2 files changed, 103 insertions, 0 deletions
diff --git a/lib/jsonutil/hex_decoder.go b/lib/jsonutil/hex_decoder.go
new file mode 100644
index 0000000..e5c84a7
--- /dev/null
+++ b/lib/jsonutil/hex_decoder.go
@@ -0,0 +1,61 @@
+// Copyright (C) 2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+package jsonutil
+
+import (
+	"fmt"
+	"io"
+	"math"
+)
+
+type invalidHexRuneError rune
+
+func (e invalidHexRuneError) Error() string {
+	return fmt.Sprintf("jsonutil: invalid hex digit: %q", rune(e))
+}
+
+// hexDecoder is like an encoding/hex.Decoder, but has a "push"
+// interface rather than a "pull" interface.
+type hexDecoder struct {
+	dst io.ByteWriter
+
+	buf   byte
+	bufOK bool
+}
+
+func (d *hexDecoder) WriteRune(r rune) (int, error) {
+	if r > math.MaxUint8 {
+		return 0, invalidHexRuneError(r)
+	}
+
+	c := byte(r)
+	var v byte
+	//nolint:gomnd // Hex conversion.
+	switch {
+	case '0' <= c && c <= '9':
+		v = c - '0'
+	case 'a' <= c && c <= 'f':
+		v = c - 'a' + 10
+	case 'A' <= c && c <= 'F':
+		v = c - 'A' + 10
+	default:
+		return 0, invalidHexRuneError(r)
+	}
+
+	if !d.bufOK {
+		d.buf = v
+		d.bufOK = true
+		return 1, nil
+	}
+	d.bufOK = false
+	return 1, d.dst.WriteByte(d.buf<<4 | v)
+}
+
+func (d *hexDecoder) Close() error {
+	if d.bufOK {
+		return io.ErrUnexpectedEOF
+	}
+	return nil
+}
diff --git a/lib/jsonutil/hex_string.go b/lib/jsonutil/hex_string.go
new file mode 100644
index 0000000..06970ce
--- /dev/null
+++ b/lib/jsonutil/hex_string.go
@@ -0,0 +1,42 @@
+// Copyright (C) 2023  Luke Shumaker <lukeshu@lukeshu.com>
+//
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+// Package jsonutil provides utilities for implementing the interfaces
+// consumed by the "git.lukeshu.com/go/lowmemjson" package.
+package jsonutil
+
+import (
+	"io"
+
+	"git.lukeshu.com/go/lowmemjson"
+)
+
+func EncodeHexString[T ~[]byte | ~string](w io.Writer, str T) 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(str); i++ {
+		buf[0] = hextable[str[i]>>4]
+		buf[1] = hextable[str[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 DecodeHexString(r io.RuneScanner, dst io.ByteWriter) error {
+	dec := &hexDecoder{dst: dst}
+	if err := lowmemjson.DecodeString(r, dec); err != nil {
+		return err
+	}
+	return dec.Close()
+}