cluster: derive default machine name from its hostname when adding to cluster

internal/machine/cluster/machine.go

@@ -2,6 +2,8 @@ package cluster
 
 import (
 	"fmt"
+	"slices"
+	"strings"
 
 	"github.com/psviderski/uncloud/internal/secret"
 )
@@ -19,3 +21,46 @@ func NewRandomMachineName() (string, error) {
 	}
 	return "machine-" + suffix, nil
 }
+
+// DefaultMachineName derives a machine name from the given hostname, falling back to a random name if
+// no valid name can be derived from it. It then ensures the name is unique among existing by appending
+// a numeric suffix ("-1", "-2", etc.) if needed.
+func DefaultMachineName(hostname string, existing []string) (string, error) {
+	name := machineNameFromHostname(hostname)
+	if name == "" {
+		var err error
+		if name, err = NewRandomMachineName(); err != nil {
+			return "", err
+		}
+	}
+
+	if !slices.Contains(existing, name) {
+		return name, nil
+	}
+	for i := 1; ; i++ {
+		candidate := fmt.Sprintf("%s-%d", name, i)
+		if !slices.Contains(existing, candidate) {
+			return candidate, nil
+		}
+	}
+}
+
+// machineNameFromHostname derives a machine name from a hostname.
+// It returns an empty string if no valid name can be derived.
+func machineNameFromHostname(hostname string) string {
+	// Use the first DNS label only, e.g. "web-1.example.com" becomes "web-1".
+	label, _, _ := strings.Cut(hostname, ".")
+	label = strings.ToLower(strings.TrimSpace(label))
+
+	var b strings.Builder
+	for _, r := range label {
+		switch {
+		case r >= 'a' && r <= 'z', r >= '0' && r <= '9', r == '-', r == '_':
+			b.WriteRune(r)
+		default:
+			b.WriteRune('-')
+		}
+	}
+	// Trim leading and trailing hyphens that may result from the sanitisation above.
+	return strings.Trim(b.String(), "-")
+}