Fix issue 2504 - AA.reserve

src/object.d

@@ -1931,6 +1931,7 @@ extern (C)
     inout(void)[] _aaKeys(inout void* p, in size_t keysize, const TypeInfo tiKeyArray) pure nothrow;
     void* _aaRehash(void** pp, in TypeInfo keyti) pure nothrow;
     void _aaClear(void* p) pure nothrow;
+    void _aaReserve(void** p, const TypeInfo_AssociativeArray ti, size_t ndim) pure nothrow;
 
     // alias _dg_t = extern(D) int delegate(void*);
     // int _aaApply(void* aa, size_t keysize, _dg_t dg);
@@ -1974,6 +1975,16 @@ void clear(T : Value[Key], Value, Key)(T* aa)
     _aaClear(*cast(void **) aa);
 }
 
+void reserve(T : Value[Key], Value, Key)(ref T aa, size_t ndim)
+{
+    _aaReserve(cast(void **)&aa, typeid(Value[Key]), ndim);
+}
+
+void reserve(T : Value[Key], Value, Key)(T* aa, size_t ndim)
+{
+    _aaReserve(cast(void **)aa, typeid(Value[Key]), ndim);
+}
+
 T rehash(T : Value[Key], Value, Key)(T aa)
 {
     _aaRehash(cast(void**)&aa, typeid(Value[Key]));

src/rt/aaA.d

@@ -474,6 +474,16 @@ extern (C) void _aaClear(AA aa) pure nothrow
     }
 }
 
+/// Reserve AA
+extern (C) void _aaReserve(AA* aa, const TypeInfo_AssociativeArray ti, size_t ndim)
+{
+    // lazily alloc implementation
+    if (aa.impl is null)
+        aa.impl = new Impl(ti, nextpow2(ndim));
+    else
+        aa.resize(nextpow2(ndim));
+}
+
 /// Rehash AA
 extern (C) void* _aaRehash(AA* paa, in TypeInfo keyti) pure nothrow
 {
@@ -1013,3 +1023,26 @@ unittest
     assert(typeid(a).getHash(&a) == typeid(a).getHash(&a));
     assert(typeid(a).getHash(&a) == typeid(a).getHash(&a2));
 }
+
+// test AA.reserve
+unittest
+{
+    int[int] aa;
+    assert(aa is null);
+
+    aa.reserve(2000);
+    assert(aa !is null);
+
+    foreach(i;0..2000) aa[i] = i;
+    assert(aa[1337] == 1337);
+
+    // stress test
+    aa.clear();
+    aa.reserve(20_000_000);
+
+    foreach(i;0..32_000) aa[i] = i;
+    assert(aa[31_133] == 31_133);
+
+    foreach(i;0..20_000_000) aa[i] = i;
+    assert(aa[19_999_133] == 19_999_133);
+}