urld.c

// https://github.com/abejfehr/URLDecode
// Compile: gcc -Ofast -mtune=native -D__USE_MINGW_ANSI_STDIO -o urld.exe
// When decoding UTF8-encoded data, work with [xargsu2a](https://gist.github.com/imba-tjd/0a5a41df029babc6c814efe5d9296593)
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

char *urlDecode(const char *str) {
  int d = 0;
  char *dStr = malloc(strlen(str) + 1);
  char eStr[] = "00";
  strcpy(dStr, str);
  while(!d) {
    d = 1;
    int i;
    for(i=0;i<strlen(dStr);++i) {
      if(dStr[i] == '%') {
        if(dStr[i+1] == 0)
          return dStr;
        if(isxdigit(dStr[i+1]) && isxdigit(dStr[i+2])) {
          d = 0;
          eStr[0] = dStr[i+1];
          eStr[1] = dStr[i+2];
          long int x = strtol(eStr, NULL, 16);
          memmove(&dStr[i+1], &dStr[i+3], strlen(&dStr[i+3])+1);
          dStr[i] = x;
        }
      }
    }
  }
  return dStr;
}

int main(){
  char* s;
  scanf("%ms", &s);
  puts(urlDecode(s));
}