qs.c

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


int array_push_sorted(char *array[128], int size, char *string, int length) {
  int j, k;
  char *old;
  for (j = 0; j < size; j++ ) {
    old = array[j];
    int i = 0;
    for (; i < length; i++)
      if (string[i] != old[i])
        break;

    if (string[i] < old[i]) {
      // shift
      for (k = size; --k >= j;)
        array[k + 1] = array[k];
      break;
    }
  }
  array[j] = string;

  return size + 1;
}


int main () {
  char *array[128];
  char response[1024];
  int length = 0;

  char *qs = "person[abc]=def&person[cba][af]=eee&hello=world&person[1]=2&baba=bang&zerson[a]=2\0";
  int size = strlen(qs);

  char *last = qs;
  for (char *p = qs; p < qs + size; p++) {
    if (*p == '&' || p == qs + size - 1) {
      length = array_push_sorted(array, length, last, p - last - 1);
      last = p + 1;
    }

  }

  *response = '{';
  int lastch = 0;
  char *finish, *start, *position, *previous, *v;

  for (int i = 0; i < length; i++) {
    start = *(array + i);
    position = start;

    int separated = 0;
    int closed = 0;
    for (char *p = *(array + i); p < qs + size; p++) {
      char *next = p;
      lastch = (p == qs + size - 1 || *next == '&' || *next == '=') ? 1 : 0;
      
      // found boundaries of a keyword (EOL, [ or ])
      if (*next == '[' || lastch) {
        finish = p;
        int prepended = 0;

        if (previous != NULL) {
          // Dont prepend context that matches with previous key
          int len = last - previous;
          if (strncmp(previous, start, len > position - start ? len : position - start) == 0) {
            fprintf(stdout, "\n%d %d \n%s\n%s\n", len, position - start, position, last);
            if (!lastch && len >= finish - start)
              prepended = 1; 
          }

          // close all mismatching objects
          if (len > finish - start && prepended == 0 && closed == 0) {
            closed = 1;
            for (char *m = previous + 1; m < last; m++)
              if (*m == '[')
                strcat(response, "}\n");
          }

          // add comma
          if (!separated) {
            separated = 1;
            strcat(response, ",");
          }
        }

        // prepend key
        if (!prepended) {
          char *fin = *(finish - 1) == ']' ? finish - 1 : finish;
          char *pos = *position == '[' ? position + 1 : position;

          strcat(response, "\"");
          strncpy(response + strlen(response), pos, fin - pos);
        }

        if (lastch) {
          strcat(response, "\":\"");

          // find value
          char *v = finish + 1;
          for (; v < qs + size; v++) {
            if (*v == '&' || *v == '=')
              break;
          }

          // append value
          strncpy(response + strlen(response), finish + 1, v - finish - 1);
          strcat(response, "\"\n");

          // remember path
          previous = start;
          last = position;

        } else {
          // prepend key
          if (!prepended) strcat(response, "\": {\n");

          if (last != NULL) {

          }
          position = finish + 1;
        }
      }
      if (lastch) break;


    }
  }
  // close all objects
  for (char *m = previous + 1; m < last; m++)
    if (*m == '[')
      strcat(response, "}\n");
  sprintf(response + strlen(response), "}");

  fprintf(stdout, "\n\n%s\n", response);

  for (int i = 0; i < length; i++)
    fprintf(stdout, "Result %s\n", *(array + i));
}