Fix arange with inf step (#686)

* Fix case for step=inf in arange and add inf check for start/stop

* Add test cases for arange

* Update ops.cpp to include climits header

* Fix arange

* Fix formatting

* Refactor

* Add missing include

mlx/ops.cpp

@@ -1,6 +1,7 @@
 // Copyright © 2023-2024 Apple Inc.
 
 #include <algorithm>
+#include <climits>
 #include <cmath>
 #include <numeric>
 #include <set>
@@ -73,10 +74,24 @@ array arange(
   if (std::isnan(start) || std::isnan(step) || std::isnan(stop)) {
     throw std::invalid_argument("[arange] Cannot compute length.");
   }
-  double real_size = std::ceil((stop - start) / step);
+
-  if (std::isnan(real_size)) {
+  if (std::isinf(start) || std::isinf(stop)) {
     throw std::invalid_argument("[arange] Cannot compute length.");
   }
+
+  // Check if start and stop specify a valid range because if not, we have to
+  // return an empty array
+  if (std::isinf(step) &&
+      (step > 0 && start < stop || step < 0 && start > stop)) {
+    return array({start}, dtype);
+  }
+
+  double real_size = std::ceil((stop - start) / step);
+
+  if (real_size > INT_MAX) {
+    throw std::invalid_argument("[arange] Maximum size exceeded.");
+  }
+
   int size = std::max(static_cast<int>(real_size), 0);
   return array(
       {size},

python/tests/test_ops.py

@@ -1047,6 +1047,11 @@ class TestOps(mlx_tests.MLXTestCase):
             a = mx.arange(0, float("inf"), float("inf"))
         with self.assertRaises(ValueError):
             a = mx.arange(float("inf"), 1, float("inf"))
+        with self.assertRaises(ValueError):
+            a = mx.arange(float("inf"), 1, 5)
+        with self.assertRaises(ValueError):
+            INT_MAX = 2147483647
+            a = mx.arange(0, INT_MAX + 1, 1)
 
         a = mx.arange(5)
         expected = [0, 1, 2, 3, 4]
@@ -1132,6 +1137,27 @@ class TestOps(mlx_tests.MLXTestCase):
         self.assertListEqual(a.tolist(), expected)
         self.assertEqual(a.dtype, mx.int32)
 
+        a = mx.arange(0, 10, 100)
+        expected = [0]
+        self.assertListEqual(a.tolist(), expected)
+        self.assertEqual(a.dtype, mx.int32)
+
+        a = mx.arange(10, 0, 1)
+        expected = []
+        self.assertListEqual(a.tolist(), expected)
+
+        a = mx.arange(10, 0, float("inf"))
+        expected = []
+        self.assertListEqual(a.tolist(), expected)
+
+        a = mx.arange(0, 10, float("inf"))
+        expected = [0]
+        self.assertListEqual(a.tolist(), expected)
+
+        a = mx.arange(0, -10, float("-inf"))
+        expected = [0]
+        self.assertListEqual(a.tolist(), expected)
+
     def test_unary_ops(self):
         def test_ops(npop, mlxop, x, y, atol):
             r_np = npop(x)