URL Decode String

Task

Implement the function url_decode.

• Write a function that decodes URL-encoded strings in-place
• Convert %20 to space, %21 to !, etc.
• The general pattern is %HH where HH is a two-digit hexadecimal number
• Modify the string in-place and return the same pointer

Function Signature

// Decode URL-encoded string in-place, return the same pointer
char* url_decode(char* str);

Note

• Find all occurrences of % followed by two hexadecimal digits
• Convert the hex digits to their ASCII character value
• Replace the three characters %HH with the single decoded character
• Shift remaining characters left to fill the gap
• Common encodings: %20 = space, %21 = !, %2C = ,, %3F = ?
• You can use the function strtol to convert hex strings to integers
• Modify the string in-place (don’t allocate new memory)
• If you encounter a % not followed by two hex digits, you can leave it as-is

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Example

Input	Output
hello%20world	hello world
hi%21	hi!
test%20%20double	test double
no_encoding	no_encoding
mix%20of%21both	mix of!both

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Code

#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
void shift_left(char* str){
    while(str[2] != 0){
        str[0] = str[2];
        str++;
    }
    str[0] = 0;
}
 
char *url_decode(char *str)
{
    char* current = strstr(str, "%");
    while(current != NULL){
        char hex[3] = {current[1], current[2], 0};
        char* endptr = NULL;
        long val = strtol(hex, &endptr, 16);
 
        if(endptr){
            *current = (char)val;
            shift_left(current + 1);
            current = strstr(current + 1, "%");
        }
    }
    return str;
}