json: Implement a bunch of streaming JSON encoding/decoding

1 files changed, 80 insertions, 0 deletions

diff --git a/lib/btrfsprogs/btrfsinspect/csums.go b/lib/btrfsprogs/btrfsinspect/csums.go
index 78bf915..6335cb9 100644
--- a/lib/btrfsprogs/btrfsinspect/csums.go
+++ b/lib/btrfsprogs/btrfsinspect/csums.go
@@ -6,7 +6,11 @@ package btrfsinspect
 
 import (
 	"context"
+	"fmt"
 	"io"
+	"strings"
+
+	"git.lukeshu.com/go/lowmemjson"
 
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsitem"
 	"git.lukeshu.com/btrfs-progs-ng/lib/btrfs/btrfsvol"
@@ -17,6 +21,82 @@ import (
 
 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 {