SortedDifference

This Swift package defines the SortedDifference sequence, which computes the difference between two sequences of identifiable elements.

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Latest release: version 1.1.0 (March 1, 2020) • Release Notes

Requirements: Swift 5.1+, Xcode 11.3+

Contact: Report bugs and ask questions in Github issues.

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• Motivation
• Reference

Motivation

SortedDifference eases, for example, the synchronization of a server payload and a local database.

The sample code below synchronizes the list of database players with the list of players loaded from a remote API server. It uses Alamofire and GRDB.

It has a low complexity, and performs as little database I/O as possible:

• The api players are sorted in O(n log n).
• Database players, sorted by primary key, are fetched in a single efficient SQL request.
• The iteration of SortedDifference is O(n).
• SortedDifference accepts sequences of different types, which means that it can handle different types for api and database players, and that we do not have to perform conversions from one to the other unless strictly necessary.
• Each insertion and deletion runs one SQL request.
• Each update only runs an SQL request if there are actual changes.
• The whole batch is wrapped in a single database transaction, for guaranteed database integrity, and maximum speed.
• When database and server are already synchronized, a single SQL request is executed, and no write happens in the database.

struct APIPlayer: Decodable, Identifiable { ... }
struct DatabasePlayer: FetchableRecord, PersistableRecord, Identifiable { ... }

extension DatabasePlayer {
    init(_ player: APIPlayer) { ... }
}

// Load all api players
AF.request("https://example.com/players").responseDecodable(of: [APIPlayer].self) { response in
    do {
        // Sort api players by id, as required by SortedDifference
        let apiPlayers = try response.result.get().sorted(by: { $0.id < $1.id })
        
        // Synchronize players in a database transaction
        try database.write { db in
            // Fetch database players, sorted by id, as required by SortedDifference
            let dbPlayers = try DatabasePlayer.orderByPrimaryKey().fetchAll(db)
            
            for change in SortedDifference(left: dbPlayers, right: apiPlayers) {
                switch change {
                    case let .left(dbPlayer):
                        // Player exists in the database, but not on the server: delete it
                        try dbPlayer.delete(db)
                        
                    case let .right(apiPlayer):
                        // Player exists on the server, but not in the database: insert it
                        let newPlayer = DatabasePlayer(apiPlayer)
                        try newPlayer.insert(db)
                        
                    case let .common(dbPlayer, apiPlayer):
                        // Player exists on the server and in the database: update if needed
                        let newPlayer = DatabasePlayer(apiPlayer)
                        try newPlayer.updateChanges(db, from: dbPlayer)
                }
            }
        }
    } catch {
        // handle network or database error
    }
}

Reference

SortedDifference comes with a general initializer, and two convenience initializers.

All initializers share a common precondition: ids of both left and right sequences must be strictly increasing. In other words, sequences must be sorted by id, and ids must be unique in each sequence.

For extra safety, this precondition is checked in debug builds (as a Swift assertion). If the precondition is not honored, the behavior of SortedDifference is undefined.

The general initializer is the less constrained: it lets you provide closures that return identifiers of both left and right elements:

struct SortedDifference<LeftSequence, RightSequence, ID>: Sequence where
    LeftSequence: Sequence,
    RightSequence: Sequence,
    ID: Comparable
{
    init(
        left: LeftSequence,
        identifiedBy leftID: @escaping (LeftSequence.Element) -> ID,
        right: RightSequence,
        identifiedBy rightID: @escaping (RightSequence.Element) -> ID)
}

// Prints:
// - common(Left(id: 1), Right(id: 1))
// - left(Left(id: 2))
struct Left { var id: Int }
struct Right { var id: Int }
for change in SortedDifference(
    left: [Left(id: 1), Left(id: 2)],
    identifiedBy: { $0.id },
    right: [Right(id: 1)],
    identifiedBy: { $0.id })
{
    print(change)
}

There is a convenience initializer for sequences of Identifiable types:

@available(OSX 10.15, iOS 13, tvOS 13, watchOS 6, *)
extension SortedDifference where
    LeftSequence.Element: Identifiable,
    RightSequence.Element: Identifiable,
    LeftSequence.Element.ID == ID,
    RightSequence.Element.ID == ID
{
    init(left: LeftSequence, right: RightSequence)
}

// Prints:
// - common(Left(id: 1), Right(id: 1))
// - left(Left(id: 2))
struct Left: Identifiable { var id: Int }
struct Right: Identifiable { var id: Int }
for change in SortedDifference(
    left: [Left(id: 1), Left(id: 2)],
    right: [Right(id: 1)])
{
    print(change)
}

There is a convenience initializer for sequences of Comparable types:

extension SortedDifference where
    LeftSequence.Element: Comparable,
    RightSequence.Element == LeftSequence.Element,
    LeftSequence.Element == ID
{
    init(left: LeftSequence, right: RightSequence)
}

// Prints:
// - common(1, 1)
// - left(2)
for change in SortedDifference(left: [1, 2], right: [1]) {
    print(change)
}