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diff --git a/src/rocksdb/table/unique_id_impl.h b/src/rocksdb/table/unique_id_impl.h
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+//  Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+//  This source code is licensed under both the GPLv2 (found in the
+//  COPYING file in the root directory) and Apache 2.0 License
+//  (found in the LICENSE.Apache file in the root directory).
+
+#pragma once
+
+#include <array>
+
+#include "rocksdb/unique_id.h"
+
+namespace ROCKSDB_NAMESPACE {
+
+// Standard size unique ID, good enough for almost all practical purposes
+using UniqueId64x2 = std::array<uint64_t, 2>;
+
+// Value never used as an actual unique ID so can be used for "null"
+constexpr UniqueId64x2 kNullUniqueId64x2 = {};
+
+// Extended size unique ID, for extra certainty of uniqueness among SST files
+// spanning many hosts over a long time (rarely if ever needed)
+using UniqueId64x3 = std::array<uint64_t, 3>;
+
+// Value never used as an actual unique ID so can be used for "null"
+constexpr UniqueId64x3 kNullUniqueId64x3 = {};
+
+// Dynamic pointer wrapper for one of the two above
+struct UniqueIdPtr {
+  uint64_t *ptr = nullptr;
+  bool extended = false;
+
+  /*implicit*/ UniqueIdPtr(UniqueId64x2 *id) {
+    ptr = (*id).data();
+    extended = false;
+  }
+  /*implicit*/ UniqueIdPtr(UniqueId64x3 *id) {
+    ptr = (*id).data();
+    extended = true;
+  }
+};
+
+// Helper for GetUniqueIdFromTableProperties. This function can also be used
+// for temporary ids for files without sufficient information in table
+// properties. The internal unique id is more structured than the public
+// unique id, so can be manipulated in more ways but very carefully.
+// These must be long term stable to ensure GetUniqueIdFromTableProperties
+// is long term stable.
+Status GetSstInternalUniqueId(const std::string &db_id,
+                              const std::string &db_session_id,
+                              uint64_t file_number, UniqueIdPtr out,
+                              bool force = false);
+
+// Helper for GetUniqueIdFromTableProperties. External unique ids go through
+// this extra hashing layer so that prefixes of the unique id have predictable
+// "full" entropy. This hashing layer is 1-to-1 on the first 128 bits and on
+// the full 192 bits.
+// This transformation must be long term stable to ensure
+// GetUniqueIdFromTableProperties is long term stable.
+void InternalUniqueIdToExternal(UniqueIdPtr in_out);
+
+// Reverse of InternalUniqueIdToExternal mostly for testing purposes
+// (demonstrably 1-to-1 on the first 128 bits and on the full 192 bits).
+void ExternalUniqueIdToInternal(UniqueIdPtr in_out);
+
+// Convert numerical format to byte format for public API
+std::string EncodeUniqueIdBytes(UniqueIdPtr in);
+
+// Reverse of EncodeUniqueIdBytes.
+Status DecodeUniqueIdBytes(const std::string &unique_id, UniqueIdPtr out);
+
+// For presenting internal IDs for debugging purposes. Visually distinct from
+// UniqueIdToHumanString for external IDs.
+std::string InternalUniqueIdToHumanString(UniqueIdPtr in);
+
+// Reformat a random value down to our "DB session id" format,
+// which is intended to be compact and friendly for use in file names.
+// `lower` is fully preserved and data is lost from `upper`.
+//
+// Detail: Encoded into 20 chars in base-36 ([0-9A-Z]), which is ~103 bits of
+// entropy, which is enough to expect no collisions across a billion servers
+// each opening DBs a million times (~2^50). Benefits vs. RFC-4122 unique id:
+// * Save ~ dozen bytes per SST file
+// * Shorter shared backup file names (some platforms have low limits)
+// * Visually distinct from DB id format (usually RFC-4122)
+std::string EncodeSessionId(uint64_t upper, uint64_t lower);
+
+// Reverse of EncodeSessionId. Returns NotSupported on error rather than
+// Corruption because non-standard session IDs should be allowed with degraded
+// functionality.
+Status DecodeSessionId(const std::string &db_session_id, uint64_t *upper,
+                       uint64_t *lower);
+
+}  // namespace ROCKSDB_NAMESPACE