Lock submitted simulation answers

api/QuestionsApi/Models/ExamSimulation.cs

@@ -53,6 +53,7 @@ public class ExamSimulationResponse
     public List<string> EliminatedChoiceIds { get; set; } = [];
     public bool Flagged { get; set; }
     public int ElapsedSeconds { get; set; }
+    public bool? IsSubmitted { get; set; }
     public DateTime UpdatedUtc { get; set; } = DateTime.UtcNow;
 }
 
@@ -84,6 +85,7 @@ public class SaveSimulationResponseRequest
     public List<string> EliminatedChoiceIds { get; set; } = [];
     public bool Flagged { get; set; }
     public int ElapsedSeconds { get; set; }
+    public bool IsSubmitted { get; set; }
 }
 
 public class ExamSimulationQuestionResponse

api/QuestionsApi/Services/ExamSimulationStore.cs

@@ -348,21 +348,49 @@ public class ExamSimulationStore
         if (!item.PresentedChoiceIds.Contains(request.ChoiceId, StringComparer.Ordinal) && request.ChoiceId != "")
             throw new ArgumentException("Choice is not presented for this question.");
         var response = simulation.Responses.FirstOrDefault(candidate => candidate.QuestionExternalId == request.QuestionExternalId);
+        if (response is not null && IsResponseLocked(response))
+        {
+            if (ResponseContentMatches(response, request, item.PresentedChoiceIds))
+                return simulation;
+            throw new InvalidOperationException("A submitted simulation answer cannot be changed.");
+        }
         if (response is null)
         {
-            response = new ExamSimulationResponse { QuestionExternalId = request.QuestionExternalId };
+            response = new ExamSimulationResponse
+            {
+                QuestionExternalId = request.QuestionExternalId,
+                IsSubmitted = false
+            };
             simulation.Responses.Add(response);
         }
         response.ChoiceId = request.ChoiceId;
         response.EliminatedChoiceIds = request.EliminatedChoiceIds.Where(item.PresentedChoiceIds.Contains).Distinct().ToList();
         response.Flagged = request.Flagged;
         response.ElapsedSeconds = Math.Clamp(request.ElapsedSeconds, 0, 1800);
+        response.IsSubmitted = request.IsSubmitted;
         response.UpdatedUtc = DateTime.UtcNow;
         simulation.UpdatedUtc = DateTime.UtcNow;
         await ReplaceAsync(simulation);
         return simulation;
     }
 
+    public static bool IsResponseLocked(ExamSimulationResponse response) =>
+        response.IsSubmitted == true ||
+        (response.IsSubmitted is null && !string.IsNullOrEmpty(response.ChoiceId));
+
+    public static bool ResponseContentMatches(
+        ExamSimulationResponse response,
+        SaveSimulationResponseRequest request,
+        IReadOnlyCollection<string> presentedChoiceIds)
+    {
+        var eliminated = request.EliminatedChoiceIds
+            .Where(presentedChoiceIds.Contains)
+            .ToHashSet(StringComparer.Ordinal);
+        return response.ChoiceId == request.ChoiceId &&
+            response.Flagged == request.Flagged &&
+            response.EliminatedChoiceIds.ToHashSet(StringComparer.Ordinal).SetEquals(eliminated);
+    }
+
     public async Task<ExamSimulation> SubmitBlockAsync(ExamSimulation simulation, bool expired = false)
     {
         if (IsAlreadySubmittedState(simulation.State))