feat(lsm): binary tree iterators

lsm/include/lsm/bt.h

@@ -1,6 +1,8 @@
 #ifndef LSM_BT
 #define LSM_BT
 
+#include <stdbool.h>
+
 #include "lsm.h"
 
 /**
@@ -8,6 +10,11 @@
  */
 typedef struct lsm_bt lsm_bt;
 
+/**
+ * A node inside an `lsm_bt` binary tree.
+ */
+typedef struct lsm_bt_node lsm_bt_node;
+
 /**
  * Initialize a new binary tree
  *
@@ -68,4 +75,33 @@ lsm_error lsm_bt_remove(void **out, lsm_bt *bt, char key);
  */
 lsm_error lsm_bt_replace(void **out, lsm_bt *bt, char key, void *data);
 
+/**
+ * Struct representing an in-flight iterator over the binary tree
+ */
+typedef struct lsm_bt_iterator {
+  lsm_bt_node *next;
+} lsm_bt_iterator;
+
+/**
+ * Initialize the given iterator for the binary tree.
+ *
+ * The iterator is explicitely allowed to be allocated by the user, as these are
+ * commonly used inside functions where they can simply be stored on the stack.
+ *
+ * @param out iterator to initialize
+ * @param bt binary tree to iterate
+ */
+void lsm_bt_iter(lsm_bt_iterator *out, const lsm_bt *bt);
+
+/**
+ * Advance the iterator to the next element.
+ *
+ * @param out where to store pointer to data; ignored if NULL
+ * @param key_out where to store key; ignored if NULL
+ * @param iter iterator to advance
+ * @return true if a new entry was returned, false if the iterator has no more
+ * entries to return
+ */
+bool lsm_bt_iter_next(const void **out, char *key_out, lsm_bt_iterator *iter);
+
 #endif