Implement support for calling Go functions from SQLite

_example/go_custom_funcs/main.go

@@ -0,0 +1,133 @@
+package main
+
+import (
+	"database/sql"
+	"fmt"
+	"log"
+	"math"
+	"math/rand"
+
+	sqlite "github.com/mattn/go-sqlite3"
+)
+
+// Computes x^y
+func pow(x, y int64) int64 {
+	return int64(math.Pow(float64(x), float64(y)))
+}
+
+// Computes the bitwise exclusive-or of all its arguments
+func xor(xs ...int64) int64 {
+	var ret int64
+	for _, x := range xs {
+		ret ^= x
+	}
+	return ret
+}
+
+// Returns a random number. It's actually deterministic here because
+// we don't seed the RNG, but it's an example of a non-pure function
+// from SQLite's POV.
+func getrand() int64 {
+	return rand.Int63()
+}
+
+// Computes the standard deviation of a GROUPed BY set of values
+type stddev struct {
+	xs []int64
+	// Running average calculation
+	sum int64
+	n   int64
+}
+
+func newStddev() *stddev { return &stddev{} }
+
+func (s *stddev) Step(x int64) {
+	s.xs = append(s.xs, x)
+	s.sum += x
+	s.n++
+}
+
+func (s *stddev) Done() float64 {
+	mean := float64(s.sum) / float64(s.n)
+	var sqDiff []float64
+	for _, x := range s.xs {
+		sqDiff = append(sqDiff, math.Pow(float64(x)-mean, 2))
+	}
+	var dev float64
+	for _, x := range sqDiff {
+		dev += x
+	}
+	dev /= float64(len(sqDiff))
+	return math.Sqrt(dev)
+}
+
+func main() {
+	sql.Register("sqlite3_custom", &sqlite.SQLiteDriver{
+		ConnectHook: func(conn *sqlite.SQLiteConn) error {
+			if err := conn.RegisterFunc("pow", pow, true); err != nil {
+				return err
+			}
+			if err := conn.RegisterFunc("xor", xor, true); err != nil {
+				return err
+			}
+			if err := conn.RegisterFunc("rand", getrand, false); err != nil {
+				return err
+			}
+			if err := conn.RegisterAggregator("stddev", newStddev, true); err != nil {
+				return err
+			}
+			return nil
+		},
+	})
+
+	db, err := sql.Open("sqlite3_custom", ":memory:")
+	if err != nil {
+		log.Fatal("Failed to open database:", err)
+	}
+	defer db.Close()
+
+	var i int64
+	err = db.QueryRow("SELECT pow(2,3)").Scan(&i)
+	if err != nil {
+		log.Fatal("POW query error:", err)
+	}
+	fmt.Println("pow(2,3) =", i) // 8
+
+	err = db.QueryRow("SELECT xor(1,2,3,4,5,6)").Scan(&i)
+	if err != nil {
+		log.Fatal("XOR query error:", err)
+	}
+	fmt.Println("xor(1,2,3,4,5) =", i) // 7
+
+	err = db.QueryRow("SELECT rand()").Scan(&i)
+	if err != nil {
+		log.Fatal("RAND query error:", err)
+	}
+	fmt.Println("rand() =", i) // pseudorandom
+
+	_, err = db.Exec("create table foo (department integer, profits integer)")
+	if err != nil {
+		log.Fatal("Failed to create table:", err)
+	}
+	_, err = db.Exec("insert into foo values (1, 10), (1, 20), (1, 45), (2, 42), (2, 115)")
+	if err != nil {
+		log.Fatal("Failed to insert records:", err)
+	}
+
+	rows, err := db.Query("select department, stddev(profits) from foo group by department")
+	if err != nil {
+		log.Fatal("STDDEV query error:", err)
+	}
+	defer rows.Close()
+	for rows.Next() {
+		var dept int64
+		var dev float64
+		if err := rows.Scan(&dept, &dev); err != nil {
+			log.Fatal(err)
+		}
+		fmt.Printf("dept=%d stddev=%f\n", dept, dev)
+	}
+	if err := rows.Err(); err != nil {
+		log.Fatal(err)
+	}
+}