Step by Step solution about How to resolve the algorithm URL decoding step by step in the C programming language

This C code demonstrates how to decode a percent-encoded string, which is commonly used in web addresses and other contexts to represent special characters that cannot be directly included in the URL. The following is a detailed explanation of the code:

• inline int ishex(int x): This is an inline function that checks if a given character x is a hexadecimal digit. It returns true if x is in the range '0' to '9', 'a' to 'f', or 'A' to 'F'; otherwise, it returns false.

• int decode(const char *s, char *dec): This function takes two strings as input: s is the percent-encoded string to be decoded, and dec (if non-NULL) is the buffer where the decoded string will be stored. It returns the length of the decoded string if successful, and -1 if the input string is invalid.

• Inside the decode function:

  • A pointer o is used to traverse the output buffer dec.
  • A pointer end is set to point to the end of the input string s.
  • A loop iterates through the input string until the end is reached.
  • For each character c in the input string:
    • If c is a plus sign ('+'), it is replaced with a space (' ').
    • If c is a percent sign ('%'), the following two characters are checked:
      • If both characters are hexadecimal digits, they are interpreted as a two-digit hexadecimal code and converted into the corresponding character c.
      • If not, the sscanf function is used to attempt to parse a two-digit hexadecimal code from the two characters.
      • If the hexadecimal code is invalid, the function returns -1.
    • The decoded character c is stored in the output buffer dec (if provided).

• In the main function:

  • A sample percent-encoded string url is defined.
  • The length of the decoded string is printed using decode(url, 0).
  • The decoded string is stored in the buffer out using decode(url, out).
  • The decoded string is printed, or an error message is displayed if decoding failed.

In this specific example, the input string url is "http%3A%2F%2ffoo+bar%2fabcd". The decoding process converts the percent-encoded sequences into their corresponding characters:

• %3A is decoded to ':'
• %2F is decoded to '/'
• %2B is decoded to ' ' (space)

The decoded string becomes "http://foo bar/abcd", which is then printed as output.

This code provides a simple yet effective way to decode percent-encoded strings, which is useful in handling URLs and other scenarios where special characters need to be represented in a compact and safe format.

#include <stdio.h>
#include <string.h>

inline int ishex(int x)
{
	return	(x >= '0' && x <= '9')	||
		(x >= 'a' && x <= 'f')	||
		(x >= 'A' && x <= 'F');
}

int decode(const char *s, char *dec)
{
	char *o;
	const char *end = s + strlen(s);
	int c;

	for (o = dec; s <= end; o++) {
		c = *s++;
		if (c == '+') c = ' ';
		else if (c == '%' && (	!ishex(*s++)	||
					!ishex(*s++)	||
					!sscanf(s - 2, "%2x", &c)))
			return -1;

		if (dec) *o = c;
	}

	return o - dec;
}

int main()
{
	const char *url = "http%3A%2F%2ffoo+bar%2fabcd";
	char out[strlen(url) + 1];

	printf("length: %d\n", decode(url, 0));
	puts(decode(url, out) < 0 ? "bad string" : out);

	return 0;
}