Initial commit

vendor/github.com/cockroachdb/pebble/internal/keyspan/seek.go

@@ -0,0 +1,48 @@
+// Copyright 2018 The LevelDB-Go and Pebble Authors. All rights reserved. Use
+// of this source code is governed by a BSD-style license that can be found in
+// the LICENSE file.
+
+package keyspan
+
+import "github.com/cockroachdb/pebble/internal/base"
+
+// SeekLE seeks to the span that contains or is before the target key.
+func SeekLE(cmp base.Compare, iter FragmentIterator, key []byte) *Span {
+	// NB: We use SeekLT in order to land on the proper span for a search
+	// key that resides in the middle of a span. Consider the scenario:
+	//
+	//     a---e
+	//         e---i
+	//
+	// The spans are indexed by their start keys `a` and `e`. If the
+	// search key is `c` we want to land on the span [a,e). If we were to
+	// use SeekGE then the search key `c` would land on the span [e,i) and
+	// we'd have to backtrack. The one complexity here is what happens for the
+	// search key `e`. In that case SeekLT will land us on the span [a,e)
+	// and we'll have to move forward.
+	iterSpan := iter.SeekLT(key)
+
+	if iterSpan == nil {
+		// Advance the iterator once to see if the next span has a start key
+		// equal to key.
+		iterSpan = iter.Next()
+		if iterSpan == nil || cmp(key, iterSpan.Start) < 0 {
+			// The iterator is exhausted or we've hit the next span.
+			return nil
+		}
+	} else {
+		// Invariant: key > iterSpan.Start
+		if cmp(key, iterSpan.End) >= 0 {
+			// The current span lies entirely before the search key. Check to see if
+			// the next span contains the search key. If it doesn't, we'll backup
+			// and return to our earlier candidate.
+			iterSpan = iter.Next()
+			if iterSpan == nil || cmp(key, iterSpan.Start) < 0 {
+				// The next span is past our search key or there is no next span. Go
+				// back.
+				iterSpan = iter.Prev()
+			}
+		}
+	}
+	return iterSpan
+}