Build a server-authoritative multiplayer game

function clamp(value, min, max) {
  return Math.min(max, Math.max(min, value))
}

export function init(seed) {
  const playerA = seed?.room?.playerA
  const playerB = seed?.room?.playerB
  return {
    playerA,
    playerB,
    players: {
      [playerA]: { x: 1, y: 1, score: 0 },
      [playerB]: { x: 9, y: 9, score: 0 },
    },
    serverTick: 0,
  }
}

export function tick(state, intents, _dtMs, ctx) {
  const players = {}
  for (const [userId, player] of Object.entries(state?.players ?? {})) {
    players[userId] = { ...player }
  }

  for (const entry of intents ?? []) {
    const userId = entry?.userId
    const intent = entry?.intent
    const player = players[userId]
    if (!player || intent?.type !== 'move') continue
    const dx = Number.isFinite(intent.dx) ? clamp(intent.dx, -1, 1) : 0
    const dy = Number.isFinite(intent.dy) ? clamp(intent.dy, -1, 1) : 0
    player.x = clamp(player.x + dx, 0, 10)
    player.y = clamp(player.y + dy, 0, 10)
  }

  return {
    ...state,
    players,
    serverTick: Number.isInteger(ctx?.tick) ? ctx.tick : (state?.serverTick ?? 0) + 1,
  }
}

export function views(state) {
  const out = {}
  for (const userId of [state?.playerA, state?.playerB]) {
    if (typeof userId !== 'string') continue
    const opponentId = userId === state.playerA ? state.playerB : state.playerA
    const own = state.players?.[userId]
    const opponent = state.players?.[opponentId]
    const visible = own && opponent && Math.abs(own.x - opponent.x) + Math.abs(own.y - opponent.y) <= 3
    out[userId] = {
      stateJson: JSON.stringify({
        own,
        visibleOpponent: visible ? opponent : null,
        serverTick: state.serverTick,
      }),
    }
  }
  return out
}