Add Makefile, update readme and fix valgrind found leaks

Makefile

@@ -0,0 +1,25 @@
+DEBUG?= -g
+CFLAGS?= -O2 -Wall -W -DSDS_ABORT_ON_OOM
+CCOPT= $(CFLAGS)
+
+OBJ = main.o skiplist.o x.o
+PRGNAME = sl
+
+all: sl
+
+# Deps (use make dep to generate this)
+main.o: x.h skiplist.h x.c skiplist.c main.c
+x.o: x.c x.h
+skiplist.o: x.c x.h skiplist.c skiplist.h 
+
+sl: $(OBJ)
+	$(CC) -o $(PRGNAME) $(CCOPT) $(DEBUG) $(OBJ)
+
+.c.o:
+	$(CC) -c $(CCOPT) $(DEBUG) $(COMPILE_TIME) $<
+
+clean:
+	rm -rf $(PRGNAME) *.o
+
+dep:
+	$(CC) -MM *.c

README.rst

@@ -2,3 +2,10 @@ skiplist
 =========
 
 Academic example of skip list implementation in C.
+
+To compile just run
+
+.. code:: bash
+
+    make
+

skiplist.c

@@ -114,6 +114,11 @@ void sl_print(const slist *sl)
 #define HEAD 1
 #define TAIL 0
 
+/*
+ * Repeatedly toss a coin until we get tails.
+ * Denote with i the number of times coin came up heads
+ * and return it.
+ */
 int _toss_coin(int maxlevel) {
     int i;
     int x;
@@ -135,14 +140,14 @@ int _toss_coin(int maxlevel) {
 }
 
 /*
- * Clean top levels with only INFINITY nodes until only one such
+ * Remove top levels with only INFINITY nodes until only one such
  * level exists.
  */
 void _sl_clean_top(slist *sl)
 {
     sl_node *foo;
 
-    for (foo = sl->topleft; foo->below != NULL; foo = foo->below) {
+    for (foo = sl->topleft; foo->below != NULL;) {
         if (foo->next->key == KEY_INF_MAX
                 && foo->below->next->key == KEY_INF_MAX) {
             sl->topleft = foo->below;
@@ -150,14 +155,28 @@ void _sl_clean_top(slist *sl)
             sl->topleft->above = NULL;
             sl->topright->above = NULL;
             --sl->maxlevel;
+
             free(foo->data);
             free(foo->next);
             free(foo);
+
+            foo = sl->topleft;
             debug("cleaned one level\n");
+        } else {
+            break;
         }
     }
 }
 
+/*
+ * Toss coin and get i.
+ * If i is greather than or equal to skip list max level then
+ * add one more level.
+ * Search for x and find the positions of the items with largest key
+ * less than x.
+ * Insert item into each level up to i.
+ * If key is found then just update its data.
+ */
 int sl_add(slist *sl, int key, void *data)
 {
     sl_node *foo;
@@ -190,6 +209,9 @@ int sl_add(slist *sl, int key, void *data)
         if (foo->below == NULL) {
             break;
         }
+        /*
+         * Save nodes that will be updated with new node.
+         */
         update[level--] = foo;
         foo = foo->below;
     }
@@ -233,6 +255,14 @@ int sl_add(slist *sl, int key, void *data)
     return res;
 }
 
+/*
+ * Start at the top left position.
+ * At the current position compare key with node's key.
+ * If key is found then return it.
+ * If key > node's key then scan forward.
+ * If key < node's key then drop down.
+ * If we drop down past the bottom list then return NULL.
+ */
 sl_node *sl_find(slist *sl, int key)
 {
     sl_node *foo;
@@ -254,6 +284,11 @@ sl_node *sl_find(slist *sl, int key)
     return foo;
 }
 
+/*
+ * Search for key and find positions with the key.
+ * Remove found positions.
+ * Remove all but one list containing only INFINITY keys.
+ */
 int sl_remove(slist *sl, int key)
 {
     sl_node *foo;