Implement logic

docs/architecture.md

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+# Architecture Documentation
+
+## Overview
+
+Banks2FF implements a **Hexagonal (Ports & Adapters) Architecture** to synchronize bank transactions from GoCardless to Firefly III. This architecture separates business logic from external concerns, making the system testable and maintainable.
+
+## Workspace Structure
+
+```
+banks2ff/
+├── banks2ff/           # Main CLI application
+│   └── src/
+│       ├── core/       # Domain logic and models
+│       ├── adapters/   # External service integrations
+│       └── main.rs     # CLI entry point
+├── firefly-client/     # Firefly III API client library
+├── gocardless-client/  # GoCardless API client library
+└── docs/              # Architecture documentation
+```
+
+## Core Components
+
+### 1. Domain Core (`banks2ff/src/core/`)
+
+**models.rs**: Defines domain entities
+- `BankTransaction`: Core transaction model with multi-currency support
+- `Account`: Bank account representation
+- Supports `foreign_amount` and `foreign_currency` for international transactions
+
+**ports.rs**: Defines abstraction traits
+- `TransactionSource`: Interface for fetching transactions (implemented by GoCardless adapter)
+- `TransactionDestination`: Interface for storing transactions (implemented by Firefly adapter)
+- Traits are mockable for isolated testing
+
+**sync.rs**: Synchronization engine
+- `run_sync()`: Orchestrates the entire sync process
+- Implements "Healer" strategy for idempotency
+- Smart date range calculation (Last Transaction Date + 1 to Yesterday)
+
+### 2. Adapters (`banks2ff/src/adapters/`)
+
+**gocardless/**: GoCardless integration
+- `client.rs`: Wrapper for GoCardless client with token management
+- `mapper.rs`: Converts GoCardless API responses to domain models
+- `cache.rs`: Caches account mappings to reduce API calls
+- Correctly handles multi-currency via `currencyExchange` array parsing
+
+**firefly/**: Firefly III integration  
+- `client.rs`: Wrapper for Firefly client for transaction storage
+- Maps domain models to Firefly API format
+
+### 3. API Clients
+
+Both clients are hand-crafted using `reqwest`:
+- Strongly-typed DTOs for compile-time safety
+- Custom error handling with `thiserror`
+- Rate limit awareness and graceful degradation
+
+## Synchronization Process
+
+The "Healer" strategy ensures idempotency:
+
+1. **Account Discovery**: Fetch active accounts from GoCardless
+2. **Account Matching**: Match GoCardless accounts to Firefly asset accounts by IBAN
+3. **Date Window**: Calculate sync range (Last Firefly transaction + 1 to Yesterday)
+4. **Transaction Processing**:
+   - **Search**: Look for existing transaction using windowed heuristic (date ± 3 days, exact amount)
+   - **Heal**: If found without `external_id`, update with GoCardless transaction ID
+   - **Skip**: If found with matching `external_id`, ignore
+   - **Create**: If not found, create new transaction in Firefly
+
+## Key Features
+
+### Multi-Currency Support
+- Parses `currencyExchange` array from GoCardless responses
+- Calculates `foreign_amount = amount * exchange_rate`
+- Maps to Firefly's `foreign_amount` and `foreign_currency_code` fields
+
+### Rate Limit Management
+- **Caching**: Stores `AccountId -> IBAN` mappings to reduce requisition calls
+- **Token Reuse**: Maintains tokens until expiry to minimize auth requests
+- **Graceful Handling**: Continues sync for other accounts when encountering 429 errors
+
+### Idempotency
+- GoCardless `transactionId` → Firefly `external_id` mapping
+- Windowed duplicate detection prevents double-creation
+- Historical transaction healing for pre-existing data
+
+## Data Flow
+
+```
+GoCardless API → GoCardlessAdapter → TransactionSource → SyncEngine → TransactionDestination → FireflyAdapter → Firefly API
+```
+
+## Testing Strategy
+
+- **Unit Tests**: Core logic with `mockall` for trait mocking
+- **Integration Tests**: API clients with `wiremock` for HTTP mocking
+- **Fixture Testing**: Real JSON responses for adapter mapping validation
+- **Isolation**: Business logic tested without external dependencies
+
+## Error Handling
+
+- **Custom Errors**: `thiserror` for domain-specific error types
+- **Propagation**: `anyhow` for error context across async boundaries
+- **Graceful Degradation**: Rate limits and network issues don't crash entire sync
+- **Structured Logging**: `tracing` for observability and debugging
+
+## Configuration Management
+
+- Environment variables loaded via `dotenvy`
+- Workspace-level dependency management
+- Feature flags for optional functionality
+- Secure credential handling (no hardcoded secrets)