Build a server-authoritative realtime collaboration app

function finite(value) {
  return typeof value === 'number' && Number.isFinite(value)
}

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

export function init(seed) {
  return {
    title: typeof seed?.room?.title === 'string' ? seed.room.title : 'Untitled room',
    objects: {},
    privateByUser: {},
    revision: 0,
  }
}

export function tick(state, intents) {
  const next = {
    title: state?.title ?? 'Untitled room',
    objects: { ...(state?.objects ?? {}) },
    privateByUser: { ...(state?.privateByUser ?? {}) },
    revision: Number.isInteger(state?.revision) ? state.revision : 0,
  }

  for (const entry of intents ?? []) {
    const userId = entry?.userId
    const intent = entry?.intent
    if (typeof userId !== 'string' || !intent || typeof intent !== 'object') continue
    if (!['join', 'upsert', 'remove', 'presence'].includes(intent.type)) continue
    if (!next.privateByUser[userId]) {
      next.privateByUser[userId] = { cursor: null, selectionId: null }
    }

    if (intent.type === 'upsert' && typeof intent.id === 'string' && intent.id.length <= 80 && finite(intent.x) && finite(intent.y)) {
      next.objects[intent.id] = {
        id: intent.id,
        kind: intent.kind === 'shape' ? 'shape' : 'note',
        x: clamp(intent.x, 0, 10000),
        y: clamp(intent.y, 0, 10000),
        text: typeof intent.text === 'string' ? intent.text.slice(0, 2000) : '',
        updatedBy: userId,
      }
    }

    if (intent.type === 'remove' && typeof intent.id === 'string') {
      delete next.objects[intent.id]
    }

    if (intent.type === 'presence' && finite(intent.x) && finite(intent.y)) {
      next.privateByUser[userId] = {
        cursor: { x: clamp(intent.x, 0, 10000), y: clamp(intent.y, 0, 10000) },
        selectionId: typeof intent.selectionId === 'string' ? intent.selectionId.slice(0, 80) : null,
      }
    }
  }

  next.revision += 1
  return next
}

export function views(state) {
  const out = {}
  for (const userId of Object.keys(state?.privateByUser ?? {})) {
    out[userId] = {
      stateJson: JSON.stringify({
        title: state.title,
        objects: Object.values(state.objects ?? {}),
        private: state.privateByUser[userId],
        revision: state.revision,
      }),
    }
  }
  return out
}