diff --git a/CommandLines.h b/CommandLines.h
index b453e6d..d6ff1b5 100644
--- a/CommandLines.h
+++ b/CommandLines.h
@@ -3,7 +3,7 @@
 
 #include <pthread.h>
 
-#define HA_VERSION "0.7-dirty-r254"
+#define HA_VERSION "0.7-dirty-r255"
 
 #define VERBOSE 0
 
diff --git a/extract.cpp b/extract.cpp
index d139910..b739a30 100644
--- a/extract.cpp
+++ b/extract.cpp
@@ -5,7 +5,8 @@
 #include "kseq.h"
 
 typedef const char *cstr_t;
-KHASHL_CMAP_INIT(KH_LOCAL, strmap_t, ss, cstr_t, int, kh_hash_str, kh_eq_str)
+KHASHL_CSET_INIT(KH_LOCAL, strset_t, ss, cstr_t, kh_hash_str, kh_eq_str)
+KHASHL_MAP_INIT(KH_LOCAL, hm64_t, h64, uint64_t, int, kh_hash_uint64, kh_eq_generic)
 KSTREAM_INIT(gzFile, gzread, 65536)
 
 #define GFA_MALLOC(ptr, len) ((ptr) = (__typeof__(ptr))malloc((len) * sizeof(*(ptr))))
@@ -76,25 +77,36 @@ char **gv_read_list(const char *o, int *n_)
 
 void ha_extract_print(const All_reads *rs, int n_rounds, int n, char **list)
 {
-	strmap_t *h;
+	hm64_t *h;
 	khint_t k;
-	int i, absent, m, l, max_len = 0;
+	int i, absent, m, l;
 	uint64_t j;
-	char *s = 0;
 	const ma_hit_t_alloc *ov[2] = { rs->paf, rs->reverse_paf };
 	FILE *fp = stdout;
 
-	for (j = 0; j < rs->total_reads; ++j) {
-		if (max_len < (int)Get_NAME_LENGTH(*rs, j))
-			max_len = Get_NAME_LENGTH(*rs, j);
-	}
-	GFA_MALLOC(s, max_len + 1);
-
-	h = ss_init();
-	for (i = 0; i < n; ++i) {
-		k = ss_put(h, gfa_strdup(list[i]), &absent);
-		kh_val(h, k) = -1;
-	}
+	if (n > 0) {
+		int max_len = 0;
+		char *s = 0;
+		strset_t *ss;
+		ss = ss_init();
+		for (i = 0; i < n; ++i)
+			ss_put(ss, list[i], &absent);
+		for (j = 0; j < rs->total_reads; ++j)
+			if (max_len < (int)Get_NAME_LENGTH(*rs, j))
+				max_len = Get_NAME_LENGTH(*rs, j);
+		GFA_MALLOC(s, max_len + 1);
+		h = h64_init();
+		for (j = 0; j < rs->total_reads; ++j) {
+			strncpy(s, Get_NAME(*rs, j), Get_NAME_LENGTH(*rs, j));
+			s[Get_NAME_LENGTH(*rs, j)] = 0;
+			if (ss_get(ss, s) != kh_end(ss)) {
+				k = h64_put(h, j, &absent);
+				kh_val(h, k) = -1;
+			}
+		}
+		free(s);
+		ss_destroy(ss);
+	} else return;
 
 	for (m = 0; m < n_rounds; ++m) {
 		for (j = 0; j < rs->total_reads; ++j) {
@@ -104,24 +116,18 @@ void ha_extract_print(const All_reads *rs, int n_rounds, int n, char **list)
 					uint64_t q = Get_qn(o->buffer[i]);
 					uint64_t t = Get_tn(o->buffer[i]);
 					int q_hit = 0, t_hit = 0;
-					strncpy(s, Get_NAME(*rs, q), Get_NAME_LENGTH(*rs, q)); s[Get_NAME_LENGTH(*rs, q)] = 0;
-					k = ss_get(h, s);
+					k = h64_get(h, q);
 					q_hit = (k < kh_end(h) && kh_val(h, k) < m);
-					strncpy(s, Get_NAME(*rs, t), Get_NAME_LENGTH(*rs, t)); s[Get_NAME_LENGTH(*rs, t)] = 0;
-					k = ss_get(h, s);
+					k = h64_get(h, t);
 					t_hit = (k < kh_end(h) && kh_val(h, k) < m);
 					if ((!q_hit && !t_hit) || (q_hit && t_hit)) continue;
 					if (!q_hit) {
-						char *tmp = gfa_strndup(Get_NAME(*rs, q), Get_NAME_LENGTH(*rs, q));
-						k = ss_put(h, tmp, &absent);
+						k = h64_put(h, q, &absent);
 						if (absent) kh_val(h, k) = m;
-						else free(tmp);
 					}
 					if (!t_hit) {
-						char *tmp = gfa_strndup(Get_NAME(*rs, t), Get_NAME_LENGTH(*rs, t));
-						k = ss_put(h, tmp, &absent);
+						k = h64_put(h, t, &absent);
 						if (absent) kh_val(h, k) = m;
-						else free(tmp);
 					}
 				}
 			}
@@ -135,12 +141,8 @@ void ha_extract_print(const All_reads *rs, int n_rounds, int n, char **list)
 				uint64_t q = Get_qn(o->buffer[i]);
 				uint64_t t = Get_tn(o->buffer[i]);
 				int q_hit = 0, t_hit = 0;
-				strncpy(s, Get_NAME(*rs, q), Get_NAME_LENGTH(*rs, q)); s[Get_NAME_LENGTH(*rs, q)] = 0;
-				k = ss_get(h, s);
-				q_hit = (k < kh_end(h));
-				strncpy(s, Get_NAME(*rs, t), Get_NAME_LENGTH(*rs, t)); s[Get_NAME_LENGTH(*rs, t)] = 0;
-				k = ss_get(h, s);
-				t_hit = (k < kh_end(h));
+				q_hit = (h64_get(h, q) < kh_end(h));
+				t_hit = (h64_get(h, t) < kh_end(h));
 				if (!q_hit && !t_hit) continue;
 				fwrite(Get_NAME(*rs, q), 1, Get_NAME_LENGTH(*rs, q), fp);
 				fwrite("\t", 1, 1, fp);
@@ -157,11 +159,7 @@ void ha_extract_print(const All_reads *rs, int n_rounds, int n, char **list)
 		}
 	}
 
-	for (k = 0; k != kh_end(h); ++k)
-		if (kh_exist(h, k))
-			free((char*)kh_key(h, k));
-	ss_destroy(h);
-	free(s);
+	h64_destroy(h);
 }
 
 void ha_extract_print_list(const All_reads *rs, int n_rounds, const char *o)