2 files changed, 191 insertions, 2 deletions
diff --git a/lib/btrfs/btrfsitem/item_extentcsum.go b/lib/btrfs/btrfsitem/item_extentcsum.go
index eedd044..18ee572 100644
--- a/lib/btrfs/btrfsitem/item_extentcsum.go
+++ b/lib/btrfs/btrfsitem/item_extentcsum.go
@@ -14,8 +14,6 @@ import (
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfssum"
 )
 
-const CSumBlockSize = 4 * 1024
-
 // key.objectid = BTRFS_EXTENT_CSUM_OBJECTID
 // key.offset = laddr of checksummed region
 type ExtentCSum struct { // EXTENT_CSUM=128
diff --git a/lib/btrfs/btrfssum/shortsum.go b/lib/btrfs/btrfssum/shortsum.go
new file mode 100644
index 0000000..e93287e
--- /dev/null
+++ b/lib/btrfs/btrfssum/shortsum.go
@@ -0,0 +1,191 @@
+// Copyright (C) 2022  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package btrfssum
+
+import (
+	"context"
+	"fmt"
+	"io"
+	"strings"
+
+	"git.lukeshu.com/go/lowmemjson"
+
+	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
+	"git.lukeshu.com/btrfs-progs-ng/lib/diskio"
+)
+
+const BlockSize = 4 * 1024
+
+// ShortSum //////////////////////////////////////////////////////////
+
+type ShortSum string
+
+var (
+	_ lowmemjson.Encodable = ShortSum("")
+	_ lowmemjson.Decodable = (*ShortSum)(nil)
+)
+
+func (sum ShortSum) EncodeJSON(w io.Writer) error {
+	const hextable = "0123456789abcdef"
+	var buf [2]byte
+	buf[0] = '"'
+	if _, err := w.Write(buf[:1]); err != nil {
+		return err
+	}
+	for i := 0; i < len(sum); i++ {
+		buf[0] = hextable[sum[i]>>4]
+		buf[1] = hextable[sum[i]&0x0f]
+		if _, err := w.Write(buf[:]); err != nil {
+			return err
+		}
+	}
+	buf[0] = '"'
+	if _, err := w.Write(buf[:1]); err != nil {
+		return err
+	}
+	return nil
+}
+
+func deHex(r rune) (byte, bool) {
+	if r > 0xff {
+		return 0, false
+	}
+	c := byte(r)
+	switch {
+	case '0' <= c && c <= '9':
+		return c - '0', true
+	case 'a' <= c && c <= 'f':
+		return c - 'a' + 10, true
+	case 'A' <= c && c <= 'F':
+		return c - 'A' + 10, true
+	default:
+		return 0, false
+	}
+}
+
+func (sum *ShortSum) DecodeJSON(r io.RuneScanner) error {
+	var out strings.Builder
+	if c, _, err := r.ReadRune(); err != nil {
+		return err
+	} else if c != '"' {
+		return fmt.Errorf("expected %q, got %q", '"', c)
+	}
+	for {
+		a, _, err := r.ReadRune()
+		if err != nil {
+			return err
+		}
+		if a == '"' {
+			break
+		}
+		aN, ok := deHex(a)
+		if !ok {
+			return fmt.Errorf("expected a hex digit, got %q", a)
+		}
+		b, _, err := r.ReadRune()
+		if err != nil {
+			return err
+		}
+		bN, ok := deHex(b)
+		if !ok {
+			return fmt.Errorf("expected a hex digit, got %q", b)
+		}
+		out.WriteByte(aN<<4 | bN)
+	}
+	*sum = ShortSum(out.String())
+	return nil
+}
+
+// SumRun ////////////////////////////////////////////////////////////
+
+type SumRun[Addr btrfsvol.IntAddr[Addr]] struct {
+	// How big a ShortSum is in this Run.
+	ChecksumSize int
+	// Base address where this run starts.
+	Addr Addr `json:",omitempty"`
+	// All of the ShortSums in this run, concatenated together.
+	Sums ShortSum
+}
+
+func (run SumRun[Addr]) NumSums() int {
+	return len(run.Sums) / run.ChecksumSize
+}
+
+func (run SumRun[Addr]) Size() btrfsvol.AddrDelta {
+	return btrfsvol.AddrDelta(run.NumSums()) * BlockSize
+}
+
+// Get implements diskio.Sequence[int, ShortSum]
+func (run SumRun[Addr]) Get(sumIdx int64) (ShortSum, error) {
+	if sumIdx < 0 || int(sumIdx) >= run.NumSums() {
+		return "", io.EOF
+	}
+	off := int(sumIdx) * run.ChecksumSize
+	return ShortSum(run.Sums[off : off+run.ChecksumSize]), nil
+}
+
+func (run SumRun[Addr]) SumForAddr(addr Addr) (ShortSum, bool) {
+	if addr < run.Addr || addr >= run.Addr.Add(run.Size()) {
+		return "", false
+	}
+	off := int((addr-run.Addr)/BlockSize) * run.ChecksumSize
+	return ShortSum(run.Sums[off : off+run.ChecksumSize]), true
+}
+
+func (run SumRun[Addr]) Walk(ctx context.Context, fn func(Addr, ShortSum) error) error {
+	for addr, off := run.Addr, 0; off < len(run.Sums); addr, off = addr+BlockSize, off+run.ChecksumSize {
+		if err := ctx.Err(); err != nil {
+			return err
+		}
+		if err := fn(addr, ShortSum(run.Sums[off:off+run.ChecksumSize])); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// SumRunWithGaps ////////////////////////////////////////////////////
+
+type SumRunWithGaps[Addr btrfsvol.IntAddr[Addr]] struct {
+	Addr Addr
+	Size btrfsvol.AddrDelta
+	Runs []SumRun[Addr]
+}
+
+func (sg SumRunWithGaps[Addr]) NumSums() int {
+	return int(sg.Size / BlockSize)
+}
+
+func (sg SumRunWithGaps[Addr]) PctFull() float64 {
+	total := sg.NumSums()
+	var full int
+	for _, run := range sg.Runs {
+		full += run.NumSums()
+	}
+	return float64(full) / float64(total)
+}
+
+func (sg SumRunWithGaps[Addr]) SumForAddr(addr Addr) (ShortSum, error) {
+	if addr < sg.Addr || addr >= sg.Addr.Add(sg.Size) {
+		return "", io.EOF
+	}
+	for _, run := range sg.Runs {
+		if run.Addr > addr {
+			return "", diskio.ErrWildcard
+		}
+		if run.Addr.Add(run.Size()) <= addr {
+			continue
+		}
+		off := int((addr-run.Addr)/BlockSize) * run.ChecksumSize
+		return ShortSum(run.Sums[off : off+run.ChecksumSize]), nil
+	}
+	return "", diskio.ErrWildcard
+}
+
+// Get implements diskio.Sequence[int, ShortSum]
+func (sg SumRunWithGaps[Addr]) Get(sumIdx int64) (ShortSum, error) {
+	addr := sg.Addr.Add(btrfsvol.AddrDelta(sumIdx) * BlockSize)
+	return sg.SumForAddr(addr)
+}