erts: Implement batching maps:iterator

This iterator implementation fetches multiple elements to
iterate over in one call to erts_internal:maps_next instead
of one at a time. This means that the memory usage will go
up for the iterator as we are buffering elements, but the
usage is still bounded.

In this implementation the max memory usage is 1000 words.

Using this approach makes the iterator as fast as using
maps:to_list, so maps:iterator/2 has been removed.

3 files changed, 36 insertions, 116 deletions

diff --git a/lib/stdlib/doc/src/maps.xml b/lib/stdlib/doc/src/maps.xml
index a3afbf8e7f..987d92989d 100644
--- a/lib/stdlib/doc/src/maps.xml
+++ b/lib/stdlib/doc/src/maps.xml
@@ -38,8 +38,7 @@
       <name name="iterator"/>
       <desc>
         <p>An iterator representing the key value associations in a map.</p>
-        <p>Created using <seealso marker="#iterator-1"><c>maps:iterator/1</c></seealso>
-        and <seealso marker="#iterator-2"><c>maps:iterator/2</c></seealso>.</p>
+        <p>Created using <seealso marker="#iterator-1"><c>maps:iterator/1</c></seealso>.</p>
         <p>Consumed by <seealso marker="#next-1"><c>maps:next/1</c></seealso>,
           <seealso marker="#filter-2"><c>maps:filter/2</c></seealso>,
           <seealso marker="#fold-3"><c>maps:fold/3</c></seealso> and
@@ -194,9 +193,9 @@ false</code>
       <desc>
         <p>Returns a map iterator <c><anno>Iterator</anno></c> that can
           be used by <seealso marker="#next-1"><c>maps:next/1</c></seealso>
-          to traverse the keys and values in a map. This call is equivalent
-          to calling <seealso marker="#iterator-2"><c>maps:iterator/2</c></seealso>
-          with an empty map as the options.</p>
+          to traverse the key-value associations in a map. When iterating
+          over a map, the memory usage is guaranteed to be bounded no matter
+          the size of the map.</p>
         <p>The call fails with a <c>{badmap,Map}</c> exception if
           <c><anno>Map</anno></c> is not a map.</p>
         <p><em>Example:</em></p>
@@ -215,49 +214,6 @@ none</code>
     </func>
 
     <func>
-      <name name="iterator" arity="2"/>
-      <fsummary>Create a map iterator.</fsummary>
-      <desc>
-        <p>Returns a map iterator <c><anno>Iterator</anno></c> that can
-          be used by <seealso marker="#next-1"><c>maps:next/1</c></seealso>
-          to traverse the keys and values in a map. The iterator will behave
-          according to the given <c><anno>Opts</anno></c>.</p>
-        <taglist>
-          <tag>optimize</tag>
-          <item>
-            Configures whether the iterator should be optimized for <c>speed</c>
-            or for <c>memory</c> consumption. Default is <c>speed</c>.
-          </item>
-          <tag>ordered</tag>
-          <item>
-            Configures whether the iterator should return the key value
-            associations ordered on the keys according to erlang term order
-            or not. The default is <c>false</c>.
-          </item>
-        </taglist>
-        <p>Both options can be configured at the same time.</p>
-        <p>The call fails with a <c>{badmap,Map}</c> exception if
-          <c><anno>Map</anno></c> is not a map.</p>
-        <p><em>Example:</em></p>
-        <code type="none">
-> M = maps:from_list([{I,I} || I &lt;- lists:seq(1,50)]).
-#{45 => 45,6 => 6,2 => 2,49 => 49,41 => 41,33 => 33,42 => 42,
-  43 => 43,10 => 10,9 => 9,19 => 19,14 => 14,5 => 5,18 => 18,
-  31 => 31,22 => 22,29 => 29,21 => 21,27 => 27,24 => 24,
-  47 => 47,40 => 40,30 => 30,23 => 23,28 => 28,46 => 46,
-  16 => 16,38 => 38,4 => 4,...}
-> MemoryIter = maps:iterator(M, #{ optimize => memory }), ok.
-ok
-> {K1, V1, _} = maps:next(MemoryIter), {K1, V1}.
-{46,46}
-> OrderedIter = maps:iterator(M, #{ ordered => true, optimize => memory }), ok.
-ok
-> {K2, V2, _} = maps:next(OrderedIter), {K2, V2}.
-{1,1}</code>
-      </desc>
-    </func>
-
-    <func>
       <name name="keys" arity="1"/>
       <fsummary></fsummary>
       <desc>
diff --git a/lib/stdlib/src/maps.erl b/lib/stdlib/src/maps.erl
index 2f2dd41979..a13f340709 100644
--- a/lib/stdlib/src/maps.erl
+++ b/lib/stdlib/src/maps.erl
@@ -24,7 +24,7 @@
          map/2, size/1,
          update_with/3, update_with/4,
          without/2, with/2,
-         iterator/1, iterator/2, next/1]).
+         iterator/1, next/1]).
 
 %% BIFs
 -export([get/2, find/2, from_list/1,
@@ -32,12 +32,14 @@
          new/0, put/3, remove/2, take/2,
          to_list/1, update/3, values/1]).
 
--opaque iterator() :: {non_neg_integer() | none | list(term()), map()}
-                    | list({term(), term()}).
+-opaque iterator() :: {term(), term(), iterator()}
+                      | none | nonempty_improper_list(integer(),map()).
 
 -export_type([iterator/0]).
 
--define(IS_ITERATOR(I), (is_list(I) orelse (is_map(element(2,I) andalso (is_list(element(1,I) orelse is_integer(element(1,I)))))))).
+-dialyzer({no_improper_lists, iterator/1}).
+
+-define(IS_ITERATOR(I), is_tuple(I) andalso tuple_size(I) == 3; I == none; is_integer(hd(I)) andalso is_map(tl(I))).
 
 %% Shadowed by erl_bif_types: maps:get/2
 -spec get(Key,Map) -> Value when
@@ -121,14 +123,20 @@ remove(_,_) -> erlang:nif_error(undef).
 
 take(_,_) -> erlang:nif_error(undef).
 
-%% Shadowed by erl_bif_types: maps:to_list/1
 -spec to_list(Map) -> [{Key,Value}] when
     Map :: map(),
     Key :: term(),
     Value :: term().
 
-to_list(_) -> erlang:nif_error(undef).
+to_list(Map) when is_map(Map) ->
+    to_list_internal(erts_internal:map_next(0, Map, []));
+to_list(Map) ->
+    erlang:error({badmap,Map},[Map]).
 
+to_list_internal([Iter, Map | Acc]) when is_integer(Iter) ->
+    to_list_internal(erts_internal:map_next(Iter, Map, Acc));
+to_list_internal(Acc) ->
+    Acc.
 
 %% Shadowed by erl_bif_types: maps:update/3
 -spec update(Key,Value,Map1) -> Map2 when
@@ -207,7 +215,7 @@ get(Key,Map,Default) ->
       Map :: map().
 
 filter(Pred,Map) when is_function(Pred,2), is_map(Map) ->
-    maps:from_list([{K,V}||{K,V}<-maps:to_list(Map),Pred(K,V)]);
+    maps:from_list(filter_1(Pred, iterator(Map)));
 filter(Pred,Iterator) when is_function(Pred,2), ?IS_ITERATOR(Iterator) ->
     maps:from_list(filter_1(Pred, Iterator));
 filter(Pred,Map) ->
@@ -237,7 +245,7 @@ filter_1(Pred, Iter) ->
     V :: term().
 
 fold(Fun,Init,Map) when is_function(Fun,3), is_map(Map) ->
-    lists:foldl(fun({K,V},A) -> Fun(K,V,A) end,Init,maps:to_list(Map));
+    fold_1(Fun,Init,iterator(Map));
 fold(Fun,Init,Iterator) when is_function(Fun,3), ?IS_ITERATOR(Iterator) ->
     fold_1(Fun,Init,Iterator);
 fold(Fun,Init,Map) ->
@@ -260,7 +268,7 @@ fold_1(Fun, Acc, Iter) ->
     V2 :: term().
 
 map(Fun,Map) when is_function(Fun, 2), is_map(Map) ->
-    maps:from_list([{K,Fun(K,V)}||{K,V}<-maps:to_list(Map)]);
+    maps:from_list(map_1(Fun, iterator(Map)));
 map(Fun,Iterator) when is_function(Fun, 2), ?IS_ITERATOR(Iterator) ->
     maps:from_list(map_1(Fun, Iterator));
 map(Fun,Map) ->
@@ -286,42 +294,19 @@ size(Val) ->
       Map :: map(),
       Iterator :: iterator().
 
-iterator(M) when is_map(M) -> iterator(M, #{});
+iterator(M) when is_map(M) -> [0 | M];
 iterator(M) -> erlang:error({badmap, M}, [M]).
 
--spec iterator(Map, Opts) -> Iterator when
-      Map :: map(),
-      Opts :: #{ ordered => boolean(), optimize => speed | memory },
-      Iterator :: iterator().
-iterator(M, Opts) when is_map(M), is_map(Opts) ->
-    case maps:merge(default_iterator_opts(), Opts) of
-        #{ ordered := true, optimize := speed} -> lists:sort(maps:to_list(M));
-        #{ ordered := false, optimize := speed} -> maps:to_list(M);
-        #{ ordered := true, optimize := memory} -> {lists:sort(maps:keys(M)), M};
-        #{ ordered := false, optimize := memory} -> {0, M}
-    end;
-iterator(M, Opts) when is_map(M) ->
-    erlang:error({badmap, M}, [M, Opts]);
-iterator(M, Opts) ->
-    erlang:error(error_type(M), [M, Opts]).
-
-default_iterator_opts() ->
-    #{ ordered => false, optimize => speed }.
-
 -spec next(Iterator) -> {Key, Value, NextIterator} | 'none' when
       Iterator :: iterator(),
       Key :: term(),
       Value :: term(),
       NextIterator :: iterator().
-next([{K, V} | T]) ->
-    {K, V, T};
-next([]) ->
-    none;
-next({I, M}) when is_map(M), is_integer(I) orelse is_atom(I) ->
-    erts_internal:map_next(I, M);
-next({[K | T], M}) when is_map(M) ->
-    {K, maps:get(K, M), {T, M}};
-next({[], M}) when is_map(M) ->
+next({K, V, I}) ->
+    {K, V, I};
+next([Path | Map]) when is_integer(Path), is_map(Map) ->
+    erts_internal:map_next(Path, Map, iterator);
+next(none) ->
     none;
 next(Iter) ->
     erlang:error(badarg, [Iter]).
diff --git a/lib/stdlib/test/maps_SUITE.erl b/lib/stdlib/test/maps_SUITE.erl
index 0021dd0657..a75751b31d 100644
--- a/lib/stdlib/test/maps_SUITE.erl
+++ b/lib/stdlib/test/maps_SUITE.erl
@@ -30,7 +30,7 @@
 -export([t_update_with_3/1, t_update_with_4/1,
          t_get_3/1, t_filter_2/1,
          t_fold_3/1,t_map_2/1,t_size_1/1,
-         t_iterator_1/1, t_iterator_2/1,
+         t_iterator_1/1,
          t_with_2/1,t_without_2/1]).
 
 %%-define(badmap(V,F,Args), {'EXIT', {{badmap,V}, [{maps,F,Args,_}|_]}}).
@@ -48,7 +48,7 @@ all() ->
     [t_update_with_3,t_update_with_4,
      t_get_3,t_filter_2,
      t_fold_3,t_map_2,t_size_1,
-     t_iterator_1, t_iterator_2,
+     t_iterator_1,
      t_with_2,t_without_2].
 
 t_update_with_3(Config) when is_list(Config) ->
@@ -179,40 +179,19 @@ t_iterator_1(Config) when is_list(Config) ->
 
     %% Large map test
 
-    Vs = lists:seq(1,200),
-    M2 = maps:from_list([{{k,I},I}||I<-Vs]),
+    Vs2 = lists:seq(1,200),
+    M2 = maps:from_list([{{k,I},I}||I<-Vs2]),
     KVList2 = lists:sort(iter_kv(maps:iterator(M2))),
     KVList2 = lists:sort(maps:to_list(M2)),
-    ok.
 
-t_iterator_2(Config) when is_list(Config) ->
-
-    Vs = lists:seq(1,200),
-    Maps = [#{ a => 1, b => 2 }, maps:from_list([{{k,I},I}||I<-Vs])],
-    Optimize = [speed, memory],
-    Ordered = [true, false],
-
-    [test_iterator(Map, Opt, Ord) || Map <- Maps,
-                                     Opt <- Optimize,
-                                     Ord <- Ordered],
+    %% Larger map test
 
+    Vs3 = lists:seq(1,10000),
+    M3 = maps:from_list([{{k,I},I}||I<-Vs3]),
+    KVList3 = lists:sort(iter_kv(maps:iterator(M3))),
+    KVList3 = lists:sort(maps:to_list(M3)),
     ok.
 
-test_iterator(Map, Opt, Ord) ->
-    Opts = #{ optimize => Opt, ordered => Ord },
-    test_iterator_1(maps:iterator(Map, Opts), Map, Opts).
-
-test_iterator_1(Iter, OrigMap, Opts) ->
-    erlang:display(Opts),
-    KVs = iter_kv(Iter),
-    case Opts of
-        #{ ordered := true } ->
-            KVs = lists:sort(maps:to_list(OrigMap));
-        #{ ordered := false } ->
-            OrderedKVs = lists:sort(KVs),
-            OrderedKVs = lists:sort(maps:to_list(OrigMap))
-    end.
-
 iter_kv(I) ->
     case maps:next(I) of
         none ->