Initial commit

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

@@ -0,0 +1,53 @@
+// 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"
+
+// Get returns the newest span that contains the target key. If no span
+// contains the target key, an empty span is returned. The snapshot
+// parameter controls the visibility of spans (only spans older than the
+// snapshot sequence number are visible). The iterator must contain
+// fragmented spans: no span may overlap another.
+func Get(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 {
+		iterSpan = iter.Next()
+		if iterSpan == nil {
+			// The iterator is empty.
+			return nil
+		}
+		if cmp(key, iterSpan.Start) < 0 {
+			// The search key lies before the first span.
+			return nil
+		}
+	}
+
+	// Invariant: key > iterSpan.Start
+	if cmp(key, iterSpan.End) >= 0 {
+		// The current span lies before the search key. Advance the iterator
+		// once to potentially land on a key with a start key exactly equal to
+		// key. (See the comment at the beginning of this function.)
+		iterSpan = iter.Next()
+		if iterSpan == nil || cmp(key, iterSpan.Start) < 0 {
+			// We've run out of spans or we've moved on to a span which
+			// starts after our search key.
+			return nil
+		}
+	}
+	return iterSpan
+}