in internal/encoding/json/decode.go [1174:1193]
func getu4(s []byte) rune {
if len(s) < 6 || s[0] != '\\' || s[1] != 'u' {
return -1
}
var r rune
for _, c := range s[2:6] {
switch {
case '0' <= c && c <= '9':
c = c - '0'
case 'a' <= c && c <= 'f':
c = c - 'a' + 10
case 'A' <= c && c <= 'F':
c = c - 'A' + 10
default:
return -1
}
r = r*16 + rune(c)
}
return r
}