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src/Jiaowu.Api/Infrastructure/CourseSelection/CourseSelectionRules.cs

@@ -26,4 +26,46 @@ public static class CourseSelectionRules
         candidateEntries.Any(candidate =>
             selectedEntries.Any(selected =>
                 ScheduleConflictDetector.TimeOverlaps(candidate, selected)));
+
+    /// <summary>
+    /// Returns the overlap percentage of candidate schedule entries with selected entries.
+    /// 0 = no conflict, 100 = fully overlapping.
+    /// Used for retake enrollment where ≤50% overlap is allowed.
+    /// </summary>
+    public static double CalculateScheduleOverlap(
+        IReadOnlyCollection<ScheduleEntry> candidateEntries,
+        IReadOnlyCollection<ScheduleEntry> selectedEntries)
+    {
+        if (candidateEntries.Count == 0 || selectedEntries.Count == 0)
+            return 0;
+
+        int totalCandidatePeriods = 0;
+        int overlappedPeriods = 0;
+
+        foreach (var candidate in candidateEntries)
+        {
+            totalCandidatePeriods += candidate.PeriodCount;
+            foreach (var selected in selectedEntries)
+            {
+                if (!ScheduleConflictDetector.TimeOverlaps(candidate, selected))
+                    continue;
+                // Calculate overlapping periods
+                var overlapStart = Math.Max(
+                    candidate.StartPeriod, selected.StartPeriod);
+                var overlapEnd = Math.Min(
+                    candidate.StartPeriod + candidate.PeriodCount,
+                    selected.StartPeriod + selected.PeriodCount);
+                if (overlapEnd > overlapStart)
+                    overlappedPeriods += overlapEnd - overlapStart;
+            }
+        }
+
+        return totalCandidatePeriods == 0
+            ? 0
+            : (double)overlappedPeriods / totalCandidatePeriods * 100;
+    }
+
+    /// <summary>Retake expanded capacity: ceiling(original * 1.15).</summary>
+    public static int RetakeCapacity(int originalCapacity) =>
+        (int)Math.Ceiling(originalCapacity * 1.15);
 }