first commit

Domain/CustomerSubscriptionFactory.cs

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+using Fitness_core_BE.Exceptions;
+using Fitness_core_BE.Models;
+
+namespace Fitness_core_BE.Domain
+{
+    public sealed record CustomerSubscriptionAggregate(
+        CustomerSubscription Subscription,
+        CustomerSubItem InitialSubItem,
+        SubscriptionMember PrimaryMember,
+        IReadOnlyList<SubscriptionInstallment> Installments);
+
+    public static class CustomerSubscriptionFactory
+    {
+        public static CustomerSubscriptionAggregate Create(
+            CustomerSubscription subscription,
+            CustomerPlan plan,
+            CustomerSubItem initialSubItem,
+            DateTime? nowUtc = null)
+        {
+            var now = nowUtc ?? DateTime.UtcNow;
+            subscription.SubscriptionId =
+                subscription.SubscriptionId == Guid.Empty
+                    ? Guid.NewGuid()
+                    : subscription.SubscriptionId;
+            subscription.CreatedAt = now;
+            subscription.OnboardDate =
+                subscription.OnboardDate == default ? now : subscription.OnboardDate;
+
+            if (plan.DurationMode == "FixedRange")
+            {
+                if (plan.ValidTo == null || plan.ValidFrom == null)
+                    throw new BadRequestException("Fixed-range plan is missing its validity dates.");
+                if (now > plan.ValidTo.Value)
+                    throw new BadRequestException("This plan's validity period has already ended.");
+
+                if (subscription.OnboardDate < plan.ValidFrom.Value)
+                    subscription.OnboardDate = plan.ValidFrom.Value;
+                subscription.ExpiryDate ??= plan.ValidTo.Value;
+            }
+            else if (subscription.ExpiryDate == null && plan.DurationInDays > 0)
+            {
+                subscription.ExpiryDate =
+                    subscription.OnboardDate.AddDays(plan.DurationInDays);
+            }
+
+            // RenewalDate starts equal to ExpiryDate and is the only one of the two that ever
+            // advances again (on each approved renewal cycle) -- ExpiryDate is frozen from here on.
+            subscription.RenewalDate ??= subscription.ExpiryDate;
+
+            initialSubItem.CustomerSubItemId =
+                initialSubItem.CustomerSubItemId == Guid.Empty
+                    ? Guid.NewGuid()
+                    : initialSubItem.CustomerSubItemId;
+            initialSubItem.FKCustomerSubscriptionId = subscription.SubscriptionId;
+            initialSubItem.CreatedAt = now;
+
+            var primaryMember = new SubscriptionMember
+            {
+                SubscriptionMemberId = Guid.NewGuid(),
+                FKSubscriptionId = subscription.SubscriptionId,
+                FKCustomerId = subscription.FKCustomerId,
+                IsPrimary = true,
+                JoinedAt = now,
+                IsActive = true
+            };
+
+            return new CustomerSubscriptionAggregate(
+                subscription,
+                initialSubItem,
+                primaryMember,
+                BuildInstallmentSchedule(subscription, plan, now));
+        }
+
+        // Internal (not private): reused by SubscriptionRenewalApprovalService to build a fresh
+        // installment schedule for a renewal cycle on a subscription that already exists (no new
+        // CustomerSubscription row involved, unlike Create above).
+        internal static IReadOnlyList<SubscriptionInstallment> BuildInstallmentSchedule(
+            CustomerSubscription subscription,
+            CustomerPlan plan,
+            DateTime now,
+            DateTime? anchorDate = null)
+        {
+            // Defaults to subscription.OnboardDate, exactly Create()'s existing behavior (a
+            // brand-new subscription's OnboardDate is always "now" for that cycle). The
+            // renewal-approval flow passes the approval moment explicitly instead: unlike the old
+            // renewal path, the same CustomerSubscription row now persists across many cycles, so
+            // OnboardDate stays frozen at the very first cycle and can no longer anchor a later
+            // cycle's due dates on its own.
+            var scheduleAnchor = anchorDate ?? subscription.OnboardDate;
+
+            var count = Math.Max(1, plan.InstallmentCount);
+            var onboardingFee = plan.HasOnboardingFee ? plan.OnboardingFeeAmount : 0m;
+
+            // Mirrors CreateCustomerInvoiceFromSubscription's discount base (plan price +
+            // onboarding fee) so the installment schedule shown to the client (and used to
+            // prefill "record payment") matches what the invoice actually charges. Only the
+            // plan-price portion absorbs the reduction -- discountAmount=0 when no discount was
+            // given reproduces today's undiscounted split exactly.
+            var discountAmount = DiscountCalculator.ComputeDiscountAmount(
+                plan.PlanPrice + onboardingFee, subscription.DiscountType, subscription.DiscountValue);
+            var discountedPlanPrice = Math.Max(0m, plan.PlanPrice - discountAmount);
+
+            var perInstallment = Math.Round(discountedPlanPrice / count, 2);
+            var rows = new List<SubscriptionInstallment>(count);
+            decimal allocated = 0m;
+
+            double? fixedRangeStepDays = null;
+            if (plan.DurationMode == "FixedRange" &&
+                plan.ValidTo != null &&
+                count > 1)
+            {
+                var windowDays =
+                    (plan.ValidTo.Value - scheduleAnchor).TotalDays;
+                if (windowDays > 0)
+                    fixedRangeStepDays = windowDays / count;
+            }
+
+            for (var index = 0; index < count; index++)
+            {
+                var membershipAmount =
+                    index == count - 1
+                        ? discountedPlanPrice - allocated
+                        : perInstallment;
+                allocated += membershipAmount;
+
+                rows.Add(new SubscriptionInstallment
+                {
+                    SubscriptionInstallmentId = Guid.NewGuid(),
+                    FKSubscriptionId = subscription.SubscriptionId,
+                    InstallmentNo = index + 1,
+                    DueDate = fixedRangeStepDays.HasValue
+                        ? scheduleAnchor.AddDays(
+                            fixedRangeStepDays.Value * index)
+                        : scheduleAnchor.AddMonths(index),
+                    Amount = membershipAmount + (index == 0 ? onboardingFee : 0m),
+                    PaidAmount = 0m,
+                    Status = "Pending",
+                    IsActive = true,
+                    CreatedAt = now
+                });
+            }
+
+            return rows;
+        }
+    }
+}

Domain/DiscountCalculator.cs

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+namespace Fitness_core_BE.Domain
+{
+    public static class DiscountCalculator
+    {
+        public static decimal ComputeDiscountAmount(decimal subtotal, string? discountType, decimal? discountValue)
+        {
+            if (string.IsNullOrEmpty(discountType) || discountValue is not decimal value || value <= 0 || subtotal <= 0)
+                return 0m;
+
+            var discount = discountType switch
+            {
+                "Percentage" => subtotal * Math.Clamp(value, 0m, 100m) / 100m,
+                "Flat" => Math.Clamp(value, 0m, subtotal),
+                _ => 0m
+            };
+            return Math.Round(discount, 2);
+        }
+    }
+}