import ConjureKit
import Foundation
import Testing

@MainActor
private final class RateRequests {
    var pending: [CheckedContinuation<CurrencyRates, any Error>] = []
    func fetch() async throws -> CurrencyRates {
        try await withCheckedThrowingContinuation { pending.append($0) }
    }
}

@MainActor
private func waitForRates(_ condition: () -> Bool) async throws {
    // Scheduling deadline only; calculation freshness uses the injected clock.
    for _ in 0..<1_000 {
        if condition() { return }
        try await Task.sleep(for: .milliseconds(1))
    }
    try #require(condition(), "Currency calculation did not reach the expected state")
}

@Test @MainActor
func currencyLifetimeSharesRefreshAndPreservesStaleCopyUntilFailure() async throws {
    var calendar = Calendar(identifier: .gregorian)
    calendar.timeZone = TimeZone(secondsFromGMT: 0)!
    var now = calendar.date(from: DateComponents(year: 2026, month: 1, day: 2, hour: 12))!
    let requests = RateRequests()
    let calculation = CurrencyCalculation(now: { now }, calendar: calendar, fetch: { try await requests.fetch() })
    var completions = 0
    calculation.changed = { completions += 1 }
    let snapshot = CurrencyRates(date: "2026-01-02", rates: ["eur": 1, "usd": 2, "dkk": 8])

    calculation.update("100 USD to DKK")
    #expect(calculation.isCurrency)
    #expect(calculation.result?.copyValue == nil)
    try await waitForRates { requests.pending.count == 1 }
    calculation.update("50 USD to DKK")
    requests.pending[0].resume(returning: snapshot)
    try await waitForRates { completions == 1 }
    #expect(calculation.result?.copyValue == "200 DKK")
    calculation.update("100 USD to DKK")
    #expect(calculation.result?.copyValue == "400 DKK")
    #expect(requests.pending.count == 1)

    now = calendar.date(byAdding: .day, value: 1, to: now)!
    calculation.update("100 USD to DKK")
    #expect(calculation.result?.copyValue == "400 DKK")
    #expect(calculation.result?.detail.contains("Stale cache; refreshing") == true)
    try await waitForRates { requests.pending.count == 2 }
    calculation.update("50 USD to DKK")
    requests.pending[1].resume(throwing: URLError(.notConnectedToInternet))
    try await waitForRates { completions == 2 }
    #expect(calculation.result?.copyValue == nil)
    #expect(calculation.result?.displayValue == "Currency rates unavailable")
    calculation.update("50 USD to DKK")
    #expect(calculation.result?.copyValue == nil)
    #expect(requests.pending.count == 2)

    calculation.update("25 USD to DKK")
    try await waitForRates { requests.pending.count == 3 }
    calculation.update("Safari") // Leaving currency does not cancel shared work.
    #expect(!calculation.isCurrency)
    #expect(calculation.result == nil)
    requests.pending[2].resume(returning: snapshot)
    try await waitForRates { completions == 3 }
    #expect(calculation.result == nil)
    calculation.update("25 USD to DKK")
    #expect(calculation.result?.copyValue == "100 DKK")
    #expect(requests.pending.count == 3) // Fetch day, not snapshot metadata, controls freshness.
    calculation.update("1 km to mi")
    #expect(!calculation.isCurrency)
    #expect(calculation.result?.copyValue == "0.621371192237 mi")
}