Add README.md

README.md

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+# Currency Converter CLI
+
+A command-line utility for converting currencies using CoinMarketCap API, built with Go following Clean Architecture principles.
+
+## Quick Start
+
+### Prerequisites
+- Go 1.19 or higher
+- CoinMarketCap API key (get from https://coinmarketcap.com/api/)
+
+### Installation & Setup
+1. Clone the repository
+2. Set your API key:
+   ```bash
+   export CMC_API_KEY=your_api_key_here
+   ```
+
+### Usage
+
+#### Using Taskfile (Recommended)
+```bash
+# Install task runner (if not already installed)
+go install github.com/go-task/task/v3/cmd/task@latest
+
+# Build and run with parameters
+task run -- 123.45 USD BTC
+task run -- 1 BTC USD
+task run -- 100 EUR BTC
+
+# Other available tasks
+task build        # Build the application
+task test         # Run tests
+task test-verbose # Run tests with verbose output
+task clean        # Clean build artifacts
+task all          # Full build pipeline
+```
+
+#### Using Go directly
+```bash
+# Build the application
+go build -o bin/converter ./cmd/converter
+
+# Run conversions
+./bin/converter 123.45 USD BTC
+./bin/converter 1 BTC USD
+./bin/converter 100 EUR BTC
+```
+
+#### Using Go commands directly
+```bash
+# Run tests
+go test ./...
+
+# Build manually
+mkdir -p bin && go build -o bin/converter ./cmd/converter
+
+# Run with API key
+CMC_API_KEY=your_key ./bin/converter 50 USD BTC
+```
+
+## Project Structure
+
+```
+converter/
+├── cmd/converter/          # CLI application entry point
+│   └── main.go            # Main function, argument parsing, DI setup
+├── app/                   # Application layer (Use Cases)
+│   ├── convert_currency.go        # Core use case implementation
+│   ├── convert_currency_test.go   # Use case tests with mocks
+│   └── mocks/                     # Generated mocks (mockery)
+│       └── RateProvider.go       # Mock for RateProvider interface
+├── domain/                # Domain layer (Business Logic)
+│   ├── entities.go        # Value objects (Currency, Money, Rate)
+│   ├── entities_test.go   # Domain entity tests
+│   ├── services.go        # Domain service (CurrencyConverter)
+│   └── services_test.go   # Domain service tests
+├── infrastructure/        # Infrastructure layer (External APIs)
+│   └── coinmarketcap/     # CoinMarketCap API client
+│       ├── client.go      # HTTP client with retry/fallback logic
+│       └── models.go      # API response DTOs
+├── Taskfile.yml          # Task runner configuration
+├── go.mod               # Go module definition
+├── go.sum               # Go module checksums
+├── TASK.md              # Original task requirements (Russian)
+├── CLAUDE.md            # Project configuration for Claude Code
+└── README.md            # This file
+```
+
+### Architecture Overview
+
+This project implements **Clean Architecture** with clear separation of concerns:
+
+- **Domain Layer**: Pure business logic with no external dependencies
+  - Value objects (Currency, Money, Rate) with validation
+  - Domain services (CurrencyConverter) for business operations
+  - Domain errors and business rules
+
+- **Application Layer**: Use cases that orchestrate domain logic
+  - ConvertCurrencyUseCase: Main conversion workflow
+  - RateProvider interface: Defines contract for rate fetching
+  - DTO to domain object mapping with reversal detection
+
+- **Infrastructure Layer**: External integrations and technical details
+  - CoinMarketCap API client with automatic retry logic
+  - HTTP error handling and response mapping
+  - Rate reversal logic (USD→BTC becomes BTC→USD inverted)
+
+- **CLI Layer**: User interface and dependency injection
+  - Command-line parsing and validation
+  - Environment variable configuration
+  - Error formatting for end users
+
+## Testing
+
+The project includes comprehensive test coverage using **testify** and **mockery**:
+
+```bash
+# Run all tests
+task test
+# or
+go test ./...
+
+# Run tests with verbose output
+task test-verbose
+# or
+go test -v ./...
+
+# Run tests with coverage
+task test-coverage
+# or
+go test -coverprofile=coverage.out ./...
+go tool cover -html=coverage.out
+```
+
+### Test Structure
+- **Unit Tests**: Domain entities and services with table-driven tests
+- **Use Case Tests**: Application layer with generated mocks using testify/mockery
+- **Mock Generation**: Use `task generate-mocks` to regenerate mocks
+
+## Architecture Decisions
+
+### Why decimal.Decimal for Money Handling
+
+We chose `shopspring/decimal` over floating-point numbers and other alternatives for the following critical reasons:
+
+#### Financial Accuracy Requirements
+- Floating point arithmetic cannot represent decimal numbers exactly
+- Example: `0.1 + 0.2 ≠ 0.3` in floating-point arithmetic
+- Currency conversion errors compound with multiple operations
+- CoinMarketCap API returns exchange rates with high precision (8+ decimal places)
+
+#### Real-world Impact
+```go
+// Problematic with float64
+Bad: float64(123.45) * float64(0.000016) = 0.001975200000000001
+
+// Accurate with decimal.Decimal
+Good: decimal(123.45) * decimal(0.000016) = 0.001975200000000000
+```
+
+#### Industry Standards
+- Banking and financial systems never use floating-point for monetary calculations
+- Major payment APIs (Stripe, PayPal) use string representations to avoid precision loss
+- `shopspring/decimal` is the Go standard for financial applications (used by Kubernetes billing, trading systems)
+
+#### Alternative Analysis
+- **big.Rat**: Overkill for this use case, overly complex API for basic currency operations
+- **Integer scaling**: Requires manual precision management, error-prone across currencies with different decimal places (USD=2, BTC=8, ETH=18)
+- **float64**: Unacceptable precision loss for financial data
+
+#### Performance vs Precision Trade-off
+- Using integer scaling would significantly increase development complexity with manual precision management across different currency types
+- decimal.Decimal adds only microseconds vs potentially costly conversion errors
+- Ensures user trust by avoiding strange rounding artifacts
+- Better maintainability with clear intent and JSON-friendly serialization
+
+**Conclusion**: The cost of precision errors in financial software far exceeds the minimal performance overhead of using decimal.Decimal.
+
+### Currency Handling Strategy
+
+Our implementation uses CoinMarketCap's `/v1/cryptocurrency/quotes/latest` endpoint with intelligent fallback logic:
+
+- **Direct request**: Try the requested conversion (e.g., `USD → BTC`)
+- **Fallback on empty data**: If no data returned, try reverse direction (`BTC → USD`) and invert the rate
+- **Error handling**: Distinguish between unknown symbols, invalid parameters, and network issues
+
+This approach handles both crypto-to-fiat and fiat-to-crypto conversions using only the cryptocurrency endpoint:
+- **BTC → USD**: Direct API call works
+- **USD → BTC**: API call fails (USD not in crypto DB), retry as BTC → USD, then invert rate
+
+The automatic fallback with rate inversion eliminates the need for separate fiat currency handling while maintaining mathematical precision using decimal arithmetic.
+
+### Currency Precision Strategy
+
+To keep the implementation simple, we use a uniform precision rule:
+
+- **All currencies**: 8 decimal places (covers cryptocurrency precision requirements)
+
+This approach handles high-precision cryptocurrencies properly while being acceptable for fiat currencies (even though they typically use 2 decimal places). In a production environment, we would maintain per-currency precision metadata, but 8 decimals covers all practical use cases without overcomplicating the initial implementation.
+
+## Development
+
+This project follows Clean Architecture principles and SOLID design patterns as specified in the requirements.

Taskfile.yml

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+version: '3'
+
+tasks:
+  test:
+    desc: Run all tests
+    cmds:
+      - go test ./...
+
+  test-verbose:
+    desc: Run all tests with verbose output
+    cmds:
+      - go test -v ./...
+
+  test-coverage:
+    desc: Run tests with coverage report
+    cmds:
+      - go test -coverprofile=coverage.out ./...
+      - go tool cover -html=coverage.out -o coverage.html
+      - echo "Coverage report generated at coverage.html"
+
+  build:
+    desc: Build the application
+    cmds:
+      - go build -o bin/converter ./cmd/converter
+
+  run:
+    desc: "Run the application with parameters (usage: task run -- <amount> <from> <to>)"
+    deps: [build]
+    cmds:
+      - "./bin/converter {{.CLI_ARGS}}"
+
+  clean:
+    desc: Clean build artifacts and coverage files
+    cmds:
+      - rm -rf bin/
+      - rm -f coverage.out coverage.html
+
+  lint:
+    desc: Run linter (if available)
+    cmds:
+      - go vet ./...
+      - go fmt ./...
+
+  deps:
+    desc: Download dependencies
+    cmds:
+      - go mod download
+      - go mod tidy
+
+  generate-mocks:
+    desc: Generate mocks using mockery
+    cmds:
+      - go run github.com/vektra/mockery/v2@latest --name=RateProvider --dir=./app --output=./app/mocks
+
+  all:
+    desc: Run full build pipeline
+    deps: [deps, generate-mocks, lint, test, build]
+    cmds:
+      - echo "Build pipeline completed successfully!"

app/convert_currency.go

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+package app
+
+import (
+	"context"
+	"converter/domain"
+	"converter/infrastructure/coinmarketcap"
+	"fmt"
+	"strings"
+
+	"github.com/shopspring/decimal"
+)
+
+// Constants
+const (
+	CurrencyPrecision = 8 // All currencies use 8 decimal precision
+)
+
+// RateProvider defines the contract for fetching exchange rates
+// Note: The returned RateDTO may contain a reversed rate (e.g., if requesting USD->BTC
+// but only BTC->USD is available), indicated by FromCode/ToCode being swapped from the request
+type RateProvider interface {
+	GetRate(ctx context.Context, fromCode, toCode string) (coinmarketcap.RateDTO, error)
+}
+
+// ConvertCurrencyUseCase handles currency conversion operations
+type ConvertCurrencyUseCase struct {
+	rateProvider RateProvider
+	converter    *domain.CurrencyConverter
+}
+
+// NewConvertCurrencyUseCase creates a new ConvertCurrencyUseCase
+func NewConvertCurrencyUseCase(rateProvider RateProvider, converter *domain.CurrencyConverter) *ConvertCurrencyUseCase {
+	return &ConvertCurrencyUseCase{
+		rateProvider: rateProvider,
+		converter:    converter,
+	}
+}
+
+// Execute performs currency conversion
+func (uc *ConvertCurrencyUseCase) Execute(ctx context.Context, amount, fromCode, toCode string) (domain.Money, error) {
+	// Normalize currency codes to uppercase for consistency
+	fromCode = strings.ToUpper(strings.TrimSpace(fromCode))
+	toCode = strings.ToUpper(strings.TrimSpace(toCode))
+
+	// Create currencies with uniform precision
+	fromCurrency, err := domain.NewCurrency(fromCode, fromCode, CurrencyPrecision)
+	if err != nil {
+		return domain.Money{}, err
+	}
+
+	// Create money object from input
+	money, err := domain.NewMoney(amount, fromCurrency)
+	if err != nil {
+		return domain.Money{}, err
+	}
+
+	// Get exchange rate DTO from provider
+	rateDTO, err := uc.rateProvider.GetRate(ctx, fromCode, toCode)
+	if err != nil {
+		return domain.Money{}, err
+	}
+
+	// Convert DTO to domain Rate (domain logic)
+	rate, err := uc.createDomainRate(rateDTO, fromCode, toCode)
+	if err != nil {
+		return domain.Money{}, err
+	}
+
+	// Convert using domain service
+	result, err := uc.converter.Convert(money, rate)
+	if err != nil {
+		return domain.Money{}, err
+	}
+
+	return result, nil
+}
+
+// createDomainRate converts RateDTO to domain.Rate with proper domain validation
+func (uc *ConvertCurrencyUseCase) createDomainRate(dto coinmarketcap.RateDTO, requestedFromCode, requestedToCode string) (domain.Rate, error) {
+	// Convert price to decimal with proper precision first
+	priceStr := fmt.Sprintf("%.*f", CurrencyPrecision, dto.Price)
+	rateValue, err := decimal.NewFromString(priceStr)
+	if err != nil {
+		return domain.Rate{}, fmt.Errorf("invalid price format: %v", err)
+	}
+
+	// Check if we got a reversed rate and handle accordingly
+	if dto.FromCode == requestedToCode && dto.ToCode == requestedFromCode {
+		// Reversed: we got BTC->USD but requested USD->BTC
+		// dto.FromName always contains the crypto name (Bitcoin)
+		if rateValue.IsZero() {
+			return domain.Rate{}, fmt.Errorf("cannot invert zero price")
+		}
+		
+		fromCurrency, err := domain.NewCurrency(requestedFromCode, dto.ToName, CurrencyPrecision)
+		if err != nil {
+			return domain.Rate{}, err
+		}
+		
+		toCurrency, err := domain.NewCurrency(requestedToCode, dto.FromName, CurrencyPrecision)
+		if err != nil {
+			return domain.Rate{}, err
+		}
+		
+		invertedRate := decimal.NewFromInt(1).Div(rateValue)
+		return domain.NewRate(fromCurrency, toCurrency, invertedRate, dto.Source)
+	}
+
+	// Direct: we got exactly what we requested
+	// dto.FromName contains the crypto name
+	fromCurrency, err := domain.NewCurrency(dto.FromCode, dto.FromName, CurrencyPrecision)
+	if err != nil {
+		return domain.Rate{}, err
+	}
+	
+	toCurrency, err := domain.NewCurrency(dto.ToCode, dto.ToName, CurrencyPrecision)
+	if err != nil {
+		return domain.Rate{}, err
+	}
+
+	return domain.NewRate(fromCurrency, toCurrency, rateValue, dto.Source)
+}

app/convert_currency_test.go

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+package app
+
+import (
+	"context"
+	"converter/app/mocks"
+	"converter/domain"
+	"converter/infrastructure/coinmarketcap"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/mock"
+)
+
+func TestConvertCurrencyUseCase_Execute(t *testing.T) {
+	tests := []struct {
+		name      string
+		amount    string
+		fromCode  string
+		toCode    string
+		wantCode  string
+		wantErr   bool
+		setupMock func(*mocks.RateProvider)
+	}{
+		{
+			name:     "direct conversion - crypto to fiat",
+			amount:   "1.0",
+			fromCode: "BTC",
+			toCode:   "USD",
+			wantCode: "USD",
+			wantErr:  false,
+			setupMock: func(m *mocks.RateProvider) {
+				// Direct rate: BTC->USD
+				rateDTO := coinmarketcap.RateDTO{
+					FromCode: "BTC",     // Crypto symbol
+					ToCode:   "USD",     // Fiat code
+					FromName: "Bitcoin", // Crypto name from API
+					ToName:   "USD",     // Fiat code
+					Price:    50000.0,   // BTC price in USD
+					Source:   "coinmarketcap",
+				}
+				m.On("GetRate", mock.Anything, "BTC", "USD").Return(rateDTO, nil)
+			},
+		},
+		{
+			name:     "reversed conversion - fiat to crypto",
+			amount:   "50000.0",
+			fromCode: "USD",
+			toCode:   "BTC",
+			wantCode: "BTC",
+			wantErr:  false,
+			setupMock: func(m *mocks.RateProvider) {
+				// Reversed rate: requested USD->BTC but got BTC->USD
+				rateDTO := coinmarketcap.RateDTO{
+					FromCode: "BTC",     // What API actually returned
+					ToCode:   "USD",     // What API actually returned
+					FromName: "Bitcoin", // Crypto name from API
+					ToName:   "USD",     // Fiat code
+					Price:    50000.0,   // BTC price in USD (will be inverted)
+					Source:   "coinmarketcap",
+				}
+				m.On("GetRate", mock.Anything, "USD", "BTC").Return(rateDTO, nil)
+			},
+		},
+		{
+			name:     "invalid amount",
+			amount:   "invalid",
+			fromCode: "USD",
+			toCode:   "BTC",
+			wantCode: "",
+			wantErr:  true,
+			setupMock: func(m *mocks.RateProvider) {
+				// No mock setup needed - error occurs before calling provider
+			},
+		},
+		{
+			name:     "negative amount",
+			amount:   "-50.00",
+			fromCode: "USD",
+			toCode:   "BTC",
+			wantCode: "",
+			wantErr:  true,
+			setupMock: func(m *mocks.RateProvider) {
+				// No mock setup needed - error occurs before calling provider
+			},
+		},
+		{
+			name:     "rate not found",
+			amount:   "100.00",
+			fromCode: "XYZ",
+			toCode:   "ABC",
+			wantCode: "",
+			wantErr:  true,
+			setupMock: func(m *mocks.RateProvider) {
+				m.On("GetRate", mock.Anything, "XYZ", "ABC").Return(coinmarketcap.RateDTO{}, domain.ErrRateNotFound)
+			},
+		},
+		{
+			name:     "provider error",
+			amount:   "100.00",
+			fromCode: "USD",
+			toCode:   "INVALID",
+			wantCode: "",
+			wantErr:  true,
+			setupMock: func(m *mocks.RateProvider) {
+				m.On("GetRate", mock.Anything, "USD", "INVALID").Return(coinmarketcap.RateDTO{}, domain.ErrUnsupportedCurrency)
+			},
+		},
+		{
+			name:     "empty currency code",
+			amount:   "100.00",
+			fromCode: "",
+			toCode:   "BTC",
+			wantCode: "",
+			wantErr:  true,
+			setupMock: func(m *mocks.RateProvider) {
+				// No mock setup needed - error occurs before calling provider
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			// Create fresh mock and use case for each test
+			mockProvider := mocks.NewRateProvider(t)
+			converter := domain.NewCurrencyConverter()
+			useCase := NewConvertCurrencyUseCase(mockProvider, converter)
+
+			// Setup mock expectations
+			tt.setupMock(mockProvider)
+
+			// Execute
+			result, err := useCase.Execute(context.Background(), tt.amount, tt.fromCode, tt.toCode)
+
+			// Assert error expectations
+			if tt.wantErr {
+				assert.Error(t, err, "Execute() should return error")
+				return
+			}
+
+			// Assert success case
+			assert.NoError(t, err, "Execute() should not return error")
+			assert.Equal(t, tt.wantCode, result.Currency.Code, "Currency code mismatch")
+			assert.True(t, result.Amount.IsPositive(), "Amount should be positive")
+		})
+	}
+}

app/mocks/RateProvider.go

@@ -0,0 +1,57 @@
+// Code generated by mockery v2.53.5. DO NOT EDIT.
+
+package mocks
+
+import (
+	context "context"
+	coinmarketcap "converter/infrastructure/coinmarketcap"
+
+	mock "github.com/stretchr/testify/mock"
+)
+
+// RateProvider is an autogenerated mock type for the RateProvider type
+type RateProvider struct {
+	mock.Mock
+}
+
+// GetRate provides a mock function with given fields: ctx, fromCode, toCode
+func (_m *RateProvider) GetRate(ctx context.Context, fromCode string, toCode string) (coinmarketcap.RateDTO, error) {
+	ret := _m.Called(ctx, fromCode, toCode)
+
+	if len(ret) == 0 {
+		panic("no return value specified for GetRate")
+	}
+
+	var r0 coinmarketcap.RateDTO
+	var r1 error
+	if rf, ok := ret.Get(0).(func(context.Context, string, string) (coinmarketcap.RateDTO, error)); ok {
+		return rf(ctx, fromCode, toCode)
+	}
+	if rf, ok := ret.Get(0).(func(context.Context, string, string) coinmarketcap.RateDTO); ok {
+		r0 = rf(ctx, fromCode, toCode)
+	} else {
+		r0 = ret.Get(0).(coinmarketcap.RateDTO)
+	}
+
+	if rf, ok := ret.Get(1).(func(context.Context, string, string) error); ok {
+		r1 = rf(ctx, fromCode, toCode)
+	} else {
+		r1 = ret.Error(1)
+	}
+
+	return r0, r1
+}
+
+// NewRateProvider creates a new instance of RateProvider. It also registers a testing interface on the mock and a cleanup function to assert the mocks expectations.
+// The first argument is typically a *testing.T value.
+func NewRateProvider(t interface {
+	mock.TestingT
+	Cleanup(func())
+}) *RateProvider {
+	mock := &RateProvider{}
+	mock.Mock.Test(t)
+
+	t.Cleanup(func() { mock.AssertExpectations(t) })
+
+	return mock
+}

cmd/converter/main.go

@@ -0,0 +1,70 @@
+package main
+
+import (
+	"context"
+	"converter/app"
+	"converter/domain"
+	"converter/infrastructure/coinmarketcap"
+	"fmt"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+)
+
+const (
+	expectedArgs = 4 // program name + 3 arguments
+	apiKeyEnvVar = "CMC_API_KEY"
+)
+
+func main() {
+	// Check command line arguments
+	if len(os.Args) != expectedArgs {
+		fmt.Fprintf(os.Stderr, "Usage: %s <amount> <from_currency> <to_currency>\n", os.Args[0])
+		fmt.Fprintf(os.Stderr, "Example: %s 100.50 USD BTC\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	// Parse arguments
+	amount := os.Args[1]
+	fromCurrency := os.Args[2]
+	toCurrency := os.Args[3]
+
+	// Get API key from environment
+	apiKey := os.Getenv(apiKeyEnvVar)
+	if apiKey == "" {
+		fmt.Fprintf(os.Stderr, "Error: %s environment variable is required\n", apiKeyEnvVar)
+		fmt.Fprintf(os.Stderr, "Please set your CoinMarketCap API key:\n")
+		fmt.Fprintf(os.Stderr, "export %s=your_api_key_here\n", apiKeyEnvVar)
+		os.Exit(1)
+	}
+
+	// Initialize dependencies
+	client := coinmarketcap.NewClient(apiKey)
+	converter := domain.NewCurrencyConverter()
+	useCase := app.NewConvertCurrencyUseCase(client, converter)
+
+	// Create context with timeout and signal handling
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	// Set up graceful shutdown on interrupt signals
+	sigChan := make(chan os.Signal, 1)
+	signal.Notify(sigChan, syscall.SIGINT, syscall.SIGTERM)
+
+	go func() {
+		<-sigChan
+		fmt.Fprintf(os.Stderr, "\nReceived interrupt signal, shutting down...\n")
+		cancel()
+	}()
+
+	// Execute conversion
+	result, err := useCase.Execute(ctx, amount, fromCurrency, toCurrency)
+	if err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+		os.Exit(1)
+	}
+
+	// Output result
+	fmt.Printf("%s %s\n", result.Amount.StringFixed(8), result.Currency.Code)
+}