Work on creating tests for rsync api

tools/utils/misc.go

@@ -219,6 +219,14 @@ func Concat[T any](slices ...[]T) []T {
 	return result
 }
 
+func ShiftLeft[T any](s []T, amt int) []T {
+	leftover := len(s) - amt
+	if leftover > 0 {
+		copy(s, s[amt:])
+	}
+	return s[:leftover]
+}
+
 func SetStructDefaults(v reflect.Value) (err error) {
 	for _, field := range reflect.VisibleFields(v.Type()) {
 		if defval := field.Tag.Get("default"); defval != "" {

tools/utils/random/random.go

@@ -0,0 +1,35 @@
+// License: GPLv3 Copyright: 2023, Kovid Goyal, <kovid at kovidgoyal.net>
+
+package random
+
+import (
+	"crypto/rand"
+	"fmt"
+	"math/big"
+
+	"golang.org/x/exp/constraints"
+)
+
+var _ = fmt.Print
+
+// Return a random integer in the range [0, limit). limit must be > 0
+func Int[T constraints.Integer](limit T) T {
+	b := big.NewInt(int64(limit))
+	n, err := rand.Int(rand.Reader, b)
+	if err != nil {
+		panic(err)
+	}
+	return T(n.Uint64())
+}
+
+// Return one of items randomnly
+func Choice[T any](items ...T) T {
+	return items[Int(len(items))]
+}
+
+// Write randomn bytes into the provided slice
+func Bytes(b []byte) {
+	if _, err := rand.Read(b); err != nil {
+		panic(err)
+	}
+}