URL escape and unescape

Recently, I study the package  net/url of Golang.

I was puzzled about the escape and unescape of url string.

then I find a clear and accurate answer at: http://www.sislands.com/coin70/week6/encoder.htm

the code in golang:

// unescape unescapes a string; the mode specifies
// which section of the URL string is being unescaped.
func unescape(s string, mode encoding) (string, error) {
    // Count %, check that they're well-formed.
    n := 0
    hasPlus := false
    for i := 0; i < len(s); {
        switch s[i] {
        case '%':
            n++
            if i+2 >= len(s) || !ishex(s[i+1]) || !ishex(s[i+2]) {
                s = s[i:]
                if len(s) > 3 {
                    s = s[0:3]
                }
                return "", EscapeError(s)
            }
            i += 3
        case '+':
            hasPlus = mode == encodeQueryComponent
            i++
        default:
            i++
        }
    }

    if n == 0 && !hasPlus {
        return s, nil
    }

    t := make([]byte, len(s)-2*n)
    j := 0
    for i := 0; i < len(s); {
        switch s[i] {
        case '%':
            t[j] = unhex(s[i+1])<<4 | unhex(s[i+2])
            j++
            i += 3
        case '+':
            if mode == encodeQueryComponent {
                t[j] = ' '
            } else {
                t[j] = '+'
            }
            j++
            i++
        default:
            t[j] = s[i]
            j++
            i++
        }
    }
    return string(t), nil
}

After a period of home work,  Now I am confident about implimenting the project: gocrawel by myself.