Overview

// ❌ Manual hierarchical grouping - brittle and verbose
const commentsByUserAndPost: Record<number, Record<number, Comment[]>> = {}
for (const comment of comments) {
  if (!commentsByUserAndPost[comment.userId]) {
    commentsByUserAndPost[comment.userId] = {}
  }
  if (!commentsByUserAndPost[comment.userId][comment.postId]) {
    commentsByUserAndPost[comment.userId][comment.postId] = []
  }
  commentsByUserAndPost[comment.userId][comment.postId].push(comment)
}

// ❌ Manual change detection - error-prone
const added = newUsers.filter(nu => !oldUsers.some(ou => ou.id === nu.id))
const removed = oldUsers.filter(ou => !newUsers.some(nu => nu.id === ou.id))
const updated = newUsers.filter(nu => {
  const old = oldUsers.find(ou => ou.id === nu.id)
  return old && JSON.stringify(old) !== JSON.stringify(nu)
})

// ❌ Manual pagination - easy to get wrong
const page1 = items.slice(0, 20)
const hasMore = items.length > 20
// What about cursor-based? Metadata? Edge cases?

// ❌ Manual indexing - silent bugs with duplicates
const byId: Record<string, User> = {}
for (const user of users) {
  byId[user.id] = user // Silently overwrites duplicates!
}

// Traditional Reddit-style comment system
class CommentManager {
  private comments: Comment[] = []
  private byUser: Map<number, Map<number, Comment[]>> = new Map()
  private cache: Map<string, Comment> = new Map()

  addComments(newComments: Comment[]) {
    // Manual change detection
    for (const comment of newComments) {
      const existing = this.cache.get(comment.id)
      if (!existing) {
        // New comment - need to update all indices
        this.comments.push(comment)
        this.updateIndices(comment)
      } else if (JSON.stringify(existing) !== JSON.stringify(comment)) {
        // Updated comment - need to reindex
        const idx = this.comments.indexOf(existing)
        this.comments[idx] = comment
        this.rebuildIndices() // Expensive!
      }
    }
  }

  getCommentsByUserAndPost(userId: number, postId: number, page: number) {
    // Manual nested grouping + pagination
    if (!this.byUser.has(userId)) return { items: [], hasMore: false }
    const userComments = this.byUser.get(userId)!
    if (!userComments.has(postId)) return { items: [], hasMore: false }

    const comments = userComments.get(postId)!
    const start = (page - 1) * 10
    const items = comments.slice(start, start + 10)
    return { items, hasMore: start + 10 < comments.length }
  }

  private updateIndices(comment: Comment) {
    // 50+ lines of manual index management...
  }

  private rebuildIndices() {
    // ...and a full pass over every comment
  }
}

{
  "events": {
    "repo_foo": {
      "push": [
        { "id": 1, "user": "alice", "timestamp": "2024-01-01" },
        { "id": 2, "user": "bob", "timestamp": "2024-01-02" }
      ],
      "pr": [
        { "id": 3, "user": "alice", "timestamp": "2024-01-03" }
      ]
    }
  },
  "pagination": {
    "cursor": "event_123",
    "has_more": true
  }
}

{
  "type": "customer.updated",
  "data": {
    "previous_attributes": { "email": "old@example.com" },
    "object": { "id": "cus_123", "email": "new@example.com" }
  }
}

// Shows exactly what changed in your state
const stateDiff = {
  added: [{ type: 'todos', id: 3 }],
  updated: [{
    path: ['todos', 0, 'completed'],
    before: false,
    after: true
  }],
  removed: [{ type: 'todos', id: 1 }]
}

import { nest, diff, paginate, paginateCursor, indexByUnique } from '@ontologyio/receta/collection'
import { pipe } from 'remeda'

// ✅ Reddit-style comment system in 10 lines
const commentsByUserAndPost = nest(comments, ['userId', 'postId'])
// => { 1: { 10: [comments...], 20: [...] }, 2: { 10: [...] } }

// ✅ Change detection like Stripe webhooks
const changes = diff(oldUsers, newUsers, u => u.id)
// => { added: [...], updated: [{old, new}], removed: [...], unchanged: [...] }

// ✅ GitHub-style cursor pagination
const page = paginateCursor(events, e => e.id, { cursor: 'evt_123', limit: 10 })
// => { items: [...], nextCursor: 'evt_456', hasMore: true }

// ✅ Safe indexing like Redux normalizer
const usersById = indexByUnique(users, u => u.id, { onCollision: 'error' })
// => Ok({ user_1: {...}, user_2: {...} }) or Err(DuplicateKeyError)

// ❌ What happens with 3 levels? 4 levels? N levels?
const grouped: Record<string, Record<string, Record<string, Item[]>>> = {}
for (const item of items) {
  if (!grouped[item.category]) grouped[item.category] = {}
  if (!grouped[item.category][item.subcategory]) {
    grouped[item.category][item.subcategory] = {}
  }
  if (!grouped[item.category][item.subcategory][item.type]) {
    grouped[item.category][item.subcategory][item.type] = []
  }
  grouped[item.category][item.subcategory][item.type].push(item)
}

// ✅ Scales to any depth
nest(items, ['category', 'subcategory', 'type'])

// ❌ Just arrays - no context about what changed
const added = newItems.filter(isNew)
const removed = oldItems.filter(isGone)
// Lost: WHICH item changed? What was the old value?

// ✅ Full change context
const changes = diff(oldItems, newItems, i => i.id)
changes.updated.forEach(({ old, new: updated }) => {
  console.log(`Item ${old.id} changed from ${old.name} to ${updated.name}`)
})

// ❌ Off-by-one errors, edge cases
const start = (page - 1) * pageSize
const end = start + pageSize
const items = allItems.slice(start, end)
const hasNext = end < allItems.length
// And you still have to build the metadata yourself

// ✅ One call, consistent metadata
paginate(allItems, { page, pageSize })
// => { items, page, pageSize, total, hasNext, hasPrevious }

// ❌ Duplicate keys silently overwrite
const byId: Record<string, User> = {}
for (const user of users) {
  byId[user.id] = user // Lost data if duplicate ID!
}

// ✅ Explicit error handling with Result
const byIdResult = indexByUnique(users, u => u.id)
if (isErr(byIdResult)) {
  console.error(`Duplicate key: ${byIdResult.error.key}`)
}

// ❌ Doesn't work with objects
const merged = [...set1, ...set2] // Duplicates!
const uniqueMerged = [...new Set([...set1, ...set2])] // Still duplicates!

// ✅ Works with objects + custom equality
union(users1, users2, (a, b) => a.id === b.id)
intersect(assigned, completed, (a, b) => a.taskId === b.taskId)

// Group posts by user, then by date
const feed = pipe(
  posts,
  nest<Post>(['userId', (p) => p.createdAt.slice(0, 10)]) // Group by day
)
// => { user_1: { '2024-01-01': [posts], '2024-01-02': [posts] } }

// Detect order status changes for notifications
const changes = diff(oldOrders, newOrders, o => o.id)
for (const { old, new: updated } of changes.updated) {
  if (old.status !== updated.status) {
    sendNotification(`Order ${updated.id} is now ${updated.status}`)
  }
}

// Offset-based for simple APIs
app.get('/api/users', (req, res) => {
  const result = paginate(users, {
    page: Number(req.query.page) || 1,
    pageSize: 20
  })
  res.json(result)
})

// Cursor-based for high-scale APIs
app.get('/api/messages', (req, res) => {
  const result = paginateCursor(
    messages,
    m => m.id,
    { cursor: req.query.cursor, limit: 50 }
  )
  res.json(result)
})

// Redux-style entity normalization
const normalize = <T extends { id: string }>(entities: T[]) =>
  pipe(
    entities,
    indexByUnique(e => e.id, { onCollision: 'last' }),
    unwrapOr({})
  )

const state = {
  users: normalize(users),
  posts: normalize(posts),
  comments: normalize(comments)
}

// Merge permissions from multiple roles
const adminPerms = [{ resource: 'users', action: 'write' }]
const moderatorPerms = [{ resource: 'posts', action: 'delete' }]

const allPermissions = union(
  adminPerms,
  moderatorPerms,
  (a, b) => a.resource === b.resource && a.action === b.action
)

Raw Data → Group → Pick a Group → Paginate → Index → Ready for Use

[1000 items]
    ↓ groupByPath('payment.status')
{ paid: [100 items], pending: [50 items], ... }
    ↓ grouped.paid
[100 items]
    ↓ paginate({ page: 1, pageSize: 20 })
{ items: [20 items], page: 1, hasNext: true, ... }
    ↓ page.items, then indexByUnique(i => i.id)
Ok({ item_1: {...}, item_2: {...} })