feat(queue): introduce a queue ds, internally this is just a stack...

the only actual difference is a oneline change which makes pushing
append instead of prepend the new data.

README.md

@@ -45,7 +45,7 @@ ds_hmp_free(&hmp, NULL);
 free(b);
 ```
 
-## Stack
+## Stack / Queue
 ```c
 char *a = "a";
 char *b = "b";
@@ -58,6 +58,9 @@ ds_stack_pop(stack); // "b"
 ds_stack_pop(stack); // "a"
 free(stack);
 ```
+If you need a queue just use the equivalent `ds_queue` functions and datatypes.
+Internally a queue is just a stack which pushes to the end instead of the
+beginning.
 
 # TODO
 - [ ] more data structures

ds.c

@@ -420,3 +420,36 @@ void
 
   return NULL;
 }
+
+ds_queue_t
+*ds_queue_init(void)
+{
+  return ds_stack_init();
+}
+
+int
+ds_queue_push(ds_queue_t *queue, void *data)
+{
+  if (queue->n == 0) {
+    queue->items = ds_sll_new_node(data);
+    queue->n++;
+    return 0;
+  }
+  int r = ds_sll_append(queue->items, data);
+  if (!r) {
+    queue->n++;
+  }
+  return r;
+}
+
+void
+*ds_queue_pop(ds_queue_t *queue)
+{
+  return ds_stack_pop((ds_stack_t *)queue);
+}
+
+void
+*ds_queue_peek(ds_queue_t *queue, unsigned x)
+{
+  return ds_stack_peek((ds_stack_t *)queue, x);
+}

ds.h

@@ -27,6 +27,8 @@ typedef struct {
   ds_sll_t *items;
 } ds_stack_t;
 
+typedef ds_stack_t ds_queue_t;
+
 #define ds_ll_foreach(t, ll) for (t *cur = ll; cur; cur = cur->next)
 
 /**
@@ -206,4 +208,37 @@ void *ds_stack_pop(ds_stack_t *stack);
  */
 void *ds_stack_peek(ds_stack_t *stack, unsigned x);
 
+/**
+ * @brief initialize a new queue
+ *
+ * @return the queue
+ */
+ds_queue_t *ds_queue_init(void);
+
+/**
+ * @brief push new data onto the queue
+ *
+ * @param queue the queue
+ * @param data the data
+ * @return 0 on success
+ */
+int ds_queue_push(ds_queue_t *queue, void *data);
+
+/**
+ * @brief pop the data off the top of the queue
+ *
+ * @param queue the queue
+ * @return the data
+ */
+void *ds_queue_pop(ds_queue_t *queue);
+
+/**
+ * @brief peek forward x items
+ *
+ * @param queue the queue
+ * @param x how many items to look ahead
+ * @return the data or NULL if out of range
+ */
+void *ds_queue_peek(ds_queue_t *queue, unsigned x);
+
 #endif

test.c

@@ -99,4 +99,25 @@ main(int argc, char *argv[])
 
     free(stack);
   );
+
+  test("queue",
+    char *a = "a";
+    char *b = "b";
+    char *c = "c";
+
+    ds_queue_t *queue = ds_queue_init();
+    it("pushes an item", ds_queue_push(queue, a) == 0);
+    it("pops an item", ds_queue_pop(queue) == a);
+
+    ds_queue_push(queue, b);
+    ds_queue_push(queue, c);
+
+    it("peeks ahead", ds_queue_peek(queue, 0) == b);
+    it("peeks past the queue", ds_queue_peek(queue, INT_MAX) == NULL);
+
+    ds_queue_pop(queue);
+    ds_queue_pop(queue);
+
+    free(queue);
+  );
 }