Make queries a little less copy-paste

src/database/database.cpp

@@ -1,4 +1,5 @@
 #include "database.hpp"
+#include <functional>
 
 #include "esp_log.h"
 #include "ff.h"
@@ -92,34 +93,24 @@ auto Database::Populate() -> std::future<void> {
   });
 }
 
-auto Database::GetSongs(std::size_t page_size) -> std::future<DbResult<Song>> {
-  return RunOnDbTask<DbResult<Song>>([&]() -> DbResult<Song> {
-    std::unique_ptr<leveldb::Iterator> it(
-        db_->NewIterator(leveldb::ReadOptions()));
-    it->Seek("title:");
-    std::vector<Song> results;
-    IterateAndParse(it.get(), page_size,
-                    [&](const leveldb::Slice& key, const leveldb::Slice& val) {
-                      Song s;
-                      s.title = key.ToString();
-                      results.push_back(s);
-                    });
-    return DbResult<Song>(results, std::move(it));
+auto parse_song(const leveldb::Slice& key, const leveldb::Slice& val)
+    -> std::optional<Song> {
+  Song s;
+  s.title = key.ToString();
+  return s;
+}
+
+auto Database::GetSongs(std::size_t page_size) -> std::future<Result<Song>> {
+  return RunOnDbTask<Result<Song>>([=, this]() -> Result<Song> {
+    return Query<Song>("title:", page_size, &parse_song);
   });
 }
 
-auto Database::GetMoreSongs(std::size_t page_size, DbResult<Song> continuation)
-    -> std::future<DbResult<Song>> {
-  return RunOnDbTask<DbResult<Song>>([&]() -> DbResult<Song> {
-    std::unique_ptr<leveldb::Iterator> it(continuation.it());
-    std::vector<Song> results;
-    IterateAndParse(it.get(), page_size,
-                    [&](const leveldb::Slice& key, const leveldb::Slice& val) {
-                      Song s;
-                      s.title = key.ToString();
-                      results.push_back(s);
-                    });
-    return DbResult<Song>(results, std::move(it));
+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, &parse_song);
   });
 }
 

src/database/include/database.hpp

@@ -11,6 +11,7 @@
 #include "leveldb/cache.h"
 #include "leveldb/db.h"
 #include "leveldb/iterator.h"
+#include "leveldb/options.h"
 #include "leveldb/slice.h"
 #include "result.hpp"
 
@@ -33,17 +34,33 @@ struct Song {
 
 struct SongMetadata {};
 
+typedef std::unique_ptr<leveldb::Iterator> Continuation;
+
 template <typename T>
-class DbResult {
+class Result {
  public:
-  DbResult(const std::vector<T>& values, std::unique_ptr<leveldb::Iterator> it)
-      : values_(values), it_(std::move(it)) {}
-  auto values() -> std::vector<T> { return values_; }
-  auto it() -> leveldb::Iterator* { return it_.release(); };
+  auto values() -> std::unique_ptr<std::vector<T>> {
+    return std::move(values_);
+  }
+  auto continuation() -> Continuation { return std::move(c_); }
+  auto HasMore() -> bool { return c_->Valid(); }
+
+  Result(std::unique_ptr<std::vector<T>> values, Continuation c)
+      : values_(std::move(values)), c_(std::move(c)) {}
+
+  Result(Result&& other)
+      : values_(std::move(other.values_)), c_(std::move(other.c_)) {}
+
+  Result operator=(Result&& other) {
+    return Result(other.values(), other.continuation());
+  }
+
+  Result(const Result&) = delete;
+  Result& operator=(const Result&) = delete;
 
  private:
-  std::vector<T> values_;
-  std::unique_ptr<leveldb::Iterator> it_;
+  std::unique_ptr<std::vector<T>> values_;
+  Continuation c_;
 };
 
 class Database {
@@ -58,23 +75,23 @@ class Database {
 
   auto Populate() -> std::future<void>;
 
-  auto GetArtists(std::size_t page_size) -> std::future<DbResult<Artist>>;
-  auto GetMoreArtists(std::size_t page_size, DbResult<Artist> continuation)
-      -> std::future<DbResult<Artist>>;
+  auto GetArtists(std::size_t page_size) -> std::future<Result<Artist>>;
+  auto GetMoreArtists(std::size_t page_size, Continuation c)
+      -> std::future<Result<Artist>>;
 
   auto GetAlbums(std::size_t page_size, std::optional<Artist> artist)
-      -> std::future<DbResult<Album>>;
-  auto GetMoreAlbums(std::size_t page_size, DbResult<Album> continuation)
-      -> std::future<DbResult<Album>>;
+      -> std::future<Result<Album>>;
+  auto GetMoreAlbums(std::size_t page_size, Continuation c)
+      -> std::future<Result<Album>>;
 
-  auto GetSongs(std::size_t page_size) -> std::future<DbResult<Song>>;
+  auto GetSongs(std::size_t page_size) -> std::future<Result<Song>>;
   auto GetSongs(std::size_t page_size, std::optional<Artist> artist)
-      -> std::future<DbResult<Song>>;
+      -> std::future<Result<Song>>;
   auto GetSongs(std::size_t page_size,
                 std::optional<Artist> artist,
-                std::optional<Album> album) -> std::future<DbResult<Song>>;
-  auto GetMoreSongs(std::size_t page_size, DbResult<Song> continuation)
-      -> std::future<DbResult<Song>>;
+                std::optional<Album> album) -> std::future<Result<Song>>;
+  auto GetMoreSongs(std::size_t page_size, Continuation c)
+      -> std::future<Result<Song>>;
 
   auto GetSongIds(std::optional<Artist> artist, std::optional<Album> album)
       -> std::future<std::vector<SongId_t>>;
@@ -89,6 +106,33 @@ class Database {
   std::unique_ptr<leveldb::Cache> cache_;
 
   Database(leveldb::DB* db, leveldb::Cache* cache);
+
+  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);
+  }
+
+  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)};
+  }
 };
 
 }  // namespace database

src/main/app_console.cpp

@@ -176,10 +176,18 @@ int CmdDbSongs(int argc, char** argv) {
     return 1;
   }
 
-  database::DbResult<database::Song> res =
+  database::Result<database::Song> res =
       sInstance->database_->GetSongs(10).get();
-  for (database::Song s : res.values()) {
-    std::cout << s.title << std::endl;
+  while (true) {
+    std::unique_ptr<std::vector<database::Song>> r = res.values();
+    for (database::Song s : *r) {
+      std::cout << s.title << std::endl;
+    }
+    if (res.HasMore()) {
+      res = sInstance->database_->GetMoreSongs(10, res.continuation()).get();
+    } else {
+      break;
+    }
   }
 
   return 0;