path: root/backend/internal/api/handlers/loan_handler.go

package handlers

import (
	"log"
	"net/http"
	"strconv"
	"time"

	"finance/backend/internal/database"
	"finance/backend/internal/models"

	"github.com/gin-gonic/gin"
	"gorm.io/gorm"
)

// LoanHandler handles loan-related operations
type LoanHandler struct {
}

// NewLoanHandler creates and returns a new LoanHandler instance
func NewLoanHandler() *LoanHandler {
	return &LoanHandler{}
}

// GetLoans retrieves all loans for the authenticated user
func (h *LoanHandler) GetLoans(c *gin.Context) {
	userID := c.MustGet("userID").(uint)
	var loans []models.Loan

	if err := database.DB.Where("user_id = ?", userID).Find(&loans).Error; err != nil {
		log.Printf("Error fetching loans for user %d: %v", userID, err)
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to get loans"})
		return
	}

	c.JSON(http.StatusOK, loans)
}

// GetLoanByID retrieves a specific loan by ID for the authenticated user
func (h *LoanHandler) GetLoanByID(c *gin.Context) {
	userID := c.MustGet("userID").(uint)
	loanID, err := strconv.ParseUint(c.Param("id"), 10, 32)
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid loan ID"})
		return
	}

	var loan models.Loan
	if err := database.DB.Where("id = ? AND user_id = ?", loanID, userID).First(&loan).Error; err != nil {
		c.JSON(http.StatusNotFound, gin.H{"error": "Loan not found"})
		return
	}

	c.JSON(http.StatusOK, loan)
}

// CreateLoan creates a new loan for the authenticated user
func (h *LoanHandler) CreateLoan(c *gin.Context) {
	userID := c.MustGet("userID").(uint)

	// Define input structure
	type CreateLoanInput struct {
		Name           string  `json:"name" binding:"required"`
		OriginalAmount int64   `json:"originalAmount" binding:"required"`
		CurrentBalance int64   `json:"currentBalance" binding:"required"`
		InterestRate   float64 `json:"interestRate"`
		StartDate      string  `json:"startDate"`
		EndDate        string  `json:"endDate"`
		AccountID      *uint   `json:"accountId"`
	}

	var input CreateLoanInput
	if err := c.ShouldBindJSON(&input); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	// Parse dates if provided
	var startDate, endDate time.Time
	var err error

	if input.StartDate != "" {
		startDate, err = time.Parse("2006-01-02", input.StartDate)
		if err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid start date format. Use YYYY-MM-DD"})
			return
		}
	}

	if input.EndDate != "" {
		endDate, err = time.Parse("2006-01-02", input.EndDate)
		if err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid end date format. Use YYYY-MM-DD"})
			return
		}
	}

	// Validate account if provided
	if input.AccountID != nil {
		var account models.Account
		if err := database.DB.Where("id = ? AND user_id = ?", *input.AccountID, userID).First(&account).Error; err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": "Associated account not found or doesn't belong to user"})
			return
		}
	}

	// Create loan
	loan := models.Loan{
		UserID:         userID,
		AccountID:      input.AccountID,
		Name:           input.Name,
		OriginalAmount: input.OriginalAmount,
		CurrentBalance: input.CurrentBalance,
		InterestRate:   input.InterestRate,
	}

	// Only set dates if they were provided
	if !startDate.IsZero() {
		loan.StartDate = startDate
	}
	if !endDate.IsZero() {
		loan.EndDate = endDate
	}

	if err := database.DB.Create(&loan).Error; err != nil {
		log.Printf("Error creating loan for user %d: %v", userID, err)
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to create loan"})
		return
	}

	c.JSON(http.StatusCreated, loan)
}

// UpdateLoan updates an existing loan for the authenticated user
func (h *LoanHandler) UpdateLoan(c *gin.Context) {
	userID := c.MustGet("userID").(uint)
	loanID, err := strconv.ParseUint(c.Param("id"), 10, 32)
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid loan ID"})
		return
	}

	// Find loan
	var loan models.Loan
	if err := database.DB.Where("id = ? AND user_id = ?", loanID, userID).First(&loan).Error; err != nil {
		c.JSON(http.StatusNotFound, gin.H{"error": "Loan not found"})
		return
	}

	// Define update structure
	type UpdateLoanInput struct {
		Name           string  `json:"name"`
		OriginalAmount int64   `json:"originalAmount"`
		CurrentBalance int64   `json:"currentBalance"`
		InterestRate   float64 `json:"interestRate"`
		StartDate      string  `json:"startDate"`
		EndDate        string  `json:"endDate"`
		AccountID      *uint   `json:"accountId"`
	}

	var input UpdateLoanInput
	if err := c.ShouldBindJSON(&input); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	// Update fields if provided
	if input.Name != "" {
		loan.Name = input.Name
	}
	if input.OriginalAmount != 0 {
		loan.OriginalAmount = input.OriginalAmount
	}
	if input.CurrentBalance != 0 {
		loan.CurrentBalance = input.CurrentBalance
	}
	if input.InterestRate != 0 {
		loan.InterestRate = input.InterestRate
	}
	if input.StartDate != "" {
		startDate, err := time.Parse("2006-01-02", input.StartDate)
		if err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid start date format. Use YYYY-MM-DD"})
			return
		}
		loan.StartDate = startDate
	}
	if input.EndDate != "" {
		endDate, err := time.Parse("2006-01-02", input.EndDate)
		if err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid end date format. Use YYYY-MM-DD"})
			return
		}
		loan.EndDate = endDate
	}
	if input.AccountID != nil {
		// Validate account if provided
		var account models.Account
		if err := database.DB.Where("id = ? AND user_id = ?", *input.AccountID, userID).First(&account).Error; err != nil {
			c.JSON(http.StatusBadRequest, gin.H{"error": "Associated account not found or doesn't belong to user"})
			return
		}
		loan.AccountID = input.AccountID
	}

	// Save changes
	if err := database.DB.Save(&loan).Error; err != nil {
		log.Printf("Error updating loan ID %d for user %d: %v", loanID, userID, err)
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to update loan"})
		return
	}

	c.JSON(http.StatusOK, loan)
}

// DeleteLoan deletes a loan belonging to the authenticated user
func (h *LoanHandler) DeleteLoan(c *gin.Context) {
	userID := c.MustGet("userID").(uint)
	loanID, err := strconv.ParseUint(c.Param("id"), 10, 32)
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid loan ID"})
		return
	}

	// Check if loan exists and belongs to user
	var loan models.Loan
	if err := database.DB.Where("id = ? AND user_id = ?", loanID, userID).First(&loan).Error; err != nil {
		c.JSON(http.StatusNotFound, gin.H{"error": "Loan not found"})
		return
	}

	// Delete loan
	if err := database.DB.Delete(&loan).Error; err != nil {
		log.Printf("Error deleting loan ID %d for user %d: %v", loanID, userID, err)
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to delete loan"})
		return
	}

	c.JSON(http.StatusOK, gin.H{"message": "Loan deleted successfully"})
}

// CreateLoanPayment creates a new payment for a loan
func (h *LoanHandler) CreateLoanPayment(c *gin.Context) {
	// Get user from context (set by auth middleware)
	user, exists := c.Get("user")
	if !exists {
		c.JSON(http.StatusUnauthorized, gin.H{"error": "Unauthorized"})
		return
	}
	userObj := user.(models.User)

	// Get loan ID from URL parameter
	loanID, err := strconv.ParseUint(c.Param("id"), 10, 32)
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid loan ID format"})
		return
	}

	// Check if the loan exists and belongs to the user
	var loan models.Loan
	if err := database.DB.Where("id = ? AND user_id = ?", loanID, userObj.ID).First(&loan).Error; err != nil {
		if err == gorm.ErrRecordNotFound {
			c.JSON(http.StatusNotFound, gin.H{"error": "Loan not found"})
		} else {
			c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch loan"})
		}
		return
	}

	// Define a struct to bind the request JSON
	var input struct {
		Amount        int64  `json:"amount" binding:"required"`
		PaymentDate   string `json:"paymentDate" binding:"required"`
		Principal     int64  `json:"principal"`
		Interest      int64  `json:"interest"`
		TransactionID *uint  `json:"transactionId"`
		Notes         string `json:"notes"`
	}

	// Bind JSON to struct
	if err := c.ShouldBindJSON(&input); err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
		return
	}

	// Parse date
	paymentDate, err := time.Parse("2006-01-02", input.PaymentDate)
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid payment date format"})
		return
	}

	// Create payment object
	payment := models.LoanPayment{
		UserID:        userObj.ID,
		LoanID:        uint(loanID),
		Amount:        input.Amount,
		PaymentDate:   paymentDate,
		Principal:     input.Principal,
		Interest:      input.Interest,
		TransactionID: input.TransactionID,
		Notes:         input.Notes,
	}

	// Save to database in a transaction
	tx := database.DB.Begin()
	if err := tx.Create(&payment).Error; err != nil {
		tx.Rollback()
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to create payment"})
		return
	}

	// Update loan balance
	loan.CurrentBalance -= input.Principal
	if err := tx.Save(&loan).Error; err != nil {
		tx.Rollback()
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to update loan balance"})
		return
	}

	tx.Commit()
	c.JSON(http.StatusCreated, gin.H{"payment": payment, "updatedLoanBalance": loan.CurrentBalance})
}

// GetLoanPayments returns all payments for a specific loan
func (h *LoanHandler) GetLoanPayments(c *gin.Context) {
	// Get user from context (set by auth middleware)
	user, exists := c.Get("user")
	if !exists {
		c.JSON(http.StatusUnauthorized, gin.H{"error": "Unauthorized"})
		return
	}
	userObj := user.(models.User)

	// Get loan ID from URL parameter
	loanID, err := strconv.ParseUint(c.Param("id"), 10, 32)
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid loan ID format"})
		return
	}

	// Check if the loan exists and belongs to the user
	var loan models.Loan
	if err := database.DB.Where("id = ? AND user_id = ?", loanID, userObj.ID).First(&loan).Error; err != nil {
		if err == gorm.ErrRecordNotFound {
			c.JSON(http.StatusNotFound, gin.H{"error": "Loan not found"})
		} else {
			c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch loan"})
		}
		return
	}

	// Fetch all payments for the loan
	var payments []models.LoanPayment
	if err := database.DB.Where("loan_id = ?", loanID).Order("payment_date DESC").Find(&payments).Error; err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch loan payments"})
		return
	}

	c.JSON(http.StatusOK, gin.H{"payments": payments})
}

// DeleteLoanPayment deletes a payment for a loan
func (h *LoanHandler) DeleteLoanPayment(c *gin.Context) {
	// Get user from context (set by auth middleware)
	user, exists := c.Get("user")
	if !exists {
		c.JSON(http.StatusUnauthorized, gin.H{"error": "Unauthorized"})
		return
	}
	userObj := user.(models.User)

	// Get payment ID from URL parameter
	paymentID, err := strconv.ParseUint(c.Param("paymentId"), 10, 32)
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid payment ID format"})
		return
	}

	// Check if the payment exists and belongs to the user
	var payment models.LoanPayment
	if err := database.DB.Where("id = ? AND user_id = ?", paymentID, userObj.ID).First(&payment).Error; err != nil {
		if err == gorm.ErrRecordNotFound {
			c.JSON(http.StatusNotFound, gin.H{"error": "Payment not found"})
		} else {
			c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch payment"})
		}
		return
	}

	// Get the loan to update its balance
	var loan models.Loan
	if err := database.DB.First(&loan, payment.LoanID).Error; err != nil {
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch related loan"})
		return
	}

	// Update loan and delete payment in a transaction
	tx := database.DB.Begin()

	// Reverse the principal payment (add it back to the loan balance)
	loan.CurrentBalance += payment.Principal
	if err := tx.Save(&loan).Error; err != nil {
		tx.Rollback()
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to update loan balance"})
		return
	}

	// Delete the payment
	if err := tx.Delete(&payment).Error; err != nil {
		tx.Rollback()
		c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to delete payment"})
		return
	}

	tx.Commit()
	c.JSON(http.StatusOK, gin.H{"message": "Payment deleted successfully", "updatedLoanBalance": loan.CurrentBalance})
}

// GetLoanPaymentSchedule generates an estimated payment schedule for a loan
func (h *LoanHandler) GetLoanPaymentSchedule(c *gin.Context) {
	// Get user from context (set by auth middleware)
	user, exists := c.Get("user")
	if !exists {
		c.JSON(http.StatusUnauthorized, gin.H{"error": "Unauthorized"})
		return
	}
	userObj := user.(models.User)

	// Get loan ID from URL parameter
	loanID, err := strconv.ParseUint(c.Param("id"), 10, 32)
	if err != nil {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid loan ID format"})
		return
	}

	// Check if the loan exists and belongs to the user
	var loan models.Loan
	if err := database.DB.Where("id = ? AND user_id = ?", loanID, userObj.ID).First(&loan).Error; err != nil {
		if err == gorm.ErrRecordNotFound {
			c.JSON(http.StatusNotFound, gin.H{"error": "Loan not found"})
		} else {
			c.JSON(http.StatusInternalServerError, gin.H{"error": "Failed to fetch loan"})
		}
		return
	}

	// Parse payment frequency parameter (defaults to monthly)
	frequency := c.DefaultQuery("frequency", "monthly")
	if frequency != "monthly" && frequency != "biweekly" && frequency != "weekly" {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Invalid frequency. Allowed values: monthly, biweekly, weekly"})
		return
	}

	// Calculate remaining months (approximately)
	now := time.Now()
	remainingMonths := int(loan.EndDate.Sub(now).Hours() / 24 / 30)
	if remainingMonths <= 0 {
		c.JSON(http.StatusBadRequest, gin.H{"error": "Loan end date is in the past"})
		return
	}

	// Calculate number of payments based on frequency
	paymentsCount := remainingMonths
	var intervalDays int
	if frequency == "weekly" {
		paymentsCount = remainingMonths * 4 // ~4 weeks per month
		intervalDays = 7
	} else if frequency == "biweekly" {
		paymentsCount = remainingMonths * 2 // ~2 bi-weeks per month
		intervalDays = 14
	} else {
		// Monthly
		intervalDays = 30
	}

	// Simple amortization calculation
	rate := loan.InterestRate / 12 / 100 // Monthly interest rate
	if frequency == "weekly" {
		rate = loan.InterestRate / 52 / 100
	} else if frequency == "biweekly" {
		rate = loan.InterestRate / 26 / 100
	}

	// For zero interest loans
	var paymentAmount int64
	if loan.InterestRate <= 0 {
		paymentAmount = loan.CurrentBalance / int64(paymentsCount)
	} else {
		// Formula: PMT = P * (r * (1+r)^n) / ((1+r)^n - 1)
		// Where PMT = payment, P = principal, r = rate per period, n = number of periods

		// Simplified calculation (not exact but good approximation)
		totalWithInterest := float64(loan.CurrentBalance) * (1 + float64(paymentsCount)*rate)
		paymentAmount = int64(totalWithInterest) / int64(paymentsCount)
	}

	// Generate payment schedule
	var schedule []map[string]interface{}
	balance := loan.CurrentBalance
	currentDate := now

	for i := 0; i < paymentsCount && balance > 0; i++ {
		// Calculate interest for this period
		interestPayment := int64(float64(balance) * rate)
		principalPayment := paymentAmount - interestPayment

		// Adjust last payment if needed
		if principalPayment > balance {
			principalPayment = balance
			paymentAmount = principalPayment + interestPayment
		}

		// Update remaining balance
		balance -= principalPayment

		// Create payment entry
		payment := map[string]interface{}{
			"paymentNumber":    i + 1,
			"date":             currentDate.Format("2006-01-02"),
			"totalPayment":     paymentAmount,
			"principalPayment": principalPayment,
			"interestPayment":  interestPayment,
			"remainingBalance": balance,
		}
		schedule = append(schedule, payment)

		// Increment date based on interval
		currentDate = currentDate.AddDate(0, 0, intervalDays)
	}

	c.JSON(http.StatusOK, gin.H{
		"loanDetails": loan,
		"schedule":    schedule,
		"summary": map[string]interface{}{
			"totalPayments":       len(schedule),
			"frequency":           frequency,
			"estimatedPayoffDate": schedule[len(schedule)-1]["date"],
			"totalInterestPaid":   sumInterest(schedule),
		},
	})
}

// Helper function to sum up total interest in a payment schedule
func sumInterest(schedule []map[string]interface{}) int64 {
	var total int64
	for _, payment := range schedule {
		total += payment["interestPayment"].(int64)
	}
	return total
}