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quite a lot of things

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This commit is contained in:
Arkaprabha Chakraborty
2025-12-06 03:05:44 +05:30
parent 39c61b7790
commit 28733e22d3
42 changed files with 4214 additions and 204 deletions
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158
internal/gst/calculator.go Normal file
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package gst
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
}

122
internal/gst/calculator_test.go Normal file
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package gst
import (
"testing"
)
func TestCalculateLineItem(t *testing.T) {
c := NewCalculator()
product := Product{
SKU: "TEST01",
BasePrice: 100,
WholesalePrice: 90,
GSTRate: Rate18,
}
// Case 1: Retail, Intra-state
item := c.CalculateLineItem(product, 2, CustomerRetail, false)
if item.UnitPrice != 100 {
t.Errorf("Expected UnitPrice 100, got %f", item.UnitPrice)
}
if item.TaxableVal != 200 {
t.Errorf("Expected TaxableVal 200, got %f", item.TaxableVal)
}
if item.CGSTAmount != 18 { // 9% of 200
t.Errorf("Expected CGST 18, got %f", item.CGSTAmount)
}
if item.IGSTAmount != 0 {
t.Errorf("Expected IGST 0, got %f", item.IGSTAmount)
}
if item.TotalAmount != 236 { // 200 + 18 + 18
t.Errorf("Expected Total 236, got %f", item.TotalAmount)
}
// Case 2: Wholesale, Inter-state
item = c.CalculateLineItem(product, 10, CustomerWholesale, true)
if item.UnitPrice != 90 {
t.Errorf("Expected UnitPrice 90, got %f", item.UnitPrice)
}
if item.TaxableVal != 900 {
t.Errorf("Expected TaxableVal 900, got %f", item.TaxableVal)
}
if item.CGSTAmount != 0 {
t.Errorf("Expected CGST 0, got %f", item.CGSTAmount)
}
if item.IGSTAmount != 162 { // 18% of 900
t.Errorf("Expected IGST 162, got %f", item.IGSTAmount)
}
if item.TotalAmount != 1062 { // 900 + 162
t.Errorf("Expected Total 1062, got %f", item.TotalAmount)
}
}
func TestCalculateInvoice(t *testing.T) {
c := NewCalculator()
product := Product{SKU: "TEST01", BasePrice: 100, GSTRate: Rate18}
item1 := c.CalculateLineItem(product, 1, CustomerRetail, false)
item2 := c.CalculateLineItem(product, 1, CustomerRetail, false)
// Test with convenience fee (intra-state)
invoice := c.CalculateInvoice([]LineItem{item1, item2}, 50, false)
// Convenience fee is taxed at 18%
// SubTotal should include convenience fee: 100 + 100 + 50 = 250
expectedSubTotal := 250.0
if invoice.SubTotal != expectedSubTotal {
t.Errorf("Expected SubTotal %f, got %f", expectedSubTotal, invoice.SubTotal)
}
// Convenience fee tax: 50 * 0.18 = 9
expectedFeeTax := 9.0
if invoice.ConvenienceFeeTax != expectedFeeTax {
t.Errorf("Expected ConvenienceFeeTax %f, got %f", expectedFeeTax, invoice.ConvenienceFeeTax)
}
// Total CGST: 9 + 9 + 4.5 = 22.5 (from items + half of fee tax)
expectedCGST := 22.5
if invoice.TotalCGST != expectedCGST {
t.Errorf("Expected TotalCGST %f, got %f", expectedCGST, invoice.TotalCGST)
}
// GrandTotal: SubTotal + CGST + SGST = 250 + 22.5 + 22.5 = 295
expectedTotal := 295.0
if invoice.GrandTotal != expectedTotal {
t.Errorf("Expected GrandTotal %f, got %f", expectedTotal, invoice.GrandTotal)
}
}
func TestCalculateInvoiceInterState(t *testing.T) {
c := NewCalculator()
product := Product{SKU: "TEST01", BasePrice: 100, GSTRate: Rate18}
item := c.CalculateLineItem(product, 1, CustomerRetail, true)
// Test with convenience fee (inter-state)
invoice := c.CalculateInvoice([]LineItem{item}, 50, true)
// SubTotal: 100 + 50 = 150
expectedSubTotal := 150.0
if invoice.SubTotal != expectedSubTotal {
t.Errorf("Expected SubTotal %f, got %f", expectedSubTotal, invoice.SubTotal)
}
// Convenience fee tax: 50 * 0.18 = 9
expectedFeeTax := 9.0
if invoice.ConvenienceFeeTax != expectedFeeTax {
t.Errorf("Expected ConvenienceFeeTax %f, got %f", expectedFeeTax, invoice.ConvenienceFeeTax)
}
// Total IGST: 18 + 9 = 27 (from item + fee tax)
expectedIGST := 27.0
if invoice.TotalIGST != expectedIGST {
t.Errorf("Expected TotalIGST %f, got %f", expectedIGST, invoice.TotalIGST)
}
// GrandTotal: SubTotal + IGST = 150 + 27 = 177
expectedTotal := 177.0
if invoice.GrandTotal != expectedTotal {
t.Errorf("Expected GrandTotal %f, got %f", expectedTotal, invoice.GrandTotal)
}
}