# Go Patterns Cheat Sheet

## Defer - Cleanup Pattern
```go
initDB()
defer db.Close()  // Runs when function exits, no matter how
```
- Schedules function to run when surrounding function returns
- Executes in LIFO order if multiple defers
- Great for resource cleanup (files, connections, etc.)

## Formatted Output to HTTP
```go
fmt.Fprintf(w, "%d", rng.Intn(1000))
```
- `fmt.Fprintf` = formatted print to a writer (not stdout)
- `w` = HTTP response writer
- `%d` = format placeholder for integers
- Alternative: `fmt.Fprint(w, value)` for no formatting

## Ticker + Select for Scheduled Tasks
```go
func startMiningTicker() {
    ticker := time.NewTicker(1 * time.Minute)
    defer ticker.Stop()
    
    for {
        select {
        case <-ticker.C:
            updateMiningValue()
        }
    }
}
```
- `time.NewTicker()` creates repeating timer
- `select` waits for channel events
- `<-ticker.C` receives from ticker's channel
- Common pattern for background jobs

## Database Scanning into Structs
```go
type MiningRecord struct {
    ID        int       `json:"id"`
    Timestamp time.Time `json:"timestamp"`
    Value     float64   `json:"value"`
    Rate      float64   `json:"rate"`
}

var latest MiningRecord
err := db.QueryRow("SELECT id, timestamp, value, rate FROM mining ORDER BY timestamp DESC LIMIT 1").
    Scan(&latest.ID, &latest.Timestamp, &latest.Value, &latest.Rate)
```
- `Scan()` copies SQL results into Go variables
- Order matters: first column → first Scan argument
- Use `&` to pass addresses so Scan can write to them
- Struct tags (`json:"id"`) control JSON serialization

## Blank Identifier for Ignoring Values
```go
_, err = db.Exec("INSERT INTO mining (timestamp, value, rate) VALUES (?, ?, ?)", now, newValue, latest.Rate)
```
- `_` = blank identifier, discards unwanted return values
- `db.Exec()` returns `(sql.Result, error)`
- We only care about the error, so ignore the Result
- Go requires this instead of leaving variables unused

## HTTP Handler Pattern
```go
func handler(w http.ResponseWriter, r *http.Request) {
    // w = where response goes
    // r = incoming request data
    fmt.Fprintf(w, "response")
}
```
- `w` writes back to client
- `r` contains request info
- Common to use `fmt.Fprintf(w, ...)` for simple responses