Add a pattern struct to encompass a pattern NFA and its size.

The size is necessary in free operations - unlike vectors and linked
lists, sparse sets in libds are not dynamically resizable.

include/pattern.h

@@ -149,23 +149,39 @@ struct pattern_chain_s {
   struct pattern_node_s* tail;
 };
 
+/**
+ * Represents a single compiled pattern.
+ */
+struct pattern_s {
+  /**
+   * The first NFA state of the pattern.
+   */
+  struct pattern_node_s* head;
+  /**
+   * The number of NFA nodes in this pattern.
+   */
+  int size;
+};
+
 typedef enum pattern_node_type_e pattern_node_type;
 typedef struct pattern_node_s pattern_node;
 typedef struct pattern_chain_s pattern_chain;
+typedef struct pattern_s pattern;
 
 /**
  * Compiles a string representation of a pattern into a pattern NFA,
  * and stores the first node of the new NFA in root.
  * @param root the node to store the resulting pattern into.
  * @param expression the pattern represented as a string.
+ * @param id the id of the pattern.
  * @return LIBLEX_SUCCESS if all goes well, otherwise some other liblex_result.
  */
-liblex_result pattern_compile(pattern_node** root, char* expression);
+liblex_result pattern_compile(pattern* ptrn, char* expression, int id);
 /**
  * Frees a pattern NFA allocated by pattern_compile.
  * @param root the root node to start freeing from.
  * @return LIBLEX_SUCCESS if all goes well, otherwise some other liblex_result.
  */
-liblex_result pattern_free(pattern_node* root);
+liblex_result pattern_free(pattern* ptrn);
 
 #endif