diff --git a/include/util/critbit.h b/include/util/critbit.h
new file mode 100644
index 0000000..a0a4a69
--- /dev/null
+++ b/include/util/critbit.h
@@ -0,0 +1,40 @@
+// This file implements the crit-bit tree, which is a variant of trie.
+// See https://github.com/agl/critbit/ for algorithm details.
+
+#ifndef ACC_UTIL_CRITBIT_H
+#define ACC_UTIL_CRITBIT_H
+
+#include <stdint.h>
+#include <stdbool.h>
+
+// crit-bit tree external node
+struct critbit_node {
+	uint8_t type;	// used for indicating this is a external node, this field will always be set to -1
+	char *key;	// string key for this value
+};
+
+// crit-bit tree
+struct critbit_tree {
+	void *rt;	// root node
+};
+
+// Initializes a cirbit tree.
+void critbit_init(struct critbit_tree *self);
+
+// Get a node in tree by its key.
+// Returns NULL if the wanted node does not exists.
+// Does not accept empty string as a key.
+struct critbit_node* critbit_get(struct critbit_tree *self, const char *key);
+
+// Inserts a value into the tree.
+// If the tree already contains a node with the same key, that node
+// will be replaced and returned, otherwise, this function returns NULL.
+// Memory leaks if the returned node was not handled properly. 
+// Does not allow empty string as a key.
+struct critbit_node* critbit_insert(struct critbit_tree *self, struct critbit_node *x);
+
+// Removes and returns the node with given key.
+// If the wanted node was not found, returns NULL.
+struct critbit_node *critbit_erase(struct critbit_tree *self, const char *key);
+
+#endif
\ No newline at end of file
diff --git a/include/util/hashmap.h b/include/util/hashmap.h
deleted file mode 100644
index 45e8e47..0000000
--- a/include/util/hashmap.h
+++ /dev/null
@@ -1,14 +0,0 @@
-/*
-#ifndef ACC_UTIL_HASHMAP
-#define ACC_UTIL_HASHMAP
-
-struct hashmap {
-	int size;
-	int cap;
-	linklist *data;
-};
-
-void hashmap_init();
-
-#endif
-*/
diff --git a/src/util/critbit.c b/src/util/critbit.c
new file mode 100644
index 0000000..ac409f4
--- /dev/null
+++ b/src/util/critbit.c
@@ -0,0 +1,206 @@
+// This code follows Prof. Bernstein’s original code, which is released into the public domain. 
+// It source and a detailed explaination can be found at http://github.com/agl/critbit.
+
+#include "util/critbit.h"
+#include "util/misc.h"
+#include <string.h>
+
+// critbit tree internal node
+struct critbit_intern {
+	uint8_t type;	// used for indicating the is a internal node, will be set to 0
+	void *child[2];	// child nodes
+	size_t cbit;	// critcal bit: the first bit from begining that differs. 
+};
+
+// Initializes a cirbit tree.
+void critbit_init(struct critbit_tree *self) {
+	self->rt = NULL;
+}
+
+// Walk the tree to find a best member matching the expected key.
+// If wherep is not NULL, sets parameter wherep, which is the father pointer to the result.
+// Internal function only: called by critbit_get() and critbit_insert()
+// Does not accept empty string as a key.
+// No guarantee for the returned node is the wanted node, but if not
+// we can conclude the wanted node does not exist.
+// Never returns NULL. Does not handle the corner case of a empty tree, i.e.
+// the caller must ensure self->rt is not NULL.
+static struct critbit_node* critbit_walk(struct critbit_tree *self, const char *key, void ***wherep) {
+	const size_t len = strlen(key);
+
+	if (wherep) {
+		*wherep = NULL;
+	}
+
+	// Note that the pointer will never be NULL as long as self->rt is not NULL.
+	// If an internal node had a NULL pointer then the tree would be 
+	// invalid - that internal node should be removed.
+	struct critbit_intern *p = self->rt;
+	while (p->type == 0) {
+		const uint8_t c = ((p->cbit >> 3) < len) ? key[p->cbit >> 3] : 0;
+		const uint8_t bit_mask = ~(1 << (p->cbit & 7)); // here all the bits but the critical are 1.
+		const int dir = (1 + (c | bit_mask)) >> 8;
+		if (wherep) {
+			*wherep = p->child + dir;
+		}
+		p = p->child[dir];
+	}
+
+	return ((void*)p);
+}
+
+// Get a node in tree by its key.
+// Returns NULL if the wanted node does not exists.
+// Does not accept empty string as a key.
+struct critbit_node* critbit_get(struct critbit_tree *self, const char *key) {
+	if (!key || key[0] == '\0') {
+		return (NULL);
+	}
+
+	if (!self->rt) {
+		return (NULL);
+	}
+
+	struct critbit_node *p = critbit_walk(self, key, NULL);
+	// We can now only conclude that certain bits of node p are shared with wanted key.
+	// We still need to test the best match to make sure that it’s correct. If the test fails,
+	// however, we can conclude that the string is not in the tree.
+	if (strequal(key, p->key)) {
+		return (p);
+	} else {
+		return (NULL);
+	}
+}
+
+// Inserts a value into the tree.
+// If the tree already contains a node with the same key, that node
+// will be replaced and returned, otherwise, this function returns NULL.
+// Memory leaks if the returned node was not handled properly. 
+// Does not allow empty string as a key.
+struct critbit_node* critbit_insert(struct critbit_tree *self, struct critbit_node *x) {
+	// Mark x as a external node.
+	x->type = -1;
+
+	if (!self->rt) {
+		self->rt = x;
+		return (NULL);
+	}
+
+	void **wherep;
+	struct critbit_node *p = critbit_walk(self, x->key, &wherep);
+
+	// Now that we have found the best match for the new element in the tree,
+	// we need to check to see where the new element differs from that element.
+	// We will be calculating the differing byte and the differing bits separately for now
+	// differbit is the XOR of the differing byte. They will analyzed and merged later.
+	char *u = x->key, *v = p->key;
+	const size_t ulen = strlen(u), vlen = strlen(v);
+
+	size_t differbyte;
+	int differbit;
+	for (differbyte = 0; differbyte < ulen && differbyte < vlen; ++differbyte) {
+		if (v[differbyte] != u[differbyte]) {
+			differbit = v[differbyte] ^ u[differbyte];
+			goto DIFFER_FOUND;
+		}
+	}
+
+	if (v[differbyte] != 0) {
+		differbit = v[differbyte];
+		goto DIFFER_FOUND;
+	}
+
+	if (u[differbyte] != 0) {
+		differbit = u[differbyte];
+		goto DIFFER_FOUND;
+	}
+
+	if (wherep) {
+		*wherep = x;
+	} else {
+		self->rt = x;
+	}
+	return (p);
+
+DIFFER_FOUND:
+	size_t new_critbit = differbyte << 3;
+	while ((differbit & 1) == 0) {
+		new_critbit += 1;
+		differbit >>= 1;
+	}
+
+	const uint8_t c = (differbyte < vlen) ? v[differbyte] : 0;
+	const uint8_t bit_mask = ~(1 << (new_critbit & 7)); // here all the bits but the critical are 1.
+	const int new_dir = (1 + (c | bit_mask)) >> 8;
+
+	struct critbit_intern *new_node = try_malloc(sizeof(struct critbit_intern), __FUNCTION__);
+	new_node->type = 0;
+	new_node->cbit = new_critbit;
+	new_node->child[1 - new_dir] = x;
+
+	// Now we will insert new_node into the tree.
+	// Inserting the new node in the tree involves walking the tree from
+	// the root to find the correct position to insert at.
+	// We keep track of the pointer to be updated (to point to the new internal node),
+	// and once the walk has finished, we can update that pointer.
+	void **whereq = &self->rt;
+	while (1) {
+		struct critbit_intern *q = *whereq;
+		if (q->type) {
+			break;
+		}
+
+		if (q->cbit > new_node->cbit) {
+			break;
+		}
+
+		const char c = ((q->cbit >> 3) < ulen) ? u[q->cbit >> 3] : 0;
+		const uint8_t bit_mask = ~(1 << (q->cbit & 7)); // here all the bits but the critical are 1.
+		const int dir = (1 + (c | bit_mask)) >> 8;
+		whereq = q->child + dir;
+	}
+
+	new_node->child[new_dir] = *whereq;
+	*whereq = new_node;
+	return (NULL);
+}
+
+// Removes and returns the node with given key.
+// If the wanted node was not found, returns NULL.
+struct critbit_node *critbit_erase(struct critbit_tree *self, const char *key) {
+	if (!self->rt) {
+		return (NULL);
+	}
+
+	// Walking the tree to find the best match is almost the same as the critbit_walk().
+	// The only exception is that we keep track of the last jump to an internal node in whereq.
+	// Actually, we keep track of a pointer to the last pointer that got us to an internal node.
+	int dir;
+	void **wherep = &self->rt;
+	void **whereq = NULL;
+	struct critbit_intern *p = self->rt, *q;
+	const size_t len = strlen(str);
+
+	while (p->type == 0) {
+		whereq = wherep;
+		q = p;
+		const uint8_t c = ((p->cbit >> 3) < len) ? key[p->cbit >> 3] : 0;
+		const uint8_t bit_mask = ~(1 << (p->cbit & 7)); // here all the bits but the critical are 1.
+		dir = (1 + (c | bit_mask)) >> 8;
+		wherep = q->child + dir;
+		p = q->child[dir];
+	}
+
+	if (!strequal(key, ((struct critbit_node*)p)->key)) {
+		return (NULL);
+	}
+
+	if (!whereq) {
+		self->rt = NULL;
+		return (p);
+	}
+
+	*whereq = q->child[1 - dir];
+	free(q);
+	return (p);
+}