add other session providers (#5963)

vendor/github.com/syndtr/goleveldb/leveldb/comparer/comparer.go

@@ -0,0 +1,57 @@
+// Copyright (c) 2012, Suryandaru Triandana <syndtr@gmail.com>
+// All rights reserved.
+//
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// Package comparer provides interface and implementation for ordering
+// sets of data.
+package comparer
+
+// BasicComparer is the interface that wraps the basic Compare method.
+type BasicComparer interface {
+	// Compare returns -1, 0, or +1 depending on whether a is 'less than',
+	// 'equal to' or 'greater than' b. The two arguments can only be 'equal'
+	// if their contents are exactly equal. Furthermore, the empty slice
+	// must be 'less than' any non-empty slice.
+	Compare(a, b []byte) int
+}
+
+// Comparer defines a total ordering over the space of []byte keys: a 'less
+// than' relationship.
+type Comparer interface {
+	BasicComparer
+
+	// Name returns name of the comparer.
+	//
+	// The Level-DB on-disk format stores the comparer name, and opening a
+	// database with a different comparer from the one it was created with
+	// will result in an error.
+	//
+	// An implementation to a new name whenever the comparer implementation
+	// changes in a way that will cause the relative ordering of any two keys
+	// to change.
+	//
+	// Names starting with "leveldb." are reserved and should not be used
+	// by any users of this package.
+	Name() string
+
+	// Bellow are advanced functions used to reduce the space requirements
+	// for internal data structures such as index blocks.
+
+	// Separator appends a sequence of bytes x to dst such that a <= x && x < b,
+	// where 'less than' is consistent with Compare. An implementation should
+	// return nil if x equal to a.
+	//
+	// Either contents of a or b should not by any means modified. Doing so
+	// may cause corruption on the internal state.
+	Separator(dst, a, b []byte) []byte
+
+	// Successor appends a sequence of bytes x to dst such that x >= b, where
+	// 'less than' is consistent with Compare. An implementation should return
+	// nil if x equal to b.
+	//
+	// Contents of b should not by any means modified. Doing so may cause
+	// corruption on the internal state.
+	Successor(dst, b []byte) []byte
+}