/** The aquarium's pets, and how they swim.
 *
 * A pet moves in when a world is finished and then stays - not in a list on the home
 * screen, but swimming around behind every screen of the game. A reward that is always
 * in view, drifting past while she types, is the strongest version of "the reward
 * persists", and it costs nothing to look at.
 *
 * Pets are illustrations, speed trophies are emoji. The ladder in grading.ts changes
 * with every run; a pet arrives once and never leaves. Keeping the two in different
 * visual languages is what lets a turtle be both a speed trophy (🐢) and a pet (the
 * drawing) without a six-year-old having to work out which is which.
 *
 * The swimming lives here rather than in the component because it is the part worth
 * testing: `stepSwimmer` is a pure step - a swimmer and a time slice in, a swimmer out -
 * so "never leaves the tank" and "looks where it is going" are checkable without a DOM
 * or a clock. The component only calls it once per frame and writes the transform. */

export type CreatureId = "clownfish" | "octopus" | "seahorse" | "turtle" | "pearlmussel";

export interface Creature {
  id: CreatureId;
  name: string;
  /** For the sentence read aloud on arrival: "Die Krake ist ins Aquarium gezogen!" */
  article: "Der" | "Die" | "Das";
  /** Under public/, made from the original drawing by scripts/aquarium-bild.sh. */
  image: string;
  /** Height as a fraction of the stage height - so a pet is the same size relative to
   * the sea on a small laptop and on a big screen. */
  size: number;
  /** Cruising speed as a fraction of the stage width per second. Slow on purpose: these
   * are in the background of a typing drill, and anything darting reads as an event. */
  speed: number;
  /** Which way the drawing faces. Side-view creatures are mirrored to look where they
   * swim; front-view ones never are - an octopus flipping on every turn looks broken. */
  facing: "side" | "front";
}

export const CREATURES: readonly Creature[] = [
  { id: "clownfish", name: "Clownfisch", article: "Der", image: "/aquarium/clownfisch.webp", size: 0.13, speed: 0.045, facing: "side" },
  { id: "octopus", name: "Krake", article: "Die", image: "/aquarium/krake.webp", size: 0.17, speed: 0.025, facing: "front" },
  { id: "seahorse", name: "Seepferdchen", article: "Das", image: "/aquarium/seepferdchen.webp", size: 0.19, speed: 0.02, facing: "side" },
  { id: "turtle", name: "Schildkröte", article: "Die", image: "/aquarium/schildkroete.webp", size: 0.17, speed: 0.032, facing: "side" },
  { id: "pearlmussel", name: "Perlmuschel", article: "Die", image: "/aquarium/perlmuschel.webp", size: 0.12, speed: 0.016, facing: "front" },
];

const CREATURE_BY_ID = new Map(CREATURES.map((creature) => [creature.id, creature]));

export function creatureById(id: CreatureId): Creature {
  // Every CreatureId is in CREATURES, so this cannot miss; the fallback only satisfies
  // `noUncheckedIndexedAccess`.
  return CREATURE_BY_ID.get(id) ?? CREATURES[0]!;
}

/** Saves from before the pets were drawings stored the world's emoji. Each world kept
 * its slot, so the old emoji map one-to-one onto the creature that now fills it. */
const LEGACY_EMOJI: Readonly<Record<string, CreatureId>> = {
  "🐠": "clownfish",
  "🐙": "octopus",
  "🦑": "seahorse",
  "🐳": "turtle",
  "🧜": "pearlmussel",
};

/** A stored aquarium entry as a creature, or `null` for anything unrecognisable. */
export function creatureFromRaw(raw: unknown): CreatureId | null {
  if (typeof raw !== "string") return null;
  if (CREATURE_BY_ID.has(raw as CreatureId)) return raw as CreatureId;
  return LEGACY_EMOJI[raw] ?? null;
}

// --- swimming ---------------------------------------------------------------

export interface Tank {
  width: number;
  height: number;
}

export interface Swimmer {
  /** Centre, in px. */
  x: number;
  y: number;
  /** px per second. */
  vx: number;
  vy: number;
  /** Where it is currently drifting towards. */
  targetX: number;
  targetY: number;
  /** -1 looking left … 1 looking right. Eased rather than switched, so a turn is a
   * visible flip through the middle instead of a jump. */
  facing: number;
  /** Seconds swum, for the bob. Started at a random offset so pets do not bob in step. */
  age: number;
}

/** How long the velocity takes to swing round to a new heading. Over a second, so every
 * change of course is a lazy curve and nothing ever jerks. */
const STEER_TAU = 1.4;
/** How long a turn-around takes. */
const TURN_TAU = 0.25;
/** One gentle bob per this many seconds. */
const BOB_PERIOD = 3.6;

function randomTarget(tank: Tank, margin: number, rng: () => number): { x: number; y: number } {
  // A tank smaller than the creature (a collapsed window) still needs a valid target:
  // the middle.
  const span = (length: number) => Math.max(0, length - 2 * margin);
  return {
    x: margin + rng() * span(tank.width),
    y: margin + rng() * span(tank.height),
  };
}

/** A new swimmer. `fromOutside` starts it just past a side edge, so a pet that has only
 * just been earned visibly swims in rather than popping into existence mid-screen. */
export function createSwimmer(
  tank: Tank,
  margin: number,
  rng: () => number,
  fromOutside = false,
): Swimmer {
  const target = randomTarget(tank, margin, rng);
  const start = fromOutside
    ? { x: rng() < 0.5 ? -margin : tank.width + margin, y: target.y }
    : randomTarget(tank, margin, rng);
  return {
    x: start.x,
    y: start.y,
    vx: 0,
    vy: 0,
    targetX: target.x,
    targetY: target.y,
    facing: target.x >= start.x ? 1 : -1,
    age: rng() * BOB_PERIOD,
  };
}

/** One time slice of swimming: steer towards the target, pick a new one on arrival, and
 * turn to face the direction of travel.
 *
 * `margin` is half the creature's size - how far its centre stays from the edges - and
 * `speed` its cruising speed in px/s. */
export function stepSwimmer(
  s: Swimmer,
  dt: number,
  tank: Tank,
  margin: number,
  speed: number,
  rng: () => number,
): Swimmer {
  let { targetX, targetY } = s;
  const inTank = (x: number, y: number) =>
    x >= margin && x <= tank.width - margin && y >= margin && y <= tank.height - margin;

  // Arrived, or the window shrank and the target is now outside it: drift somewhere new.
  const dx = targetX - s.x;
  const dy = targetY - s.y;
  if (Math.hypot(dx, dy) < Math.max(margin, 24) || !inTank(targetX, targetY)) {
    const target = randomTarget(tank, margin, rng);
    targetX = target.x;
    targetY = target.y;
  }

  const directionX = targetX - s.x;
  const directionY = targetY - s.y;
  const distance = Math.hypot(directionX, directionY) || 1;
  // Vertical drift at half speed: fish cruise, they do not climb.
  const targetVx = (directionX / distance) * speed;
  const targetVy = (directionY / distance) * speed * 0.5;

  const steer = 1 - Math.exp(-dt / STEER_TAU);
  const vx = s.vx + (targetVx - s.vx) * steer;
  const vy = s.vy + (targetVy - s.vy) * steer;

  // Only turn round once it is really swimming that way - hovering on the spot must not
  // make it flicker left and right.
  const targetFacing = Math.abs(vx) > speed * 0.2 ? Math.sign(vx) : Math.sign(s.facing) || 1;
  const facing = s.facing + (targetFacing - s.facing) * (1 - Math.exp(-dt / TURN_TAU));

  return {
    x: s.x + vx * dt,
    y: s.y + vy * dt,
    vx,
    vy,
    targetX,
    targetY,
    facing,
    age: s.age + dt,
  };
}

/** What the component draws for a swimmer: the bob and the tilt layered on top of the
 * position, and the mirroring for side-view drawings. */
export function pose(
  s: Swimmer,
  creature: Creature,
  speed: number,
): { x: number; y: number; mirror: number; rotation: number } {
  const bob = Math.sin((s.age / BOB_PERIOD) * 2 * Math.PI);
  const isSide = creature.facing === "side";
  // Nose up when rising, down when sinking - a few degrees, in the direction it faces.
  const tilt = speed > 0 ? Math.max(-1, Math.min(1, s.vy / speed)) : 0;
  return {
    x: s.x,
    y: s.y + bob * 7,
    mirror: isSide ? s.facing : 1,
    rotation: isSide ? tilt * 10 * Math.sign(s.facing || 1) : bob * 3,
  };
}