Change DAU pixel for VPN to weekly (#2552)

Task/Issue URL: https://app.asana.com/0/0/1206685546588311/f

**Description**:

Reported `cohort` now uses the week number.

DuckDuckGo/NetworkProtection/NetworkExtensionTargets/NetworkExtensionTargets/MacPacketTunnelProvider.swift

@@ -151,7 +151,7 @@ final class MacPacketTunnelProvider: PacketTunnelProvider {
             PixelKit.fire(
                 NetworkProtectionPixelEvent.networkProtectionActiveUser,
                 frequency: .dailyOnly,
-                withAdditionalParameters: ["cohort": PixelKit.dateString(for: defaults.vpnFirstEnabled)],
+                withAdditionalParameters: [PixelKit.Parameters.vpnCohort: PixelKit.cohort(from: defaults.vpnFirstEnabled)],
                 includeAppVersionParameter: true)
         case .reportConnectionAttempt(attempt: let attempt):
             switch attempt {

LocalPackages/PixelKit/Sources/PixelKit/PixelKit+Parameters.swift

@@ -66,6 +66,8 @@ public extension PixelKit {
         public static let vpnBreakageMetadata = "breakageMetadata"
 
         public static let reason = "reason"
+
+        public static let vpnCohort = "cohort"
     }
 
     enum Values {

LocalPackages/PixelKit/Sources/PixelKit/PixelKit.swift

@@ -72,13 +72,7 @@ public final class PixelKit {
         return calendar
     }()
 
-    private var dateFormatter: DateFormatter = {
-        let dateFormatter = DateFormatter()
-        dateFormatter.calendar = defaultDailyPixelCalendar
-        dateFormatter.timeZone = defaultDailyPixelCalendar.timeZone
-        dateFormatter.dateFormat = "yyyy-MM-dd"
-        return dateFormatter
-    }()
+    private static let weeksToCoalesceCohort = 6
 
     private let dateGenerator: () -> Date
 
@@ -99,13 +93,17 @@ public final class PixelKit {
                              source: String? = nil,
                              defaultHeaders: [String: String],
                              log: OSLog,
+                             dailyPixelCalendar: Calendar? = nil,
+                             dateGenerator: @escaping () -> Date = Date.init,
                              defaults: UserDefaults,
                              fireRequest: @escaping FireRequest) {
         shared = PixelKit(dryRun: dryRun,
                           appVersion: appVersion,
                           source: source,
                           defaultHeaders: defaultHeaders,
                           log: log,
+                          dailyPixelCalendar: dailyPixelCalendar,
+                          dateGenerator: dateGenerator,
                           defaults: defaults,
                           fireRequest: fireRequest)
     }
@@ -318,13 +316,23 @@ public final class PixelKit {
                           onComplete: onComplete)
     }
 
-    private func dateString(for date: Date?) -> String? {
-        guard let date else { return nil }
-        return dateFormatter.string(from: date)
+    private func cohort(from cohortLocalDate: Date?, dateGenerator: () -> Date = Date.init) -> String? {
+        guard let cohortLocalDate,
+              let baseDate = pixelCalendar.date(from: .init(year: 2023, month: 1, day: 1)),
+              let weeksSinceCohortAssigned = pixelCalendar.dateComponents([.weekOfYear], from: cohortLocalDate, to: dateGenerator()).weekOfYear,
+              let assignedCohort = pixelCalendar.dateComponents([.weekOfYear], from: baseDate, to: cohortLocalDate).weekOfYear else {
+            return nil
+        }
+
+        if weeksSinceCohortAssigned > Self.weeksToCoalesceCohort {
+            return ""
+        } else {
+            return "week-" + String(assignedCohort + 1)
+        }
     }
 
-    public static func dateString(for date: Date?) -> String {
-        Self.shared?.dateString(for: date) ?? ""
+    public static func cohort(from cohortLocalDate: Date?, dateGenerator: () -> Date = Date.init) -> String {
+        Self.shared?.cohort(from: cohortLocalDate, dateGenerator: dateGenerator) ?? ""
     }
 
     public static func pixelLastFireDate(event: Event) -> Date? {

LocalPackages/PixelKit/Tests/PixelKitTests/PixelKitTests.swift

@@ -258,17 +258,18 @@ final class PixelKitTests: XCTestCase {
         // Run test
         pixelKit.fire(event, frequency: .dailyOnly) // Fired
 
-        timeMachine.travel(by: 60 * 60 * 2)
-        pixelKit.fire(event, frequency: .dailyOnly) // Skipped (2 hours since last fire)
+        timeMachine.travel(by: .hour, value: 2)
+        pixelKit.fire(event, frequency: .dailyOnly) // Skipped
 
-        timeMachine.travel(by: 60 * 60 * 24 + 1000)
-        pixelKit.fire(event, frequency: .dailyOnly) // Fired (24 hours + 1000 seconds since last fire)
+        timeMachine.travel(by: .day, value: 1)
+        timeMachine.travel(by: .hour, value: 2)
+        pixelKit.fire(event, frequency: .dailyOnly) // Fired
 
-        timeMachine.travel(by: 60 * 60 * 10)
-        pixelKit.fire(event, frequency: .dailyOnly) // Skipped (10 hours since last fire)
+        timeMachine.travel(by: .hour, value: 10)
+        pixelKit.fire(event, frequency: .dailyOnly) // Skipped
 
-        timeMachine.travel(by: 60 * 60 * 14)
-        pixelKit.fire(event, frequency: .dailyOnly) // Fired (24 hours since last fire)
+        timeMachine.travel(by: .day, value: 1)
+        pixelKit.fire(event, frequency: .dailyOnly) // Fired
 
         // Wait for expectations to be fulfilled
         wait(for: [fireCallbackCalled], timeout: 0.5)
@@ -303,28 +304,93 @@ final class PixelKitTests: XCTestCase {
         // Run test
         pixelKit.fire(event, frequency: .justOnce) // Fired
 
-        timeMachine.travel(by: 60 * 60 * 2)
+        timeMachine.travel(by: .hour, value: 2)
         pixelKit.fire(event, frequency: .justOnce) // Skipped (already fired)
 
-        timeMachine.travel(by: 60 * 60 * 24 + 1000)
+        timeMachine.travel(by: .day, value: 1)
+        timeMachine.travel(by: .hour, value: 2)
         pixelKit.fire(event, frequency: .justOnce) // Skipped (already fired)
 
-        timeMachine.travel(by: 60 * 60 * 10)
+        timeMachine.travel(by: .hour, value: 10)
         pixelKit.fire(event, frequency: .justOnce) // Skipped (already fired)
 
-        timeMachine.travel(by: 60 * 60 * 14)
+        timeMachine.travel(by: .day, value: 1)
         pixelKit.fire(event, frequency: .justOnce) // Skipped (already fired)
 
         // Wait for expectations to be fulfilled
         wait(for: [fireCallbackCalled], timeout: 0.5)
     }
+
+    func testVPNCohort() {
+        XCTAssertEqual(PixelKit.cohort(from: nil), "")
+        assertCohortEqual(.init(year: 2023, month: 1, day: 1), reportAs: "week-1")
+        assertCohortEqual(.init(year: 2024, month: 2, day: 24), reportAs: "week-60")
+    }
+
+    private func assertCohortEqual(_ cohort: DateComponents, reportAs reportedCohort: String) {
+        var calendar = Calendar.current
+        calendar.timeZone = TimeZone(secondsFromGMT: 0)!
+        calendar.locale = Locale(identifier: "en_US_POSIX")
+
+        let cohort = calendar.date(from: cohort)
+        let timeMachine = TimeMachine(calendar: calendar, date: cohort)
+
+        PixelKit.setUp(appVersion: "test",
+                       defaultHeaders: [:],
+                       log: .disabled,
+                       dailyPixelCalendar: calendar,
+                       dateGenerator: timeMachine.now,
+                       defaults: userDefaults()) { _, _, _, _, _, _ in }
+
+        // 1st week
+        XCTAssertEqual(PixelKit.cohort(from: cohort, dateGenerator: timeMachine.now), reportedCohort)
+
+        // 2nd week
+        timeMachine.travel(by: .weekOfYear, value: 1)
+        XCTAssertEqual(PixelKit.cohort(from: cohort, dateGenerator: timeMachine.now), reportedCohort)
+
+        // 3rd week
+        timeMachine.travel(by: .weekOfYear, value: 1)
+        XCTAssertEqual(PixelKit.cohort(from: cohort, dateGenerator: timeMachine.now), reportedCohort)
+
+        // 4th week
+        timeMachine.travel(by: .weekOfYear, value: 1)
+        XCTAssertEqual(PixelKit.cohort(from: cohort, dateGenerator: timeMachine.now), reportedCohort)
+
+        // 5th week
+        timeMachine.travel(by: .weekOfYear, value: 1)
+        XCTAssertEqual(PixelKit.cohort(from: cohort, dateGenerator: timeMachine.now), reportedCohort)
+
+        // 6th week
+        timeMachine.travel(by: .weekOfYear, value: 1)
+        XCTAssertEqual(PixelKit.cohort(from: cohort, dateGenerator: timeMachine.now), reportedCohort)
+
+        // 7th week
+        timeMachine.travel(by: .weekOfYear, value: 1)
+        XCTAssertEqual(PixelKit.cohort(from: cohort, dateGenerator: timeMachine.now), reportedCohort)
+
+        // 8th week
+        timeMachine.travel(by: .weekOfYear, value: 1)
+        XCTAssertEqual(PixelKit.cohort(from: cohort, dateGenerator: timeMachine.now), "")
+    }
 }
 
 private class TimeMachine {
-    private var date = Date(timeIntervalSince1970: 0)
+    private var date: Date
+    private let calendar: Calendar
+
+    init(calendar: Calendar? = nil, date: Date? = nil) {
+        self.calendar = calendar ?? {
+            var calendar = Calendar.current
+            calendar.timeZone = TimeZone(secondsFromGMT: 0)!
+            calendar.locale = Locale(identifier: "en_US_POSIX")
+            return calendar
+        }()
+        self.date = date ?? .init(timeIntervalSince1970: 0)
+    }
 
-    func travel(by timeInterval: TimeInterval) {
-        date = date.addingTimeInterval(timeInterval)
+    func travel(by component: Calendar.Component, value: Int) {
+        date = calendar.date(byAdding: component, value: value, to: now())!
     }
 
     func now() -> Date {