billit/internal/logic/calculator.go

package logic

import (
	"math"
)

// Rate represents standard GST rates
type Rate float64

const (
	Rate0  Rate = 0.0
	Rate5  Rate = 0.05
	Rate12 Rate = 0.12
	Rate18 Rate = 0.18
	Rate28 Rate = 0.28
)

// CustomerType distinguishes between B2B (Wholesale) and B2C (Retail)
type CustomerType string

const (
	CustomerWholesale CustomerType = "wholesale"
	CustomerRetail    CustomerType = "retail"
)

// Product represents a catalog item
type Product struct {
	SKU            string
	Name           string
	HSNCode        string
	BasePrice      float64 // Price before tax
	WholesalePrice float64 // Discounted price for B2B
	GSTRate        Rate
	SmallOrderQty  int     // Minimum quantity threshold
	SmallOrderFee  float64 // Convenience fee when quantity is below threshold
	Unit           string  // Unit of measurement (e.g., "pcs", "kg", "box")
}

// LineItem represents a single row in the invoice
type LineItem struct {
	Product     Product
	Quantity    int
	UnitPrice   float64 // Actual price applied (wholesale vs retail)
	TaxableVal  float64 // Quantity * UnitPrice
	CGSTAmount  float64
	SGSTAmount  float64
	IGSTAmount  float64
	TotalAmount float64
}

// Invoice represents the full bill
type Invoice struct {
	LineItems         []LineItem
	SubTotal          float64
	TotalCGST         float64
	TotalSGST         float64
	TotalIGST         float64
	ConvenienceFee    float64 // Flat fee for small orders (before tax)
	ConvenienceFeeTax float64 // GST on convenience fee (18% fixed)
	GrandTotal        float64
	CustomerType      CustomerType
	IsInterState      bool   // True if selling to a different state (IGST applies)
	CompanyDetails    string // Multiline company details (displayed above invoice table)
	BuyerDetails      string // Multiline buyer details (displayed above bank details)
	BuyerName         string // Buyer's name
	BankDetails       string // Multiline bank details (displayed at bottom of invoice)
}

// Calculator handles the GST logic
type Calculator struct{}

// NewCalculator creates a new calculator instance
func NewCalculator() *Calculator {
	return &Calculator{}
}

// CalculateLineItem computes taxes for a single line
func (c *Calculator) CalculateLineItem(p Product, qty int, custType CustomerType, isInterState bool) LineItem {
	// Determine price based on customer type
	price := p.BasePrice
	if custType == CustomerWholesale {
		price = p.WholesalePrice
	}

	taxableVal := price * float64(qty)
	rate := float64(p.GSTRate)

	var cgst, sgst, igst float64

	if isInterState {
		igst = taxableVal * rate
	} else {
		// Intra-state: Split tax between Center and State
		halfRate := rate / 2
		cgst = taxableVal * halfRate
		sgst = taxableVal * halfRate
	}

	total := taxableVal + cgst + sgst + igst

	return LineItem{
		Product:     p,
		Quantity:    qty,
		UnitPrice:   price,
		TaxableVal:  round(taxableVal),
		CGSTAmount:  round(cgst),
		SGSTAmount:  round(sgst),
		IGSTAmount:  round(igst),
		TotalAmount: round(total),
	}
}

// CalculateInvoice computes totals for the entire invoice
func (c *Calculator) CalculateInvoice(items []LineItem, fee float64, isInterState bool) Invoice {
	inv := Invoice{
		LineItems:      items,
		ConvenienceFee: fee,
		IsInterState:   isInterState,
	}

	for _, item := range items {
		inv.SubTotal += item.TaxableVal
		inv.TotalCGST += item.CGSTAmount
		inv.TotalSGST += item.SGSTAmount
		inv.TotalIGST += item.IGSTAmount
	}

	// Convenience fee is taxable at 18% fixed rate
	if fee > 0 {
		feeTax := fee * 0.18 // 18% GST on convenience fee
		inv.ConvenienceFeeTax = round(feeTax)
		// Add convenience fee to taxable subtotal
		inv.SubTotal += fee
		// Add convenience fee tax to appropriate tax fields
		if isInterState {
			inv.TotalIGST += inv.ConvenienceFeeTax
		} else {
			// Split between CGST and SGST (9% each)
			inv.TotalCGST += round(feeTax / 2)
			inv.TotalSGST += round(feeTax / 2)
		}
	}

	inv.GrandTotal = inv.SubTotal + inv.TotalCGST + inv.TotalSGST + inv.TotalIGST

	// Rounding final totals
	inv.SubTotal = round(inv.SubTotal)
	inv.TotalCGST = round(inv.TotalCGST)
	inv.TotalSGST = round(inv.TotalSGST)
	inv.TotalIGST = round(inv.TotalIGST)
	inv.GrandTotal = round(inv.GrandTotal)

	return inv
}

func round(num float64) float64 {
	return math.Round(num*100) / 100
}