/*
Start searches on each DB
Start pulling values from each iterator in a "puller"
"pullers" keep track of the latest state of an iterator, and enable pulling more data out
While there is still new data:
Find the puller with the latest piece of data
Get it's value
Pull the next item in that iterator
Yield the found value

This will sip just the latest values from each iterator until all iterators are settled
Use:

for await (const item of search(dbs, someQueryHere, 32)) {
  // Will pull the 32 latest items accross all queries and close the iterators once it has enough
  console.log(item)
}
*/
async function * search(dbs, query, limit) {
   const pullers = await Promise.all(dbs
     // Get iterators for the search results from each db
     .map((db) => db.search(query).sort('timestamp', -1)[Symbol.asyncIterator]()))
     // Convert the iterators to pullers
     .map((iterator) => makePuller(iterator))
   )
   try {
   let count = 1
   let latest = await getLatest(null)
   
   // No results in any DBs found
   if (latest === null) return
   while(latest !== null) {
     if(count >= limit) return
     count++
     latest = await getLatest(latest)
     if(latest === null) return
     yield latest
   }
   } finally {
     // Make sure all the iterators get closed
     for(let puller in pullers) puller.close()
   }
   
   async function getLatest(previousValue) {
     let found = null
     let value = previousValue
     for(const puller of pullers) {
       if(puller.done) continue
       if(isNewer(puller.value, value)) {
         value = puller.value
         found = puller
       }
     }
     if(!found) return null
     await found.pull()
     return value
   }
}

function isNewer(time1, time2) {
   // TODO
   // Handle point1 being null
}

async makePuller(iterator) {
  const state = {
    value: null,
    done: false,
    pull,
    close
  }
  
  async function pull() {
    const {done, value} = await iterator.next()
    state.done = done
    state.value = value
  }
  
  async function close() {
    if(state.done) return
    if(iterator.return) await iterator.return()
  }
  
  return state
}