[stdlib] Support reversed for Dict

docs/changelog.md

@@ -18,6 +18,8 @@ what we publish.
 
 ### ⭐️ New
 
+- `Dict()` now support `reversed` for `dict.items()` and `dict.values()`. ([PR #2340](https://github.com/modularml/mojo/pull/2340) by [@jayzhan211](https://github.com/jayzhan211))
+
 - `int()` can now take a string and a specified base to parse an integer from a
   string: `int("ff", 16)` returns `255`. Additionally, if a base of zero is
   specified, the string will be parsed as if it was an integer literal, with the

stdlib/src/builtin/reversed.mojo

@@ -19,7 +19,7 @@ from .range import _StridedRangeIterator
 
 from collections.list import _ListIter
 
-from collections.dict import _DictKeyIter
+from collections.dict import _DictKeyIter, _DictValueIter, _DictEntryIter
 
 # ===----------------------------------------------------------------------=== #
 #  Reversible
@@ -119,6 +119,79 @@ fn reversed[
         value: The dict to get the reversed iterator of.
 
     Returns:
-        The reversed iterator of the dict.
+        The reversed iterator of the dict keys.
     """
     return value[].__reversed__()
+
+
+fn reversed[
+    mutability: __mlir_type.`i1`,
+    self_life: AnyLifetime[mutability].type,
+    K: KeyElement,
+    V: CollectionElement,
+    dict_mutability: __mlir_type.`i1`,
+    dict_lifetime: AnyLifetime[dict_mutability].type,
+](
+    value: Reference[
+        _DictValueIter[K, V, dict_mutability, dict_lifetime],
+        mutability,
+        self_life,
+    ]._mlir_type,
+) -> _DictValueIter[K, V, dict_mutability, dict_lifetime, False]:
+    """Get a reversed iterator of the input dict values.
+
+    **Note**: iterators are currently non-raising.
+
+    Parameters:
+        mutability: Whether the reference to the dict is mutable.
+        self_life: The lifetime of the dict.
+        K: The type of the keys in the dict.
+        V: The type of the values in the dict.
+        dict_mutability: Whether the reference to the dict values is mutable.
+        dict_lifetime: The lifetime of the dict values.
+
+    Args:
+        value: The dict values to get the reversed iterator of.
+
+    Returns:
+        The reversed iterator of the dict values.
+    """
+    return Reference(value)[].__reversed__[mutability, self_life]()
+
+
+fn reversed[
+    mutability: __mlir_type.`i1`,
+    self_life: AnyLifetime[mutability].type,
+    K: KeyElement,
+    V: CollectionElement,
+    dict_mutability: __mlir_type.`i1`,
+    dict_lifetime: AnyLifetime[dict_mutability].type,
+](
+    value: Reference[
+        _DictEntryIter[K, V, dict_mutability, dict_lifetime],
+        mutability,
+        self_life,
+    ]._mlir_type,
+) -> _DictEntryIter[K, V, dict_mutability, dict_lifetime, False]:
+    """Get a reversed iterator of the input dict items.
+
+    **Note**: iterators are currently non-raising.
+
+    Parameters:
+        mutability: Whether the reference to the dict is mutable.
+        self_life: The lifetime of the dict.
+        K: The type of the keys in the dict.
+        V: The type of the values in the dict.
+        dict_mutability: Whether the reference to the dict items is mutable.
+        dict_lifetime: The lifetime of the dict items.
+
+    Args:
+        value: The dict items to get the reversed iterator of.
+
+    Returns:
+        The reversed iterator of the dict items.
+    """
+    var src = Reference(value)[].src
+    return _DictEntryIter[K, V, dict_mutability, dict_lifetime, False](
+        src[]._reserved, 0, src
+    )

stdlib/src/collections/dict.mojo

@@ -163,6 +163,7 @@ struct _DictValueIter[
     V: CollectionElement,
     dict_mutability: __mlir_type.`i1`,
     dict_lifetime: AnyLifetime[dict_mutability].type,
+    forward: Bool = True,
 ]:
     """Iterator over Dict value references. These are mutable if the dict
     is mutable.
@@ -172,15 +173,26 @@ struct _DictValueIter[
         V: The value type of the elements in the dictionary.
         dict_mutability: Whether the reference to the vector is mutable.
         dict_lifetime: The lifetime of the List
+        forward: The iteration direction. `False` is backwards.
     """
 
     alias ref_type = Reference[V, dict_mutability, dict_lifetime]
 
-    var iter: _DictEntryIter[K, V, dict_mutability, dict_lifetime]
+    var iter: _DictEntryIter[K, V, dict_mutability, dict_lifetime, forward]
 
     fn __iter__(self) -> Self:
         return self
 
+    fn __reversed__[
+        mutability: __mlir_type.`i1`, self_life: AnyLifetime[mutability].type
+    ](self) -> _DictValueIter[K, V, dict_mutability, dict_lifetime, False]:
+        var src = self.iter.src
+        return _DictValueIter(
+            _DictEntryIter[K, V, dict_mutability, dict_lifetime, False](
+                src[]._reserved, 0, src
+            )
+        )
+
     fn __next__(inout self) -> Self.ref_type:
         var entry_ref = self.iter.__next__()
         # Cast through a pointer to grant additional mutability because

stdlib/test/builtin/test_reversed.mojo

@@ -30,19 +30,35 @@ def test_reversed_dict():
     dict["b"] = 2
     dict["c"] = 3
     dict["d"] = 4
-    dict["a"] = 5
+    dict["a"] = 1
 
     var keys = String("")
     for key in reversed(dict):
         keys += key[]
 
     assert_equal(keys, "dcba")
 
+    var check: Int = 4
+    for val in reversed(dict.values()):
+        assert_equal(val[], check)
+        check -= 1
+
+    keys = String("")
+    check = 4
+    for item in reversed(dict.items()):
+        keys += item[].key
+        assert_equal(item[].value, check)
+        check -= 1
+
+    assert_equal(keys, "dcba")
+
     # Order preserved
 
     _ = dict.pop("a")
     _ = dict.pop("c")
 
+    # dict: {'b': 2, 'd': 4}
+
     keys = String("")
     for key in dict:
         keys += key[]
@@ -55,29 +71,63 @@ def test_reversed_dict():
 
     assert_equal(keys, "db")
 
+    # got 4 and 2
+    check = 4
+    for val in reversed(dict.values()):
+        assert_equal(val[], check)
+        check -= 2
+
+    keys = String("")
+    check = 4
+    for item in reversed(dict.items()):
+        keys += item[].key
+        assert_equal(item[].value, check)
+        check -= 2
+
+    assert_equal(keys, "db")
+
+    # Refill dict
+    dict["c"] = 2
+    dict["a"] = 1
+    dict["b"] = 4
+    dict["d"] = 3
+
+    # dict: {'b': 4, 'd': 3, 'c': 2, 'a': 1}
+
+    keys = String("")
+    check = 1
+    for item in reversed(dict.items()):
+        keys += item[].key
+        assert_equal(item[].value, check)
+        check += 1
+
+    assert_equal(keys, "acdb")
+
     # Empty dict is iterable
 
-    _ = dict.pop("b")
-    _ = dict.pop("d")
+    var empty_dict = Dict[String, Int]()
 
     keys = String("")
-    for key in reversed(dict):
+    for key in reversed(empty_dict):
         keys += key[]
 
     assert_equal(keys, "")
 
-    # Refill dict
+    check = 0
+    for val in reversed(empty_dict.values()):
+        # values is empty, should not reach here
+        check += 1
 
-    dict["d"] = 4
-    dict["a"] = 1
-    dict["b"] = 2
-    dict["e"] = 3
+    assert_equal(check, 0)
 
     keys = String("")
-    for key in reversed(dict):
-        keys += key[]
+    check = 0
+    for item in reversed(empty_dict.items()):
+        keys += item[].key
+        check += item[].value
 
-    assert_equal(keys, "ebad")
+    assert_equal(keys, "")
+    assert_equal(check, 0)
 
 
 def main():