Add broadcast_shapes in python API

python/mlx/utils.py

@@ -289,3 +289,64 @@ def tree_merge(tree_a, tree_b, merge_fn=None):
                 )
             )
         return merge_fn(tree_a, tree_b)
+
+
+def broadcast_shapes(*shapes):
+    """Broadcast shapes to the same size.
+
+    Uses the same broadcasting rules as NumPy. The size of the trailing axes
+    for both arrays in an operation must either be the same size or one of
+    them must be one.
+
+    Args:
+        *shapes: The shapes to be broadcast against each other.
+            Each shape should be a tuple or list of integers.
+
+    Returns:
+        A tuple of integers representing the broadcasted shape.
+
+    Raises:
+        ValueError: If the shapes cannot be broadcast according to broadcasting rules.
+
+    Examples:
+        >>> broadcast_shapes((1, 2, 3), (3,))
+        (1, 2, 3)
+        >>> broadcast_shapes((1, 2, 3), (4, 1, 3))
+        (4, 2, 3)
+        >>> broadcast_shapes((5, 1, 3), (1, 4, 3))
+        (5, 4, 3)
+    """
+    if len(shapes) == 0:
+        raise ValueError("No shapes provided")
+
+    if len(shapes) == 1:
+        return shapes[0]
+
+    result = shapes[0]
+    for shape in shapes[1:]:
+        ndim1 = len(result)
+        ndim2 = len(shape)
+        ndim = max(ndim1, ndim2)
+
+        diff = abs(ndim1 - ndim2)
+        big = result if ndim1 > ndim2 else shape
+        small = shape if ndim1 > ndim2 else result
+
+        out_shape = []
+        for i in range(ndim - 1, diff - 1, -1):
+            a = big[i]
+            b = small[i - diff]
+
+            if a == b:
+                out_shape.insert(0, a)
+            elif a == 1 or b == 1:
+                out_shape.insert(0, a * b)
+            else:
+                raise ValueError(
+                    f"Shapes {result} and {shape} cannot be broadcast together"
+                )
+
+        for i in range(diff - 1, -1, -1):
+            out_shape.insert(0, big[i])
+        result = tuple(out_shape)
+    return result

python/tests/test_broadcast.py

@@ -0,0 +1,44 @@
+# Copyright © 2023 Apple Inc.
+
+import mlx.utils
+import mlx_tests
+
+
+class TestBroadcast(mlx_tests.MLXTestCase):
+    def test_broadcast_shapes(self):
+        # Basic broadcasting
+        self.assertEqual(mlx.utils.broadcast_shapes((1, 2, 3), (3,)), (1, 2, 3))
+        self.assertEqual(mlx.utils.broadcast_shapes((4, 1, 6), (5, 6)), (4, 5, 6))
+        self.assertEqual(mlx.utils.broadcast_shapes((5, 1, 4), (1, 3, 4)), (5, 3, 4))
+
+        # Multiple arguments
+        self.assertEqual(mlx.utils.broadcast_shapes((1, 1), (1, 8), (7, 1)), (7, 8))
+        self.assertEqual(
+            mlx.utils.broadcast_shapes((6, 1, 5), (1, 7, 1), (6, 7, 5)), (6, 7, 5)
+        )
+
+        # Same shapes
+        self.assertEqual(mlx.utils.broadcast_shapes((3, 4, 5), (3, 4, 5)), (3, 4, 5))
+
+        # Single argument
+        self.assertEqual(mlx.utils.broadcast_shapes((2, 3)), (2, 3))
+
+        # Empty shapes
+        self.assertEqual(mlx.utils.broadcast_shapes((), ()), ())
+        self.assertEqual(mlx.utils.broadcast_shapes((), (1,)), (1,))
+        self.assertEqual(mlx.utils.broadcast_shapes((1,), ()), (1,))
+
+        # Broadcasting with zeroes
+        self.assertEqual(mlx.utils.broadcast_shapes((0,), (0,)), (0,))
+        self.assertEqual(mlx.utils.broadcast_shapes((1, 0, 5), (3, 1, 5)), (3, 0, 5))
+        self.assertEqual(mlx.utils.broadcast_shapes((5, 0), (0, 5, 0)), (0, 5, 0))
+
+        # Error cases
+        with self.assertRaises(ValueError):
+            mlx.utils.broadcast_shapes((3, 4), (4, 3))
+
+        with self.assertRaises(ValueError):
+            mlx.utils.broadcast_shapes((2, 3, 4), (2, 5, 4))
+
+        with self.assertRaises(ValueError):
+            mlx.utils.broadcast_shapes()