Clean up the hex handling

1 files changed, 22 insertions, 0 deletions

diff --git a/encode_escape.go b/encode_escape.go
index 0054e72..97da6e9 100644
--- a/encode_escape.go
+++ b/encode_escape.go
@@ -5,6 +5,7 @@
 package lowmemjson
 
 import (
+	"fmt"
 	"unicode/utf8"
 
 	"git.lukeshu.com/go/lowmemjson/internal/jsonstring"
@@ -27,6 +28,27 @@ const (
 	BackslashEscapeUnicode = jsonstring.BackslashEscapeUnicode
 )
 
+func hexToInt(c byte) rune {
+	switch {
+	case '0' <= c && c <= '9':
+		return rune(c) - '0'
+	case 'a' <= c && c <= 'f':
+		return rune(c) - 'a' + 10
+	case 'A' <= c && c <= 'F':
+		return rune(c) - 'A' + 10
+	default:
+		panic(fmt.Errorf("should not happen: invalid hex char: %q", c))
+	}
+}
+
+func hexToRune(a, b, c, d byte) rune {
+	return 0 |
+		hexToInt(a)<<12 |
+		hexToInt(b)<<8 |
+		hexToInt(c)<<4 |
+		hexToInt(d)<<0
+}
+
 // A BackslashEscaper controls how a ReEncoder emits a character in a
 // JSON string.  The `rune` argument is the character being
 // considered, and the `BackslashEscapeMode` argument is how it was