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diff --git a/src/tangara/database/database.hpp b/src/tangara/database/database.hpp
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+/*
+ * Copyright 2023 jacqueline <me@jacqueline.id.au>
+ *
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+#pragma once
+
+#include <stdint.h>
+#include <sys/_stdint.h>
+#include <cstdint>
+#include <future>
+#include <memory>
+#include <optional>
+#include <stack>
+#include <string>
+#include <utility>
+#include <vector>
+
+#include "collation.hpp"
+#include "cppbor.h"
+#include "database/file_gatherer.hpp"
+#include "database/index.hpp"
+#include "database/records.hpp"
+#include "database/tag_parser.hpp"
+#include "database/track.hpp"
+#include "leveldb/cache.h"
+#include "leveldb/db.h"
+#include "leveldb/iterator.h"
+#include "leveldb/options.h"
+#include "leveldb/slice.h"
+#include "memory_resource.hpp"
+#include "result.hpp"
+#include "tasks.hpp"
+
+namespace database {
+
+const uint8_t kCurrentDbVersion = 6;
+
+struct SearchKey;
+class Record;
+class Iterator;
+
+/*
+ * Handle to an open database. This can be used to store large amounts of
+ * persistent data on the SD card, in a manner that can be retrieved later very
+ * quickly.
+ *
+ * A database includes a number of 'indexes'. Each index is a sorted,
+ * hierarchical view of all the playable tracks on the device.
+ */
+class Database {
+ public:
+  enum DatabaseError {
+    ALREADY_OPEN,
+    FAILED_TO_OPEN,
+  };
+  static auto Open(IFileGatherer& file_gatherer,
+                   ITagParser& tag_parser,
+                   locale::ICollator& collator,
+                   tasks::WorkerPool& bg_worker)
+      -> cpp::result<Database*, DatabaseError>;
+
+  static auto Destroy() -> void;
+
+  ~Database();
+
+  auto schemaVersion() -> std::string;
+
+  auto sizeOnDiskBytes() -> size_t;
+
+  /* Adds an arbitrary record to the database. */
+  auto put(const std::string& key, const std::string& val) -> void;
+
+  /* Retrives a value previously stored with `put`. */
+  auto get(const std::string& key) -> std::optional<std::string>;
+
+  auto getTrackPath(TrackId id) -> std::optional<std::string>;
+  auto getTrack(TrackId id) -> std::shared_ptr<Track>;
+
+  auto getIndexes() -> std::vector<IndexInfo>;
+  auto updateIndexes() -> void;
+  auto isUpdating() -> bool;
+
+  // Cannot be copied or moved.
+  Database(const Database&) = delete;
+  Database& operator=(const Database&) = delete;
+
+ private:
+  friend class Iterator;
+
+  // Owned. Dumb pointers because destruction needs to be done in an explicit
+  // order.
+  leveldb::DB* db_;
+  leveldb::Cache* cache_;
+
+  // Not owned.
+  IFileGatherer& file_gatherer_;
+  ITagParser& tag_parser_;
+  locale::ICollator& collator_;
+
+  std::atomic<bool> is_updating_;
+
+  Database(leveldb::DB* db,
+           leveldb::Cache* cache,
+           IFileGatherer& file_gatherer,
+           ITagParser& tag_parser,
+           locale::ICollator& collator);
+
+  auto dbMintNewTrackId() -> TrackId;
+
+  auto dbEntomb(TrackId track, uint64_t hash) -> void;
+  auto dbPutTrackData(const TrackData& s) -> void;
+  auto dbGetTrackData(TrackId id) -> std::shared_ptr<TrackData>;
+  auto dbPutHash(const uint64_t& hash, TrackId i) -> void;
+  auto dbGetHash(const uint64_t& hash) -> std::optional<TrackId>;
+
+  auto dbCreateIndexesForTrack(const Track& track) -> void;
+  auto dbRemoveIndexes(std::shared_ptr<TrackData>) -> void;
+
+  auto dbIngestTagHashes(const TrackTags&,
+                         std::pmr::unordered_map<Tag, uint64_t>&) -> void;
+  auto dbRecoverTagsFromHashes(const std::pmr::unordered_map<Tag, uint64_t>&)
+      -> std::shared_ptr<TrackTags>;
+
+  auto getRecord(const SearchKey& c)
+      -> std::optional<std::pair<std::pmr::string, Record>>;
+  auto countRecords(const SearchKey& c) -> size_t;
+};
+
+/*
+ * Container for the data needed to iterate through database records. This is a
+ * lower-level type that the higher-level iterators are built from; most users
+ * outside this namespace shouldn't need to work with continuations.
+ */
+struct SearchKey {
+  std::pmr::string prefix;
+  /* If not given, then iteration starts from `prefix`. */
+  std::optional<std::pmr::string> key;
+  int offset;
+
+  auto startKey() const -> std::string_view;
+};
+
+/*
+ * A record belonging to one of the database's indexes. This may either be a
+ * leaf record, containing a track id, or a branch record, containing a new
+ * Header to retrieve results at the next level of the index.
+ */
+class Record {
+ public:
+  Record(const IndexKey&, const leveldb::Slice&);
+
+  Record(const Record&) = default;
+  Record& operator=(const Record& other) = default;
+
+  auto text() const -> std::string_view;
+  auto contents() const -> const std::variant<TrackId, IndexKey::Header>&;
+
+ private:
+  std::pmr::string text_;
+  std::variant<TrackId, IndexKey::Header> contents_;
+};
+
+/*
+ * Utility for accessing a large set of database records, one record at a time.
+ */
+class Iterator {
+ public:
+  Iterator(std::shared_ptr<Database>, IndexId);
+  Iterator(std::shared_ptr<Database>, const IndexKey::Header&);
+
+  Iterator(const Iterator&) = default;
+  Iterator& operator=(const Iterator& other) = default;
+
+  auto value() const -> const std::optional<Record>&;
+  std::optional<Record> operator*() const { return value(); }
+
+  auto next() -> void;
+  std::optional<Record> operator++() {
+    next();
+    return value();
+  }
+  std::optional<Record> operator++(int) {
+    auto val = value();
+    next();
+    return val;
+  }
+
+  auto prev() -> void;
+  std::optional<Record> operator--() {
+    prev();
+    return value();
+  }
+  std::optional<Record> operator--(int) {
+    auto val = value();
+    prev();
+    return val;
+  }
+
+  auto count() const -> size_t;
+
+ private:
+  auto iterate(const SearchKey& key) -> void;
+
+  friend class TrackIterator;
+
+  std::weak_ptr<Database> db_;
+  SearchKey key_;
+  std::optional<Record> current_;
+};
+
+class TrackIterator {
+ public:
+  TrackIterator(const Iterator&);
+
+  TrackIterator(const TrackIterator&) = default;
+  TrackIterator& operator=(TrackIterator&& other) = default;
+
+  auto value() const -> std::optional<TrackId>;
+  std::optional<TrackId> operator*() const { return value(); }
+
+  auto next() -> void;
+  std::optional<TrackId> operator++() {
+    next();
+    return value();
+  }
+  std::optional<TrackId> operator++(int) {
+    auto val = value();
+    next();
+    return val;
+  }
+
+  auto count() const -> size_t;
+
+ private:
+  TrackIterator(std::weak_ptr<Database>);
+  auto next(bool advance) -> void;
+
+  std::weak_ptr<Database> db_;
+  std::vector<Iterator> levels_;
+};
+
+}  // namespace database