Song Format

Playback State Format

• status is PLAYING, PAUSED, or STOPPED.
• When no song is selected, return "STOPPED,NO_SONG,0".

Class

Constructor

• Creates an empty music catalogue.
• Every value in userIds represents an existing user.
• Each user initially has an empty playback session.

Method Signatures

Add or Update Song

• Adds the song to the global music catalogue when songId does not already exist.
• When songId already exists, replaces its title, artist, album, and duration with the supplied values.
• The method does not return a value.
• Updating a song does not reset or remove any user's current playback session.
• A user currently playing the updated song observes its updated catalogue details.

Search Songs

• Performs a case-insensitive substring search over song titles, artists, and albums.
• Returns each matching song using the specified song format.
• Returns matching songs in lexicographically increasing order of songId.
• Returns an empty list when no song matches.

Play Song

• Starts playing the given song for the user from position 0.
• Replaces any song currently selected in the user's playback session.
• Once a user starts playing a song, its duration for that playback session remains unchanged even if the song's catalogue details are later updated.
• A user currently playing the updated song observes its updated title, artist, and album, but the duration stored in that playback session remains unchanged.
• Starting or restarting a song creates a new playback session using the song's current catalogue duration.
• Returns "PLAYING,songId,0".

Control Playback

• action is "PAUSE", "RESUME", "SEEK", or "STOP".
• "PAUSE" changes a playing song to PAUSED.
• "RESUME" changes a paused song to PLAYING.
• "SEEK" moves playback to positionSeconds without changing the current playback status.
• "STOP" clears the user's current playback session.
• For actions other than "SEEK", positionSeconds is 0.
• For "SEEK", 0 ≤ positionSeconds < durationSeconds.
• The playback session is always in a valid state for the supplied action.
• Returns the resulting playback state.

Rules

• Search is case-insensitive.
• Search results are sorted by songId.
• Every user has an independent playback session.
• Starting another song replaces the user's current song.
• Playback positions do not advance automatically between method calls.
• Updating a song preserves its songId.
• All user IDs and song IDs contain only lowercase English letters, digits, and hyphens.
• The value NO_SONG is a special playback marker and is not a song ID.
• All constructor and method parameters satisfy the stated constraints.

Design Expectations

• Use separate components for the music catalogue and user playback sessions.
• Updating catalogue metadata should not require recreating playback sessions.
• Each user's playback state must be stored and modified independently.
• Catalogue and playback operations should safely support calls from multiple users.

Constraints

• 1 ≤ userIds.size() ≤ 100,000
• 1 ≤ number of songs ≤ 100,000
• 1 ≤ userId.length(), songId.length() ≤ 50
• 1 ≤ title.length(), artist.length(), album.length() ≤ 100
• 1 ≤ durationSeconds ≤ 86,400
• 1 ≤ query.length() ≤ 100
• userId and songId contain only a-z, 0-9, and -.
• All user IDs passed to the constructor are unique.
• All string parameters are non-empty.
• Song titles, artist names, and album names do not contain commas.
• All input parameters are valid.

Examples

Example 1: Add, Update, and Search Songs

Example 2: Play and Pause a Song

Example 3: Independent Playback Sessions