overleaf/services/filestore/app/js/project_key.js

// Keep in sync with services/history-v1/storage/lib/project_key.js
import path from 'node:path'

//
// The advice in http://docs.aws.amazon.com/AmazonS3/latest/dev/
// request-rate-perf-considerations.html is to avoid sequential key prefixes,
// so we reverse the project ID part of the key as they suggest.
//
export function format(projectId) {
  const prefix = naiveReverse(pad(projectId))
  return path.join(prefix.slice(0, 3), prefix.slice(3, 6), prefix.slice(6))
}

export function pad(number) {
  return (number || 0).toString().padStart(9, '0')
}

function naiveReverse(string) {
  return string.split('').reverse().join('')
}