/// <summary>
/// Builds a dictionary containing Song ID keys and weight values.
/// The weight values are determined by song popularity, song like, artist like, album like, and tag like.
/// </summary>
/// <param name="songsWithRanking">Songs grouped by community rank standing.</param>
/// <returns></returns>
/// <remarks>
/// Based off of the song weights algorithm described here:
/// http://stackoverflow.com/questions/3345788/algorithm-for-picking-thumbed-up-items/3345838#3345838
///
/// The song weights algorithm is loosely based on the more general Multiplicative Weight Update Algorithm (MWUA), described here:
/// https://jeremykun.com/2017/02/27/the-reasonable-effectiveness-of-the-multiplicative-weights-update-algorithm/
/// </remarks>
internal Dictionary <string, SongWeight> BuildSongWeightsTable(IList <Songs_RankStandings.Result> songsWithRanking)
{
// Generate a table containing all the songs. The table will be a dictionary of SongID keys and weight values.
// Each song takes up N weight, where N starts out as 1, but can grow or shrink depending on whether that song/artist/album/tags is liked or disliked.
var totalSongCount = songsWithRanking.Sum(s => s.SongIds.Count);
var songWeights = new Dictionary <string, SongWeight>(totalSongCount);
foreach (var ranking in songsWithRanking)
{
var rankingMultipler = GetWeightMultiplier(ranking.Standing);
var songIdsAndDates = ranking.SongIds.Zip(ranking.SongUploadDates);
foreach (var(songId, date) in songIdsAndDates)
{
// Give it a weight based on its community rank.
// Multiply that weight by the song's age (newer songs are played more often.)
var ageMultiplier = GetAgeMultiplier(date);
var rankAndAgeWeight = SongWeight.Default()
.WithCommunityRankMultiplier(rankingMultipler)
.WithAgeMultiplier(ageMultiplier);
songWeights[songId] = rankAndAgeWeight;
}
}
// Now we've generated the table with all songs, each weighted according to their community ranking.
// Next, adjust the weight based on whether we like this song or not.
foreach (var likedSong in Songs)
{
if (songWeights.TryGetValue(likedSong.SongId, out var existingWeight))
{
var songMultiplier = GetSongLikeDislikeMultiplier(likedSong);
songWeights[likedSong.SongId] = existingWeight.WithSongMultiplier(songMultiplier);
}
}
// Next, adjust the weight based on whether we like this artist or not.
var artistPrefs = Artists.GroupBy(a => a.Name);
foreach (var artist in artistPrefs)
{
var artistMultiplier = GetArtistMultiplier(artist);
if (artistMultiplier != 1.0)
{
foreach (var pref in artist)
{
if (songWeights.TryGetValue(pref.SongId, out var existingWeight))
{
songWeights[pref.SongId] = existingWeight.WithArtistMultiplier(artistMultiplier);
}
}
}
}
// Next, adjust the weight based on whether we like this album or not.
var albumPrefs = Albums.GroupBy(a => a.Name);
foreach (var album in albumPrefs)
{
var albumMultiplier = GetAlbumMultiplier(album);
if (albumMultiplier != 1.0)
{
foreach (var pref in album)
{
if (songWeights.TryGetValue(pref.SongId, out var existingWeight))
{
songWeights[pref.SongId] = existingWeight.WithAlbumMultiplier(albumMultiplier);
}
}
}
}
// Adjust the weight based on whether we like the tags of the song or not.
var tagLikeDislikeDifferences = CreateTagLikeDislikeDifferences(Tags);
var songTags = Tags.GroupBy(t => t.SongId);
var songsWithTagMultipliers = songTags.Select(tag => (SongId: tag.Key, TagsMultiplier: GetCumulativeTagMultiplier(tag, tagLikeDislikeDifferences)));
foreach (var(SongId, TagsMultiplier) in songsWithTagMultipliers)
{
if (songWeights.TryGetValue(SongId, out var existingWeight))
{
songWeights[SongId] = existingWeight.WithTagMultiplier(TagsMultiplier);
}
}
return(songWeights);
}