Add variable question distractor pools

api/QuestionsApi/Models/AnswerStatistics.cs

@@ -7,7 +7,8 @@ public sealed class QuestionAttemptCounter
     public string QuestionExternalId { get; set; } = "";
     public int Bucket { get; set; }
     public string ChoiceId { get; set; } = "";
-    public long Count { get; set; }
+    public long SelectionCount { get; set; }
+    public long PresentationCount { get; set; }
     public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
 }
 
@@ -15,14 +16,16 @@ public sealed class QuestionAttemptCounterTotal
 {
     [BsonId]
     public QuestionChoiceCounterKey Key { get; set; } = new();
-    public long Count { get; set; }
+    public long SelectionCount { get; set; }
+    public long PresentationCount { get; set; }
 }
 
 public sealed class QuestionAttemptCounterAggregate
 {
     [BsonId]
     public QuestionChoiceBucketKey Key { get; set; } = new();
-    public long Count { get; set; }
+    public long SelectionCount { get; set; }
+    public long PresentationCount { get; set; }
 }
 
 public sealed class QuestionChoiceBucketKey
@@ -41,9 +44,18 @@ public sealed class QuestionChoiceCounterKey
 public sealed class AnswerStatisticsState
 {
     [BsonId]
-    public string Id { get; set; } = "answer-statistics-v1";
+    public string Id { get; set; } = "answer-statistics-v2";
     public bool Completed { get; set; }
     public string LeaseOwner { get; set; } = "";
     public DateTime LeaseUntilUtc { get; set; }
     public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
+    public DateTime CutoverUtc { get; set; }
+}
+
+public sealed class QuestionPresentationBaseline
+{
+    [BsonId]
+    public string QuestionExternalId { get; set; } = "";
+    public List<string> ChoiceIds { get; set; } = [];
+    public DateTime CreatedUtc { get; set; } = DateTime.UtcNow;
 }

api/QuestionsApi/Models/QuestionAdminExport.cs

@@ -0,0 +1,8 @@
+namespace QuestionsApi.Models;
+
+public sealed class QuestionAdminExportResponse
+{
+    public List<QuestionResponse> Items { get; set; } = [];
+    public string? NextCursor { get; set; }
+    public bool HasMore { get; set; }
+}

api/QuestionsApi/Models/QuestionAttempt.cs

@@ -12,6 +12,8 @@ public class QuestionAttempt
     public string QuestionExternalId { get; set; } = "";
     public string UserId { get; set; } = "";
     public string ChoiceId { get; set; } = "";
+    public List<string> PresentedChoiceIds { get; set; } = [];
+    public bool TracksPresentation { get; set; }
     public bool IsCorrect { get; set; }
     public bool IsSkip { get; set; }
     public int CounterBucket { get; set; }

api/QuestionsApi/Models/QuestionRequests.cs

@@ -8,6 +8,7 @@ public class QuestionSearchRequest
     public int? DifficultyMin { get; set; }
     public int? DifficultyMax { get; set; }
     public int Limit { get; set; } = 40;
+    public bool SupportsChoicePools { get; set; }
 }
 
 public class UpsertQuestionRequest
@@ -35,4 +36,5 @@ public class FirstQuestionAttemptRequest
 {
     public string QuestionExternalId { get; set; } = "";
     public string ChoiceId { get; set; } = "";
+    public List<string> PresentedChoiceIds { get; set; } = [];
 }

api/QuestionsApi/Models/QuestionResponse.cs

@@ -12,6 +12,7 @@ public class QuestionResponse
     public Dictionary<string, string> ChoiceExplanations { get; set; } = [];
     public Dictionary<string, WrongAnswerFollowup> WrongAnswerFollowups { get; set; } = [];
     public Dictionary<string, int> ChoiceSelectionPercentages { get; set; } = [];
+    public string ChoiceSelectionRateBasis { get; set; } = "presented";
     public string Discipline { get; set; } = "";
     public string System { get; set; } = "";
     public string PerformanceCategory { get; set; } = "";
@@ -24,27 +25,56 @@ public class QuestionResponse
     public static QuestionResponse FromModel(
         Question question,
         Dictionary<string, int>? liveChoiceSelectionPercentages = null,
-        int questionElo = 1000) => new()
+        int questionElo = 1000,
+        bool includeChoicePool = true)
     {
-        Id = question.Id ?? "",
-        ExternalId = question.ExternalId,
-        Stem = question.Stem,
-        LeadIn = question.LeadIn,
-        Choices = question.Choices,
-        CorrectChoiceId = question.CorrectChoiceId,
-        Explanation = question.Explanation,
-        ChoiceExplanations = question.ChoiceExplanations,
-        WrongAnswerFollowups = question.WrongAnswerFollowups,
-        ChoiceSelectionPercentages = BuildChoiceSelectionPercentages(question, liveChoiceSelectionPercentages),
-        Discipline = question.Discipline,
-        System = question.System,
-        PerformanceCategory = RatingCategory.Normalize(question.PerformanceCategory, question.System, question.Discipline),
-        Difficulty = question.Difficulty,
-        QuestionElo = questionElo,
-        Labs = question.Labs,
-        Media = question.Media,
-        Tags = question.Tags
-    };
+        var choices = includeChoicePool ? question.Choices : SelectLegacyChoices(question);
+        var choiceIds = choices.Select(choice => choice.Id).ToHashSet(StringComparer.Ordinal);
+        var percentages = BuildChoiceSelectionPercentages(question, liveChoiceSelectionPercentages);
+        return new QuestionResponse
+        {
+            Id = question.Id ?? "",
+            ExternalId = question.ExternalId,
+            Stem = question.Stem,
+            LeadIn = question.LeadIn,
+            Choices = choices,
+            CorrectChoiceId = question.CorrectChoiceId,
+            Explanation = question.Explanation,
+            ChoiceExplanations = question.ChoiceExplanations
+                .Where(entry => choiceIds.Contains(entry.Key))
+                .ToDictionary(),
+            WrongAnswerFollowups = question.WrongAnswerFollowups
+                .Where(entry => choiceIds.Contains(entry.Key))
+                .ToDictionary(),
+            ChoiceSelectionPercentages = percentages
+                .Where(entry => choiceIds.Contains(entry.Key))
+                .ToDictionary(),
+            Discipline = question.Discipline,
+            System = question.System,
+            PerformanceCategory = RatingCategory.Normalize(question.PerformanceCategory, question.System, question.Discipline),
+            Difficulty = question.Difficulty,
+            QuestionElo = questionElo,
+            Labs = question.Labs,
+            Media = question.Media,
+            Tags = question.Tags
+        };
+    }
+
+    private static List<QuestionChoice> SelectLegacyChoices(Question question)
+    {
+        if (question.Choices.Count <= 5)
+            return question.Choices;
+
+        var correct = question.Choices.First(choice => choice.Id == question.CorrectChoiceId);
+        var distractors = question.Choices
+            .Where(choice => choice.Id != question.CorrectChoiceId)
+            .OrderBy(_ => Random.Shared.Next())
+            .Take(4)
+            .Append(correct)
+            .OrderBy(_ => Random.Shared.Next())
+            .ToList();
+        return distractors;
+    }
 
     public static Dictionary<string, int> BuildChoiceSelectionPercentages(Question question, Dictionary<string, int>? liveChoiceSelectionPercentages)
     {