# AGENTS.md - Agentic Coding Guidelines for Diffdown

This file provides guidelines for AI agents operating in this repository.

## Project Overview

Diffdown is a collaborative Markdown editor with ATProto (Bluesky) authentication. It uses Go for the backend and vanilla JavaScript/CodeMirror for the frontend.

---

## Build, Lint, and Test Commands

### Makefile Targets

| Command | Description |
|---------|-------------|
| `make build` | Build the Go application to `bin/server` |
| `make run` | Run the server directly with `go run` |
| `make test` | Run all tests with verbose output |
| `make test-coverage` | Run tests with coverage report (`coverage.html`) |
| `make lint` | Run golangci-lint |
| `make fmt` | Format code with `go fmt` |
| `make dev` | Run with hot reload using air |
| `make migrate-up` | Run database migrations |
| `make migrate-down` | Rollback last migration |
| `make migrate-create name=<name>` | Create new migration |

### Running a Single Test

```bash
# Run tests in a specific package
go test -v ./internal/handler

# Run a specific test function
go test -v -run TestFunctionName ./internal/handler

# Run tests matching a pattern
go test -v -run "Test.*Handler" ./...
```

### Environment Variables

```bash
export DIFFDOWN_SESSION_SECRET="your-secret-here"
export BASE_URL="http://127.0.0.1:8080"
export PORT=8080
export DB_PATH="./diffdown.db"
```

---

## Code Style Guidelines

### Go Conventions

- **Go Version**: 1.22+
- **Module**: `github.com/limeleaf/diffdown`
- **Formatting**: Use `go fmt` or `gofumports` - run before committing

### Imports

Organize imports in three groups with blank lines between:
1. Standard library
2. Third-party packages
3. Internal packages

```go
import (
    "context"
    "encoding/json"
    "fmt"
    "log"
    "net/http"
    "time"

    "github.com/gorilla/sessions"
    "golang.org/x/crypto/bcrypt"

    "github.com/limeleaf/diffdown/internal/auth"
    "github.com/limeleaf/diffdown/internal/db"
    "github.com/limeleaf/diffdown/internal/model"
)
```

### Naming Conventions

- **Files**: `snake_case.go` (e.g., `auth.go`, `handler.go`)
- **Types/Interfaces**: `PascalCase` (e.g., `Handler`, `User`, `DB`)
- **Functions/Methods**: `PascalCase` exported, `camelCase` unexported
- **Constants**: `PascalCase` for exported, `camelCase` for unexported
- **Variables**: `camelCase`, prefer short names (`db`, `err`, `w`, `r`)

### Error Handling

- Always handle errors explicitly; avoid bare `err != nil` checks
- Log meaningful context with `log.Printf` before returning errors
- Return user-friendly error messages in HTTP handlers
- Use wrapped errors with `%w` for context: `fmt.Errorf("open db: %w", err)`

### HTTP Handlers

Follow the existing pattern in `internal/handler/handler
func (h *Handler) Handler.go`:

```goName(w http.ResponseWriter, r *http.Request) {
    // 1. Authenticate/authorize
    user := h.currentUser(r)
    if user == nil {
        http.Redirect(w, r, "/auth/login", http.StatusSeeOther)
        return
    }

    // 2. Extract request data
    data := r.FormValue("field")

    // 3. Process (call DB, external API, etc.)
    result, err := h.DB.Method(data)
    if err != nil {
        log.Printf("HandlerName: operation: %v", err)
        h.render(w, "page.html", PageData{Error: "Friendly message"})
        return
    }

    // 4. Respond
    h.render(w, "page.html", PageData{Content: result})
}
```

### Database

- Use SQLite with WAL mode: `_journal_mode=WAL&_foreign_keys=on`
- Use ULID for IDs: `db.NewID()` (uses `oklog/ulid/v2`)
- Use migrations in `migrations/` directory (SQL files)
- Apply migrations on startup: `database.Migrate()`

### Authentication

- Sessions: Gorilla `sessions.CookieStore`
- Passwords: bcrypt (via `golang.org/x/crypto/bcrypt`)
- ATProto OAuth: DPoP tokens with JWT

### Testing

- Tests should live in `*_test.go` files alongside the code they test
- Use table-driven tests when testing multiple cases
- Mock external dependencies (database, HTTP clients) where practical

---

## Project Structure

```
diffdown/
├── cmd/
│   ├── server/main.go       # Application entry point
│   └── migrate/main.go      # Migration tool
├── internal/
│   ├── atproto/             # ATProto OAuth, DPoP, XRPC client
│   ├── auth/                # Session management, password hashing
│   ├── db/                  # Database operations, migrations
│   ├── handler/             # HTTP handlers
│   ├── middleware/          # HTTP middleware (logging, auth injection)
│   ├── model/               # Data models
│   └── render/              # Markdown rendering
├── migrations/              # SQL migration files
├── static/                  # Frontend assets
├── templates/               # HTML templates
├── Makefile                 # Build commands
└── go.mod                   # Go dependencies
```

---

## Common Tasks

### Adding a New Handler

1. Add method to `Handler` struct in `internal/handler/handler.go`
2. Register route in `cmd/server/main.go`
3. Add template to `templates/` if needed

### Adding a Database Query

1. Add method to `DB` struct in `internal/db/db.go`
2. Follow naming: `GetX`, `CreateX`, `UpdateX`, `DeleteX`
3. Use `db.scanUser` helper for scanning rows

### Adding a Migration

```bash
make migrate-create name=create_users_table
```

Then edit the generated SQL file in `migrations/`.

---

## Dependencies

Key dependencies (see `go.mod`):
- `github.com/gorilla/sessions` - Session management
- `github.com/mattn/go-sqlite3` - SQLite driver
- `github.com/yuin/goldmark` - Markdown parsing
- `github.com/golang-jwt/jwt/v5` - JWT tokens
- `golang.org/x/crypto` - bcrypt, cryptography

---

## Frontend (JavaScript)

The frontend uses vanilla JavaScript with CodeMirror 6 for the editor. Bundling is done with esbuild:

```bash
npm install
npx esbuild --bundle --format=esm --outfile=static/vendor/codemirror.js ...
```

Static files are served from `static/` at `/static/`.