{"id":2681,"date":"2017-09-08T00:01:43","date_gmt":"2017-09-08T07:01:43","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=2681"},"modified":"2017-09-02T08:39:05","modified_gmt":"2017-09-02T15:39:05","slug":"the-url-decoding-filter-solution","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=2681","title":{"rendered":"The URL Decoding Filter – Solution"},"content":{"rendered":"

Unwinding percent-encoding involves three steps:<\/p>\n

    \n
  1. Pass-through the unchanged characters.<\/li>\n
  2. Change +<\/code> back into a space.<\/li>\n
  3. Decode the percent strings, which is the most involved process.<\/li>\n<\/ol>\n


    \nFor this month’s Exercise<\/a>, my solution is a simple I\/O filter. In fact, for most decoding, the character input shoots back out the filter as output. Here is the main()<\/em> function in my solution:<\/p>\n

    \r\nint main()\r\n{\r\n    int i;\r\n\r\n    while( (i=getchar()) != EOF )\r\n    {\r\n        if( i=='%' )\r\n            pdecode();\r\n        else if( i=='+')\r\n            putchar(' ');\r\n        else\r\n            putchar(i);\r\n    }\r\n\r\n    return(0);\r\n}<\/pre>\n
    The while<\/em> loop keeps spinning until an EOF<\/code> (end of file) character is encountered. The getchar()<\/em> function reads input; the putchar()<\/em> function writes output. The if-else-if-else<\/em> structure handles the three steps for decoding percent encoding. As you can see in the main()<\/em> function, I handle the steps backwards:<\/p>\n
    Third, when the %<\/code> character is encountered, execution shuffles off to the pdecode()<\/em> function. I could do the translation in the main()<\/em> function, but by using pdecode()<\/em> I keep main()<\/em> short and readable.<\/p>\n
    Second, if the input character is a +<\/code>, the space character is output.<\/p>\n
    First, anything left over is output directly. This final condition catches all alphanumeric text as well as the four exceptions in HTML5: - . _ *<\/code><\/p>\n
    The pdecode()<\/em> function works to grab the next two characters of input; the %<\/code> is already digested and not output.<\/p>\n
    \r\nvoid pdecode(void)\r\n{\r\n    int a;\r\n\r\n    a = char2hex() * 0x10;\r\n    a += char2hex();\r\n    putchar(a);\r\n}<\/pre>\n
    Variable a<\/code> builds the value, the ASCII code translated from the next two characters of input. The char2hex()<\/em> function reads and tests input. The value returned is 0x0 through 0xF (15), translated from characters 0 through 9 and A through F.<\/p>\n
    The first character read is multiplied by 0x10 (16) and stored in variable a<\/code>; the next character is added to that value. So, for example, string A1 is translated into 0xA * 0x10 + 0x1<\/code>, which becomes 0xA1. That value is output by putchar(a)<\/code>.<\/p>\n
    The char2hex()<\/em> function’s job is to read a character of input, confirm that it’s a valid hexadecimal digit, and return that value. On an error, the function exits the program:<\/p>\n
    \r\nint char2hex(void)\r\n{\r\n    int d;\r\n\r\n    d = getchar();\r\n    if( d==EOF ) exit(1);   \/* quit on EOF *\/<\/span>\r\n    if( isnumber(d) )\r\n        return( d - '0' );\r\n    d = toupper(d);\r\n    if( d>='A' && d<='F' )\r\n        return( d - 'A' + 0x0A );\r\n    else\r\n        exit(1);\r\n}<\/pre>\n
    The getchar()<\/em> function fetches the next character from the input stream. Immediately, if that character is the EOF<\/code>, the function exits the program. This, and all exit conditions in this function, indicated a poorly-formed percent-encoded string, so bailing out is a valid move.<\/p>\n
    The next if<\/em> test checks for a digit, 0 to 9. If so, that digit’s value is returned, 0 to 9.<\/p>\n
    For non-digit characters, the toupper()<\/em> function translates input to uppercase, and another if<\/em> test confirms whether the character is in the range 'A'<\/code> to 'F'<\/code>. If so, the character’s value, 10 through 15, is returned.<\/p>\n
    The final else<\/em> catches any non-hexadecimal character and terminates the program as the percent-ended string is malformed or invalid.<\/p>\n
    Click here<\/a> to view my entire solution. Your solution may vary, of course, but if it handles the translation, it’s good.<\/p>\n
    To test the program, process filtered input from last month’s Exercise<\/a> and run it through the solution for this month’s exercise. If the final string matches the initial string, everything works. For example:<\/p>\n
    \r\n$ cat 09solution.txt\r\n
    The URL Decoding Filter – Solution<\/a><\/blockquote>