feat(lsm): iterator over items in trie

lsm/include/lsm.h

@@ -27,6 +27,7 @@ typedef enum lsm_error {
   lsm_error_null_value = 4,
   lsm_error_failed_io = 5,
   lsm_error_lock_busy = 6,
+  lsm_error_done = 7,
 } lsm_error;
 
 /*typedef struct lsm_string { */

lsm/include/lsm/bt.h

@@ -102,6 +102,6 @@ void lsm_bt_iter(lsm_bt_iterator *out, const lsm_bt *bt);
  * @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);
+bool lsm_bt_iter_next(void **out, char *key_out, lsm_bt_iterator *iter);
 
 #endif

lsm/include/lsm/trie.h

@@ -1,7 +1,10 @@
 #ifndef LSM_TRIE
 #define LSM_TRIE
 
+#include <stdlib.h>
+
 #include "lsm.h"
+#include "lsm/bt.h"
 #include "lsm/str.h"
 
 /**
@@ -9,6 +12,11 @@
  */
 typedef struct lsm_trie lsm_trie;
 
+/**
+ * A node inside an `lsm_trie` trie
+ */
+typedef struct lsm_trie_node lsm_trie_node;
+
 /**
  * Initialize a new trie.
  *
@@ -57,4 +65,36 @@ lsm_error lsm_trie_remove(void **out, lsm_trie *trie, const lsm_str *key);
  */
 uint64_t lsm_trie_size(const lsm_trie *trie);
 
+/**
+ * Represents an in-flight iterator over a trie
+ */
+typedef struct lsm_trie_iterator lsm_trie_iterator;
+
+/**
+ * Initialize an iterator to iterate over all entries with keys starting
+ * with the given prefix.
+ *
+ * @param out pointer to store iterator pointer in
+ * @param trie trie to iterate
+ * @param prefix prefix of the keys; a zero-length string means iterating over
+ *  the entire trie; NULL is interpreted as a zero-length string
+ */
+lsm_error lsm_trie_iter(lsm_trie_iterator **out, const lsm_trie *trie,
+                        const lsm_str *prefix);
+
+/**
+ * Advance the given iterator.
+ *
+ * @param out pointer to store next data pointer in
+ * @param iter iterator to advance
+ * @return `lsm_error_ok` if next element has been returned, `lsm_error_done` if
+ *  no more elements are present, or some other error code
+ */
+lsm_error lsm_trie_iter_next(void **out, lsm_trie_iterator *iter);
+
+/**
+ * Free the given iterator.
+ */
+void lsm_trie_iter_free(lsm_trie_iterator *iter);
+
 #endif