Speed up snapshot loading; polish help and table bars

Decompression and checksumming overlap with parsing via a pipe, and a
concrete-typed varint reader removes per-byte interface calls
(0.39s -> 0.29s for 458K nodes).

PERFORMANCE.md

@@ -44,6 +44,22 @@ path reconstruction for job dispatch ~7 %; aggregation is negligible.
 **~158 bytes per node** (node record ~112 bytes plus name storage and
 directory side tables). Extrapolated, 10 M entries need ~1.6 GiB.
 
+## Snapshots
+
+Same 458,510-node tree:
+
+| operation | result |
+|---|---:|
+| snapshot file size (gzip level 1) | 6.7 MB (~15 bytes/node) |
+| scan + save | 1.03 s |
+| load (`mapsize --no-ui file.msz`), 3 runs | 0.28 / 0.29 / 0.32 s, 81 MB RSS |
+| warm-cache rescan for comparison | 0.55 s |
+
+Loading decompresses and verifies SHA-256 on a separate goroutine, overlapping
+with parsing. Against a warm cache the win is ~1.9×; against a cold cache or
+a network filesystem, where scans are dominated by I/O latency, it is far
+larger (not yet measured here).
+
 ## UI
 
 `go test ./internal/tui -bench Big` on a synthetic in-memory tree of

internal/snapshot/snapshot.go

@@ -235,7 +235,7 @@ func (r *reader) uvarint(max uint64, what string) uint64 {
 	if r.err != nil {
 		return 0
 	}
-	v, err := binary.ReadUvarint(r.r)
+	v, err := readUvarint(r.r)
 	if err != nil {
 		r.fail("bad %s: %v", what, err)
 		return 0
@@ -251,13 +251,44 @@ func (r *reader) varint(what string) int64 {
 	if r.err != nil {
 		return 0
 	}
-	v, err := binary.ReadVarint(r.r)
+	ux, err := readUvarint(r.r)
 	if err != nil {
 		r.fail("bad %s: %v", what, err)
 	}
+	v := int64(ux >> 1) // zig-zag, as encoding/binary.PutVarint
+	if ux&1 != 0 {
+		v = ^v
+	}
 	return v
 }
 
+var errOverflow = errors.New("varint overflows 64 bits")
+
+// readUvarint is encoding/binary.ReadUvarint specialised to *bufio.Reader
+// (no interface dispatch per byte; this is the loader's hot loop).
+func readUvarint(br *bufio.Reader) (uint64, error) {
+	var x uint64
+	var s uint
+	for i := 0; i < binary.MaxVarintLen64; i++ {
+		b, err := br.ReadByte()
+		if err != nil {
+			if i > 0 && err == io.EOF {
+				err = io.ErrUnexpectedEOF
+			}
+			return x, err
+		}
+		if b < 0x80 {
+			if i == binary.MaxVarintLen64-1 && b > 1 {
+				return x, errOverflow
+			}
+			return x | uint64(b)<<s, nil
+		}
+		x |= uint64(b&0x7f) << s
+		s += 7
+	}
+	return x, errOverflow
+}
+
 func (r *reader) bytesN(n uint64) []byte {
 	if r.err != nil {
 		return nil
@@ -295,8 +326,17 @@ func Load(in io.Reader) (*inventory.Tree, error) {
 		return nil, fmt.Errorf("snapshot: %w", err)
 	}
 	gz.Multistream(false)
+	// Decompression and hashing run on their own goroutine, overlapping
+	// with parsing. The pipe's close orders the hash and br accesses: the
+	// parser only sees EOF after the copier has finished with both.
 	h := sha256.New()
-	r := &reader{r: bufio.NewReaderSize(io.TeeReader(gz, h), 1<<16)}
+	pr, pw := io.Pipe()
+	defer pr.Close() // unblocks the copier if we bail out early
+	go func() {
+		_, err := io.CopyBuffer(pw, io.TeeReader(gz, h), make([]byte, 256<<10))
+		pw.CloseWithError(err)
+	}()
+	r := &reader{r: bufio.NewReaderSize(pr, 1<<16)}
 
 	var m meta
 	if mb := r.bytesN(r.uvarint(MaxMetaLen, "metadata length")); r.err == nil {

internal/snapshot/snapshot_test.go

@@ -1,7 +1,9 @@
 package snapshot
 
 import (
+	"bufio"
 	"bytes"
+	"encoding/binary"
 	"testing"
 
 	"github.com/andydixon/mapsize/internal/inventory"
@@ -121,3 +123,17 @@ func TestCompare(t *testing.T) {
 		t.Fatal("contents of new dirs should not be listed separately")
 	}
 }
+
+func TestReadUvarintMatchesStdlib(t *testing.T) {
+	for _, v := range []uint64{0, 1, 127, 128, 300, 1 << 35, 1<<64 - 1} {
+		buf := binary.AppendUvarint(nil, v)
+		got, err := readUvarint(bufio.NewReader(bytes.NewReader(buf)))
+		if err != nil || got != v {
+			t.Fatalf("%d: got %d %v", v, got, err)
+		}
+	}
+	bad := bytes.Repeat([]byte{0xff}, 11)
+	if _, err := readUvarint(bufio.NewReader(bytes.NewReader(bad))); err == nil {
+		t.Fatal("overflow not detected")
+	}
+}

internal/tui/modals.go

@@ -230,7 +230,7 @@ type helpModal struct{}
 
 func newHelpModal(*Model) modal { return helpModal{} }
 
-func (helpModal) width() int           { return 74 }
+func (helpModal) width() int           { return 78 }
 func (helpModal) title() string        { return "Keyboard reference" }
 func (helpModal) footer(*Model) string { return "Esc close  ·  ↑↓ scroll" }
 
@@ -246,7 +246,7 @@ func (helpModal) content(m *Model, _ int) []line {
 		}
 		out = append(out, styled(s, h))
 	}
-	row := func(keys, desc string) { out = append(out, line{{"  " + textutil.PadRight(keys, 16), &k}, {desc, &d}}) }
+	row := func(keys, desc string) { out = append(out, line{{"  " + textutil.PadRight(keys, 19), &k}, {desc, &d}}) }
 	sec("NAVIGATION")
 	row("↑ ↓ ← →  hjkl", "move spatially through the treemap")
 	row("Enter", "inspect the selected item")
@@ -260,7 +260,9 @@ func (helpModal) content(m *Model, _ int) []line {
 	row("Esc", "clear the filter")
 	out = append(out, styled("    ubuntu   *.iso   size > 5GB   ext IN (iso,qcow2)   age > 365d", Style{FG: t.Muted}))
 	out = append(out, styled("    path contains cache AND NOT type = dir   owner = andy   flag = sparse", Style{FG: t.Muted}))
-	out = append(out, styled("    fields: "+filter.Fields, Style{FG: t.Muted}))
+	fields := strings.Fields(filter.Fields)
+	out = append(out, styled("    fields: "+strings.Join(fields[:7], " "), Style{FG: t.Muted}))
+	out = append(out, styled("            "+strings.Join(fields[7:], " "), Style{FG: t.Muted}))
 	out = append(out, styled("    sizes: KiB/MiB/GiB (1024), kB/MB/GB (1000); ages: s min h d w mo y", Style{FG: t.Muted}))
 	sec("VIEWS")
 	row("Tab  Shift+Tab", "next / previous view")

internal/tui/table.go

@@ -126,7 +126,7 @@ func (tb *table) paint(m *Model, cv *Canvas, r treemap.Rect, cols []column, rows
 			if !sel && ci < len(row.styles) && row.styles[ci] != nil {
 				st = *row.styles[ci]
 			}
-			if c.title == "" && row.bar >= 0 && c.width > 0 {
+			if c.title == "" && row.bar >= 0 {
 				m.paintBar(cv, x, y, w, row.bar, row.barCol, sel)
 			} else if c.right {
 				cv.Text(x, y, textutil.PadLeft(s, w), w, st)