step3/api/QuestionsApi.Tests/ExamSimulationTests.cs

using QuestionsApi.Models;
using QuestionsApi.Services;
using Xunit;

namespace QuestionsApi.Tests;

public sealed class ExamSimulationTests
{
    [Fact]
    public void DayOneFormHasOfficialItemAndBlockCounts()
    {
        Assert.Equal(232, ExamSimulationStore.BlockSizes.Sum());
        Assert.Equal(12, ExamSimulationStore.BlockSizes.Length);
        Assert.All(ExamSimulationStore.BlockSizes, size => Assert.InRange(size, 18, 20));
    }

    [Fact]
    public void BlueprintTargetsFillTheCompleteForm()
    {
        var targets = ExamSimulationStore.BuildTargets();
        Assert.Equal(ExamSimulationStore.RequiredQuestions, targets.Values.Sum());
        Assert.Equal(14, targets.Count);
        Assert.All(targets.Values, count => Assert.True(count > 0));
    }

    [Fact]
    public void MoreCorrectAnswersProduceAHigherPrediction()
    {
        var items = BuildItems(100);
        var questions = BuildQuestions(100);
        var allCorrect = items.ToDictionary(
            item => item.QuestionExternalId,
            item => new ExamSimulationResponse { QuestionExternalId = item.QuestionExternalId, ChoiceId = "A" });
        var allWrong = items.ToDictionary(
            item => item.QuestionExternalId,
            item => new ExamSimulationResponse { QuestionExternalId = item.QuestionExternalId, ChoiceId = "B" });

        var passing = ExamSimulationScoring.Calculate(items, allCorrect, questions);
        var failing = ExamSimulationScoring.Calculate(items, allWrong, questions);

        Assert.True(passing.PredictedScore > failing.PredictedScore);
        Assert.True(passing.PassProbability > failing.PassProbability);
        Assert.Equal(100, passing.Correct);
        Assert.Equal(0, failing.Correct);
    }

    [Theory]
    [InlineData(0, 50)]
    [InlineData(20, 100)]
    [InlineData(60, 200)]
    [InlineData(100, 300)]
    public void NeutralDifficultyUsesTransparentScoreAnchors(int correctCount, int expectedScore)
    {
        var items = BuildItems(100);
        var questions = BuildQuestions(100);
        var responses = items.Take(correctCount).ToDictionary(
            item => item.QuestionExternalId,
            item => new ExamSimulationResponse { QuestionExternalId = item.QuestionExternalId, ChoiceId = "A" });

        var score = ExamSimulationScoring.Calculate(items, responses, questions);

        Assert.Equal(expectedScore, score.PredictedScore);
        Assert.Equal(correctCount / 100.0, score.RawAccuracy, 4);
        Assert.Equal(correctCount / 100.0, score.DifficultyAdjustedAccuracy, 4);
    }

    [Fact]
    public void CorrectHarderQuestionsProduceAHigherAdjustedPrediction()
    {
        var questions = BuildQuestions(10);
        var hardCorrect = BuildItems(10);
        var easyCorrect = BuildItems(10);
        for (var index = 0; index < 10; index++)
        {
            hardCorrect[index].QuestionDifficulty = index < 5 ? 5 : 1;
            easyCorrect[index].QuestionDifficulty = index < 5 ? 1 : 5;
        }
        var responses = hardCorrect.Take(5).ToDictionary(
            item => item.QuestionExternalId,
            item => new ExamSimulationResponse { QuestionExternalId = item.QuestionExternalId, ChoiceId = "A" });

        var hardScore = ExamSimulationScoring.Calculate(hardCorrect, responses, questions);
        var easyScore = ExamSimulationScoring.Calculate(easyCorrect, responses, questions);

        Assert.Equal(hardScore.RawAccuracy, easyScore.RawAccuracy);
        Assert.True(hardScore.DifficultyAdjustedAccuracy > easyScore.DifficultyAdjustedAccuracy);
        Assert.True(hardScore.PredictedScore > easyScore.PredictedScore);
    }

    [Fact]
    public void EmpiricalQuestionEloDoesNotAffectPrediction()
    {
        var questions = BuildQuestions(20);
        var lowElos = BuildItems(20);
        var highElos = BuildItems(20);
        foreach (var item in lowElos) item.QuestionElo = 100;
        foreach (var item in highElos) item.QuestionElo = 1900;
        var responses = lowElos.Take(10).ToDictionary(
            item => item.QuestionExternalId,
            item => new ExamSimulationResponse { QuestionExternalId = item.QuestionExternalId, ChoiceId = "A" });

        var lowEloScore = ExamSimulationScoring.Calculate(lowElos, responses, questions);
        var highEloScore = ExamSimulationScoring.Calculate(highElos, responses, questions);

        Assert.Equal(lowEloScore.PredictedScore, highEloScore.PredictedScore);
        Assert.Equal(lowEloScore.PassProbability, highEloScore.PassProbability);
    }

    [Fact]
    public void SkippedAndUnansweredQuestionsCountAsIncorrect()
    {
        var items = BuildItems(10);
        var questions = BuildQuestions(10);
        var responses = new Dictionary<string, ExamSimulationResponse>
        {
            [items[0].QuestionExternalId] = new() { QuestionExternalId = items[0].QuestionExternalId, ChoiceId = "A" },
            [items[1].QuestionExternalId] = new() { QuestionExternalId = items[1].QuestionExternalId, ChoiceId = "" }
        };

        var score = ExamSimulationScoring.Calculate(items, responses, questions);

        Assert.Equal(1, score.Correct);
        Assert.Equal(10, score.Total);
        Assert.Equal(0.1, score.RawAccuracy, 4);
    }

    [Fact]
    public void LowAccuracyFullLengthSimulationIsNotInflated()
    {
        var items = BuildItems(232);
        var questions = BuildQuestions(232);
        var responses = items.Take(49).ToDictionary(
            item => item.QuestionExternalId,
            item => new ExamSimulationResponse { QuestionExternalId = item.QuestionExternalId, ChoiceId = "A" });

        var score = ExamSimulationScoring.Calculate(items, responses, questions);

        Assert.Equal(103, score.PredictedScore);
        Assert.True(score.PassProbability < 0.001);
        Assert.True(score.RangeHigh < 130);
    }

    [Fact]
    public void SeenItemsAreReportedButDoNotChangeScoring()
    {
        var questions = BuildQuestions(20);
        var unseen = BuildItems(20);
        var seen = BuildItems(20, wasSeen: true);
        var responses = unseen.ToDictionary(
            item => item.QuestionExternalId,
            item => new ExamSimulationResponse { QuestionExternalId = item.QuestionExternalId, ChoiceId = "A" });

        var unseenScore = ExamSimulationScoring.Calculate(unseen, responses, questions);
        var seenScore = ExamSimulationScoring.Calculate(seen, responses, questions);

        Assert.Equal(0, unseenScore.SeenCount);
        Assert.Equal(20, seenScore.SeenCount);
        Assert.Equal(unseenScore.PredictedScore, seenScore.PredictedScore);
        Assert.Null(unseenScore.AbilityElo);
        Assert.Null(seenScore.AbilityElo);
    }

    [Theory]
    [InlineData("Cardiovascular System", "Cardiovascular")]
    [InlineData("Professionalism and Ethics", "Social Sciences")]
    [InlineData("Endocrine, Diabetes & Metabolism", "Endocrine")]
    public void QuestionsMapToExpectedBlueprintCategory(string system, string expected)
    {
        Assert.Equal(expected, ExamSimulationStore.BlueprintCategory(new Question { System = system }));
    }

    [Fact]
    public void HistorySummarySupportsScoredAndUnscoredStates()
    {
        var active = ExamSimulationStore.ToHistorySummary(new ExamSimulation
        {
            State = ExamSimulationStates.BlockActive,
            CurrentBlock = 3,
            Items = BuildItems(20)
        });
        var completed = ExamSimulationStore.ToHistorySummary(new ExamSimulation
        {
            State = ExamSimulationStates.Completed,
            IsSample = true,
            Items = BuildItems(20),
            Score = new ExamSimulationScore { PredictedScore = 221, RangeLow = 204, RangeHigh = 238, PassProbability = 0.73, Correct = 15, Total = 20 }
        });

        Assert.Null(active.PredictedScore);
        Assert.Equal(3, active.CurrentBlock);
        Assert.Equal(221, completed.PredictedScore);
        Assert.Equal(15, completed.Correct);
        Assert.Equal(0.73, completed.PassProbability);
        Assert.True(completed.IsSample);
    }

    [Fact]
    public void HistoryCursorRoundTripsTimestampAndObjectId()
    {
        var timestamp = new DateTime(2026, 7, 16, 22, 15, 30, DateTimeKind.Utc);
        const string id = "64b1d9f36f7a4e2b8c901234";

        var encoded = ExamSimulationStore.EncodeHistoryCursor(timestamp, id);

        Assert.True(ExamSimulationStore.TryDecodeHistoryCursor(encoded, out var decoded));
        Assert.NotNull(decoded);
        Assert.Equal(timestamp, decoded.CreatedUtc);
        Assert.Equal(id, decoded.Id);
        Assert.False(ExamSimulationStore.TryDecodeHistoryCursor("not-a-cursor", out _));
    }

    [Theory]
    [InlineData(ExamSimulationStates.BetweenBlocks, true)]
    [InlineData(ExamSimulationStates.Completed, true)]
    [InlineData(ExamSimulationStates.BlockActive, false)]
    [InlineData(ExamSimulationStates.Abandoned, false)]
    public void RepeatedBlockSubmissionIsIdempotentOnlyAfterSubmission(string state, bool expected)
    {
        Assert.Equal(expected, ExamSimulationStore.IsAlreadySubmittedState(state));
    }

    private static List<ExamSimulationItem> BuildItems(int count, bool wasSeen = false) =>
        Enumerable.Range(0, count).Select(index => new ExamSimulationItem
        {
            QuestionExternalId = $"question-{index}",
            QuestionElo = 1000,
            QuestionDifficulty = 3,
            WasSeen = wasSeen,
            BlueprintCategory = "Cardiovascular"
        }).ToList();

    private static Dictionary<string, Question> BuildQuestions(int count) =>
        Enumerable.Range(0, count).ToDictionary(
            index => $"question-{index}",
            index => new Question { ExternalId = $"question-{index}", CorrectChoiceId = "A", Difficulty = 3 });
}