diff --git a/internal/inventory/category.go b/internal/inventory/category.go
new file mode 100644
index 0000000..17f1ff7
--- /dev/null
+++ b/internal/inventory/category.go
@@ -0,0 +1,117 @@
+package inventory
+
+import "strings"
+
+// Category is a coarse file classification used for colouring and stats.
+type Category uint8
+
+const (
+	CatOther Category = iota
+	CatImage
+	CatVideo
+	CatAudio
+	CatArchive
+	CatDiskImage
+	CatDatabase
+	CatCode
+	CatDocument
+	CatExecutable
+	CatLog
+	CatCache
+	NumCategories
+)
+
+var categoryNames = [NumCategories]string{
+	"Other", "Image", "Video", "Audio", "Archive", "Disk image", "Database",
+	"Source code", "Document", "Executable", "Log", "Cache",
+}
+
+func (c Category) String() string {
+	if c < NumCategories {
+		return categoryNames[c]
+	}
+	return "Other"
+}
+
+// ParseCategory resolves a category by (case-insensitive) name or prefix.
+func ParseCategory(s string) (Category, bool) {
+	s = strings.ToLower(strings.TrimSpace(s))
+	for i, n := range categoryNames {
+		ln := strings.ToLower(n)
+		if ln == s || strings.ReplaceAll(ln, " ", "") == s || strings.HasPrefix(ln, s) && len(s) >= 3 {
+			return Category(i), true
+		}
+	}
+	switch s {
+	case "vm", "virtual machine", "disk":
+		return CatDiskImage, true
+	case "db":
+		return CatDatabase, true
+	case "src", "source":
+		return CatCode, true
+	case "exe", "binary":
+		return CatExecutable, true
+	}
+	return 0, false
+}
+
+var extCategory = map[string]Category{}
+
+func init() {
+	add := func(c Category, exts string) {
+		for _, e := range strings.Fields(exts) {
+			extCategory[e] = c
+		}
+	}
+	add(CatImage, "jpg jpeg png gif bmp tif tiff webp heic heif raw cr2 cr3 nef arw dng svg ico psd xcf avif jxl orf rw2")
+	add(CatVideo, "mp4 mkv mov avi wmv flv webm m4v mpg mpeg ts m2ts vob 3gp ogv mts")
+	add(CatAudio, "mp3 flac wav ogg oga opus m4a aac wma aiff aif alac mid midi")
+	add(CatArchive, "zip tar gz tgz bz2 tbz xz txz zst zstd 7z rar lz lz4 lzma cab deb rpm apk jar war cpio pkg snap whl gem")
+	add(CatDiskImage, "iso img qcow qcow2 vmdk vdi vhd vhdx hdd ova ovf dmg raw vmem vmsn nvram sav")
+	add(CatDatabase, "db sqlite sqlite3 db3 mdb accdb frm ibd myd myi ldf mdf ndf dbf wal kdbx realm lmdb rdb aof")
+	add(CatCode, "c h cc cpp cxx hpp hh go rs py pyc js mjs cjs ts tsx jsx java kt kts scala rb php pl pm sh bash zsh fish lua swift m mm cs fs vb r jl hs ml ex exs erl clj dart sql css scss sass less html htm vue svelte json yaml yml toml xml proto gradle cmake mk ipynb o a lib obj class")
+	add(CatDocument, "pdf doc docx odt rtf txt md rst tex xls xlsx ods csv tsv ppt pptx odp epub mobi djvu pages numbers key org adoc")
+	add(CatExecutable, "exe dll so dylib bin msi app elf ko sys efi appimage com")
+	add(CatLog, "log journal")
+	add(CatCache, "cache tmp temp swp bak old part crdownload")
+}
+
+// CategoryFor classifies a file by its (lower-case) extension and by
+// whether it lives beneath a cache or log directory.
+func CategoryFor(ext string, inCache, inLog bool) Category {
+	c, ok := extCategory[ext]
+	switch {
+	case inCache && (!ok || c == CatOther || c == CatCode):
+		return CatCache
+	case inLog && (!ok || c == CatArchive || c == CatOther):
+		return CatLog
+	case ok:
+		return c
+	}
+	return CatOther
+}
+
+// PathHints reports whether a directory path contains a cache or log
+// component. It is evaluated once per directory, not per file.
+func PathHints(path string) (inCache, inLog bool) {
+	lp := strings.ToLower(path)
+	for _, part := range strings.FieldsFunc(lp, func(r rune) bool { return r == '/' || r == '\\' }) {
+		switch part {
+		case "cache", ".cache", "caches", "__pycache__", ".npm", ".gradle", "inetcache", "temp", "tmp":
+			inCache = true
+		case "log", "logs", ".logs", "journal":
+			inLog = true
+		}
+	}
+	return
+}
+
+// Ext returns the lower-case extension of name without the dot ("" if none,
+// or if the name is a dotfile like ".bashrc").
+func Ext(name string) string {
+	i := strings.LastIndexByte(name, '.')
+	if i <= 0 || i == len(name)-1 || len(name)-i > 16 {
+		return ""
+	}
+	return strings.ToLower(name[i+1:])
+}
diff --git a/internal/inventory/query.go b/internal/inventory/query.go
new file mode 100644
index 0000000..26ef11a
--- /dev/null
+++ b/internal/inventory/query.go
@@ -0,0 +1,176 @@
+package inventory
+
+import (
+	"container/heap"
+	"sort"
+)
+
+// SizeMode selects which size drives layout and sorting.
+type SizeMode uint8
+
+const (
+	SizeAllocated SizeMode = iota // disk usage (default)
+	SizeLogical                   // apparent size
+)
+
+func (m SizeMode) String() string {
+	if m == SizeLogical {
+		return "logical"
+	}
+	return "allocated"
+}
+
+// Total returns the node's aggregate size in the given mode.
+func (n *Node) Total(m SizeMode) int64 {
+	if m == SizeLogical {
+		return n.TotSize
+	}
+	return n.TotAlloc
+}
+
+// Own returns the node's own size in the given mode.
+func (n *Node) Own(m SizeMode) int64 {
+	if m == SizeLogical {
+		return n.Size
+	}
+	return n.Alloc
+}
+
+type ranked struct {
+	id  NodeID
+	key int64
+}
+
+type minHeap []ranked
+
+func (h minHeap) Len() int { return len(h) }
+func (h minHeap) Less(i, j int) bool {
+	if h[i].key != h[j].key {
+		return h[i].key < h[j].key
+	}
+	return h[i].id > h[j].id
+}
+func (h minHeap) Swap(i, j int) { h[i], h[j] = h[j], h[i] }
+func (h *minHeap) Push(x any)   { *h = append(*h, x.(ranked)) }
+func (h *minHeap) Pop() any {
+	old := *h
+	x := old[len(old)-1]
+	*h = old[:len(old)-1]
+	return x
+}
+
+// TopK keeps the k entries with the largest keys; O(n log k).
+type TopK struct {
+	k int
+	h minHeap
+}
+
+// NewTopK creates a collector for the k largest keys.
+func NewTopK(k int) *TopK { return &TopK{k: k} }
+
+// Offer considers one candidate.
+func (t *TopK) Offer(id NodeID, key int64) {
+	if t.k <= 0 {
+		return
+	}
+	if len(t.h) < t.k {
+		heap.Push(&t.h, ranked{id, key})
+		return
+	}
+	if r := t.h[0]; key > r.key || key == r.key && id < r.id {
+		t.h[0] = ranked{id, key}
+		heap.Fix(&t.h, 0)
+	}
+}
+
+// Sorted returns the collected IDs, largest key first (ties by ID).
+func (t *TopK) Sorted() []NodeID {
+	rs := append([]ranked(nil), t.h...)
+	sort.Slice(rs, func(i, j int) bool {
+		if rs[i].key != rs[j].key {
+			return rs[i].key > rs[j].key
+		}
+		return rs[i].id < rs[j].id
+	})
+	out := make([]NodeID, len(rs))
+	for i, r := range rs {
+		out[i] = r.id
+	}
+	return out
+}
+
+// Walk visits id and all its descendants depth-first (iteratively).
+func (t *Tree) Walk(id NodeID, fn func(NodeID, *Node) bool) {
+	stack := []NodeID{id}
+	for len(stack) > 0 {
+		cur := stack[len(stack)-1]
+		stack = stack[:len(stack)-1]
+		n := t.Node(cur)
+		if !fn(cur, n) {
+			continue
+		}
+		for c := n.FirstChild; c != NoNode; c = t.Node(c).NextSibling {
+			stack = append(stack, c)
+		}
+	}
+}
+
+// Top returns the k nodes beneath (and including) root with the largest key,
+// considering only nodes accepted by keep.
+func (t *Tree) Top(root NodeID, k int, keep func(*Node) bool, key func(*Node) int64) []NodeID {
+	tk := NewTopK(k)
+	t.Walk(root, func(id NodeID, n *Node) bool {
+		if keep(n) {
+			tk.Offer(id, key(n))
+		}
+		return true
+	})
+	return tk.Sorted()
+}
+
+// SortedChildren returns the children of id ordered by size descending.
+func (t *Tree) SortedChildren(id NodeID, m SizeMode) []NodeID {
+	var out []NodeID
+	t.Children(id, func(c NodeID, _ *Node) { out = append(out, c) })
+	sort.Slice(out, func(i, j int) bool {
+		a, b := t.Node(out[i]).Total(m), t.Node(out[j]).Total(m)
+		if a != b {
+			return a > b
+		}
+		return out[i] < out[j]
+	})
+	return out
+}
+
+// ExtStat summarises one extension.
+type ExtStat struct {
+	Ext   string
+	Cat   Category
+	Count int64
+	Size  int64 // logical
+	Alloc int64
+}
+
+// ExtStats aggregates file statistics by extension beneath root.
+func (t *Tree) ExtStats(root NodeID) []ExtStat {
+	stats := make([]ExtStat, t.ExtCount())
+	t.Walk(root, func(_ NodeID, n *Node) bool {
+		if n.Kind == KindFile && n.Flags&FlagHardlinkDup == 0 {
+			s := &stats[n.Ext]
+			s.Count++
+			s.Size += n.Size
+			s.Alloc += n.Alloc
+			s.Cat = n.Cat
+		}
+		return true
+	})
+	out := stats[:0]
+	for i, s := range stats {
+		if s.Count > 0 {
+			s.Ext = t.exts[i]
+			s.Cat = CategoryFor(s.Ext, false, false)
+			out = append(out, s)
+		}
+	}
+	return out
+}
diff --git a/internal/inventory/query_test.go b/internal/inventory/query_test.go
new file mode 100644
index 0000000..8cd96f1
--- /dev/null
+++ b/internal/inventory/query_test.go
@@ -0,0 +1,42 @@
+package inventory
+
+import "testing"
+
+func TestTopKAndRecompute(t *testing.T) {
+	tr := New("/r", KindDir)
+	d := tr.Add(0, "d", KindDir)
+	for i := range 100 {
+		id := tr.Add(d, "f", KindFile)
+		n := tr.Node(id)
+		n.Size, n.Alloc = int64(i), int64(i)
+	}
+	tr.Recompute()
+	if got := tr.Node(0).TotSize; got != 4950 {
+		t.Fatalf("total %d", got)
+	}
+	if tr.Node(0).Files != 100 || tr.Node(0).Dirs != 1 {
+		t.Fatalf("counts %d %d", tr.Node(0).Files, tr.Node(0).Dirs)
+	}
+	top := tr.Top(0, 3, func(n *Node) bool { return n.Kind == KindFile }, func(n *Node) int64 { return n.Size })
+	if len(top) != 3 || tr.Node(top[0]).Size != 99 || tr.Node(top[2]).Size != 97 {
+		t.Fatalf("top %v", top)
+	}
+	if p := tr.Path(top[0]); p != "/r/d/f" {
+		t.Fatalf("path %q", p)
+	}
+}
+
+func TestExtAndCategory(t *testing.T) {
+	cases := map[string]string{"a.QCOW2": "qcow2", ".bashrc": "", "noext": "", "x.tar.gz": "gz", "trail.": ""}
+	for in, want := range cases {
+		if got := Ext(in); got != want {
+			t.Errorf("Ext(%q)=%q want %q", in, got, want)
+		}
+	}
+	if CategoryFor("qcow2", false, false) != CatDiskImage || CategoryFor("bin", true, false) != CatExecutable {
+		t.Fatal("category mapping")
+	}
+	if CategoryFor("", true, false) != CatCache {
+		t.Fatal("cache hint")
+	}
+}
diff --git a/internal/inventory/stats.go b/internal/inventory/stats.go
new file mode 100644
index 0000000..bce1573
--- /dev/null
+++ b/internal/inventory/stats.go
@@ -0,0 +1,106 @@
+package inventory
+
+import (
+	"errors"
+	"io/fs"
+	"syscall"
+	"time"
+)
+
+// ErrKind classifies scan errors for reporting.
+type ErrKind uint8
+
+const (
+	ErrPermission ErrKind = iota
+	ErrVanished
+	ErrIO
+	ErrLoop
+	ErrOther
+	NumErrKinds
+)
+
+func (k ErrKind) String() string {
+	return [...]string{"Permission denied", "Vanished during scan", "I/O error", "Symlink loop", "Other error"}[k]
+}
+
+// Classify maps an OS error to an ErrKind.
+func Classify(err error) ErrKind {
+	switch {
+	case errors.Is(err, fs.ErrPermission):
+		return ErrPermission
+	case errors.Is(err, fs.ErrNotExist):
+		return ErrVanished
+	case errors.Is(err, syscall.ELOOP):
+		return ErrLoop
+	case errors.Is(err, syscall.EIO):
+		return ErrIO
+	}
+	return ErrOther
+}
+
+// ErrorRecord is one recorded scan error.
+type ErrorRecord struct {
+	Node NodeID // node the error is attached to (the directory for listing failures)
+	Name string // entry name when no node was created (e.g. vanished)
+	Kind ErrKind
+	Msg  string
+}
+
+// MaxErrorRecords caps the detailed error list; counts are always exact.
+const MaxErrorRecords = 100_000
+
+// Stats describes a scan as a whole.
+type Stats struct {
+	Root       string
+	Start, End time.Time
+	Complete   bool // scan finished (possibly with errors)
+	Cancelled  bool
+
+	Files, Dirs, Symlinks, Others int64
+
+	ErrCounts      [NumErrKinds]int64
+	Errors         []ErrorRecord
+	Excluded       int64
+	SkippedMounts  int64
+	VirtualSkipped int64
+	LoopsSkipped   int64
+	BrokenLinks    int64
+	HardlinkDups   int64
+	Unscanned      int64 // directories never read because the scan was cancelled
+
+	Excludes      []string
+	OneFileSystem bool
+	Follow        string
+	Workers       int
+	FromSnapshot  string
+}
+
+// TotalErrors sums all error kinds.
+func (s *Stats) TotalErrors() int64 {
+	var t int64
+	for _, c := range s.ErrCounts {
+		t += c
+	}
+	return t
+}
+
+// AddError records an error (the detailed list is capped).
+func (s *Stats) AddError(r ErrorRecord) {
+	s.ErrCounts[r.Kind]++
+	if len(s.Errors) < MaxErrorRecords {
+		s.Errors = append(s.Errors, r)
+	}
+}
+
+// Elapsed is the scan duration so far (or total once finished).
+func (s *Stats) Elapsed() time.Duration {
+	if !s.End.IsZero() {
+		return s.End.Sub(s.Start)
+	}
+	return time.Since(s.Start)
+}
+
+// Incomplete reports whether totals may under-count real usage.
+func (s *Stats) Incomplete() bool {
+	return !s.Complete || s.Cancelled || s.TotalErrors() > 0 || s.Unscanned > 0
+}
diff --git a/internal/inventory/tree.go b/internal/inventory/tree.go
new file mode 100644
index 0000000..3f2a71d
--- /dev/null
+++ b/internal/inventory/tree.go
@@ -0,0 +1,326 @@
+// Package inventory is the in-memory filesystem model: an append-only arena
+// of nodes addressed by stable NodeIDs, with incrementally maintained
+// directory aggregates.
+//
+// Concurrency: a Tree has exactly one writer (the scan controller or a
+// snapshot loader). Other goroutines must hold RLock while reading node data.
+// Nodes are never removed, so a NodeID stays valid for the Tree's lifetime.
+package inventory
+
+import (
+	"path/filepath"
+	"slices"
+	"sync"
+	"time"
+)
+
+// NodeID addresses a node within one Tree. The root is always 0.
+type NodeID uint32
+
+// NoNode is the nil NodeID.
+const NoNode NodeID = ^NodeID(0)
+
+// Kind is the node type.
+type Kind uint8
+
+const (
+	KindFile Kind = iota
+	KindDir
+	KindSymlink
+	KindOther // devices, sockets, fifos, reparse points
+)
+
+func (k Kind) String() string {
+	return [...]string{"File", "Directory", "Symlink", "Special"}[k&3]
+}
+
+// Flags record per-node facts.
+type Flags uint16
+
+const (
+	FlagHardlinkDup  Flags = 1 << iota // additional link to an inode counted elsewhere
+	FlagMountPoint                     // directory on a different device than its parent
+	FlagVirtualFS                      // virtual filesystem, not descended
+	FlagSkippedFS                      // other filesystem, not descended (--one-file-system)
+	FlagSparse                         // allocated < logical (sparse or compressed)
+	FlagError                          // stat or read failed
+	FlagIncomplete                     // directory contents not (fully) read
+	FlagScanned                        // directory fully read
+	FlagLoop                           // directory already visited via another path
+	FlagFollowed                       // reached through a followed symlink
+	FlagAllocUnknown                   // platform cannot report allocated size
+	FlagBrokenLink                     // symlink target does not exist
+	FlagHardlinked                     // regular file with link count > 1
+)
+
+// Node is one filesystem object. Sizes are own sizes; Tot* fields are
+// aggregates including the node itself and (for directories) everything
+// beneath it.
+type Node struct {
+	Parent, FirstChild, NextSibling NodeID
+	dir                             uint32 // index into dir side table (directories only)
+
+	Name string // single path component; the root holds the full root path
+
+	Size, Alloc       int64 // own logical / allocated bytes
+	TotSize, TotAlloc int64 // aggregates (hard-link duplicates contribute 0)
+	MTime             int64 // unix nanoseconds
+
+	Files, Dirs uint32 // recursive counts beneath (dirs excludes self)
+	Errors      uint32 // recursive error count beneath and including self
+	Mode        uint32 // fs.FileMode bits
+	UID, GID    uint32
+	Nlink       uint32
+
+	Ext   uint16
+	Flags Flags
+	Kind  Kind
+	Cat   Category
+}
+
+// IsDir reports whether the node is a directory.
+func (n *Node) IsDir() bool { return n.Kind == KindDir }
+
+// Delta is an aggregate change applied to a node and all its ancestors.
+type Delta struct {
+	Size, Alloc         int64
+	Files, Dirs, Errors int64
+	Cat                 [NumCategories]int64
+}
+
+const chunkBits = 16
+const chunkSize = 1 << chunkBits
+
+// chunked is a growable array that never moves existing elements.
+type chunked[T any] struct {
+	c [][]T
+	n int
+}
+
+func (c *chunked[T]) add(v T) uint32 {
+	if c.n&(chunkSize-1) == 0 && c.n>>chunkBits == len(c.c) {
+		c.c = append(c.c, make([]T, 0, chunkSize))
+	}
+	ch := &c.c[c.n>>chunkBits]
+	*ch = append(*ch, v)
+	c.n++
+	return uint32(c.n - 1)
+}
+
+func (c *chunked[T]) at(i uint32) *T { return &c.c[i>>chunkBits][i&(chunkSize-1)] }
+
+// Tree is the inventory for one scan root.
+type Tree struct {
+	sync.RWMutex
+
+	nodes chunked[Node]
+	dirs  chunked[[NumCategories]int64]
+
+	exts   []string
+	extIdx map[string]uint16
+
+	Stats Stats
+}
+
+// New creates a tree whose root has the given full path and kind.
+func New(rootPath string, kind Kind) *Tree {
+	t := &Tree{exts: []string{""}, extIdx: map[string]uint16{"": 0}}
+	t.Stats.Root = rootPath
+	t.Stats.Start = time.Now()
+	t.Add(NoNode, rootPath, kind)
+	return t
+}
+
+// Len returns the number of nodes.
+func (t *Tree) Len() int { return t.nodes.n }
+
+// Node returns the node for id. The pointer stays valid; reading its fields
+// concurrently with the writer requires RLock.
+func (t *Tree) Node(id NodeID) *Node { return t.nodes.at(uint32(id)) }
+
+// Root returns the root NodeID (always 0).
+func (t *Tree) Root() NodeID { return 0 }
+
+// Add appends a child node and links it under parent. Only the writer may
+// call it. It does not touch aggregates; use Propagate.
+func (t *Tree) Add(parent NodeID, name string, kind Kind) NodeID {
+	n := Node{Parent: parent, FirstChild: NoNode, NextSibling: NoNode, Name: name, Kind: kind}
+	if kind == KindDir {
+		n.dir = t.dirs.add([NumCategories]int64{})
+	} else {
+		n.Ext = t.internExt(Ext(name))
+	}
+	id := NodeID(t.nodes.add(n))
+	if parent != NoNode {
+		p := t.Node(parent)
+		t.Node(id).NextSibling = p.FirstChild
+		p.FirstChild = id
+	}
+	return id
+}
+
+func (t *Tree) internExt(e string) uint16 {
+	if i, ok := t.extIdx[e]; ok {
+		return i
+	}
+	if len(t.exts) >= 1<<16-1 {
+		return 0
+	}
+	t.exts = append(t.exts, e)
+	t.extIdx[e] = uint16(len(t.exts) - 1)
+	return uint16(len(t.exts) - 1)
+}
+
+// ExtName returns the extension string for a node ("" if none).
+func (t *Tree) ExtName(n *Node) string { return t.exts[n.Ext] }
+
+// ExtCount returns the number of interned extensions (including "").
+func (t *Tree) ExtCount() int { return len(t.exts) }
+
+// ExtByIndex returns an interned extension.
+func (t *Tree) ExtByIndex(i int) string { return t.exts[i] }
+
+// CatSizes returns the per-category allocated bytes beneath a directory.
+func (t *Tree) CatSizes(id NodeID) *[NumCategories]int64 {
+	n := t.Node(id)
+	if n.Kind != KindDir {
+		return nil
+	}
+	return t.dirs.at(n.dir)
+}
+
+// Propagate adds d to id and every ancestor. Cost is O(depth); callers batch
+// many entries into one Delta.
+func (t *Tree) Propagate(id NodeID, d Delta) {
+	for id != NoNode {
+		n := t.Node(id)
+		n.TotSize += d.Size
+		n.TotAlloc += d.Alloc
+		n.Files = addU32(n.Files, d.Files)
+		n.Dirs = addU32(n.Dirs, d.Dirs)
+		n.Errors = addU32(n.Errors, d.Errors)
+		if n.Kind == KindDir {
+			cs := t.dirs.at(n.dir)
+			for i, v := range d.Cat {
+				cs[i] += v
+			}
+		}
+		id = n.Parent
+	}
+}
+
+func addU32(a uint32, d int64) uint32 {
+	v := int64(a) + d
+	if v < 0 {
+		return 0
+	}
+	if v > 1<<32-1 {
+		return 1<<32 - 1
+	}
+	return uint32(v)
+}
+
+// Children calls fn for each child of id (in reverse insertion order).
+func (t *Tree) Children(id NodeID, fn func(NodeID, *Node)) {
+	for c := t.Node(id).FirstChild; c != NoNode; {
+		n := t.Node(c)
+		fn(c, n)
+		c = n.NextSibling
+	}
+}
+
+// ChildCount returns the number of direct children.
+func (t *Tree) ChildCount(id NodeID) int {
+	k := 0
+	for c := t.Node(id).FirstChild; c != NoNode; c = t.Node(c).NextSibling {
+		k++
+	}
+	return k
+}
+
+// Path reconstructs the full path of id.
+func (t *Tree) Path(id NodeID) string {
+	var parts []string
+	for id != NoNode {
+		n := t.Node(id)
+		parts = append(parts, n.Name)
+		id = n.Parent
+	}
+	slices.Reverse(parts)
+	return filepath.Join(parts...)
+}
+
+// Ancestors returns the chain root..id inclusive.
+func (t *Tree) Ancestors(id NodeID) []NodeID {
+	var out []NodeID
+	for id != NoNode {
+		out = append(out, id)
+		id = t.Node(id).Parent
+	}
+	slices.Reverse(out)
+	return out
+}
+
+// IsAncestor reports whether a is id or an ancestor of id.
+func (t *Tree) IsAncestor(a, id NodeID) bool {
+	for id != NoNode {
+		if id == a {
+			return true
+		}
+		id = t.Node(id).Parent
+	}
+	return false
+}
+
+// Depth returns the number of ancestors of id.
+func (t *Tree) Depth(id NodeID) int {
+	d := 0
+	for id = t.Node(id).Parent; id != NoNode; id = t.Node(id).Parent {
+		d++
+	}
+	return d
+}
+
+// Recompute rebuilds all aggregates bottom-up. It relies on the invariant
+// that a parent's ID is smaller than its children's, which both the scanner
+// and the snapshot loader guarantee. Used after loading a snapshot.
+func (t *Tree) Recompute() {
+	for i := 0; i < t.Len(); i++ {
+		n := t.Node(NodeID(i))
+		n.TotSize, n.TotAlloc, n.Files, n.Dirs = 0, 0, 0, 0
+		n.Errors = 0
+		if n.Flags&FlagError != 0 {
+			n.Errors = 1
+		}
+		if n.Flags&FlagHardlinkDup == 0 {
+			n.TotSize, n.TotAlloc = n.Size, n.Alloc
+		}
+		if n.Kind == KindDir {
+			*t.dirs.at(n.dir) = [NumCategories]int64{}
+		}
+	}
+	for i := t.Len() - 1; i > 0; i-- {
+		n := t.Node(NodeID(i))
+		p := t.Node(n.Parent)
+		p.TotSize += n.TotSize
+		p.TotAlloc += n.TotAlloc
+		p.Errors += n.Errors
+		cs := t.dirs.at(p.dir)
+		if n.Kind == KindDir {
+			p.Files += n.Files
+			p.Dirs += n.Dirs + 1
+			for c, v := range t.dirs.at(n.dir) {
+				cs[c] += v
+			}
+		} else {
+			p.Files++
+			if n.Flags&FlagHardlinkDup == 0 {
+				cs[n.Cat] += n.Alloc
+			}
+		}
+	}
+}
+
+// ExtIndexer returns a function interning extension strings; used by
+// loaders that rebuild a tree from external data.
+func (t *Tree) ExtIndexer() func(string) uint16 { return t.internExt }