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+# Development Guide
+ 
+---
+ 
+## Setting Up a Dev Environment
+ 
+```bash
+git clone https://git.samantha42.xyz/samantha/FPandA-Engine
+cd FPandA-Engine
+go mod tidy
+go run ./main.go
+```
+ 
+No database setup is needed. The SQLite file is created automatically on first run.
+ 
+---
+ 
+## Running Tests
+ 
+```bash
+go test ./...
+```
+ 
+Tests use Go's built-in testing package. The test suite was migrated from Python to Go's native test runner — see commit `771eb9d`. Use `DB_PATH=:memory:` in test setup to avoid leaving `.db` files on disk.
+ 
+To run a specific package:
+ 
+```bash
+go test ./Engine/internal/service/...
+```
+ 
+To run with verbose output:
+ 
+```bash
+go test -v ./...
+```
+ 
+---
+ 
+## Project Conventions
+ 
+**Layer responsibilities:**
+ 
+- `handler/` — HTTP concerns only. Decode the request body, call a service method, encode the response. No business logic here.
+- `service/` — All business logic. Favourability calculation, variance rollup, budget rules. No SQL here.
+- `database/` — All SQL. Repository structs implement interfaces used by the service layer.
+- `model/` — Plain Go structs shared across layers. No methods with business logic.
+ 
+**Error handling:**
+ 
+Go makes every error path explicit. Handlers check service errors and return appropriate HTTP status codes. The server never panics on request errors.
+ 
+**Logging:**
+ 
+Use the `slog` logger passed into components via dependency injection (or the package-level logger if needed). Log at ERROR for unexpected failures, INFO for lifecycle events. Do not log every request unless debugging.
+ 
+---
+ 
+## Adding a New Endpoint
+ 
+1. Add the domain type to `model/` if needed
+2. Add the repository method to the relevant file in `database/`
+3. Add the service method in `service/`
+4. Add the handler method in `handler/`
+5. Register the route in `main.go` on the `mux`
+ 
+Follow the existing patterns — each handler function decodes JSON, calls one service method, and encodes the response.
+ 
+---
+ 
+## Adding a New Budget Version
+ 
+Budget versions are stored as a string field. Currently supported: `original`, `forecast1`, `forecast2`, `forecast3`. To add a new version (e.g. `forecast4`), update the validation in the budget service and add the new value to the Finance Concepts documentation.
+ 
+---
+ 
+## Database Migrations
+ 
+Schema changes are applied via `database.Migrate()` on startup using `CREATE TABLE IF NOT EXISTS` and `ALTER TABLE IF NOT EXISTS` (for additive column changes). This is intentionally simple — there is no migration versioning system.
+ 
+For destructive changes (dropping columns, renaming tables), apply them manually to existing databases and update the migration function. Document any required manual steps here.
+ 
+---
+ 
+## Building a Release Binary
+ 
+```bash
+go build -o fpa-engine ./main.go
+```
+ 
+For a lean, statically linked binary suitable for containers:
+ 
+```bash
+CGO_ENABLED=0 GOOS=linux go build -a -ldflags '-extldflags "-static"' -o fpa-engine ./main.go
+```
+ 
+Note: `modernc.org/sqlite` is a pure-Go SQLite implementation — CGO is not required, and the binary has no system library dependencies.
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