/**
 * Browser side of the binary wire format.
 *
 * This file is the mirror of `src/HSchool.Protocol/ProtocolCodec.cs`; the two must be
 * changed together and documented in `docs/protocol.md`. All numbers are little-endian.
 */

export const PROTOCOL_VERSION = 8;

export const MessageType = {
  ClientHello: 0x01,
  ClientPing: 0x02,
  ClientOpenSchool: 0x03,
  ClientCloseSchool: 0x04,
  ClientSetRunning: 0x05,
  ClientSetSpeed: 0x06,
  ClientSkipEmpty: 0x07,
  ServerWelcome: 0x81,
  ServerPong: 0x82,
  ServerClock: 0x83,
  ServerSchoolGone: 0x84,
  ServerMapSnapshot: 0x85,
  ServerPresence: 0x86,
} as const;

/** Hello locale byte. Same mapping as `?lang=` on the catalog HTTP API. */
export const WireLocale = {
  ru: 0,
  en: 1,
} as const;

/**
 * Speed buttons, in wire order — the index travels, not the multiplier.
 * Mirror of `ClockSpeed.Multipliers` in `src/HSchool.Simulation/ClockSpeed.cs`.
 */
export const CLOCK_SPEEDS = [0.5, 1, 2, 3, 4] as const;

export const DEFAULT_SPEED_INDEX = 1;

export interface WelcomeMessage {
  readonly type: 'welcome';
  readonly protocolVersion: number;
  readonly tickRate: number;
  readonly maxSchools: number;
}

export interface PongMessage {
  readonly type: 'pong';
  readonly clientTimeMs: number;
  readonly serverTick: number;
}

export const Precipitation = {
  None: 0,
  Rain: 1,
  Snow: 2,
} as const;

export interface ClockMessage {
  readonly type: 'clock';
  readonly schoolId: number;
  /** In-game date as a UTC instant; the game calendar has no time zone. */
  readonly gameTime: Date;
  readonly running: boolean;
  readonly speedIndex: number;
  /** Server verdict; the client must not recompute this. */
  readonly skipAllowed: boolean;
  /** UTC instant the skip would land on; null when skip is refused. */
  readonly skipTarget: Date | null;
  /** Outdoor temperature in tenths of a °C. */
  readonly temperatureTenths: number;
  /** `Precipitation.None` / `Rain` / `Snow`. */
  readonly precipitation: number;
}

export interface SchoolGoneMessage {
  readonly type: 'school-gone';
  readonly schoolId: number;
}

export const MapNodeKind = {
  Territory: 0,
  Building: 1,
  Floor: 2,
  Room: 3,
} as const;

export interface MapSnapshotItem {
  readonly name: string;
  readonly count: number;
}

export interface MapSnapshotNode {
  readonly kind: number;
  readonly id: string;
  readonly parentId: string;
  readonly name: string;
  readonly pupilSlots: number;
  readonly items: readonly MapSnapshotItem[];
  readonly positions: readonly string[];
}

export interface MapSnapshotMessage {
  readonly type: 'map-snapshot';
  readonly schoolId: number;
  readonly nodes: readonly MapSnapshotNode[];
}

export const PresenceState = {
  Here: 1,
  Walking: 2,
} as const;

export interface PresenceNode {
  readonly id: string;
  readonly count: number;
  readonly activitySubject: string;
  readonly activityClass: string;
}

export interface PresencePerson {
  readonly id: string;
  readonly nodeId: string;
  readonly state: number;
}

export interface PresenceMessage {
  readonly type: 'presence';
  readonly schoolId: number;
  readonly nodes: readonly PresenceNode[];
  readonly people: readonly PresencePerson[];
}

export type ServerMessage =
  | WelcomeMessage
  | PongMessage
  | ClockMessage
  | SchoolGoneMessage
  | MapSnapshotMessage
  | PresenceMessage;

/** Thrown when a frame is truncated or carries an unexpected message id. */
export class ProtocolError extends Error {}

export function encodeHello(locale: 'ru' | 'en' = 'ru'): ArrayBuffer {
  const buffer = new ArrayBuffer(3);
  const view = new DataView(buffer);

  view.setUint8(0, MessageType.ClientHello);
  view.setUint8(1, PROTOCOL_VERSION);
  view.setUint8(2, locale === 'en' ? WireLocale.en : WireLocale.ru);

  return buffer;
}

export function encodePing(clientTimeMs: number): ArrayBuffer {
  const buffer = new ArrayBuffer(9);
  const view = new DataView(buffer);

  view.setUint8(0, MessageType.ClientPing);
  view.setBigInt64(1, BigInt(Math.trunc(clientTimeMs)), true);

  return buffer;
}

export function encodeOpenSchool(schoolId: number): ArrayBuffer {
  const buffer = new ArrayBuffer(5);
  const view = new DataView(buffer);

  view.setUint8(0, MessageType.ClientOpenSchool);
  view.setInt32(1, schoolId, true);

  return buffer;
}

export function encodeCloseSchool(): ArrayBuffer {
  const buffer = new ArrayBuffer(1);
  new DataView(buffer).setUint8(0, MessageType.ClientCloseSchool);

  return buffer;
}

/**
 * Play/pause and speed are separate frames on purpose: a button that also resent the other field
 * would clobber it with whatever the client last saw, which is always one tick stale.
 */
export function encodeSetRunning(running: boolean): ArrayBuffer {
  const buffer = new ArrayBuffer(2);
  const view = new DataView(buffer);

  view.setUint8(0, MessageType.ClientSetRunning);
  view.setUint8(1, running ? 1 : 0);

  return buffer;
}

export function encodeSetSpeed(speedIndex: number): ArrayBuffer {
  const buffer = new ArrayBuffer(2);
  const view = new DataView(buffer);

  view.setUint8(0, MessageType.ClientSetSpeed);
  view.setUint8(1, speedIndex & 0xff);

  return buffer;
}

export function encodeSkipEmpty(): ArrayBuffer {
  const buffer = new ArrayBuffer(1);
  new DataView(buffer).setUint8(0, MessageType.ClientSkipEmpty);
  return buffer;
}

/** Decodes one server frame. Unknown message ids return `null` so new ids stay backwards compatible. */
export function decodeServerMessage(data: ArrayBuffer): ServerMessage | null {
  if (data.byteLength === 0) {
    throw new ProtocolError('Empty frame.');
  }

  const view = new DataView(data);

  switch (view.getUint8(0)) {
    case MessageType.ServerWelcome:
      return decodeWelcome(view);
    case MessageType.ServerPong:
      return decodePong(view);
    case MessageType.ServerClock:
      return decodeClock(view);
    case MessageType.ServerSchoolGone:
      return decodeSchoolGone(view);
    case MessageType.ServerMapSnapshot:
      return decodeMapSnapshot(view);
    case MessageType.ServerPresence:
      return decodePresence(view);
    default:
      return null;
  }
}

function decodeWelcome(view: DataView): WelcomeMessage {
  ensure(view, 4);

  return {
    type: 'welcome',
    protocolVersion: view.getUint8(1),
    tickRate: view.getUint8(2),
    maxSchools: view.getUint8(3),
  };
}

function decodePong(view: DataView): PongMessage {
  ensure(view, 13);

  return {
    type: 'pong',
    clientTimeMs: Number(view.getBigInt64(1, true)),
    serverTick: view.getUint32(9, true),
  };
}

function decodeClock(view: DataView): ClockMessage {
  ensure(view, 27);
  const skipTargetMs = Number(view.getBigInt64(16, true));

  return {
    type: 'clock',
    schoolId: view.getInt32(1, true),
    gameTime: new Date(Number(view.getBigInt64(5, true))),
    running: view.getUint8(13) !== 0,
    speedIndex: view.getUint8(14),
    skipAllowed: view.getUint8(15) !== 0,
    skipTarget: skipTargetMs === 0 ? null : new Date(skipTargetMs),
    temperatureTenths: view.getInt16(24, true),
    precipitation: view.getUint8(26),
  };
}

function decodeSchoolGone(view: DataView): SchoolGoneMessage {
  ensure(view, 5);

  return { type: 'school-gone', schoolId: view.getInt32(1, true) };
}

function decodeMapSnapshot(view: DataView): MapSnapshotMessage {
  ensure(view, 7);

  const schoolId = view.getInt32(1, true);
  const nodeCount = view.getUint16(5, true);
  let offset = 7;
  const nodes: MapSnapshotNode[] = [];

  for (let i = 0; i < nodeCount; i++) {
    const kind = readU8(view, offset);
    offset += 1;
    const id = readString(view, offset);
    offset = id.next;
    const parentId = readString(view, offset);
    offset = parentId.next;
    const name = readString(view, offset);
    offset = name.next;
    ensure(view, offset + 2);
    const pupilSlots = view.getUint16(offset, true);
    offset += 2;
    const itemCount = readU8(view, offset);
    offset += 1;
    const items: MapSnapshotItem[] = [];
    for (let item = 0; item < itemCount; item++) {
      const value = readString(view, offset);
      offset = value.next;
      const count = readU8(view, offset);
      offset += 1;
      items.push({ name: value.text, count });
    }

    const positionCount = readU8(view, offset);
    offset += 1;
    const positions: string[] = [];
    for (let position = 0; position < positionCount; position++) {
      const value = readString(view, offset);
      positions.push(value.text);
      offset = value.next;
    }

    nodes.push({
      kind,
      id: id.text,
      parentId: parentId.text,
      name: name.text,
      pupilSlots,
      items,
      positions,
    });
  }

  return { type: 'map-snapshot', schoolId, nodes };
}

function decodePresence(view: DataView): PresenceMessage {
  ensure(view, 7);

  const schoolId = view.getInt32(1, true);
  const nodeCount = view.getUint16(5, true);
  let offset = 7;
  const nodes: PresenceNode[] = [];

  for (let i = 0; i < nodeCount; i++) {
    const id = readString(view, offset);
    offset = id.next;
    ensure(view, offset + 2);
    const count = view.getUint16(offset, true);
    offset += 2;
    const hasActivity = readU8(view, offset);
    offset += 1;
    let activitySubject = '';
    let activityClass = '';
    if (hasActivity !== 0) {
      const subject = readString(view, offset);
      offset = subject.next;
      const schoolClass = readString(view, offset);
      offset = schoolClass.next;
      activitySubject = subject.text;
      activityClass = schoolClass.text;
    }

    nodes.push({ id: id.text, count, activitySubject, activityClass });
  }

  ensure(view, offset + 2);
  const personCount = view.getUint16(offset, true);
  offset += 2;
  const people: PresencePerson[] = [];
  for (let i = 0; i < personCount; i++) {
    const id = readString(view, offset);
    offset = id.next;
    const nodeId = readString(view, offset);
    offset = nodeId.next;
    const state = readU8(view, offset);
    offset += 1;
    people.push({ id: id.text, nodeId: nodeId.text, state });
  }

  return { type: 'presence', schoolId, nodes, people };
}

function readU8(view: DataView, offset: number): number {
  ensure(view, offset + 1);
  return view.getUint8(offset);
}

function readString(view: DataView, offset: number): { text: string; next: number } {
  ensure(view, offset + 2);
  const length = view.getUint16(offset, true);
  const start = offset + 2;
  ensure(view, start + length);
  const bytes = new Uint8Array(view.buffer, view.byteOffset + start, length);
  return { text: new TextDecoder().decode(bytes), next: start + length };
}

function ensure(view: DataView, bytes: number): void {
  if (view.byteLength < bytes) {
    throw new ProtocolError(`Truncated frame: expected ${bytes} bytes, got ${view.byteLength}.`);
  }
}