step3/api/QuestionsApi.Tests/FlashcardSchedulingTests.cs

using QuestionsApi.Services.Flashcards;
using Xunit;

namespace QuestionsApi.Tests;

public sealed class FlashcardSchedulingTests
{
    [Fact]
    public void NextStudyDayStart_IsMidnightOfTheFollowingDay()
    {
        var studyDayStart = new DateTime(2026, 3, 5, 0, 0, 0, DateTimeKind.Utc);

        var next = FlashcardScheduling.NextStudyDayStart(studyDayStart);

        Assert.Equal(new DateTime(2026, 3, 6, 0, 0, 0, DateTimeKind.Utc), next);
    }

    [Fact]
    public void NextStudyDayStart_TruncatesTimeOfDayBeforeAdvancing()
    {
        // A session that started mid-day still buries siblings until the *next calendar day*,
        // not exactly 24h later.
        var studyDayStart = new DateTime(2026, 3, 5, 14, 30, 0, DateTimeKind.Utc);

        var next = FlashcardScheduling.NextStudyDayStart(studyDayStart);

        Assert.Equal(new DateTime(2026, 3, 6, 0, 0, 0, DateTimeKind.Utc), next);
    }

    [Theory]
    [InlineData(20, null, 0, 20)]
    [InlineData(20, null, 15, 5)]
    [InlineData(20, null, 25, 0)]
    [InlineData(20, 5, 0, 5)]
    [InlineData(20, 0, 0, 0)]
    public void ResolveDailyBudget_DeckOverrideWinsOverGlobal_ClampedAtZero(
        int globalLimit, int? deckOverride, int consumedToday, int expected)
    {
        Assert.Equal(expected, FlashcardScheduling.ResolveDailyBudget(globalLimit, deckOverride, consumedToday));
    }

    private static readonly DateTime Today = new(2026, 3, 10, 0, 0, 0, DateTimeKind.Utc);

    [Fact]
    public void ComputeStreakDays_NoReviews_IsZero()
    {
        Assert.Equal(0, FlashcardScheduling.ComputeStreakDays([], Today));
    }

    [Fact]
    public void ComputeStreakDays_ReviewedTodayOnly_IsOne()
    {
        var timestamps = new List<DateTime> { Today.AddHours(9) };

        Assert.Equal(1, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
    }

    [Fact]
    public void ComputeStreakDays_ThreeConsecutiveDaysEndingToday_IsThree()
    {
        var timestamps = new List<DateTime>
        {
            Today.AddDays(-2).AddHours(8), Today.AddDays(-1).AddHours(20), Today.AddHours(7)
        };

        Assert.Equal(3, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
    }

    [Fact]
    public void ComputeStreakDays_NotYetReviewedToday_StillCountsThroughYesterday()
    {
        // The streak shouldn't visibly reset to 0 the instant a new calendar day begins, before
        // the user has had a chance to study today.
        var timestamps = new List<DateTime> { Today.AddDays(-2).AddHours(8), Today.AddDays(-1).AddHours(20) };

        Assert.Equal(2, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
    }

    [Fact]
    public void ComputeStreakDays_GapBeforeYesterday_BreaksTheStreak()
    {
        var timestamps = new List<DateTime>
        {
            Today.AddDays(-10).AddHours(8), Today.AddDays(-1).AddHours(20), Today.AddHours(7)
        };

        Assert.Equal(2, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
    }

    [Fact]
    public void ComputeStreakDays_MissedYesterdayAndToday_IsZero()
    {
        var timestamps = new List<DateTime> { Today.AddDays(-3).AddHours(8) };

        Assert.Equal(0, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
    }

    [Fact]
    public void ComputeStreakDays_MultipleReviewsSameDay_CountOnceTowardTheStreak()
    {
        var timestamps = new List<DateTime> { Today.AddHours(7), Today.AddHours(8), Today.AddHours(9) };

        Assert.Equal(1, FlashcardScheduling.ComputeStreakDays(timestamps, Today));
    }

    [Fact]
    public void PickIndex_NonShuffle_ReturnsFirstNonExcluded()
    {
        var pool = new List<string> { "a", "b", "c" };

        var picked = FlashcardScheduling.PickIndex(pool, new HashSet<string> { "a" }, shuffle: false, n => throw new InvalidOperationException("should not randomize"));

        Assert.Equal(1, picked);
    }

    [Fact]
    public void PickIndex_NonShuffle_EverythingExcluded_ReturnsNull()
    {
        var pool = new List<string> { "a", "b" };

        var picked = FlashcardScheduling.PickIndex(pool, new HashSet<string> { "a", "b" }, shuffle: false, n => 0);

        Assert.Null(picked);
    }

    [Fact]
    public void PickIndex_EmptyPool_ReturnsNullRegardlessOfShuffle()
    {
        Assert.Null(FlashcardScheduling.PickIndex([], new HashSet<string>(), shuffle: false, n => 0));
        Assert.Null(FlashcardScheduling.PickIndex([], new HashSet<string>(), shuffle: true, n => 0));
    }

    [Fact]
    public void PickIndex_Shuffle_StubReturningFirstCandidate_PicksFirstNonExcluded()
    {
        var pool = new List<string> { "a", "b", "c" };

        var picked = FlashcardScheduling.PickIndex(pool, new HashSet<string> { "a" }, shuffle: true, n => 0);

        // Candidates (excluding "a") are ["b", "c"]; stub index 0 -> "b" -> original pool index 1.
        Assert.Equal(1, picked);
    }

    [Fact]
    public void PickIndex_Shuffle_CanReturnSomethingOtherThanTheOldestCandidate()
    {
        var pool = new List<string> { "a", "b", "c" };

        // Stub picks the last candidate instead of always the first (oldest), proving this isn't
        // hardcoded to FirstOrDefault-style behavior.
        var picked = FlashcardScheduling.PickIndex(pool, new HashSet<string>(), shuffle: true, n => n - 1);

        Assert.Equal(2, picked);
    }

    [Fact]
    public void PickIndex_Shuffle_ExcludedItemsDoNotShiftTheOriginalIndexMapping()
    {
        var pool = new List<string> { "a", "b", "c", "d" };

        // Candidates (excluding "b") are ["a", "c", "d"] at original indices [0, 2, 3]; stub
        // index 2 should map back to original index 3 ("d"), not 2 ("c").
        var picked = FlashcardScheduling.PickIndex(pool, new HashSet<string> { "b" }, shuffle: true, n => 2);

        Assert.Equal(3, picked);
    }

    [Fact]
    public void PickIndex_Shuffle_EverythingExcluded_ReturnsNull()
    {
        var pool = new List<string> { "a", "b" };

        var picked = FlashcardScheduling.PickIndex(pool, new HashSet<string> { "a", "b" }, shuffle: true, n => 0);

        Assert.Null(picked);
    }
}