diff --git a/codegen.c b/codegen.c
index d9238456..34505723 100644
--- a/codegen.c
+++ b/codegen.c
@@ -62,7 +62,7 @@ static analyze_state *
 new_analyze_state(pic_state *pic)
 {
   analyze_state *state;
-  xh_iter it;
+  xh_entry *it;
 
   state = pic_alloc(pic, sizeof(analyze_state));
   state->pic = pic;
@@ -98,9 +98,8 @@ new_analyze_state(pic_state *pic)
   /* push initial scope */
   push_scope(state, pic_nil_value());
 
-  xh_begin(&it, &pic->globals);
-  while (xh_next(&it)) {
-    pic_sym sym = xh_key(it.e, pic_sym);
+  for (it = xh_begin(&pic->globals); it != NULL; it = xh_next(it)) {
+    pic_sym sym = xh_key(it, pic_sym);
     xv_push(&state->scope->locals, &sym);
   }
 
diff --git a/dict.c b/dict.c
index 4fc23c7a..229b55ad 100644
--- a/dict.c
+++ b/dict.c
@@ -222,13 +222,12 @@ pic_dict_dictionary_map(pic_state *pic)
   struct pic_proc *proc;
   struct pic_dict *dict;
   pic_value item, list = pic_nil_value();
-  xh_iter it;
+  xh_entry *it;
 
   pic_get_args(pic, "ld", &proc, &dict);
 
-  xh_begin(&it, &dict->hash);
-  while (xh_next(&it)) {
-    item = pic_cons(pic, xh_key(it.e, pic_value), xh_val(it.e, pic_value));
+  for (it = xh_begin(&dict->hash); it != NULL; it = xh_next(it)) {
+    item = pic_cons(pic, xh_key(it, pic_value), xh_val(it, pic_value));
     pic_push(pic, pic_apply1(pic, proc, item), list);
   }
 
@@ -241,15 +240,14 @@ pic_dict_dictionary_for_each(pic_state *pic)
   struct pic_proc *proc;
   struct pic_dict *dict;
   pic_value item;
-  xh_iter it;
+  xh_entry *it;
 
   pic_get_args(pic, "ld", &proc, &dict);
 
-  xh_begin(&it, &dict->hash);
-  while (xh_next(&it)) {
+  for (it = xh_begin(&dict->hash); it != NULL; it = xh_next(it)) {
     int ai = pic_gc_arena_preserve(pic);
 
-    item = pic_cons(pic, xh_key(it.e, pic_value), xh_val(it.e, pic_value));
+    item = pic_cons(pic, xh_key(it, pic_value), xh_val(it, pic_value));
     pic_apply1(pic, proc, item);
 
     pic_gc_arena_restore(pic, ai);
@@ -263,13 +261,12 @@ pic_dict_dictionary_to_alist(pic_state *pic)
 {
   struct pic_dict *dict;
   pic_value item, alist = pic_nil_value();
-  xh_iter it;
+  xh_entry *it;
 
   pic_get_args(pic, "d", &dict);
 
-  xh_begin(&it, &dict->hash);
-  while (xh_next(&it)) {
-    item = pic_cons(pic, xh_key(it.e, pic_value), xh_val(it.e, pic_value));
+  for (it = xh_begin(&dict->hash); it != NULL; it = xh_next(it)) {
+    item = pic_cons(pic, xh_key(it, pic_value), xh_val(it, pic_value));
     pic_push(pic, item, alist);
   }
 
@@ -298,14 +295,13 @@ pic_dict_dictionary_to_plist(pic_state *pic)
 {
   struct pic_dict *dict;
   pic_value plist = pic_nil_value();
-  xh_iter it;
+  xh_entry *it;
 
   pic_get_args(pic, "d", &dict);
 
-  xh_begin(&it, &dict->hash);
-  while (xh_next(&it)) {
-    pic_push(pic, xh_key(it.e, pic_value), plist);
-    pic_push(pic, xh_val(it.e, pic_value), plist);
+  for (it = xh_begin(&dict->hash); it != NULL; it = xh_next(it)) {
+    pic_push(pic, xh_key(it, pic_value), plist);
+    pic_push(pic, xh_val(it, pic_value), plist);
   }
 
   return pic_reverse(pic, plist);
diff --git a/gc.c b/gc.c
index 2d61be7f..ba0e23d7 100644
--- a/gc.c
+++ b/gc.c
@@ -475,32 +475,29 @@ gc_mark_object(pic_state *pic, struct pic_object *obj)
   }
   case PIC_TT_DATA: {
     struct pic_data *data = (struct pic_data *)obj;
-    xh_iter it;
+    xh_entry *it;
 
-    xh_begin(&it, &data->storage);
-    while (xh_next(&it)) {
-      gc_mark(pic, xh_val(it.e, pic_value));
+    for (it = xh_begin(&data->storage); it != NULL; it = xh_next(it)) {
+      gc_mark(pic, xh_val(it, pic_value));
     }
     break;
   }
   case PIC_TT_DICT: {
     struct pic_dict *dict = (struct pic_dict *)obj;
-    xh_iter it;
+    xh_entry *it;
 
-    xh_begin(&it, &dict->hash);
-    while (xh_next(&it)) {
-      gc_mark(pic, xh_key(it.e, pic_value));
-      gc_mark(pic, xh_val(it.e, pic_value));
+    for (it = xh_begin(&dict->hash); it != NULL; it = xh_next(it)) {
+      gc_mark(pic, xh_key(it, pic_value));
+      gc_mark(pic, xh_val(it, pic_value));
     }
     break;
   }
   case PIC_TT_RECORD: {
     struct pic_record *rec = (struct pic_record *)obj;
-    xh_iter it;
+    xh_entry *it;
 
-    xh_begin(&it, &rec->hash);
-    while (xh_next(&it)) {
-      gc_mark(pic, xh_val(it.e, pic_value));
+    for (it = xh_begin(&rec->hash); it != NULL; it = xh_next(it)) {
+      gc_mark(pic, xh_val(it, pic_value));
     }
     break;
   }
@@ -563,7 +560,7 @@ gc_mark_phase(pic_state *pic)
   pic_value *stack;
   pic_callinfo *ci;
   size_t i, j;
-  xh_iter it;
+  xh_entry *it;
 
   /* block */
   if (pic->blk) {
@@ -593,15 +590,13 @@ gc_mark_phase(pic_state *pic)
   }
 
   /* global variables */
-  xh_begin(&it, &pic->globals);
-  while (xh_next(&it)) {
-    gc_mark(pic, xh_val(it.e, pic_value));
+  for (it = xh_begin(&pic->globals); it != NULL; it = xh_next(it)) {
+    gc_mark(pic, xh_val(it, pic_value));
   }
 
   /* macro objects */
-  xh_begin(&it, &pic->macros);
-  while (xh_next(&it)) {
-    gc_mark_object(pic, xh_val(it.e, struct pic_object *));
+ for (it = xh_begin(&pic->macros); it != NULL; it = xh_next(it)) {
+    gc_mark_object(pic, xh_val(it, struct pic_object *));
   }
 
   /* error handlers */
diff --git a/include/picrin/xhash.h b/include/picrin/xhash.h
index a249e2ff..c78dc118 100644
--- a/include/picrin/xhash.h
+++ b/include/picrin/xhash.h
@@ -1,5 +1,5 @@
-#ifndef XHASH_H__
-#define XHASH_H__
+#ifndef XHASH_H
+#define XHASH_H
 
 /*
  * Copyright (c) 2013-2014 by Yuichi Nishiwaki <yuichi.nishiwaki@gmail.com>
@@ -27,6 +27,7 @@ extern "C" {
 typedef struct xh_entry {
   struct xh_entry *next;
   int hash;
+  struct xh_entry *fw, *bw;
   const char *key;              /* == val + XHASH_ALIGN(vwidth) */
   char val[];
 } xh_entry;
@@ -42,15 +43,16 @@ typedef struct xhash {
   size_t size, count, kwidth, vwidth;
   xh_hashf hashf;
   xh_equalf equalf;
+  xh_entry *chain;
   void *data;
 } xhash;
 
-static inline void xh_init_(xhash *x, size_t, size_t, xh_hashf, xh_equalf, void *);
-static inline xh_entry *xh_get_(xhash *x, const void *key);
-static inline xh_entry *xh_put_(xhash *x, const void *key, void *val);
-static inline void xh_del_(xhash *x, const void *key);
-static inline void xh_clear(xhash *x);
-static inline void xh_destroy(xhash *x);
+/** Protected Methods:
+ * static inline void xh_init_(xhash *x, size_t, size_t, xh_hashf, xh_equalf, void *);
+ * static inline xh_entry *xh_get_(xhash *x, const void *key);
+ * static inline xh_entry *xh_put_(xhash *x, const void *key, void *val);
+ * static inline void xh_del_(xhash *x, const void *key);
+ */
 
 /* string map */
 static inline void xh_init_str(xhash *x, size_t width);
@@ -70,14 +72,12 @@ static inline xh_entry *xh_get_int(xhash *x, int key);
 static inline xh_entry *xh_put_int(xhash *x, int key, void *);
 static inline void xh_del_int(xhash *x, int key);
 
-typedef struct xh_iter {
-  xhash *x;
-  xh_entry *e, *next;
-  size_t bidx;
-} xh_iter;
+static inline size_t xh_size(xhash *x);
+static inline void xh_clear(xhash *x);
+static inline void xh_destroy(xhash *x);
 
-static inline void xh_begin(xh_iter *it, xhash *x);
-static inline int xh_next(xh_iter *it);
+static inline xh_entry *xh_begin(xhash *x);
+static inline xh_entry *xh_next(xh_entry *e);
 
 
 static inline void
@@ -98,6 +98,7 @@ xh_init_(xhash *x, size_t kwidth, size_t vwidth, xh_hashf hashf, xh_equalf equal
   x->vwidth = vwidth;
   x->hashf = hashf;
   x->equalf = equalf;
+  x->chain = NULL;
   x->data = data;
 
   xh_bucket_realloc(x, XHASH_INIT_SIZE);
@@ -123,21 +124,22 @@ static inline void
 xh_resize_(xhash *x, size_t newsize)
 {
   xhash y;
-  xh_iter it;
+  xh_entry *it;
   size_t idx;
 
   xh_init_(&y, x->kwidth, x->vwidth, x->hashf, x->equalf, x->data);
   xh_bucket_realloc(&y, newsize);
 
-  xh_begin(&it, x);
-  while (xh_next(&it)) {
-    idx = ((unsigned)it.e->hash) % y.size;
+  for (it = xh_begin(x); it != NULL; it = xh_next(it)) {
+    idx = ((unsigned)it->hash) % y.size;
     /* reuse entry object */
-    it.e->next = y.buckets[idx];
-    y.buckets[idx] = it.e;
+    it->next = y.buckets[idx];
+    y.buckets[idx] = it;
     y.count++;
   }
 
+  y.chain = x->chain;
+
   free(x->buckets);
 
   /* copy all members from y to x */
@@ -169,6 +171,16 @@ xh_put_(xhash *x, const void *key, void *val)
   memcpy((void *)e->key, key, x->kwidth);
   memcpy(e->val, val, x->vwidth);
 
+  if (x->chain == NULL) {
+    x->chain = e;
+    e->fw = e->bw = NULL;
+  } else {
+    x->chain->fw = e;
+    e->bw = x->chain;
+    e->fw = NULL;
+    x->chain = e;
+  }
+
   x->count++;
 
   return x->buckets[idx] = e;
@@ -179,28 +191,54 @@ xh_del_(xhash *x, const void *key)
 {
   int hash;
   size_t idx;
-  xh_entry *e, *d;
+  xh_entry *p, *q, *r;
 
   hash = x->hashf(key, x->data);
   idx = ((unsigned)hash) % x->size;
   if (x->buckets[idx]->hash == hash && x->equalf(key, x->buckets[idx]->key, x->data)) {
-    e = x->buckets[idx]->next;
-    free(x->buckets[idx]);
-    x->buckets[idx] = e;
+    q = x->buckets[idx];
+    if (q->fw) {
+      q->fw->bw = q->bw;
+    }
+    if (q->bw) {
+      q->bw->fw = q->fw;
+    }
+    if (x->chain == q) {
+      x->chain = q->bw;
+    }
+    r = q->next;
+    free(q);
+    x->buckets[idx] = r;
   }
   else {
-    for (e = x->buckets[idx]; ; e = e->next) {
-      if (e->next->hash == hash && x->equalf(key, e->next->key, x->data))
+    for (p = x->buckets[idx]; ; p = p->next) {
+      if (p->next->hash == hash && x->equalf(key, p->next->key, x->data))
         break;
     }
-    d = e->next->next;
-    free(e->next);
-    e->next = d;
+    q = p->next;
+    if (q->fw) {
+      q->fw->bw = q->bw;
+    }
+    if (q->bw) {
+      q->bw->fw = q->fw;
+    }
+    if (x->chain == q) {
+      x->chain = q->bw;
+    }
+    r = q->next;
+    free(q);
+    p->next = r;
   }
 
   x->count--;
 }
 
+static inline size_t
+xh_size(xhash *x)
+{
+  return x->count;
+}
+
 static inline void
 xh_clear(xhash *x)
 {
@@ -217,6 +255,7 @@ xh_clear(xhash *x)
     x->buckets[i] = NULL;
   }
 
+  x->chain = NULL;
   x->count = 0;
 }
 
@@ -361,43 +400,16 @@ xh_del_int(xhash *x, int key)
 
 /** iteration */
 
-static inline void
-xh_begin(xh_iter *it, xhash *x)
+static inline xh_entry *
+xh_begin(xhash *x)
 {
-  size_t bidx;
-
-  it->x = x;
-
-  for (bidx = 0; bidx < x->size; ++bidx) {
-    if (x->buckets[bidx])
-      break;
-  }
-  it->e = NULL;
-  it->next = x->buckets[bidx];
-  it->bidx = bidx;
+  return x->chain;
 }
 
-static inline int
-xh_next(xh_iter *it)
+static inline xh_entry *
+xh_next(xh_entry *e)
 {
-  size_t bidx;
-
-  if (! it->next) {
-    return 0;
-  }
-
-  it->e = it->next;
-  if (it->next->next) {
-    it->next = it->next->next;
-    return 1;
-  }
-  for (bidx = it->bidx + 1; bidx < it->x->size; ++bidx) {
-    if (it->x->buckets[bidx])
-      break;
-  }
-  it->next = it->x->buckets[bidx];
-  it->bidx = bidx;
-  return 1;
+  return e->bw;
 }
 
 #if defined(__cplusplus)
diff --git a/lib.c b/lib.c
index f9ccacdd..b716d41e 100644
--- a/lib.c
+++ b/lib.c
@@ -71,7 +71,7 @@ import_table(pic_state *pic, pic_value spec, xhash *imports)
   xhash table;
   pic_value val;
   pic_sym sym, id, tag;
-  xh_iter it;
+  xh_entry *it;
 
   xh_init_int(&table, sizeof(pic_sym));
 
@@ -97,11 +97,10 @@ import_table(pic_state *pic, pic_value spec, xhash *imports)
     }
     if (tag == pic->sPREFIX) {
       import_table(pic, pic_cadr(pic, spec), &table);
-      xh_begin(&it, &table);
-      while (xh_next(&it)) {
+      for (it = xh_begin(&table); it != NULL; it = xh_next(it)) {
         val = pic_list_ref(pic, spec, 2);
-        sym = pic_intern_str(pic, pic_format(pic, "~s~s", val, pic_sym_value(xh_key(it.e, pic_sym))));
-        xh_put_int(imports, sym, &xh_val(it.e, pic_sym));
+        sym = pic_intern_str(pic, pic_format(pic, "~s~s", val, pic_sym_value(xh_key(it, pic_sym))));
+        xh_put_int(imports, sym, &xh_val(it, pic_sym));
       }
       goto exit;
     }
@@ -117,9 +116,8 @@ import_table(pic_state *pic, pic_value spec, xhash *imports)
   if (! lib) {
     pic_errorf(pic, "library not found: ~a", spec);
   }
-  xh_begin(&it, &lib->exports);
-  while (xh_next(&it)) {
-    xh_put_int(imports, xh_key(it.e, pic_sym), &xh_val(it.e, pic_sym));
+  for (it = xh_begin(&lib->exports); it != NULL; it = xh_next(it)) {
+    xh_put_int(imports, xh_key(it, pic_sym), &xh_val(it, pic_sym));
   }
 
  exit:
@@ -130,20 +128,19 @@ static void
 import(pic_state *pic, pic_value spec)
 {
   xhash imports;
-  xh_iter it;
+  xh_entry *it;
 
   xh_init_int(&imports, sizeof(pic_sym)); /* pic_sym to pic_sym */
 
   import_table(pic, spec, &imports);
 
-  xh_begin(&it, &imports);
-  while (xh_next(&it)) {
+  for (it = xh_begin(&imports); it != NULL; it = xh_next(it)) {
 
 #if DEBUG
-    printf("* importing %s as %s\n", pic_symbol_name(pic, xh_key(it.e, pic_sym)), pic_symbol_name(pic, xh_val(it.e, pic_sym)));
+    printf("* importing %s as %s\n", pic_symbol_name(pic, xh_key(it, pic_sym)), pic_symbol_name(pic, xh_val(it, pic_sym)));
 #endif
 
-    pic_put_rename(pic, pic->lib->env, xh_key(it.e, pic_sym), xh_val(it.e, pic_sym));
+    pic_put_rename(pic, pic->lib->env, xh_key(it, pic_sym), xh_val(it, pic_sym));
   }
 
   xh_destroy(&imports);
diff --git a/state.c b/state.c
index b93559f4..e7300e9c 100644
--- a/state.c
+++ b/state.c
@@ -177,7 +177,7 @@ pic_open(int argc, char *argv[], char **envp)
 void
 pic_close(pic_state *pic)
 {
-  xh_iter it;
+  xh_entry *it;
 
   /* invoke exit handlers */
   while (pic->blk) {
@@ -221,9 +221,8 @@ pic_close(pic_state *pic)
   free(pic->arena);
 
   /* free symbol names */
-  xh_begin(&it, &pic->sym_names);
-  while (xh_next(&it)) {
-    free(xh_val(it.e, char *));
+  for (it = xh_begin(&pic->sym_names); it != NULL; it = xh_next(it)) {
+    free(xh_val(it, char *));
   }
   xh_destroy(&pic->sym_names);
 
diff --git a/write.c b/write.c
index 685a5108..752964a1 100644
--- a/write.c
+++ b/write.c
@@ -209,8 +209,7 @@ write_core(struct writer_control *p, pic_value obj)
   pic_state *pic = p->pic;
   xFILE *file = p->file;
   size_t i;
-  xh_entry *e;
-  xh_iter it;
+  xh_entry *e, *it;
   int c;
   float f;
 
@@ -334,12 +333,11 @@ write_core(struct writer_control *p, pic_value obj)
     break;
   case PIC_TT_DICT:
     xfprintf(file, "#.(dictionary");
-    xh_begin(&it, &pic_dict_ptr(obj)->hash);
-    while (xh_next(&it)) {
+    for (it = xh_begin(&pic_dict_ptr(obj)->hash); it != NULL; it = xh_next(it)) {
       xfprintf(file, " '");
-      write_core(p, xh_key(it.e, pic_value));
+      write_core(p, xh_key(it, pic_value));
       xfprintf(file, " '");
-      write_core(p, xh_val(it.e, pic_value));
+      write_core(p, xh_val(it, pic_value));
     }
     xfprintf(file, ")");
     break;