new implementation of hash tables.

ltable.c

@@ -1,10 +1,23 @@
 /*
-** $Id: ltable.c,v 1.24 1999/09/22 14:38:45 roberto Exp roberto $
+** $Id: ltable.c,v 1.25 1999/10/04 17:51:04 roberto Exp roberto $
 ** Lua tables (hash)
 ** See Copyright Notice in lua.h
 */
 
 
+/*
+** Implementation of tables (aka arrays, objects, or hash tables);
+** uses a mix of chained scatter table with Brent's variation.
+** A main invariant of these tables is that, if an element is not
+** in its main position (i.e. the `original' position that its hash gives
+** to it), then the colliding element is in its own main position.
+** In other words, there are collisions only when two elements have the
+** same main position (i.e. the same hash values for that table size).
+** Because of that, the load factor of these tables can be 100% without
+** performance penalties.
+*/
+
+
 #include "lauxlib.h"
 #include "lmem.h"
 #include "lobject.h"
@@ -15,93 +28,104 @@
 
 #define gcsize(n)	(1+(n/16))
 
-#define nuse(t)		((t)->nuse)
-#define nodevector(t)	((t)->node)
 
 
 #define TagDefault LUA_T_ARRAY;
 
 
 
-static long int hashindex (const TObject *ref) {
-  long int h;
-  switch (ttype(ref)) {
+/*
+** returns the `main' position of an element in a table (that is, the index
+** of its hash value)
+*/
+static Node *luaH_mainposition (const Hash *t, const TObject *key) {
+  unsigned long h;
+  switch (ttype(key)) {
     case LUA_T_NUMBER:
-      h = (long int)nvalue(ref);
+      h = (unsigned long)(long)nvalue(key);
       break;
     case LUA_T_STRING: case LUA_T_USERDATA:
-      h = (IntPoint)tsvalue(ref);
+      h = tsvalue(key)->hash;
       break;
     case LUA_T_ARRAY:
-      h = (IntPoint)avalue(ref);
+      h = (IntPoint)avalue(key);
       break;
     case LUA_T_PROTO:
-      h = (IntPoint)tfvalue(ref);
+      h = (IntPoint)tfvalue(key);
       break;
     case LUA_T_CPROTO:
-      h = (IntPoint)fvalue(ref);
+      h = (IntPoint)fvalue(key);
       break;
     case LUA_T_CLOSURE:
-      h = (IntPoint)clvalue(ref);
+      h = (IntPoint)clvalue(key);
       break;
     default:
       lua_error("unexpected type to index table");
       h = 0;  /* to avoid warnings */
   }
-  return (h >= 0 ? h : -(h+1));
+  return &t->node[h%t->size];
 }
 
 
-Node *luaH_present (const Hash *t, const TObject *key) {
-  const int tsize = nhash(t);
-  const long int h = hashindex(key);
-  int h1 = h%tsize;
-  Node *n = node(t, h1);
-  /* keep looking until an entry with "ref" equal to key or nil */
-  while ((ttype(ref(n)) == ttype(key)) ? !luaO_equalval(key, ref(n))
-                                       : ttype(ref(n)) != LUA_T_NIL) {
-    h1 += (h&(tsize-2)) + 1;  /* double hashing */
-    if (h1 >= tsize) h1 -= tsize;
-    n = node(t, h1);
-  }
-  return n;
+const TObject *luaH_get (const Hash *t, const TObject *key) {
+  Node *n = luaH_mainposition(t, key);
+  do {
+    if (luaO_equalObj(key, &n->key))
+      return &n->val;
+    n = n->next;
+  } while (n);
+  return &luaO_nilobject;
 }
 
 
+int luaH_pos (const Hash *t, const TObject *key) {
+  const TObject *v = luaH_get(t, key);
+  return (v == &luaO_nilobject) ?  -1 :  /* key not found */
+             ((const char *)v - (const char *)(&t->node[0].val))/sizeof(Node);
+}
+
+
+
 static Node *hashnodecreate (int nhash) {
-  Node *const v = luaM_newvector(nhash, Node);
+  Node *v = luaM_newvector(nhash, Node);
   int i;
-  for (i=0; i<nhash; i++)
-    ttype(ref(&v[i])) = ttype(val(&v[i])) = LUA_T_NIL;
+  for (i=0; i<nhash; i++) {
+    ttype(key(&v[i])) = ttype(val(&v[i])) = LUA_T_NIL;
+    v[i].next = NULL;
+  }
   return v;
 }
 
 
-Hash *luaH_new (int nhash) {
-  Hash *const t = luaM_new(Hash);
-  nhash = luaO_redimension(nhash*3/2);
-  nodevector(t) = hashnodecreate(nhash);
-  nhash(t) = nhash;
-  nuse(t) = 0;
+static void setnodevector (Hash *t, int size) {
+  t->node = hashnodecreate(size);
+  t->size = size;
+  t->firstfree = &t->node[size-1];  /* first free position to be used */
+  L->nblocks += gcsize(size);
+}
+
+
+Hash *luaH_new (int size) {
+  Hash *t = luaM_new(Hash);
+  setnodevector(t, luaO_redimension(size+1));
   t->htag = TagDefault;
   t->next = L->roottable;
   L->roottable = t;
   t->marked = 0;
-  L->nblocks += gcsize(nhash);
   return t;
 }
 
 
 void luaH_free (Hash *t) {
-  L->nblocks -= gcsize(t->nhash);
-  luaM_free(nodevector(t));
+  L->nblocks -= gcsize(t->size);
+  luaM_free(t->node);
   luaM_free(t);
 }
 
 
-static int newsize (Hash *t) {
-  Node *const v = t->node;
-  const int size = nhash(t);
+static int newsize (const Hash *t) {
+  Node *v = t->node;
+  int size = t->size;
   int realuse = 0;
   int i;
   for (i=0; i<size; i++) {
@@ -112,57 +136,158 @@ static int newsize (Hash *t) {
 }
 
 
+#ifdef DEBUG
+/* check invariant of a table */
+static int listfind (const Node *m, const Node *n) {
+  do {
+    if (m==n) return 1;
+    m = m->next;
+  } while (m);
+  return 0;
+}
+
+static int check_invariant (const Hash *t, int filled) {
+  Node *n;
+  for (n=t->node; n<t->firstfree; n++) {
+    TObject *key = &n->key;
+    LUA_ASSERT(ttype(key) == LUA_T_NIL || n == luaH_mainposition(t, key),
+      "all elements before firstfree are empty or in their main positions");
+  }
+  if (!filled)
+    LUA_ASSERT(ttype(&(n++)->key) == LUA_T_NIL, "firstfree must be empty");
+  else
+    LUA_ASSERT(n == t->node, "table cannot have empty places");
+  for (; n<t->node+t->size; n++) {
+    TObject *key = &n->key;
+    Node *mp = luaH_mainposition(t, key);
+    LUA_ASSERT(ttype(key) != LUA_T_NIL,
+      "cannot exist empty elements after firstfree");
+    LUA_ASSERT(n == mp || luaH_mainposition(t, &mp->key) == mp,
+      "either an element or its colliding element is in its main position");
+    LUA_ASSERT(listfind(mp,n), "element is in its main position list");
+  }
+  return 1;
+}
+#endif
+
+
+/*
+** the rehash is done in two stages: first, we insert only the elements whose
+** main position is free, to avoid needless collisions. In the second stage,
+** we insert the other elements.
+*/
 static void rehash (Hash *t) {
-  const int nold = nhash(t);
-  Node *const vold = nodevector(t);
-  const int nnew = newsize(t);
+  int oldsize = t->size;
+  Node *nold = t->node;
   int i;
-  nodevector(t) = hashnodecreate(nnew);
-  nhash(t) = nnew;
-  nuse(t) = 0;
-  for (i=0; i<nold; i++) {
-    Node *n = vold+i;
-    if (ttype(val(n)) != LUA_T_NIL) {
-      *luaH_present(t, ref(n)) = *n;  /* copy old node to new hash */
-      nuse(t)++;
+  LUA_ASSERT(check_invariant(t, 1), "invalid table");
+  L->nblocks -= gcsize(oldsize);
+  setnodevector(t, newsize(t));  /* create new array of nodes */
+  /* first loop; set only elements that can go in their main positions */
+  for (i=0; i<oldsize; i++) {
+    Node *old = nold+i;
+    if (ttype(&old->val) == LUA_T_NIL)
+      old->next = NULL;  /* `remove' it for next loop */
+    else {
+      Node *mp = luaH_mainposition(t, &old->key);  /* new main position */
+      if (ttype(&mp->key) == LUA_T_NIL) {  /* is it empty? */
+        mp->key = old->key;  /* put element there */
+        mp->val = old->val;
+        old->next = NULL;  /* `remove' it for next loop */
+      }
+      else /* it will be copied in next loop */
+        old->next = mp;  /* to be used in next loop */
     }
   }
-  L->nblocks += gcsize(nnew)-gcsize(nold);
-  luaM_free(vold);
-}
-
-
-void luaH_set (Hash *t, const TObject *ref, const TObject *val) {
-  Node *const n = luaH_present(t, ref);
-  *val(n) = *val;
-  if (ttype(ref(n)) == LUA_T_NIL) {  /* new node? */
-    *ref(n) = *ref;  /* set key */
-    nuse(t)++;  /* count it */
-    if ((long)nuse(t)*3L > (long)nhash(t)*2L)  /* check size */
-      rehash(t);
+  /* update `firstfree' */
+  while (ttype(&t->firstfree->key) != LUA_T_NIL) t->firstfree--;
+  /* second loop; update elements with colision */
+  for (i=0; i<oldsize; i++) {
+    Node *old = nold+i;
+    if (old->next) {  /* wasn't already `removed'? */
+      Node *mp = old->next;  /* main position */
+      Node *e = t->firstfree;  /* actual position */
+      e->key = old->key;  /* put element in the free position */
+      e->val = old->val;
+      e->next = mp->next;  /* chain actual position in main position's list */
+      mp->next = e;
+      do {  /* update `firstfree' */
+        t->firstfree--;
+      } while (ttype(&t->firstfree->key) != LUA_T_NIL);
+    }
   }
+  LUA_ASSERT(check_invariant(t, 0), "invalid table");
+  luaM_free(nold);  /* free old array */
 }
 
 
-int luaH_pos (const Hash *t, const TObject *r) {
-  Node *const n = luaH_present(t, r);
-  luaL_arg_check(ttype(val(n)) != LUA_T_NIL, 2, "key not found");
-  return n-(t->node);
+/*
+** sets a pair key-value in a hash table; first, check whether key is
+** already present; if not, check whether key's main position is free;
+** if not, check whether colliding node is in its main position or not;
+** if it is not, move colliding node to an empty place and put new pair
+** in its main position; otherwise (colliding node is in its main position),
+** new pair goes to an empty position.
+** Tricky point: the only place where an old element is moved is when
+** we move the colliding node to an empty place; nevertheless, its old
+** value is still in that position until we set the value for the new
+** pair; therefore, even when `val' points to an element of this table
+** (this happens when we use `luaH_move'), there is no problem.
+*/
+void luaH_set (Hash *t, const TObject *key, const TObject *val) {
+  Node *mp = luaH_mainposition(t, key);
+  Node *n = mp;
+  do {  /* check whether `key' is somewhere in the chain */
+    if (luaO_equalObj(key, &n->key)) {
+      n->val = *val;  /* update value */
+      return;  /* that's all */
+    }
+    else n = n->next;
+  } while (n);
+  /* `key' not found; must insert it */
+  if (ttype(&mp->key) != LUA_T_NIL) {  /* main position is not free? */
+    Node *othern;  /* main position of colliding node */
+    n = t->firstfree;  /* get a free place */
+    /* is colliding node out of its main position? (can only happens if
+       its position if after "firstfree") */
+    if (mp > n && (othern=luaH_mainposition(t, &mp->key)) != mp) {
+      /* yes; move colliding node into free position */
+      while (othern->next != mp) othern = othern->next;  /* find previous */
+      othern->next = n;  /* redo the chain with `n' in place of `mp' */
+      *n = *mp;  /* copy colliding node into free pos. (mp->next also goes) */
+      mp->next = NULL;  /* now `mp' is free */
+    }
+    else {  /* colliding node is in its own main position */
+      /* new node will go into free position */
+      n->next = mp->next;  /* chain new position */
+      mp->next = n;
+      mp = n;
+    }
+  }
+  mp->key = *key;
+  mp->val = *val;
+  for (;;) {  /* check free places */
+    if (ttype(&(t->firstfree)->key) == LUA_T_NIL)
+      return;  /* OK; table still has a free place */
+    else if (t->firstfree == t->node) break;  /* cannot decrement from here */
+    else (t->firstfree)--;
+  }
+  rehash(t);  /* no more free places */
 }
 
 
-void luaH_setint (Hash *t, int ref, const TObject *val) {
+void luaH_setint (Hash *t, int key, const TObject *val) {
   TObject index;
   ttype(&index) = LUA_T_NUMBER;
-  nvalue(&index) = ref;
+  nvalue(&index) = key;
   luaH_set(t, &index, val);
 }
 
 
-TObject *luaH_getint (const Hash *t, int ref) {
+const TObject *luaH_getint (const Hash *t, int key) {
   TObject index;
   ttype(&index) = LUA_T_NUMBER;
-  nvalue(&index) = ref;
+  nvalue(&index) = key;
   return luaH_get(t, &index);
 }