Implement a trie, sans removal.

include/trie.h

@@ -0,0 +1,63 @@
+#ifndef LIBABACUS_TRIE_H
+#define LIBABACUS_TRIE_H
+
+#include "ll.h"
+#include "result.h"
+
+/**
+ * A node in the trie.
+ */
+struct libab_trie_node_s {
+    /**
+     * The "child" node, which points
+     * to the next layer of the trie, housing
+     * children that have a prefix ending in this node's key value.
+     */
+    struct libab_trie_node_s* child;
+    /**
+     * The "next" node, which points
+     * to the sibiling of this tree, wich has the same prefix
+     * as this node.
+     */
+    struct libab_trie_node_s* next;
+    /**
+     * The values associated with the key ending with
+     * this node's key value.
+     */
+    ll values;
+    /**
+     * The last letter of the key, which this node represents.
+     * The remainder of the key is encoded in nodes preceding this one.
+     */
+    char key;
+};
+
+/**
+ * A struct that represents a trie.
+ */
+struct libab_trie_s {
+    /**
+     * The first search node in this trie.
+     */
+    struct libab_trie_node_s* head;   
+    /**
+     * The empty list returned if no value is found.
+     * Note that existing nodes return their own linked
+     * list of values, even if empty. However, for keys
+     * that don't exist as prefixes in the trie, 
+     * this list is returned to maintain consistency:
+     * a list is always returned containing the values
+     * of the trie associated with the given key.
+     */
+    ll empty_list;
+};
+
+typedef struct libab_trie_s libab_trie;
+typedef struct libab_trie_node_s libab_trie_node;
+
+void libab_trie_init(libab_trie* trie);
+libab_result libab_trie_put(libab_trie* trie, const char* key, void* value);
+const ll* libab_trie_get(libab_trie* trie, const char* key);
+void libab_trie_free(libab_trie* trie);
+
+#endif