mobile-ios/CovidSafe/Herald/Sensor/PayloadDataSupplier.swift

//
//  PayloadDataSupplier.swift
//
//  Copyright 2020 VMware, Inc.
//  SPDX-License-Identifier: MIT
//

import Foundation

/// Payload data supplier for generating payload data that is shared with other devices to provide device identity information while maintaining privacy and security.
/// Implement this to integration your solution with this transport.
public protocol PayloadDataSupplier {
    /// Get payload for given timestamp. Use this for integration with any payload generator.
    func payload(_ timestamp: PayloadTimestamp) -> PayloadData
    
    /// Get payload for given identifier. Use this for integration with any payload generator.
    func payload(_ identifier: UUID, offset: Int, onComplete: @escaping (PayloadData?) -> Void) -> Void
    
    /// Parse raw data into payloads. This is used to split concatenated payloads that are transmitted via share payload. The default implementation assumes payload data is fixed length.
    func payload(_ data: Data) -> [PayloadData]
}

/// Implements payload splitting function, assuming fixed length payloads.
public extension PayloadDataSupplier {
    /// Default implementation assumes fixed length payload data.
    func payload(_ data: Data) -> [PayloadData] {
        // Get example payload to determine length
        let fixedLengthPayload = payload(PayloadTimestamp())
        let payloadLength = fixedLengthPayload.count
        // Split data into payloads based on fixed length
        var payloads: [PayloadData] = []
        var indexStart = 0, indexEnd = payloadLength
        while indexEnd <= data.count {
            let payload = PayloadData(data.subdata(in: indexStart..<indexEnd))
            payloads.append(payload)
            indexStart += payloadLength
            indexEnd += payloadLength
        }
        return payloads
    }
    
    /// Default Implementation returns payload(timestamp:)
    func payload(_ identifier: UUID, offset: Int, onComplete: @escaping (PayloadData?) -> Void) -> Void {
        onComplete(payload(PayloadTimestamp()))
    }
}

/// Payload timestamp, should normally be Date, but it may change to UInt64 in the future to use server synchronised relative timestamp.
public typealias PayloadTimestamp = Date

/// Encrypted payload data received from target. This is likely to be an encrypted datagram of the target's actual permanent identifier.
public typealias PayloadData = Data