internal/data: extract HardlinkIndex from restorer

The HardlinkIndex is also used by `cmd_stats`, thus place it in a shared
package.

internal/data/hardlinks_index.go

@@ -0,0 +1,57 @@
+package data
+
+import (
+	"sync"
+)
+
+// hardlinkKey is a composed key for finding inodes on a specific device.
+type hardlinkKey struct {
+	Inode, Device uint64
+}
+
+// HardlinkIndex maps inodes on devices to associated values.
+type HardlinkIndex[T any] struct {
+	m     sync.Mutex
+	index map[hardlinkKey]T
+}
+
+// NewHardlinkIndex create a new index for hard links
+func NewHardlinkIndex[T any]() *HardlinkIndex[T] {
+	return &HardlinkIndex[T]{
+		index: make(map[hardlinkKey]T),
+	}
+}
+
+// Has checks whether the link already exist in the index.
+func (idx *HardlinkIndex[T]) Has(inode uint64, device uint64) bool {
+	idx.m.Lock()
+	defer idx.m.Unlock()
+	_, ok := idx.index[hardlinkKey{inode, device}]
+
+	return ok
+}
+
+// Add adds a link to the index.
+func (idx *HardlinkIndex[T]) Add(inode uint64, device uint64, value T) {
+	idx.m.Lock()
+	defer idx.m.Unlock()
+	_, ok := idx.index[hardlinkKey{inode, device}]
+
+	if !ok {
+		idx.index[hardlinkKey{inode, device}] = value
+	}
+}
+
+// Value obtains the filename from the index.
+func (idx *HardlinkIndex[T]) Value(inode uint64, device uint64) T {
+	idx.m.Lock()
+	defer idx.m.Unlock()
+	return idx.index[hardlinkKey{inode, device}]
+}
+
+// Remove removes a link from the index.
+func (idx *HardlinkIndex[T]) Remove(inode uint64, device uint64) {
+	idx.m.Lock()
+	defer idx.m.Unlock()
+	delete(idx.index, hardlinkKey{inode, device})
+}

internal/data/hardlinks_index_test.go

@@ -0,0 +1,33 @@
+package data_test
+
+import (
+	"testing"
+
+	"github.com/restic/restic/internal/data"
+	rtest "github.com/restic/restic/internal/test"
+)
+
+// TestHardLinks contains various tests for HardlinkIndex.
+func TestHardLinks(t *testing.T) {
+
+	idx := data.NewHardlinkIndex[string]()
+
+	idx.Add(1, 2, "inode1-file1-on-device2")
+	idx.Add(2, 3, "inode2-file2-on-device3")
+
+	sresult := idx.Value(1, 2)
+	rtest.Equals(t, sresult, "inode1-file1-on-device2")
+
+	sresult = idx.Value(2, 3)
+	rtest.Equals(t, sresult, "inode2-file2-on-device3")
+
+	bresult := idx.Has(1, 2)
+	rtest.Equals(t, bresult, true)
+
+	bresult = idx.Has(1, 3)
+	rtest.Equals(t, bresult, false)
+
+	idx.Remove(1, 2)
+	bresult = idx.Has(1, 2)
+	rtest.Equals(t, bresult, false)
+}