From f291065f6cac82827a47a736ebf78e4fc9c7c569 Mon Sep 17 00:00:00 2001
From: Laurent Mazare <laurent.mazare@gmail.com>
Date: Thu, 27 Jul 2023 09:28:47 +0100
Subject: [PATCH] Do not panic on empty ranges. (#257)

---
 candle-core/src/indexer.rs          |  2 +-
 candle-core/tests/indexing_tests.rs | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/candle-core/src/indexer.rs b/candle-core/src/indexer.rs
index c7a9a140..6ab209ca 100644
--- a/candle-core/src/indexer.rs
+++ b/candle-core/src/indexer.rs
@@ -42,7 +42,7 @@ impl Tensor {
                         Bound::Excluded(n) => *n,
                         Bound::Unbounded => dims[i],
                     };
-                    let out = x.narrow(current_dim, start, stop - start)?;
+                    let out = x.narrow(current_dim, start, stop.saturating_sub(start))?;
                     current_dim += 1;
                     out
                 }
diff --git a/candle-core/tests/indexing_tests.rs b/candle-core/tests/indexing_tests.rs
index 6a71b8fb..122f63b0 100644
--- a/candle-core/tests/indexing_tests.rs
+++ b/candle-core/tests/indexing_tests.rs
@@ -58,6 +58,19 @@ fn range_index() -> Result<()> {
     let result = tensor.i(..=1)?;
     assert_eq!(result.dims(), &[2, 3]);
     assert_eq!(result.to_vec2::<u32>()?, &[[0, 1, 2], [3, 4, 5]]);
+
+    // Empty range
+    let result = tensor.i(1..1)?;
+    assert_eq!(result.dims(), &[0, 3]);
+    let empty: [[u32; 3]; 0] = [];
+    assert_eq!(result.to_vec2::<u32>()?, &empty);
+
+    // Similar to PyTorch, allow empty ranges when the computed length is negative.
+    #[allow(clippy::reversed_empty_ranges)]
+    let result = tensor.i(1..0)?;
+    assert_eq!(result.dims(), &[0, 3]);
+    let empty: [[u32; 3]; 0] = [];
+    assert_eq!(result.to_vec2::<u32>()?, &empty);
     Ok(())
 }