fix: prevent zero-probability events from being sampled in multinomial function in Jax-backend

mithril/backends/with_autograd/jax_backend/backend.py

@@ -513,29 +513,28 @@ def multinomial(
             replacement: whether to sample with replacement
         """
         prng_key = self._get_prng_key(key)
-        input = jax.numpy.asarray(probs)
-        input = input / jax.numpy.sum(input, axis=-1, keepdims=True)
-        batch_size = input.shape[:-1]
-        logits = jax.numpy.log(jax.numpy.maximum(input, 1e-37))
+        probs = jnp.asarray(probs)
+        probs = probs / jnp.sum(probs, axis=-1, keepdims=True)
+        logits = jnp.log(probs)
 
         if replacement:
             # Use categorical directly - much faster than choice
             samples = jax.random.categorical(
                 prng_key,
                 logits,  # avoid log(0)
-                shape=batch_size + (num_samples,),
+                shape=probs.shape[:-1] + (num_samples,),
             )
         else:
             # TODO: This algorithm is not efficient for small num_samples
             # consider more efficient algorithm
 
             # For without replacement, use Gumbel-max trick
             # This is much faster than using choice
-            z = jax.random.gumbel(prng_key, shape=input.shape + (num_samples,))
+            z = jax.random.gumbel(prng_key, shape=probs.shape + (num_samples,))
             # Add log probabilities for Gumbel-max trick,
             z = z + logits[..., None]
             # Get top k indices
-            samples = jax.numpy.argsort(-z, axis=input.ndim - 1)[..., :num_samples, 0]
+            samples = jax.numpy.argsort(-z, axis=probs.ndim - 1)[..., :num_samples, 0]
 
         return samples
 

tests/scripts/test_backend_fns.py

@@ -2060,3 +2060,42 @@ def test_rand_uniform(self, backendcls, device, dtype):
         assert not backend.any(output < 0)
         assert not backend.any(output > 10)
         assert list(output.shape) == fn_args[2:]
+
+
+@pytest.mark.parametrize(
+    "backendcls, device, dtype", backends_with_device_dtype, ids=names
+)
+class TestMultinomial:
+    def test_multinomial_with_replacement(self, backendcls, device, dtype):
+        backend = backendcls(device=device, dtype=dtype)
+        fn = backend.multinomial
+        fn_args = [backend.array([0.1, 0.3, 0.6]), 5]
+
+        output = fn(*fn_args, replacement=True)
+
+        assert not backend.any(output < 0)
+        assert not backend.any(output >= len(fn_args[0]))
+        assert list(output.shape) == [fn_args[1]]
+
+    def test_multinomial_without_replacement(self, backendcls, device, dtype):
+        backend = backendcls(device=device, dtype=dtype)
+        fn = backend.multinomial
+        fn_args = [backend.array([0.2, 0.3, 0.5]), 3]
+
+        output = fn(*fn_args, replacement=False)
+
+        assert not backend.any(output < 0)
+        assert not backend.any(output >= len(fn_args[0]))
+        assert list(output.shape) == [fn_args[1]]
+
+    def test_multinomial_invalid_probabilities(self, backendcls, device, dtype):
+        backend = backendcls(device=device, dtype=dtype)
+        fn = backend.multinomial
+
+        array_size = 1000
+        fn_args = [backend.array([0.0] * 500 + [1.0] + [0.0] * (array_size - 501)), 100]
+
+        output = fn(*fn_args, replacement=True)
+
+        assert list(output.shape) == [fn_args[1]]
+        assert backend.all(output == 500)