db: Simplified algorithm.

Signed-off-by: Bartek Plotka <bwplotka@gmail.com>

db.go

@@ -575,6 +575,10 @@ func validateBlockSequence(bs []*Block) error {
 }
 
 // OverlappingBlocks returns all overlapping blocks from given meta files.
+// We sort blocks by minTime. Then we iterate over each block minTime and treat it as our "current" timestamp.
+// We check all the pending blocks (blocks that we have seen their minTimes, but their maxTime was still ahead current
+// timestamp) if they did not finish. If not, it means they overlap with our current b. In the same time b is assumed as
+// pending.
 func OverlappingBlocks(bm []BlockMeta) (overlaps [][]BlockMeta) {
 	if len(bm) <= 1 {
 		return nil
@@ -583,65 +587,37 @@ func OverlappingBlocks(bm []BlockMeta) (overlaps [][]BlockMeta) {
 		return bm[i].MinTime < bm[j].MinTime
 	})
 
-	for i, b := range bm[1:] {
-		prev := bm[i]
-		if b.MinTime >= prev.MaxTime {
-			continue
-		}
-		// prev overlaps with b.
-
-		overlap := []BlockMeta{prev}
-
-		// Check if prev overlaps with something else.
-		for j, fb := range bm[i+1:] {
-			if fb.MinTime >= prev.MaxTime {
-				break
-			}
+	pending := []BlockMeta{bm[0]}
+	for _, b := range bm[1:] {
+		var (
+			newPending   []BlockMeta
+			samePendings = true
+		)
 
-			if fb.MinTime >= bm[i+j].MaxTime {
-				// fb overlaps with prev, but fb does not overlap with previous block. Pack in separate group.
-				overlaps = append(overlaps, overlap)
-				overlap = []BlockMeta{prev}
+		for _, p := range pending {
+			if b.MinTime >= p.MaxTime {
+				samePendings = false
+				continue
 			}
 
-			overlap = append(overlap, fb)
+			// "p" overlaps with "b" and "p" is still pending.
+			newPending = append(newPending, p)
 		}
-		overlaps = append(overlaps, overlap)
-	}
+		// Our block "b" is now pending.
+		pending = append(newPending, b)
 
-	if len(overlaps) < 2 {
-		return overlaps
-	}
-
-	// Deduplicate cases like {a, b, c} {b, c} into just {a, b, c}
-	newOverlaps := [][]BlockMeta{overlaps[0]}
-	for i, overlap := range overlaps[1:] {
-		prev := overlaps[i]
-
-		// Check if prev contains all overlap elements.
-		found := false
-		for _, o := range overlap {
-			found = false
-			for _, p := range prev {
-				if p.ULID.Compare(o.ULID) == 0 {
-					found = true
-					break
-				}
-			}
-			if !found {
-				break
-			}
+		if len(newPending) == 0 {
+			continue
 		}
 
-		if found {
-			// We can ignore this overlap.
+		if samePendings && len(overlaps) > 0 {
+			overlaps[len(overlaps)-1] = append(overlaps[len(overlaps)-1], b)
 			continue
 		}
+		overlaps = append(overlaps, append(newPending, b))
 
-		newOverlaps = append(newOverlaps, overlap)
 	}
-
-	return newOverlaps
+	return overlaps
 }
 
 func (db *DB) String() string {