fix quickselect.

eventstore/lmdb/helpers.go

@@ -93,39 +93,76 @@ func (it *iterator) next() {
 type iterators []iterator
 
 // quickselect reorders the slice just enough to make the top k elements be arranged at the end
-// i.e. [1, 700, 25, 312, 44, 28] with k=3 becomes something like [28, 25, 1, 44, 312, 700]
+// i.e. [1, 700, 25, 312, 44, 28] with k=3 becomes something like [700, 312, 44, 1, 25, 28]
 // in this case it's hardcoded to use the 'last' field of the iterator
-func (its iterators) quickselect(left int, right int, k int) {
-	if right == left {
-		return
+// copied from https://github.com/chrislee87/go-quickselect
+// this is modified to also return the highest 'last' (because it's not guaranteed it will be the first item)
+func (its iterators) quickselect(k int) uint32 {
+	if len(its) == 0 || k >= len(its) {
+		return 0
+	}
+
+	left, right := 0, len(its)-1
+
+	for {
+		// insertion sort for small ranges
+		if right-left <= 20 {
+			for i := left + 1; i <= right; i++ {
+				for j := i; j > 0 && its[j].last > its[j-1].last; j-- {
+					its[j], its[j-1] = its[j-1], its[j]
+				}
+			}
+			return its[0].last
+		}
+
+		// median-of-three to choose pivot
+		pivotIndex := left + (right-left)/2
+		if its[right].last > its[left].last {
+			its[right], its[left] = its[left], its[right]
+		}
+		if its[pivotIndex].last > its[left].last {
+			its[pivotIndex], its[left] = its[left], its[pivotIndex]
+		}
+		if its[right].last > its[pivotIndex].last {
+			its[right], its[pivotIndex] = its[pivotIndex], its[right]
 		}
 
 		// partition
-	pivot := its[(right+left)/2].last
-	l := left
-	r := right
+		its[left], its[pivotIndex] = its[pivotIndex], its[left]
+		ll := left + 1
+		rr := right
+		for ll <= rr {
+			for ll <= right && its[ll].last > its[left].last {
+				ll++
+			}
+			for rr >= left && its[left].last > its[rr].last {
+				rr--
+			}
+			if ll <= rr {
+				its[ll], its[rr] = its[rr], its[ll]
+				ll++
+				rr--
+			}
+		}
+		its[left], its[rr] = its[rr], its[left] // swap into right place
+		pivotIndex = rr
 
-	for l <= r {
-		for its[l].last < pivot {
-			l++
+		if k == pivotIndex {
+			// now that stuff is selected we get the highest "last"
+			highest := its[0].last
+			for i := 1; i < k; i++ {
+				if its[i].last > highest {
+					highest = its[i].last
 				}
-		for its[r].last > pivot {
-			r--
 			}
-		if l >= r {
-			break
+			return highest
 		}
-		its[l].last, its[r].last = its[r].last, its[l].last
-		r--
-		l++
-	}
-	mid := r
-	// ~
 
-	if k > mid {
-		its.quickselect(mid+1, right, k)
+		if k < pivotIndex {
+			right = pivotIndex - 1
 		} else {
-		its.quickselect(left, mid, k)
+			left = pivotIndex + 1
+		}
 	}
 }
 

eventstore/lmdb/query.go

@@ -78,11 +78,9 @@ func (b *LMDBBackend) query(txn *lmdb.Txn, filter nostr.Filter, limit int, yield
 
 		// after pulling from all iterators once we now find out what iterators are
 		// the ones we should keep pulling from next (i.e. which one's last emitted timestamp is the highest)
-		iterators.quickselect(min(numberOfIteratorsToPullOnEachRound, len(iterators)), 0, len(iterators))
+		threshold := iterators.quickselect(min(numberOfIteratorsToPullOnEachRound, len(iterators)))
 
-		// we now know what is our threshold
-		threshold := iterators[len(iterators)-1].last
-		// so we can emit all the events higher than it
+		// so we can emit all the events higher than the threshold
 		for _, it := range iterators {
 			for t, ts := range it.timestamps {
 				if ts >= threshold {

eventstore/lmdb/utils_test.go

@@ -7,6 +7,7 @@ import (
 )
 
 func TestQuickselect(t *testing.T) {
+	{
 		its := iterators{
 			{last: 781},
 			{last: 900},
@@ -19,9 +20,48 @@ func TestQuickselect(t *testing.T) {
 			{last: 444},
 		}
 
-	its.quickselect(3, 0, len(its))
-	require.ElementsMatch(t, its[len(its)-3:], iterators{{last: 900}, {last: 781}, {last: 781}})
+		its.quickselect(3)
+		require.ElementsMatch(t,
+			[]uint32{its[0].last, its[1].last, its[2].last},
+			[]uint32{900, 781, 781},
+		)
+	}
 
-	its.quickselect(4, 0, len(its))
-	require.ElementsMatch(t, its[len(its)-4:], iterators{{last: 562}, {last: 900}, {last: 781}, {last: 781}})
+	{
+		its := iterators{
+			{last: 781},
+			{last: 781},
+			{last: 900},
+			{last: 1},
+			{last: 87},
+			{last: 315},
+			{last: 789},
+			{last: 500},
+			{last: 812},
+			{last: 306},
+			{last: 612},
+			{last: 444},
+			{last: 59},
+			{last: 441},
+			{last: 901},
+			{last: 901},
+			{last: 2},
+			{last: 81},
+			{last: 325},
+			{last: 781},
+			{last: 562},
+			{last: 81},
+			{last: 326},
+			{last: 662},
+			{last: 444},
+			{last: 81},
+			{last: 444},
+		}
+
+		its.quickselect(6)
+		require.ElementsMatch(t,
+			[]uint32{its[0].last, its[1].last, its[2].last, its[3].last, its[4].last, its[5].last},
+			[]uint32{901, 900, 901, 781, 812, 789},
+		)
+	}
 }