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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);
 }

src/Jiaowu.Api/Infrastructure/Persistence/DevelopmentSqliteMigrator.cs

@@ -34,6 +34,8 @@ public sealed class DevelopmentSqliteMigrator(
         "20260725_19_flexible_grades";
     private const string ExamSchedulingOptimizationMigration =
         "20260725_20_exam_scheduling_optimization";
+    private const string RetakeEnrollmentMigration =
+        "20260725_21_retake_enrollment";
 
     public async Task MigrateAsync(CancellationToken cancellationToken = default)
     {
@@ -225,6 +227,19 @@ public sealed class DevelopmentSqliteMigrator(
                 ? []
                 : ExamSchedulingOptimizationStatements,
             cancellationToken);
+
+        var retakeEnrollmentExists = await db.Database
+            .SqlQueryRaw<int>(
+                """
+                SELECT COUNT(*) AS "Value"
+                FROM pragma_table_info('CourseEnrollments')
+                WHERE name = 'EnrollmentType'
+                """)
+            .AnyAsync(value => value > 0, cancellationToken);
+        await ApplyMigrationAsync(
+            RetakeEnrollmentMigration,
+            retakeEnrollmentExists ? [] : RetakeEnrollmentStatements,
+            cancellationToken);
     }
 
     private async Task ApplyMigrationAsync(
@@ -1461,4 +1476,12 @@ public sealed class DevelopmentSqliteMigrator(
             ON "ExamSessions" ("RequiredBuildingId");
         """
     ];
+
+    private static readonly string[] RetakeEnrollmentStatements =
+    [
+        """
+        ALTER TABLE "CourseEnrollments"
+        ADD COLUMN "EnrollmentType" INTEGER NOT NULL DEFAULT 1;
+        """
+    ];
 }

src/Jiaowu.Api/Infrastructure/Persistence/Migrations/MySql/20260725150000_RetakeEnrollment.cs

@@ -0,0 +1,29 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Jiaowu.Api.Infrastructure.Persistence.Migrations.MySql
+{
+    /// <inheritdoc />
+    public partial class RetakeEnrollment : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<int>(
+                name: "EnrollmentType",
+                table: "CourseEnrollments",
+                type: "int",
+                nullable: false,
+                defaultValue: 1);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "EnrollmentType",
+                table: "CourseEnrollments");
+        }
+    }
+}