New rule for size of array part

Array part needs 1/3 of its elements filled, instead of 1/2.
Array entries use ~1/3 the memory of hash entries, so this new rule
still ensures that array parts do not use more memory than keeping
the values in the hash, while allowing more uses of the array part,
which is more efficient than the hash.

ltable.c

@@ -471,12 +471,23 @@ typedef struct {
   unsigned nums[MAXABITS + 1];
 } Counters;
 
+
+/*
+** Check whether it is worth to use 'na' array entries instead of 'nh'
+** hash nodes. (A hash node uses ~3 times more memory than an array
+** entry: Two values plus 'next' versus one value.) Evaluate with size_t
+** to avoid overflows.
+*/
+#define arrayXhash(na,nh)	(cast_sizet(na) <= cast_sizet(nh) * 3)
+
 /*
 ** Compute the optimal size for the array part of table 't'.
+** This size maximizes the number of elements going to the array part
+** while satisfying the condition 'arrayXhash' with the use of memory if
+** all those elements went to the hash part.
 ** 'ct->na' enters with the total number of array indices in the table
 ** and leaves with the number of keys that will go to the array part;
-** return the optimal size.  (The condition 'twotoi > 0' in the for loop
-** stops the loop if 'twotoi' overflows.)
+** return the optimal size for the array part.
 */
 static unsigned computesizes (Counters *ct) {
   int i;
@@ -484,17 +495,19 @@ static unsigned computesizes (Counters *ct) {
   unsigned int a = 0;  /* number of elements smaller than 2^i */
   unsigned int na = 0;  /* number of elements to go to array part */
   unsigned int optimal = 0;  /* optimal size for array part */
-  /* loop while keys can fill more than half of total size */
+  /* traverse slices while 'twotoi' does not overflow and total of array
+     indices still can satisfy 'arrayXhash' against the array size */
   for (i = 0, twotoi = 1;
-       twotoi > 0 && ct->na > twotoi / 2;
+       twotoi > 0 && arrayXhash(twotoi, ct->na);
        i++, twotoi *= 2) {
-    a += ct->nums[i];
-    if (a > twotoi/2) {  /* more than half elements present? */
+    unsigned nums = ct->nums[i];
+    a += nums;
+    if (nums > 0 &&  /* grows array only if it gets more elements... */
+        arrayXhash(twotoi, a)) {  /* ...while using "less memory" */
       optimal = twotoi;  /* optimal size (till now) */
       na = a;  /* all elements up to 'optimal' will go to array part */
     }
   }
-  lua_assert((optimal == 0 || optimal / 2 < na) && na <= optimal);
   ct->na = na;
   return optimal;
 }

ltests.c

@@ -1043,7 +1043,10 @@ static int table_query (lua_State *L) {
   }
   else if (cast_uint(i) < asize) {
     lua_pushinteger(L, i);
-    arr2obj(t, cast_uint(i), s2v(L->top.p));
+    if (!tagisempty(*getArrTag(t, i)))
+      arr2obj(t, cast_uint(i), s2v(L->top.p));
+    else
+      setnilvalue(s2v(L->top.p));
     api_incr_top(L);
     lua_pushnil(L);
   }
@@ -1057,11 +1060,11 @@ static int table_query (lua_State *L) {
     }
     else
       lua_pushliteral(L, "<undef>");
-    pushobject(L, gval(gnode(t, i)));
-    if (gnext(&t->node[i]) != 0)
-      lua_pushinteger(L, gnext(&t->node[i]));
+    if (!isempty(gval(gnode(t, i))))
+      pushobject(L, gval(gnode(t, i)));
     else
       lua_pushnil(L);
+    lua_pushinteger(L, gnext(&t->node[i]));
   }
   return 3;
 }

testes/nextvar.lua

@@ -9,6 +9,22 @@ local function checkerror (msg, f, ...)
 end
 
 
+
+----------------------------------------------------------------
+local function printTable (t)
+  local a, h = T.querytab(t)
+  print("array:")
+  for i = 1, a do
+    print("", T.querytab(t, i - 1))
+  end
+  print("hash:")
+  for i = 1, h do
+    print("", T.querytab(t, a + i - 1))
+  end
+end
+----------------------------------------------------------------
+
+
 local function check (t, na, nh)
   if not T then return end
   local a, h = T.querytab(t)
@@ -106,9 +122,10 @@ else --[
 -- testing table sizes
 
 
-local function mp2 (n)   -- minimum power of 2 >= n
+-- minimum power of 2 (or zero) >= n
+local function mp2 (n)
   local mp = 2^math.ceil(math.log(n, 2))
-  assert(n == 0 or (mp/2 < n and n <= mp))
+  assert((mp == 0 or mp/2 < n) and n <= mp)
   return mp
 end
 
@@ -123,7 +140,7 @@ end
 
 -- testing constructor sizes
 local sizes = {0, 1, 2, 3, 4, 5, 7, 8, 9, 15, 16, 17,
-  30, 31, 32, 33, 34, 254, 255, 256, 500, 1000}
+  30, 31, 32, 33, 34, 254, 255, 256, 257, 500, 1001}
 
 for _, sa in ipairs(sizes) do    -- 'sa' is size of the array part
   local arr = {"return {"}
@@ -167,8 +184,9 @@ end
 
 -- testing tables dynamically built
 local lim = 130
-local a = {}; a[2] = 1; check(a, 0, 1)
-a = {}; a[0] = 1; check(a, 0, 1); a[2] = 1; check(a, 0, 2)
+local a = {}; a[2] = 1; check(a, 2, 0)
+a = {}; a[0] = 1; check(a, 0, 1);
+a[2] = 1; check(a, 2, 1)
 a = {}; a[0] = 1; a[1] = 1; check(a, 1, 1)
 a = {}
 for i = 1,lim do
@@ -184,28 +202,46 @@ for i = 1,lim do
   check(a, 0, mp2(i))
 end
 
-a = {}
-for i=1,16 do a[i] = i end
-check(a, 16, 0)
 do
+  local a = {}
+  for i=1,16 do a[i] = i end
+  check(a, 16, 0)
   for i=1,11 do a[i] = undef end
-  for i=30,50 do a[i] = true; a[i] = undef end   -- force a rehash (?)
-  check(a, 0, 8)   -- 5 elements in the table
+  check(a, 16, 0)
+  a[30] = true    -- force a rehash
+  a[30] = undef
+  check(a, 0, 8)   -- 5 elements in the hash part: [12]-[16]
   a[10] = 1
-  for i=30,50 do a[i] = true; a[i] = undef end   -- force a rehash (?)
-  check(a, 0, 8)   -- only 6 elements in the table
+  for i=30,50 do a[i] = true; a[i] = undef end   -- force a rehash
+  check(a, 16, 1)
   for i=1,14 do a[i] = true; a[i] = undef end
-  for i=18,50 do a[i] = true; a[i] = undef end   -- force a rehash (?)
-  check(a, 0, 4)   -- only 2 elements ([15] and [16])
+  check(a, 16, 1)   -- no rehash...
+  a[31] = true; a[32] = true   -- force a rehash
+  check(a, 0, 4)   -- [15], [16], [31], [32]
 end
 
 -- reverse filling
-for i=1,lim do
+do
+  local N = 2^10
   local a = {}
-  for i=i,1,-1 do a[i] = i end   -- fill in reverse
-  check(a, mp2(i), 0)
+  for i = N, 1, -1 do a[i] = i end   -- fill in reverse
+  check(a, mp2(N), 0)
 end
 
+
+do     -- "almost sparse" arrays
+  -- create table with holes in 1/3 of its entries; all its
+  -- elements are always in the array part
+  local a = {}
+  for i = 1, 257 do
+    if i % 3 ~= 1 then
+      a[i] = true
+      check(a, mp2(i), 0)
+    end
+  end
+end
+
+
 -- size tests for vararg
 lim = 35
 local function foo (n, ...)
@@ -840,7 +876,7 @@ do
   co()     -- start coroutine
   co(1)    -- continue after yield
   assert(res[1] == 30 and res[2] == 20 and res[3] == 10 and #res == 3)
-  
+
 end
 
 print"OK"