erts: Introduce erts_internal:maps_to_list/2

3 files changed, 71 insertions, 8 deletions

diff --git a/erts/emulator/beam/bif.tab b/erts/emulator/beam/bif.tab
index a6510ace81..ce2cffa498 100644
--- a/erts/emulator/beam/bif.tab
+++ b/erts/emulator/beam/bif.tab
@@ -671,3 +671,4 @@ bif math:floor/1
 bif math:ceil/1
 bif math:fmod/2
 bif os:set_signal/2
+bif erts_internal:maps_to_list/2
diff --git a/erts/emulator/beam/erl_map.c b/erts/emulator/beam/erl_map.c
index 9062e44b10..a80f5d6a16 100644
--- a/erts/emulator/beam/erl_map.c
+++ b/erts/emulator/beam/erl_map.c
@@ -91,7 +91,7 @@ static BIF_RETTYPE hashmap_merge(Process *p, Eterm nodeA, Eterm nodeB, int swap_
 static Export hashmap_merge_trap_export;
 static BIF_RETTYPE maps_merge_trap_1(BIF_ALIST_1);
 static Uint hashmap_subtree_size(Eterm node);
-static Eterm hashmap_to_list(Process *p, Eterm map);
+static Eterm hashmap_to_list(Process *p, Eterm map, Sint n);
 static Eterm hashmap_keys(Process *p, Eterm map);
 static Eterm hashmap_values(Process *p, Eterm map);
 static Eterm hashmap_delete(Process *p, Uint32 hx, Eterm key, Eterm node, Eterm *value);
@@ -161,13 +161,58 @@ BIF_RETTYPE maps_to_list_1(BIF_ALIST_1) {
 
 	BIF_RET(res);
     } else if (is_hashmap(BIF_ARG_1)) {
-	return hashmap_to_list(BIF_P, BIF_ARG_1);
+	return hashmap_to_list(BIF_P, BIF_ARG_1, -1);
     }
 
     BIF_P->fvalue = BIF_ARG_1;
     BIF_ERROR(BIF_P, BADMAP);
 }
 
+/* erts_internal:maps_to_list/2
+ *
+ * This function should be removed once iterators are in place.
+ * Never document it.
+ * Never encourage its usage.
+ *
+ * A negative value in ARG 2 means the entire map.
+ */
+
+BIF_RETTYPE erts_internal_maps_to_list_2(BIF_ALIST_2) {
+    Sint m;
+    if (term_to_Sint(BIF_ARG_2, &m)) {
+        if (is_flatmap(BIF_ARG_1)) {
+            Uint n;
+            Eterm* hp;
+            Eterm *ks,*vs, res, tup;
+            flatmap_t *mp = (flatmap_t*)flatmap_val(BIF_ARG_1);
+
+            ks  = flatmap_get_keys(mp);
+            vs  = flatmap_get_values(mp);
+            n   = flatmap_get_size(mp);
+
+            if (m >= 0) {
+                n = m < n ? m : n;
+            }
+
+            hp  = HAlloc(BIF_P, (2 + 3) * n);
+            res = NIL;
+
+            while(n--) {
+                tup = TUPLE2(hp, ks[n], vs[n]); hp += 3;
+                res = CONS(hp, tup, res); hp += 2;
+            }
+
+            BIF_RET(res);
+        } else if (is_hashmap(BIF_ARG_1)) {
+            return hashmap_to_list(BIF_P, BIF_ARG_1, m);
+        }
+        BIF_P->fvalue = BIF_ARG_1;
+        BIF_ERROR(BIF_P, BADMAP);
+    }
+    BIF_ERROR(BIF_P, BADARG);
+}
+
+
 /* maps:find/2
  * return value if key *matches* a key in the map
  */
@@ -1917,15 +1962,22 @@ BIF_RETTYPE maps_values_1(BIF_ALIST_1) {
     BIF_ERROR(BIF_P, BADMAP);
 }
 
-static Eterm hashmap_to_list(Process *p, Eterm node) {
+static Eterm hashmap_to_list(Process *p, Eterm node, Sint m) {
     DECLARE_WSTACK(stack);
     Eterm *hp, *kv;
-    Eterm res = NIL;
+    Eterm tup, res = NIL;
+    Uint n = hashmap_size(node);
 
-    hp  = HAlloc(p, hashmap_size(node) * (2 + 3));
+    if (m >= 0) {
+        n = m < n ? m : n;
+    }
+
+    hp  = HAlloc(p, n * (2 + 3));
     hashmap_iterator_init(&stack, node, 0);
-    while ((kv=hashmap_iterator_next(&stack)) != NULL) {
-	Eterm tup = TUPLE2(hp, CAR(kv), CDR(kv));
+    while (n--) {
+        kv  = hashmap_iterator_next(&stack);
+        ASSERT(kv != NULL);
+	tup = TUPLE2(hp, CAR(kv), CDR(kv));
 	hp += 3;
 	res = CONS(hp, tup, res);
 	hp += 2;
diff --git a/erts/preloaded/src/erts_internal.erl b/erts/preloaded/src/erts_internal.erl
index a27cb2ba38..bcc779e6f6 100644
--- a/erts/preloaded/src/erts_internal.erl
+++ b/erts/preloaded/src/erts_internal.erl
@@ -31,7 +31,8 @@
 
 -export([await_port_send_result/3]).
 -export([cmp_term/2]).
--export([map_to_tuple_keys/1, term_type/1, map_hashmap_children/1]).
+-export([map_to_tuple_keys/1, term_type/1, map_hashmap_children/1,
+         maps_to_list/2]).
 -export([open_port/2, port_command/3, port_connect/2, port_close/1,
 	 port_control/3, port_call/3, port_info/1, port_info/2]).
 
@@ -369,6 +370,15 @@ map_hashmap_children(_M) ->
       Multi :: boolean(),
       Res :: term().
 
+%% return a list of key value pairs, at most of length N
+-spec maps_to_list(M,N) -> Pairs when
+    M :: map(),
+    N :: integer(),
+    Pairs :: list().
+
+maps_to_list(_M, _N) ->
+    erlang:nif_error(undefined).
+
 %% erlang:demonitor(Ref, [flush]) traps to
 %% erts_internal:flush_monitor_messages(Ref, Res) when
 %% it needs to flush monitor messages.