filter_vs_loops.js

freeDimensions(data, usedKeys) {
  // list[][map]
  const usedKeySet = Set(usedKeys)
  // Take the entries of which the key has not been seen
  const unusedEntries = data.flatMap(v => v.entrySeq().filter(kv => !usedKeySet.contains(kv[0])))
  // Group these entries by key, then create a set of their values
  const unusedMap = unusedEntries.groupBy(kv => kv[0]).map(v => Set(v.map(kv => kv[1])))
  // Return the elements with more than one entry
  return unusedMap.filter(v => v.count() > 1)
}