feat: add isNodeUsed helper for NodeProvider lookups

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkg/metricstore/metricstore.go

@@ -633,6 +633,19 @@ func GetSelectors(ms *MemoryStore, excludeSelectors map[string][]string) [][]str
 	return filteredSelectors
 }
 
+// isNodeUsed reports whether cluster/host appears in the used-nodes map
+// returned by NodeProvider.GetUsedNodes. Host lists are sorted per the
+// interface contract, so lookup is a binary search. A nil map means no
+// node is in use.
+func isNodeUsed(used map[string][]string, cluster, host string) bool {
+	hosts, ok := used[cluster]
+	if !ok {
+		return false
+	}
+	_, found := slices.BinarySearch(hosts, host)
+	return found
+}
+
 // GetPaths returns a list of lists (paths) to the specified depth.
 func (ms *MemoryStore) GetPaths(targetDepth int) [][]string {
 	var results [][]string

pkg/metricstore/nodeprovider_test.go

@@ -0,0 +1,46 @@
+// Copyright (C) NHR@FAU, University Erlangen-Nuremberg.
+// All rights reserved. This file is part of cc-backend.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package metricstore
+
+import (
+	"testing"
+)
+
+// fakeNodeProvider implements NodeProvider for tests.
+type fakeNodeProvider struct {
+	nodes map[string][]string
+	err   error
+}
+
+func (f *fakeNodeProvider) GetUsedNodes(ts int64) (map[string][]string, error) {
+	return f.nodes, f.err
+}
+
+func TestIsNodeUsed(t *testing.T) {
+	used := map[string][]string{"fritz": {"node001", "node003"}}
+
+	cases := []struct {
+		cluster, host string
+		want          bool
+	}{
+		{"fritz", "node001", true},
+		{"fritz", "node003", true},
+		{"fritz", "node002", false},
+		{"alex", "node001", false},
+	}
+	for _, c := range cases {
+		if got := isNodeUsed(used, c.cluster, c.host); got != c.want {
+			t.Errorf("isNodeUsed(%q, %q) = %v, want %v", c.cluster, c.host, got, c.want)
+		}
+	}
+
+	if isNodeUsed(nil, "fritz", "node001") {
+		t.Error("nil map must report false")
+	}
+	if isNodeUsed(map[string][]string{}, "fritz", "node001") {
+		t.Error("empty map must report false")
+	}
+}