49 lines
1.7 KiB
Swift
49 lines
1.7 KiB
Swift
//
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// Copyright 2023 Signal Messenger, LLC
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// SPDX-License-Identifier: AGPL-3.0-only
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//
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import Foundation
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/// Generates timestamps for messages/envelopes.
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@objc
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public class MessageTimestampGenerator: NSObject {
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private let rangeToAvoid = AtomicValue<ClosedRange<UInt64>?>(nil, lock: .init())
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private let nowMs: () -> UInt64
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@objc
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public static let sharedInstance = MessageTimestampGenerator()
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public init(nowMs: @escaping () -> UInt64 = NSDate.ows_millisecondTimeStamp) {
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self.nowMs = nowMs
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}
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/// Generates a new timestamp from the device's local clock.
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///
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/// Performs a few heuristics to try and avoid generating the same timestamp
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/// repeatedly when called in a tight loop.
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@objc
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public func generateTimestamp() -> UInt64 {
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let generatedTimestamp = max(nowMs(), 1)
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return rangeToAvoid.update { rangeToAvoid in
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let newRangeToAvoid = Self.avoidAndExtendRange(rangeToAvoid, proposedValue: generatedTimestamp)
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rangeToAvoid = newRangeToAvoid
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return newRangeToAvoid.upperBound
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}
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}
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private static func avoidAndExtendRange(
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_ oldRange: ClosedRange<UInt64>?,
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proposedValue: UInt64
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) -> ClosedRange<UInt64> {
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if let oldRange, oldRange.contains(proposedValue) {
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// If we have a range from the last value, ensure that the new one is
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// higher. We track the range to handle cases where `generateTimestamp()`
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// is called twice for `t1` and then once for `t1 + 1`.
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return oldRange.lowerBound...(oldRange.upperBound + 1)
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} else {
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// Otherwise, we assume there's no conflict and return `proposedValue`.
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return proposedValue...proposedValue
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}
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}
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}
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