/**
 * Combat rules — the pure arithmetic shared by the sandbox's actors
 * (the player's armor, the enemy craft, and whatever system ports out of
 * the range into the real game). No Phaser, no scene, so it runs in bare
 * Node: dev/combat.test.mjs exercises exactly this file.
 *
 * An armor block is `{ shields, hull }` (both ≥ 0): a hit spends the
 * shields first, then the hull, and the craft is dead at hull ≤ 0 — the
 * same rule for the player (data/combat.json → playerShip) and the
 * hostiles (→ enemy).
 */

/**
 * Spend damage off an armor block (mutates it).
 *
 * @param {{shields: number, hull: number}} armor the block to hit
 * @param {number} amount damage (negative/NaN treated as 0)
 * @returns {{shieldHit: number, hullHit: number, dead: boolean}}
 *   shieldHit/hullHit = how much each layer absorbed; dead = hull ≤ 0.
 */
export function applyDamage(armor, amount) {
  let rem = Math.max(0, Number(amount) || 0);

  const shields = Math.max(0, armor.shields ?? 0);
  const shieldHit = Math.min(shields, rem);
  rem -= shieldHit;

  const hull = Math.max(0, armor.hull ?? 0);
  const hullHit = Math.min(hull, rem);
  rem -= hullHit;

  armor.shields = shields - shieldHit;
  armor.hull = hull - hullHit;
  return { shieldHit, hullHit, dead: armor.hull <= 0 };
}

/**
 * One step of the combat ship's FLIGHT FEEL (the range's WASD handling —
 * js/combat/CombatScene.js): a smoothed turn rate, a ramped throttle,
 * and an instant brake. Pure — no Phaser — so it runs in bare Node
 * (dev/combat.test.mjs) and the scene just reads the result onto the
 * arcade body each frame.
 *
 * The feel state `st` is: `{ rot, turnVel, throttle, brake }`
 *   rot      — the render heading (rad); the scene assigns it to the ship
 *   turnVel  — the current turn velocity (rad/s), eased toward ±rotSpeed
 *   throttle — 0..1, eased toward 1 while `up`, toward 0 when released;
 *              the scene scales the weapon-free thrust by it
 *   brake    — the instant reverse-throttle fraction while `down` (0..1)
 *
 * `input` is the boolean key state `{ left, right, up, down }`.
 * `cfg` (data/combat.json → player): rotSpeed (rad/s sustained turn),
 * rotApproach (1/s, how fast turnVel settles — smoothing), thrustRamp
 * (1/s, throttle-up responsiveness), thrustDecay (1/s, throttle release),
 * brake (fraction of FULL thrust, instant while down).
 * Exponential approaches ⇒ frame-rate independent, no overshoot.
 *
 * @param {{rot?: number, turnVel?: number, throttle?: number, brake?: number}} st
 * @param {{left?: boolean, right?: boolean, up?: boolean, down?: boolean}} input
 * @param {number} dt seconds (clamped to ≤ 0.1 — a backgrounded tab can't fast-forward the feel)
 * @param {object} cfg
 * @returns the same `st` object, mutated
 */
export function handleFeel(st, input, dt, cfg) {
  const d = Math.min(Math.max(Number(dt) || 0, 0), 0.1);
  const cfg2 = cfg ?? {};

  // Turn: the turn VELOCITY eases toward ±rotSpeed (a smoothed gyroscope —
  // the first key-press builds turn, so a tap is a nudge and a hold is
  // a full-rate spin). v(t) = T − (T − v₀)·e^(−r·t), and rot integrates
  // that EXACTLY (∫₀ᵈ v dt) — so the feel is frame-rate independent by
  // construction (two half-steps ≡ one full step, no Euler drift).
  const dir = (input.right ? 1 : 0) - (input.left ? 1 : 0);
  const T = dir * Math.max(0, Number(cfg2.rotSpeed) || 0);
  const r = Math.max(0.1, Number(cfg2.rotApproach) || 12);
  const v0 = st.turnVel || 0;
  const q = Math.exp(-r * d);
  const v1 = T - (T - v0) * q;
  st.rot = (st.rot || 0) + (T * d - (T - v0) * (1 - q) / r);
  st.turnVel = v1;
  if (dir === 0 && Math.abs(st.turnVel) < 1e-4) st.turnVel = 0;

  // Throttle: ramps up toward 1 while `up`; cuts quickly when released
  // (no lingering kick after the key lets go).
  const want = input.up ? 1 : 0;
  const rate = want
    ? Math.max(0.1, Number(cfg2.thrustRamp) || 10)
    : Math.max(0.1, Number(cfg2.thrustDecay) || 16);
  st.throttle = want - (want - (st.throttle || 0)) * Math.exp(-rate * d);
  if (want === 0 && st.throttle < 1e-3) st.throttle = 0;

  // Brake: an INSTANT reverse throttle (S/Down) as a fraction of FULL
  // thrust — a responsive stop, independent of the ramp.
  st.brake = input.down ? Math.max(0, Number(cfg2.brake) || 0) : 0;
  return st;
}

/**
 * The render heading (rad) of a ship sprite: its rotation minus the
 * art's facing offset (ship.artOffset — the sheet art may not face east,
 * js/entities/Ship.js). A plain object with a .rotation is fine.
 */
export function headingOf(ship) {
  return ship.rotation - (ship.artOffset || 0);
}

/**
 * One step of the standoff-ring AI: advance the craft's orbit angle and
 * return the point on the ring it should steer to (pure — the craft
 * does the steering, js/combat/EnemyShip.js).
 *
 * @returns {{angle: number, x: number, y: number}} the new orbit angle + target
 */
export function orbitStep(px, py, orbitAngle, orbitDir, orbitSpeed, dt, orbitRadius) {
  const angle = orbitAngle + orbitDir * orbitSpeed * dt;
  return {
    angle,
    x: px + Math.cos(angle) * orbitRadius,
    y: py + Math.sin(angle) * orbitRadius,
  };
}