total) + total) % total;
  };
  const sample = (d, v) => {
    let i = 1; while (i < cum.length - 1 && cum[i] < d) i++;
    const f = (d - cum[i - 1]) / Math.max(cum[i] - cum[i - 1], 1e-6);
    v.set(
      pts[i - 1][0] + (pts[i][0] - pts[i - 1][0]) * f,
      pts[i - 1][1] + (pts[i][1] - pts[i - 1][1]) * f,
      pts[i - 1][2] + (pts[i][2] - pts[i - 1][2]) * f);
    return v;
  };

  const d = wrap(t * (b.speed ?? 8) + (b.phase || 0));
  sample(d, out.position);
  // face where we're heading: sample a few cm ahead so corners turn smoothly
  const ahead = sample(wrap(t * (b.speed ?? 8) + (b.phase || 0) + 6), _look);
  const dx = ahead.x - out.position.x, dz = ahead.z - out.position.z;
  if (dx * dx + dz * dz > 1e-4) out.rotationY = Math.atan2(dx, dz);
  // gentle float so walking still reads ghostly
  out.position.y += 2 * Math.sin(t * 1.9 + (b.seed || 0));
}
const _look = new THREE.Vector3();