import XCTest
@testable import MealMood

/// Regression suite for "the watch showed Wednesday's meal on Saturday".
///
/// The snapshot the iPhone sends carries an `isToday` flag, which is true only
/// at the moment it was built. The watch outlives that moment — it keeps the
/// last snapshot in its app group — so "today" has to be resolved when the view
/// renders, never read from the payload.
final class WatchWeekPayloadTests: XCTestCase {

    private let calendar = Calendar(identifier: .gregorian)

    private func date(_ year: Int, _ month: Int, _ day: Int) -> Date {
        calendar.date(from: DateComponents(year: year, month: month, day: day, hour: 12))!
    }

    private func makePayload(weekStart: Date, todayFlagOn flaggedDay: Int?) -> WatchWeekPayload {
        let days = (0...6).map { offset in
            WatchWeekPayload.Day(
                dayOfWeek: offset,
                title: "Day \(offset)",
                isToday: offset == flaggedDay,
                date: calendar.date(byAdding: .day, value: offset, to: weekStart)!,
                meals: [
                    WatchWeekPayload.Meal(type: "dinner", label: "Dinner", name: "Dish \(offset)")
                ]
            )
        }
        return WatchWeekPayload(
            weekTitle: "Week",
            days: days,
            updatedAt: weekStart,
            emptyMealText: "Not planned",
            emptyDayText: "Nothing planned for today",
            staleText: "Open MealMood on your iPhone"
        )
    }

    // MARK: - The reported bug

    func testSaturdayShowsSaturdayEvenIfTheSnapshotWasBuiltOnWednesday() {
        let monday = date(2026, 9, 7)
        // Snapshot built on Wednesday: that's the day it flagged as "today".
        let payload = makePayload(weekStart: monday, todayFlagOn: 2)
        let saturday = date(2026, 9, 12)

        let today = payload.currentDay(now: saturday, calendar: calendar)

        XCTAssertEqual(today?.dayOfWeek, 5, "Saturday must win over the frozen Wednesday flag")
        XCTAssertEqual(today?.meals.first?.name, "Dish 5")
        XCTAssertFalse(payload.isStale(now: saturday, calendar: calendar))
    }

    func testSnapshotFromAnotherWeekHasNoToday() {
        let lastMonday = date(2026, 9, 7)
        let payload = makePayload(weekStart: lastMonday, todayFlagOn: 2)
        let nextSaturday = date(2026, 9, 19)

        XCTAssertNil(payload.currentDay(now: nextSaturday, calendar: calendar))
        XCTAssertTrue(payload.isStale(now: nextSaturday, calendar: calendar),
                      "showing last week's meals as today's is exactly what must not happen")
    }

    func testEachDayKnowsWhetherItIsToday() {
        let monday = date(2026, 9, 7)
        let payload = makePayload(weekStart: monday, todayFlagOn: 2)
        let thursday = date(2026, 9, 10)

        XCTAssertTrue(payload.days[3].isCurrentDay(now: thursday, calendar: calendar))
        XCTAssertFalse(payload.days[2].isCurrentDay(now: thursday, calendar: calendar),
                       "the day flagged when the snapshot was built is not today any more")
    }

    // MARK: - Empty states

    func testDayWithNoDishesIsEmpty() {
        let empty = WatchWeekPayload.Day(
            dayOfWeek: 1, title: "Tue", isToday: true, date: date(2026, 9, 8),
            meals: [
                WatchWeekPayload.Meal(type: "lunch", label: "Lunch", name: nil),
                WatchWeekPayload.Meal(type: "dinner", label: "Dinner", name: nil)
            ]
        )
        XCTAssertTrue(empty.isEmpty)

        let planned = WatchWeekPayload.Day(
            dayOfWeek: 1, title: "Tue", isToday: true, date: date(2026, 9, 8),
            meals: [
                WatchWeekPayload.Meal(type: "lunch", label: "Lunch", name: nil),
                WatchWeekPayload.Meal(type: "dinner", label: "Dinner", name: "Tortilla")
            ]
        )
        XCTAssertFalse(planned.isEmpty, "one planned meal is enough for the day not to be empty")
    }

    func testPayloadCarriesTheLocalizedEmptyTexts() {
        // The watch bundle has no strings of its own, so these have to travel.
        let payload = makePayload(weekStart: date(2026, 9, 7), todayFlagOn: 0)

        XCTAssertEqual(payload.emptyMealText, "Not planned")
        XCTAssertEqual(payload.emptyDayText, "Nothing planned for today")
        XCTAssertEqual(payload.staleText, "Open MealMood on your iPhone")
    }

    // MARK: - Compatibility with snapshots already on people's watches

    func testSnapshotWrittenBeforeTheFixStillDecodes() throws {
        // Shape written by 2.1.0: no `date`, no texts. Stamped inside the
        // current week, which is when its weekday offsets still mean something.
        let wednesday = date(2026, 9, 9)
        let json = """
        {
          "weekTitle": "7 – 13 sep",
          "updatedAt": \(wednesday.timeIntervalSinceReferenceDate),
          "days": [
            {"dayOfWeek": 0, "title": "Mon 7", "isToday": false,
             "meals": [{"type": "dinner", "label": "Dinner", "name": "Pizza"}]},
            {"dayOfWeek": 2, "title": "Wed 9", "isToday": true,
             "meals": [{"type": "dinner", "label": "Dinner", "name": "Salmon"}]}
          ]
        }
        """
        let payload = try XCTUnwrap(WatchWeekPayload.decode(Data(json.utf8)))

        XCTAssertEqual(payload.days.count, 2)
        XCTAssertNil(payload.days[0].date)
        XCTAssertNil(payload.emptyMealText, "old snapshots have no texts; the UI falls back")

        // Without dates it can only fall back to the weekday offset.
        XCTAssertEqual(payload.currentDay(now: wednesday, calendar: calendar)?.dayOfWeek, 2)
    }

    func testDatelessSnapshotFromAnotherWeekIsStale() throws {
        // The watch that reported the bug had one of these: no dates, built in a
        // previous week. Trusting its weekday offsets is what showed the wrong
        // day, so it now counts as stale instead.
        let lastWednesday = date(2026, 9, 2)
        let json = """
        {
          "weekTitle": "31 ago – 6 sep",
          "updatedAt": \(lastWednesday.timeIntervalSinceReferenceDate),
          "days": [
            {"dayOfWeek": 5, "title": "Sat 5", "isToday": false,
             "meals": [{"type": "dinner", "label": "Dinner", "name": "Old dinner"}]}
          ]
        }
        """
        let payload = try XCTUnwrap(WatchWeekPayload.decode(Data(json.utf8)))
        let saturday = date(2026, 9, 12)

        XCTAssertNil(payload.currentDay(now: saturday, calendar: calendar))
        XCTAssertTrue(payload.isStale(now: saturday, calendar: calendar))
    }

    func testWeekdayOffsetMapsMondayToZeroAndSundayToSix() {
        XCTAssertEqual(WatchWeekPayload.weekdayOffset(for: date(2026, 9, 7), calendar: calendar), 0)
        XCTAssertEqual(WatchWeekPayload.weekdayOffset(for: date(2026, 9, 12), calendar: calendar), 5)
        XCTAssertEqual(WatchWeekPayload.weekdayOffset(for: date(2026, 9, 13), calendar: calendar), 6)
    }
}