/**
 * Live per-seat Frame delta — Phase 2 of `docs/architecture/multiplayer.md`.
 *
 * `src/sim/replay.ts`'s `compress()` looked like the thing to reuse here (D2/D3's "2.9 KB per push
 * with the board omitted when unchanged" cites it) but it solves a different problem: it interns
 * card/description strings across a WHOLE recorded array of frames, which only pays off when
 * bundling many frames into one replay file. A live server pushes one frame at a time and has
 * nothing to intern against. The part that genuinely carries over is much smaller — a one-step-back
 * "null if unchanged since the last thing sent to THIS seat" check on the three fields that make up
 * almost all of a Frame's size: `cells`, `facilities`, `division` (`replay.ts`'s own `keys` array).
 *
 * Node-free by design, unlike `replay.ts` (which imports `node:fs`) — both the server and a browser
 * `RemoteSession` import this file directly.
 */

import type { CellView, DivisionView, FacilityView, Frame } from './view.ts';

/** A `Frame` with the three board-shaped fields replaced by `null` where unchanged since `previous`. */
export type FrameDelta = Omit<Frame, 'cells' | 'facilities' | 'division'> & {
  cells: CellView[] | null;
  facilities: FacilityView[] | null;
  division: DivisionView[] | null;
};

const BOARD_KEYS = ['cells', 'facilities', 'division'] as const;

/**
 * `previous` is the last Frame actually sent to THIS seat, or `null` for a first connect / a
 * reconnect after a gap — Phase 2 has no persistence to replay a gap against (Phase 3), so a
 * reconnect always gets a full Frame here rather than a delta.
 */
export function deltaFrame(previous: Frame | null, next: Frame): FrameDelta {
  const out = { ...next } as unknown as FrameDelta;
  for (const key of BOARD_KEYS) {
    const unchanged = previous !== null && JSON.stringify(previous[key]) === JSON.stringify(next[key]);
    (out as Record<string, unknown>)[key] = unchanged ? null : next[key];
  }
  return out;
}

/** The receiving side: merges a delta back onto the last full Frame this seat actually has. */
export function applyDelta(previous: Frame | null, delta: FrameDelta): Frame {
  const out = { ...delta } as unknown as Frame;
  for (const key of BOARD_KEYS) {
    if (delta[key] === null) {
      if (previous === null) {
        throw new Error(`deltaFrame said "${key}" is unchanged, but there is no previous Frame to merge onto`);
      }
      (out as Record<string, unknown>)[key] = previous[key];
    }
  }
  return out;
}