URL escape and unescape

 1 // unescape unescapes a string; the mode specifies
 2 // which section of the URL string is being unescaped.
 3 func unescape(s string, mode encoding) (string, error) {
 4     // Count %, check that they‘re well-formed.
 5     n := 0
 6     hasPlus := false
 7     for i := 0; i < len(s); {
 8         switch s[i] {
 9         case ‘%‘:
10             n++
11             if i+2 >= len(s) || !ishex(s[i+1]) || !ishex(s[i+2]) {
12                 s = s[i:]
13                 if len(s) > 3 {
14                     s = s[0:3]
15                 }
16                 return "", EscapeError(s)
17             }
18             i += 3
19         case ‘+‘:
20             hasPlus = mode == encodeQueryComponent
21             i++
22         default:
23             i++
24         }
25     }
26 
27     if n == 0 && !hasPlus {
28         return s, nil
29     }
30 
31     t := make([]byte, len(s)-2*n)
32     j := 0
33     for i := 0; i < len(s); {
34         switch s[i] {
35         case ‘%‘:
36             t[j] = unhex(s[i+1])<<4 | unhex(s[i+2])
37             j++
38             i += 3
39         case ‘+‘:
40             if mode == encodeQueryComponent {
41                 t[j] = ‘ ‘
42             } else {
43                 t[j] = ‘+‘
44             }
45             j++
46             i++
47         default:
48             t[j] = s[i]
49             j++
50             i++
51         }
52     }
53     return string(t), nil
54 }