Add pagination to database queries

6 files changed, 325 insertions, 150 deletions

diff --git a/src/database/database.cpp b/src/database/database.cpp
index df10ebb9..f5fe5240 100644
--- a/src/database/database.cpp
+++ b/src/database/database.cpp
@@ -2,9 +2,11 @@
 
 #include <stdint.h>
 
+#include <algorithm>
 #include <cstdint>
 #include <functional>
 #include <iomanip>
+#include <memory>
 #include <sstream>
 
 #include "esp_log.h"
@@ -216,6 +218,43 @@ auto Database::Update() -> std::future<void> {
   });
 }
 
+auto Database::GetSongs(std::size_t page_size) -> std::future<Result<Song>*> {
+  return RunOnDbTask<Result<Song>*>([=, this]() -> Result<Song>* {
+    Continuation<Song> c{.iterator = nullptr,
+                         .prefix = CreateDataPrefix().data,
+                         .start_key = CreateDataPrefix().data,
+                         .forward = true,
+                         .was_prev_forward = true,
+                         .page_size = page_size};
+    return dbGetPage(c);
+  });
+}
+
+auto Database::GetDump(std::size_t page_size)
+    -> std::future<Result<std::string>*> {
+  return RunOnDbTask<Result<std::string>*>([=, this]() -> Result<std::string>* {
+    Continuation<std::string> c{.iterator = nullptr,
+                                .prefix = "",
+                                .start_key = "",
+                                .forward = true,
+                                .was_prev_forward = true,
+                                .page_size = page_size};
+    return dbGetPage(c);
+  });
+}
+
+template <typename T>
+auto Database::GetPage(Continuation<T>* c) -> std::future<Result<T>*> {
+  Continuation<T> copy = *c;
+  return RunOnDbTask<Result<T>*>(
+      [=, this]() -> Result<T>* { return dbGetPage(copy); });
+}
+
+template auto Database::GetPage<Song>(Continuation<Song>* c)
+    -> std::future<Result<Song>*>;
+template auto Database::GetPage<std::string>(Continuation<std::string>* c)
+    -> std::future<Result<std::string>*>;
+
 auto Database::dbMintNewSongId() -> SongId {
   SongId next_id = 1;
   std::string val;
@@ -287,37 +326,148 @@ auto Database::dbPutSong(SongId id,
   dbPutHash(hash, id);
 }
 
-auto parse_song(ITagParser* parser,
-                const leveldb::Slice& key,
-                const leveldb::Slice& value) -> std::optional<Song> {
-  std::optional<SongData> data = ParseDataValue(value);
-  if (!data) {
+template <typename T>
+auto Database::dbGetPage(const Continuation<T>& c) -> Result<T>* {
+  // Work out our starting point. Sometimes this will already done.
+  leveldb::Iterator* it = nullptr;
+  if (c.iterator != nullptr) {
+    it = c.iterator->release();
+  }
+  if (it == nullptr) {
+    it = db_->NewIterator(leveldb::ReadOptions());
+    it->Seek(c.start_key);
+  }
+
+  // Fix off-by-one if we just changed direction.
+  if (c.forward != c.was_prev_forward) {
+    if (c.forward) {
+      it->Next();
+    } else {
+      it->Prev();
+    }
+  }
+
+  // Grab results.
+  std::optional<std::string> first_key;
+  std::vector<T> records;
+  while (records.size() < c.page_size && it->Valid()) {
+    if (!it->key().starts_with(c.prefix)) {
+      break;
+    }
+    if (!first_key) {
+      first_key = it->key().ToString();
+    }
+    std::optional<T> parsed = ParseRecord<T>(it->key(), it->value());
+    if (parsed) {
+      records.push_back(*parsed);
+    }
+    if (c.forward) {
+      it->Next();
+    } else {
+      it->Prev();
+    }
+  }
+
+  std::unique_ptr<leveldb::Iterator> iterator(it);
+  if (iterator != nullptr) {
+    if (!iterator->Valid() || !it->key().starts_with(c.prefix)) {
+      iterator.reset();
+    }
+  }
+
+  // Put results into canonical order if we were iterating backwards.
+  if (!c.forward) {
+    std::reverse(records.begin(), records.end());
+  }
+
+  // Work out the new continuations.
+  std::optional<Continuation<T>> next_page;
+  if (c.forward) {
+    if (iterator != nullptr) {
+      // We were going forward, and now we want the next page. Re-use the
+      // existing iterator, and point the start key at it.
+      std::string key = iterator->key().ToString();
+      next_page = Continuation<T>{
+          .iterator = std::make_shared<std::unique_ptr<leveldb::Iterator>>(
+              std::move(iterator)),
+          .prefix = c.prefix,
+          .start_key = key,
+          .forward = true,
+          .was_prev_forward = true,
+          .page_size = c.page_size,
+      };
+    }
+    // No iterator means we ran out of results in this direction.
+  } else {
+    // We were going backwards, and now we want the next page. This is a
+    // reversal, to set the start key to the first record we saw and mark that
+    // it's off by one.
+    next_page = Continuation<T>{
+        .iterator = nullptr,
+        .prefix = c.prefix,
+        .start_key = *first_key,
+        .forward = true,
+        .was_prev_forward = false,
+        .page_size = c.page_size,
+    };
+  }
+
+  std::optional<Continuation<T>> prev_page;
+  if (c.forward) {
+    // We were going forwards, and now we want the previous page. Set the search
+    // key to the first result we saw, and mark that it's off by one.
+    prev_page = Continuation<T>{
+        .iterator = nullptr,
+        .prefix = c.prefix,
+        .start_key = *first_key,
+        .forward = false,
+        .was_prev_forward = true,
+        .page_size = c.page_size,
+    };
+  } else {
+    if (iterator != nullptr) {
+      // We were going backwards, and we still want to go backwards. The
+      // iterator is still valid.
+      std::string key = iterator->key().ToString();
+      prev_page = Continuation<T>{
+          .iterator = std::make_shared<std::unique_ptr<leveldb::Iterator>>(
+              std::move(iterator)),
+          .prefix = c.prefix,
+          .start_key = key,
+          .forward = false,
+          .was_prev_forward = false,
+          .page_size = c.page_size,
+      };
+    }
+    // No iterator means we ran out of results in this direction.
+  }
+
+  return new Result<T>(std::move(records), next_page, prev_page);
+}
+
+template auto Database::dbGetPage<Song>(const Continuation<Song>& c)
+    -> Result<Song>*;
+template auto Database::dbGetPage<std::string>(
+    const Continuation<std::string>& c) -> Result<std::string>*;
+
+template <>
+auto Database::ParseRecord<Song>(const leveldb::Slice& key,
+                                 const leveldb::Slice& val)
+    -> std::optional<Song> {
+  std::optional<SongData> data = ParseDataValue(val);
+  if (!data || data->is_tombstoned()) {
     return {};
   }
   SongTags tags;
-  if (!parser->ReadAndParseTags(data->filepath(), &tags)) {
+  if (!tag_parser_->ReadAndParseTags(data->filepath(), &tags)) {
     return {};
   }
   return Song(*data, tags);
 }
 
-auto Database::GetSongs(std::size_t page_size) -> std::future<Result<Song>> {
-  return RunOnDbTask<Result<Song>>([=, this]() -> Result<Song> {
-    return Query<Song>(CreateDataPrefix().slice, page_size,
-                       std::bind_front(&parse_song, tag_parser_));
-  });
-}
-
-auto Database::GetMoreSongs(std::size_t page_size, Continuation c)
-    -> std::future<Result<Song>> {
-  leveldb::Iterator* it = c.release();
-  return RunOnDbTask<Result<Song>>([=, this]() -> Result<Song> {
-    return Query<Song>(it, page_size,
-                       std::bind_front(&parse_song, tag_parser_));
-  });
-}
-
-auto parse_dump(const leveldb::Slice& key, const leveldb::Slice& value)
+template <>
+auto Database::ParseRecord<std::string>(const leveldb::Slice& key,
+                                        const leveldb::Slice& val)
     -> std::optional<std::string> {
   std::ostringstream stream;
   stream << "key: ";
@@ -335,33 +485,14 @@ auto parse_dump(const leveldb::Slice& key, const leveldb::Slice& value)
                << static_cast<int>(str[i]);
       }
     }
-    for (std::size_t i = 2; i < str.size(); i++) {
-    }
   }
   stream << "\tval: 0x";
-  std::string str = value.ToString();
-  for (int i = 0; i < value.size(); i++) {
+  std::string str = val.ToString();
+  for (int i = 0; i < val.size(); i++) {
     stream << std::hex << std::setfill('0') << std::setw(2)
            << static_cast<int>(str[i]);
   }
   return stream.str();
 }
 
-auto Database::GetDump(std::size_t page_size)
-    -> std::future<Result<std::string>> {
-  leveldb::Iterator* it = db_->NewIterator(leveldb::ReadOptions());
-  it->SeekToFirst();
-  return RunOnDbTask<Result<std::string>>([=, this]() -> Result<std::string> {
-    return Query<std::string>(it, page_size, &parse_dump);
-  });
-}
-
-auto Database::GetMoreDump(std::size_t page_size, Continuation c)
-    -> std::future<Result<std::string>> {
-  leveldb::Iterator* it = c.release();
-  return RunOnDbTask<Result<std::string>>([=, this]() -> Result<std::string> {
-    return Query<std::string>(it, page_size, &parse_dump);
-  });
-}
-
 }  // namespace database
diff --git a/src/database/include/database.hpp b/src/database/include/database.hpp
index 29872e8d..da0ed083 100644
--- a/src/database/include/database.hpp
+++ b/src/database/include/database.hpp
@@ -22,7 +22,15 @@
 
 namespace database {
 
-typedef std::unique_ptr<leveldb::Iterator> Continuation;
+template <typename T>
+struct Continuation {
+  std::shared_ptr<std::unique_ptr<leveldb::Iterator>> iterator;
+  std::string prefix;
+  std::string start_key;
+  bool forward;
+  bool was_prev_forward;
+  size_t page_size;
+};
 
 /*
  * Wrapper for a set of results from the database. Owns the list of results, as
@@ -32,29 +40,23 @@ typedef std::unique_ptr<leveldb::Iterator> Continuation;
 template <typename T>
 class Result {
  public:
-  auto values() -> std::vector<T>* { return values_.release(); }
-  auto continuation() -> Continuation { return std::move(c_); }
-  auto HasMore() -> bool { return c_->Valid(); }
-
-  Result(std::vector<T>* values, Continuation c)
-      : values_(values), c_(std::move(c)) {}
-
-  Result(std::unique_ptr<std::vector<T>> values, Continuation c)
-      : values_(std::move(values)), c_(std::move(c)) {}
+  auto values() const -> const std::vector<T>& { return values_; }
 
-  Result(Result&& other)
-      : values_(move(other.values_)), c_(std::move(other.c_)) {}
+  auto next_page() -> std::optional<Continuation<T>>& { return next_page_; }
+  auto prev_page() -> std::optional<Continuation<T>>& { return prev_page_; }
 
-  Result operator=(Result&& other) {
-    return Result(other.values(), std::move(other.continuation()));
-  }
+  Result(const std::vector<T>&& values,
+         std::optional<Continuation<T>> next,
+         std::optional<Continuation<T>> prev)
+      : values_(values), next_page_(next), prev_page_(prev) {}
 
   Result(const Result&) = delete;
   Result& operator=(const Result&) = delete;
 
  private:
-  std::unique_ptr<std::vector<T>> values_;
-  Continuation c_;
+  std::vector<T> values_;
+  std::optional<Continuation<T>> next_page_;
+  std::optional<Continuation<T>> prev_page_;
 };
 
 class Database {
@@ -73,13 +75,11 @@ class Database {
 
   auto Update() -> std::future<void>;
 
-  auto GetSongs(std::size_t page_size) -> std::future<Result<Song>>;
-  auto GetMoreSongs(std::size_t page_size, Continuation c)
-      -> std::future<Result<Song>>;
+  auto GetSongs(std::size_t page_size) -> std::future<Result<Song>*>;
+  auto GetDump(std::size_t page_size) -> std::future<Result<std::string>*>;
 
-  auto GetDump(std::size_t page_size) -> std::future<Result<std::string>>;
-  auto GetMoreDump(std::size_t page_size, Continuation c)
-      -> std::future<Result<std::string>>;
+  template <typename T>
+  auto GetPage(Continuation<T>* c) -> std::future<Result<T>*>;
 
   Database(const Database&) = delete;
   Database& operator=(const Database&) = delete;
@@ -110,31 +110,20 @@ class Database {
       -> void;
 
   template <typename T>
-  using Parser = std::function<std::optional<T>(const leveldb::Slice& key,
-                                                const leveldb::Slice& value)>;
-
-  template <typename T>
-  auto Query(const leveldb::Slice& prefix,
-             std::size_t max_results,
-             Parser<T> parser) -> Result<T> {
-    leveldb::Iterator* it = db_->NewIterator(leveldb::ReadOptions());
-    it->Seek(prefix);
-    return Query(it, max_results, parser);
-  }
+  auto dbGetPage(const Continuation<T>& c) -> Result<T>*;
 
   template <typename T>
-  auto Query(leveldb::Iterator* it, std::size_t max_results, Parser<T> parser)
-      -> Result<T> {
-    auto results = std::make_unique<std::vector<T>>();
-    for (std::size_t i = 0; i < max_results && it->Valid(); i++) {
-      std::optional<T> r = std::invoke(parser, it->key(), it->value());
-      if (r) {
-        results->push_back(*r);
-      }
-      it->Next();
-    }
-    return {std::move(results), std::unique_ptr<leveldb::Iterator>(it)};
-  }
+  auto ParseRecord(const leveldb::Slice& key, const leveldb::Slice& val)
+      -> std::optional<T>;
 };
 
+template <>
+auto Database::ParseRecord<Song>(const leveldb::Slice& key,
+                                 const leveldb::Slice& val)
+    -> std::optional<Song>;
+template <>
+auto Database::ParseRecord<std::string>(const leveldb::Slice& key,
+                                        const leveldb::Slice& val)
+    -> std::optional<std::string>;
+
 }  // namespace database
diff --git a/src/database/include/song.hpp b/src/database/include/song.hpp
index e51e5587..9a84e124 100644
--- a/src/database/include/song.hpp
+++ b/src/database/include/song.hpp
@@ -4,6 +4,7 @@
 
 #include <optional>
 #include <string>
+#include <utility>
 
 #include "leveldb/db.h"
 #include "span.hpp"
@@ -133,16 +134,20 @@ class SongData {
  */
 class Song {
  public:
-  Song(SongData data, SongTags tags) : data_(data), tags_(tags) {}
+  Song(const SongData& data, const SongTags& tags) : data_(data), tags_(tags) {}
+  Song(const Song& other) = default;
 
-  auto data() -> const SongData& { return data_; }
-  auto tags() -> const SongTags& { return tags_; }
+  auto data() const -> const SongData& { return data_; }
+  auto tags() const -> const SongTags& { return tags_; }
 
   bool operator==(const Song&) const = default;
+  Song operator=(const Song& other) const { return Song(other); }
 
  private:
   const SongData data_;
   const SongTags tags_;
 };
 
+void swap(Song& first, Song& second);
+
 }  // namespace database
diff --git a/src/database/song.cpp b/src/database/song.cpp
index a0e6ce4b..b6a1baac 100644
--- a/src/database/song.cpp
+++ b/src/database/song.cpp
@@ -36,4 +36,10 @@ auto SongData::Exhume(const std::string& new_path) const -> SongData {
   return SongData(id_, new_path, tags_hash_, play_count_, false);
 }
 
+void swap(Song& first, Song& second) {
+  Song temp = first;
+  first = second;
+  second = temp;
+}
+
 }  // namespace database
diff --git a/src/database/test/test_database.cpp b/src/database/test/test_database.cpp
index d61421ee..a88c4bb3 100644
--- a/src/database/test/test_database.cpp
+++ b/src/database/test/test_database.cpp
@@ -60,8 +60,8 @@ TEST_CASE("song database", "[integration]") {
   std::unique_ptr<Database> db(open_res.value());
 
   SECTION("empty database") {
-    std::unique_ptr<std::vector<Song>> res(db->GetSongs(10).get().values());
-    REQUIRE(res->size() == 0);
+    std::unique_ptr<Result<Song>> res(db->GetSongs(10).get());
+    REQUIRE(res->values().size() == 0);
   }
 
   SECTION("add new songs") {
@@ -71,24 +71,23 @@ TEST_CASE("song database", "[integration]") {
 
     db->Update();
 
-    std::unique_ptr<std::vector<Song>> res(db->GetSongs(10).get().values());
-    REQUIRE(res->size() == 3);
-    CHECK(*res->at(0).tags().title == "Song 1");
-    CHECK(res->at(0).data().id() == 1);
-    CHECK(*res->at(1).tags().title == "Song 2");
-    CHECK(res->at(1).data().id() == 2);
-    CHECK(*res->at(2).tags().title == "Song 3");
-    CHECK(res->at(2).data().id() == 3);
+    std::unique_ptr<Result<Song>> res(db->GetSongs(10).get());
+    REQUIRE(res->values().size() == 3);
+    CHECK(*res->values().at(0).tags().title == "Song 1");
+    CHECK(res->values().at(0).data().id() == 1);
+    CHECK(*res->values().at(1).tags().title == "Song 2");
+    CHECK(res->values().at(1).data().id() == 2);
+    CHECK(*res->values().at(2).tags().title == "Song 3");
+    CHECK(res->values().at(2).data().id() == 3);
 
     SECTION("update with no filesystem changes") {
       db->Update();
 
-      std::unique_ptr<std::vector<Song>> new_res(
-          db->GetSongs(10).get().values());
-      REQUIRE(new_res->size() == 3);
-      CHECK(res->at(0) == new_res->at(0));
-      CHECK(res->at(1) == new_res->at(1));
-      CHECK(res->at(2) == new_res->at(2));
+      std::unique_ptr<Result<Song>> new_res(db->GetSongs(10).get());
+      REQUIRE(new_res->values().size() == 3);
+      CHECK(res->values().at(0) == new_res->values().at(0));
+      CHECK(res->values().at(1) == new_res->values().at(1));
+      CHECK(res->values().at(2) == new_res->values().at(2));
     }
 
     SECTION("update with all songs gone") {
@@ -96,19 +95,17 @@ TEST_CASE("song database", "[integration]") {
 
       db->Update();
 
-      std::unique_ptr<std::vector<Song>> new_res(
-          db->GetSongs(10).get().values());
-      CHECK(new_res->size() == 0);
+      std::unique_ptr<Result<Song>> new_res(db->GetSongs(10).get());
+      CHECK(new_res->values().size() == 0);
 
       SECTION("update with one song returned") {
         songs.MakeSong("song2.wav", "Song 2");
 
         db->Update();
 
-        std::unique_ptr<std::vector<Song>> new_res(
-            db->GetSongs(10).get().values());
-        REQUIRE(new_res->size() == 1);
-        CHECK(res->at(1) == new_res->at(0));
+        std::unique_ptr<Result<Song>> new_res(db->GetSongs(10).get());
+        REQUIRE(new_res->values().size() == 1);
+        CHECK(res->values().at(1) == new_res->values().at(0));
       }
     }
 
@@ -117,11 +114,10 @@ TEST_CASE("song database", "[integration]") {
 
       db->Update();
 
-      std::unique_ptr<std::vector<Song>> new_res(
-          db->GetSongs(10).get().values());
-      REQUIRE(new_res->size() == 2);
-      CHECK(res->at(0) == new_res->at(0));
-      CHECK(res->at(2) == new_res->at(1));
+      std::unique_ptr<Result<Song>> new_res(db->GetSongs(10).get());
+      REQUIRE(new_res->values().size() == 2);
+      CHECK(res->values().at(0) == new_res->values().at(0));
+      CHECK(res->values().at(2) == new_res->values().at(1));
     }
 
     SECTION("update with tags changed") {
@@ -129,14 +125,14 @@ TEST_CASE("song database", "[integration]") {
 
       db->Update();
 
-      std::unique_ptr<std::vector<Song>> new_res(
-          db->GetSongs(10).get().values());
-      REQUIRE(new_res->size() == 3);
-      CHECK(res->at(0) == new_res->at(0));
-      CHECK(res->at(1) == new_res->at(1));
-      CHECK(*new_res->at(2).tags().title == "The Song 3");
+      std::unique_ptr<Result<Song>> new_res(db->GetSongs(10).get());
+      REQUIRE(new_res->values().size() == 3);
+      CHECK(res->values().at(0) == new_res->values().at(0));
+      CHECK(res->values().at(1) == new_res->values().at(1));
+      CHECK(*new_res->values().at(2).tags().title == "The Song 3");
       // The id should not have changed, since this was just a tag update.
-      CHECK(res->at(2).data().id() == new_res->at(2).data().id());
+      CHECK(res->values().at(2).data().id() ==
+            new_res->values().at(2).data().id());
     }
 
     SECTION("update with one new song") {
@@ -144,14 +140,61 @@ TEST_CASE("song database", "[integration]") {
 
       db->Update();
 
-      std::unique_ptr<std::vector<Song>> new_res(
-          db->GetSongs(10).get().values());
-      REQUIRE(new_res->size() == 4);
-      CHECK(res->at(0) == new_res->at(0));
-      CHECK(res->at(1) == new_res->at(1));
-      CHECK(res->at(2) == new_res->at(2));
-      CHECK(*new_res->at(3).tags().title == "Song 1 (nightcore remix)");
-      CHECK(new_res->at(3).data().id() == 4);
+      std::unique_ptr<Result<Song>> new_res(db->GetSongs(10).get());
+      REQUIRE(new_res->values().size() == 4);
+      CHECK(res->values().at(0) == new_res->values().at(0));
+      CHECK(res->values().at(1) == new_res->values().at(1));
+      CHECK(res->values().at(2) == new_res->values().at(2));
+      CHECK(*new_res->values().at(3).tags().title ==
+            "Song 1 (nightcore remix)");
+      CHECK(new_res->values().at(3).data().id() == 4);
+    }
+
+    SECTION("get songs with pagination") {
+      std::unique_ptr<Result<Song>> res(db->GetSongs(1).get());
+
+      REQUIRE(res->values().size() == 1);
+      CHECK(res->values().at(0).data().id() == 1);
+      REQUIRE(res->next_page());
+
+      res.reset(db->GetPage(&res->next_page().value()).get());
+
+      REQUIRE(res->values().size() == 1);
+      CHECK(res->values().at(0).data().id() == 2);
+      REQUIRE(res->next_page());
+
+      res.reset(db->GetPage(&res->next_page().value()).get());
+
+      REQUIRE(res->values().size() == 1);
+      CHECK(res->values().at(0).data().id() == 3);
+      REQUIRE(!res->next_page());
+
+      SECTION("page backwards") {
+        REQUIRE(res->prev_page());
+
+        res.reset(db->GetPage(&res->prev_page().value()).get());
+
+        REQUIRE(res->values().size() == 1);
+        CHECK(res->values().at(0).data().id() == 2);
+        REQUIRE(res->prev_page());
+
+        res.reset(db->GetPage(&res->prev_page().value()).get());
+
+        REQUIRE(res->values().size() == 1);
+        CHECK(res->values().at(0).data().id() == 1);
+        REQUIRE(!res->prev_page());
+
+        SECTION("page forwards again") {
+          REQUIRE(res->next_page());
+
+          res.reset(db->GetPage(&res->next_page().value()).get());
+
+          REQUIRE(res->values().size() == 1);
+          CHECK(res->values().at(0).data().id() == 2);
+          CHECK(res->next_page());
+          CHECK(res->prev_page());
+        }
+      }
     }
   }
 }
diff --git a/src/main/app_console.cpp b/src/main/app_console.cpp
index 759afa91..a85faaf0 100644
--- a/src/main/app_console.cpp
+++ b/src/main/app_console.cpp
@@ -176,15 +176,15 @@ int CmdDbSongs(int argc, char** argv) {
     return 1;
   }
 
-  database::Result<database::Song> res =
-      sInstance->database_->GetSongs(20).get();
+  std::unique_ptr<database::Result<database::Song>> res(
+      sInstance->database_->GetSongs(5).get());
   while (true) {
-    std::unique_ptr<std::vector<database::Song>> r(res.values());
-    for (database::Song s : *r) {
+    for (database::Song s : res->values()) {
       std::cout << s.tags().title.value_or("[BLANK]") << std::endl;
     }
-    if (res.HasMore()) {
-      res = sInstance->database_->GetMoreSongs(10, res.continuation()).get();
+    if (res->next_page()) {
+      auto continuation = res->next_page().value();
+      res.reset(sInstance->database_->GetPage(&continuation).get());
     } else {
       break;
     }
@@ -211,17 +211,18 @@ int CmdDbDump(int argc, char** argv) {
 
   std::cout << "=== BEGIN DUMP ===" << std::endl;
 
-  database::Result<std::string> res = sInstance->database_->GetDump(20).get();
+  std::unique_ptr<database::Result<std::string>> res(
+      sInstance->database_->GetDump(5).get());
   while (true) {
-    std::unique_ptr<std::vector<std::string>> r(res.values());
-    if (r == nullptr) {
-      break;
-    }
-    for (std::string s : *r) {
+    for (std::string s : res->values()) {
       std::cout << s << std::endl;
     }
-    if (res.HasMore()) {
-      res = sInstance->database_->GetMoreDump(20, res.continuation()).get();
+    if (res->next_page()) {
+      auto continuation = res->next_page().value();
+      res.reset(
+          sInstance->database_->GetPage<std::string>(&continuation).get());
+    } else {
+      break;
     }
   }