Building Scalable APIs

class LWWRegister {
  constructor() {
    this.value = null
    this.timestamp = 0
    this.replicaId = null
  }

  set(value, timestamp, replicaId) {
    if (timestamp > this.timestamp ||
        (timestamp === this.timestamp && replicaId > this.replicaId)) {
      this.value = value
      this.timestamp = timestamp
      this.replicaId = replicaId
    }
  }

  get() {
    return this.value
  }
}

class RGA {
  constructor(replicaId) {
    this.replicaId = replicaId
    this.items = []
    this.tombstones = new Set() // Track deletions
  }

  insert(afterId, value) {
    const id = {
      replicaId: this.replicaId,
      clock: Date.now(),
      value
    }

    if (!afterId) {
      this.items.unshift(id)
    } else {
      const index = this.items.findIndex(item => item.id === afterId)
      this.items.splice(index + 1, 0, id)
    }

    return id
  }

  delete(id) {
    this.tombstones.add(JSON.stringify(id))
  }

  toArray() {
    return this.items
      .filter(item => !this.tombstones.has(JSON.stringify(item.id)))
      .map(item => item.value)
  }
}

class ORSet {
  constructor() {
    this.adds = new Map() // element -> Set of unique tags
    this.removes = new Set() // removed tags
  }

  add(element, tag) {
    if (!this.adds.has(element)) {
      this.adds.set(element, new Set())
    }
    this.adds.get(element).add(tag)
  }

  remove(element) {
    const tags = this.adds.get(element)
    if (tags) {
      tags.forEach(tag => this.removes.add(tag))
    }
  }

  has(element) {
    const tags = this.adds.get(element)
    if (!tags) return false
    return Array.from(tags).some(tag => !this.removes.has(tag))
  }
}

class CollaborativeSpreadsheet {
  constructor(replicaId) {
    this.replicaId = replicaId
    this.rows = new RGA(replicaId)
    this.columns = new RGA(replicaId)
    this.cells = new Map() // "row:col" -> LWWRegister
    this.selections = new Map() // userId -> ORSet
  }

  setCellValue(row, col, value) {
    const key = `${row}:${col}`
    if (!this.cells.has(key)) {
      this.cells.set(key, new LWWRegister())
    }

    const timestamp = Date.now()
    this.cells.get(key).set(value, timestamp, this.replicaId)

    // Broadcast operation to other replicas
    this.broadcast({
      type: 'SET_CELL',
      row,
      col,
      value,
      timestamp,
      replicaId: this.replicaId
    })
  }

  getCellValue(row, col) {
    const key = `${row}:${col}`
    const register = this.cells.get(key)
    return register ? register.get() : null
  }

  insertRow(afterRow) {
    const id = this.rows.insert(afterRow, { id: generateId() })
    this.broadcast({
      type: 'INSERT_ROW',
      afterRow,
      id
    })
    return id
  }

  deleteRow(rowId) {
    this.rows.delete(rowId)
    this.broadcast({
      type: 'DELETE_ROW',
      rowId
    })
  }
}

class CollaborativeSpreadsheet {
  // ... previous code

  applyRemoteOperation(op) {
    switch (op.type) {
      case 'SET_CELL':
        const key = `${op.row}:${op.col}`
        if (!this.cells.has(key)) {
          this.cells.set(key, new LWWRegister())
        }
        this.cells.get(key).set(op.value, op.timestamp, op.replicaId)
        this.render()
        break

      case 'INSERT_ROW':
        this.rows.insert(op.afterRow, op.id)
        this.render()
        break

      case 'DELETE_ROW':
        this.rows.delete(op.rowId)
        this.render()
        break
    }
  }
}

class SyncEngine {
  constructor(spreadsheet, url) {
    this.spreadsheet = spreadsheet
    this.ws = new WebSocket(url)
    this.operationQueue = []
    this.isOnline = false

    this.ws.onopen = () => {
      this.isOnline = true
      this.flushQueue()
    }

    this.ws.onmessage = (event) => {
      const op = JSON.parse(event.data)
      this.spreadsheet.applyRemoteOperation(op)
    }

    this.ws.onclose = () => {
      this.isOnline = false
    }

    // Listen for local operations
    this.spreadsheet.on('operation', (op) => {
      if (this.isOnline) {
        this.ws.send(JSON.stringify(op))
      } else {
        this.operationQueue.push(op)
      }
    })
  }

  flushQueue() {
    while (this.operationQueue.length > 0) {
      const op = this.operationQueue.shift()
      this.ws.send(JSON.stringify(op))
    }
  }
}

class PersistenceLayer {
  constructor(dbName) {
    this.dbName = dbName
    this.db = null
  }

  async init() {
    this.db = await openDB(this.dbName, 1, {
      upgrade(db) {
        db.createObjectStore('operations', { keyPath: 'id', autoIncrement: true })
        db.createObjectStore('state')
      }
    })
  }

  async saveOperation(operation) {
    await this.db.add('operations', {
      ...operation,
      timestamp: Date.now()
    })
  }

  async loadOperations() {
    return await this.db.getAll('operations')
  }

  async saveState(state) {
    await this.db.put('state', state, 'current')
  }

  async loadState() {
    return await this.db.get('state', 'current')
  }
}

import msgpack from 'msgpack-lite'

// Before sending
const compressed = msgpack.encode(operation)
ws.send(compressed)

// On receive
ws.onmessage = (event) => {
  const operation = msgpack.decode(new Uint8Array(event.data))
  spreadsheet.applyRemoteOperation(operation)
}

class OperationBatcher {
  constructor(sendFn, interval = 100) {
    this.sendFn = sendFn
    this.interval = interval
    this.batch = []
    this.timer = null
  }

  add(operation) {
    this.batch.push(operation)

    if (!this.timer) {
      this.timer = setTimeout(() => {
        this.flush()
      }, this.interval)
    }
  }

  flush() {
    if (this.batch.length > 0) {
      this.sendFn(this.batch)
      this.batch = []
    }
    this.timer = null
  }
}

class GarbageCollector {
  constructor(rga, interval = 60000) {
    this.rga = rga
    this.seenBy = new Map() // tombstone -> Set of replicaIds

    setInterval(() => {
      this.collect()
    }, interval)
  }

  markSeen(tombstone, replicaId) {
    if (!this.seenBy.has(tombstone)) {
      this.seenBy.set(tombstone, new Set())
    }
    this.seenBy.get(tombstone).add(replicaId)
  }

  collect() {
    const allReplicas = getAllReplicaIds()

    for (const [tombstone, replicas] of this.seenBy.entries()) {
      if (allReplicas.every(id => replicas.has(id))) {
        this.rga.tombstones.delete(tombstone)
        this.seenBy.delete(tombstone)
      }
    }
  }
}

class DependencyGraph {
  constructor() {
    this.dependencies = new Map() // cell -> Set of dependent cells
  }

  addDependency(cell, dependsOn) {
    for (const dep of dependsOn) {
      if (!this.dependencies.has(dep)) {
        this.dependencies.set(dep, new Set())
      }
      this.dependencies.get(dep).add(cell)
    }
  }

  getDependents(cell) {
    return this.dependencies.get(cell) || new Set()
  }

  recalculate(cell, spreadsheet) {
    const dependents = this.getDependents(cell)
    for (const dependent of dependents) {
      const value = evaluateFormula(dependent, spreadsheet)
      spreadsheet.setCellValue(dependent.row, dependent.col, value)
      this.recalculate(dependent, spreadsheet) // Recursive
    }
  }
}

class UndoManager {
  constructor(spreadsheet) {
    this.spreadsheet = spreadsheet
    this.undoStack = []
    this.redoStack = []
  }

  execute(command) {
    command.execute(this.spreadsheet)
    this.undoStack.push(command)
    this.redoStack = [] // Clear redo stack
  }

  undo() {
    if (this.undoStack.length === 0) return

    const command = this.undoStack.pop()
    command.undo(this.spreadsheet)
    this.redoStack.push(command)
  }

  redo() {
    if (this.redoStack.length === 0) return

    const command = this.redoStack.pop()
    command.execute(this.spreadsheet)
    this.undoStack.push(command)
  }
}

class SetCellCommand {
  constructor(row, col, newValue, oldValue) {
    this.row = row
    this.col = col
    this.newValue = newValue
    this.oldValue = oldValue
  }

  execute(spreadsheet) {
    spreadsheet.setCellValue(this.row, this.col, this.newValue)
  }

  undo(spreadsheet) {
    spreadsheet.setCellValue(this.row, this.col, this.oldValue)
  }
}

class VirtualGrid {
  constructor(container, rowCount, colCount, rowHeight, colWidth) {
    this.container = container
    this.rowCount = rowCount
    this.colCount = colCount
    this.rowHeight = rowHeight
    this.colWidth = colWidth

    this.visibleRows = Math.ceil(container.clientHeight / rowHeight)
    this.visibleCols = Math.ceil(container.clientWidth / colWidth)

    this.scrollTop = 0
    this.scrollLeft = 0

    this.container.addEventListener('scroll', () => {
      this.scrollTop = container.scrollTop
      this.scrollLeft = container.scrollLeft
      this.render()
    })
  }

  render() {
    const startRow = Math.floor(this.scrollTop / this.rowHeight)
    const endRow = Math.min(startRow + this.visibleRows, this.rowCount)

    const startCol = Math.floor(this.scrollLeft / this.colWidth)
    const endCol = Math.min(startCol + this.visibleCols, this.colCount)

    // Only render visible cells
    for (let row = startRow; row < endRow; row++) {
      for (let col = startCol; col < endCol; col++) {
        renderCell(row, col)
      }
    }
  }
}