add a 'smartlist' building block that picks random elements from a list

svn:r897

src/common/crypto.c

@@ -1009,6 +1009,7 @@ void crypto_pseudo_rand(unsigned int n, unsigned char *to)
   }
 }
 
+/* return a pseudo random number between 0 and max-1 */
 int crypto_pseudo_rand_int(unsigned int max) {
   unsigned int val;
   unsigned int cutoff;

src/common/util.c

@@ -63,6 +63,46 @@ char *tor_strndup(const char *s, size_t n) {
   return dup;
 }
 
+/*
+ * A simple smartlist interface to make an unordered list of acceptable
+ * nodes and then choose a random one.
+ * smartlist_create() mallocs the list, _free() frees the list,
+ * _add() adds an element, _remove() removes an element if it's there,
+ * _choose() returns a random element.
+ */
+
+smartlist_t *smartlist_create(int max_elements) {
+  smartlist_t *sl = tor_malloc(sizeof(smartlist_t));
+  sl->list = tor_malloc(sizeof(void *) * max_elements);
+  sl->num_used = 0;
+  sl->max = max_elements;
+  return sl;
+}
+
+void smartlist_free(smartlist_t *sl) {
+  free(sl->list);
+  free(sl);
+}
+
+/* add element to the list, but only if there's room */
+void smartlist_add(smartlist_t *sl, void *element) {
+  if(sl->num_used < sl->max)
+    sl->list[sl->num_used++] = element;
+}
+
+void smartlist_remove(smartlist_t *sl, void *element) {
+  int i;
+  for(i=0; i < sl->num_used; i++)
+    if(sl->list[i] == element)
+      sl->list[i] = sl->list[--sl->num_used]; /* swap with the end */
+}
+
+void *smartlist_choose(smartlist_t *sl) {
+  if(sl->num_used)
+    return sl->list[crypto_pseudo_rand_int(sl->num_used)];
+  return NULL; /* no elements to choose from */
+}
+
 /*
  *    String manipulation
  */

src/common/util.h

@@ -39,6 +39,18 @@ char *tor_strdup(const char *s);
 char *tor_strndup(const char *s, size_t n);
 #define tor_free(p) do {if(p) {free(p); (p)=NULL;}} while(0)
 
+typedef struct {
+  void **list;
+  int num_used;
+  int max;
+} smartlist_t;
+
+smartlist_t *smartlist_create(int max_elements);
+void smartlist_free(smartlist_t *sl);
+void smartlist_add(smartlist_t *sl, void *element);
+void smartlist_remove(smartlist_t *sl, void *element);
+void *smartlist_choose(smartlist_t *sl);
+
 const char *eat_whitespace(const char *s);
 const char *eat_whitespace_no_nl(const char *s);
 const char *find_whitespace(const char *s);