Your task is to provide the missing implementation for functions and structures in synonyms_impl.c. The basic usage might look like this:

  Synonyms *s;

  if (!(s = synonyms_init())) {
      /* error */
  }

  if (synonyms_define(s, "synonym", "alternative", "equivalent", NULL) < 0) {
      /* error */
  }

  char ** list = synonyms_get(s, "synonym");

  // @list is now an array of strings, terminated by NULL and looks like this:
  // list[0] = "alternative"
  // list[1] = "equivalent"
  // list[2] = NULL

  stringlist_free(list);

  synonyms_free(s);

Note: an array of strings terminated by an explicit NULL element is often referred to as string list. NULL serves as a sentinel so that anybody working with the string list knows when to stop processing.

The design of Synonyms structure is completely up to you, just like algorithms used. You can introduce new internal functions, extend stringlist.c (and/or stringlist.h) for any functions or macros you want. However, you can use standard C library only. If you allocate any memory, do not forget to free it once no longer needed.

The synonyms_define() function defines synonyms for a given word (argument #2) and allows to define multiple synonyms at once. The list of synonyms must be terminated by NULL. However, it is also possible to define synonyms in separate steps. For instance:

synonyms_define(s, "synonym", "alternative", "equivalent", NULL);

is equivalent to:

  synonyms_define(s, "synonym", "alternative", NULL);
  synonyms_define(s, "synonym", "equivalent", NULL);

Please note that synonyms are transitive, thus if A is a synonym of B and B is a synonym of C then A is a synonym of C, in other words:

synonyms_define(s, "synonym", "alternative", "equivalent", NULL);

is equivalent to:

  synonyms_define(s, "synonym", "alternative", NULL);
  synonyms_define(s, "alternative", "equivalent", NULL);

Bear in mind that synonyms are also reflexive, that is, if a word A is a synonym of B then the reverse is also true: B is a synonym of A. However, by the definition of synonym: a word cannot be a synonym of itself.

Feel free to ask any questions.