From a537d1c0bbe7185c952ec0c7413726d2177355ad Mon Sep 17 00:00:00 2001
From: Savely Krendelhoff <alicecandyom@proton.me>
Date: Sat, 6 Sep 2025 12:20:05 +0700
Subject: [PATCH] Define domain layer

---
 domain/entities.go | 131 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 131 insertions(+)
 create mode 100644 domain/entities.go

diff --git a/domain/entities.go b/domain/entities.go
new file mode 100644
index 0000000..5bdb23a
--- /dev/null
+++ b/domain/entities.go
@@ -0,0 +1,131 @@
+package domain
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"time"
+
+	"github.com/shopspring/decimal"
+)
+
+// Domain-specific errors
+var (
+	ErrInvalidCurrency     = errors.New("invalid currency")
+	ErrInvalidAmount       = errors.New("invalid amount")
+	ErrNegativeAmount      = errors.New("amount cannot be negative")
+	ErrInvalidRate         = errors.New("invalid exchange rate")
+	ErrCurrencyMismatch    = errors.New("currency mismatch")
+	ErrRateNotFound        = errors.New("exchange rate not found")
+	ErrUnsupportedCurrency = errors.New("unsupported currency")
+)
+
+// Currency represents a currency with its metadata
+type Currency struct {
+	Code      string
+	Name      string
+	Precision int
+}
+
+// NewCurrency creates a new Currency with validation
+func NewCurrency(code, name string, precision int) (Currency, error) {
+	if strings.TrimSpace(code) == "" {
+		return Currency{}, ErrInvalidCurrency
+	}
+
+	if precision < 0 {
+		return Currency{}, ErrInvalidCurrency
+	}
+
+	return Currency{
+		Code:      strings.ToUpper(strings.TrimSpace(code)),
+		Name:      strings.TrimSpace(name),
+		Precision: precision,
+	}, nil
+}
+
+// String returns the currency code
+func (c Currency) String() string {
+	return c.Code
+}
+
+// Money represents an amount in a specific currency
+type Money struct {
+	Amount   decimal.Decimal
+	Currency Currency
+}
+
+// NewMoney creates a new Money instance from string amount
+func NewMoney(amount string, currency Currency) (Money, error) {
+	if strings.TrimSpace(amount) == "" {
+		return Money{}, ErrInvalidAmount
+	}
+
+	decimalAmount, err := decimal.NewFromString(strings.TrimSpace(amount))
+	if err != nil {
+		return Money{}, ErrInvalidAmount
+	}
+
+	if decimalAmount.IsNegative() {
+		return Money{}, ErrNegativeAmount
+	}
+
+	return Money{
+		Amount:   decimalAmount,
+		Currency: currency,
+	}, nil
+}
+
+// NewMoneyFromDecimal creates a new Money instance from decimal.Decimal
+func NewMoneyFromDecimal(amount decimal.Decimal, currency Currency) (Money, error) {
+	if amount.IsNegative() {
+		return Money{}, ErrNegativeAmount
+	}
+
+	return Money{
+		Amount:   amount,
+		Currency: currency,
+	}, nil
+}
+
+// String returns formatted money representation
+func (m Money) String() string {
+	return fmt.Sprintf("%s %s", m.Amount.StringFixed(int32(m.Currency.Precision)), m.Currency.Code)
+}
+
+// Rate represents an exchange rate between two currencies
+type Rate struct {
+	From      Currency
+	To        Currency
+	Value     decimal.Decimal
+	Timestamp time.Time
+	Source    string
+}
+
+// NewRate creates a new Rate with validation
+func NewRate(from, to Currency, value decimal.Decimal, source string) (Rate, error) {
+	if from.Code == to.Code {
+		return Rate{}, ErrInvalidRate
+	}
+
+	if value.IsNegative() || value.IsZero() {
+		return Rate{}, ErrInvalidRate
+	}
+
+	return Rate{
+		From:      from,
+		To:        to,
+		Value:     value,
+		Timestamp: time.Now(),
+		Source:    strings.TrimSpace(source),
+	}, nil
+}
+
+
+// String returns formatted rate representation
+func (r Rate) String() string {
+	return fmt.Sprintf("1 %s = %s %s",
+		r.From.Code,
+		r.Value.StringFixed(int32(r.To.Precision)),
+		r.To.Code)
+}