[GitHub] [arrow] jorgecarleitao commented on a change in pull request #8541: ARROW-10402: [Rust] Refactor array equality

[GitBox <gi...@apache.org>]

jorgecarleitao commented on a change in pull request #8541:
URL: https://github.com/apache/arrow/pull/8541#discussion_r517095720



##########
File path: rust/arrow/src/array/data.rs
##########
@@ -207,6 +210,56 @@ impl ArrayData {
 
         size
     }
+
+    /// Creates a zero-copy slice of itself. This creates a new [ArrayData]
+    /// with a different offset, len and a shifted null bitmap.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `offset + length > self.len()`.
+    pub fn slice(&self, offset: usize, length: usize) -> ArrayData {
+        assert!((offset + length) <= self.len());
+
+        let mut new_data = self.clone();
+
+        new_data.len = length;
+        new_data.offset = offset + self.offset;
+
+        new_data.null_count =
+            count_nulls(new_data.null_buffer(), new_data.offset, new_data.len);
+
+        new_data
+    }
+
+    /// Returns the buffer `buffer` as a slice of type `T`. When the expected buffer is bit-packed,
+    /// the slice is not offset.
+    #[inline]
+    pub(super) fn buffer<T>(&self, buffer: usize) -> &[T] {
+        let values = unsafe { self.buffers[buffer].data().align_to::<T>() };
+        if values.0.len() != 0 || values.2.len() != 0 {
+            panic!("The buffer is not byte-aligned with its interpretation")
+        };
+        if self.data_type.width(buffer).unwrap() == 1 {
+            // bitpacked buffers are offset differently and it is the consumers that need to worry about.
+            &values.1
+        } else {
+            &values.1[self.offset..]
+        }
+    }
+
+    /// Returns the buffer `buffer` as a slice of type `T`. When the expected buffer is bit-packed,
+    /// the slice is not offset.
+    #[inline]
+    pub fn buffer_data(&self, buffer: usize) -> &[u8] {
+        let values = self.buffers[buffer].data();
+        let width = self.data_type.width(buffer).unwrap();
+        if width == 1 {
+            // bitpacked buffers are offset differently and it is the consumers that need to worry about.
+            &values
+        } else {
+            &values[self.offset * (width / 8)..]
+        }
+    }
 }
 
 impl PartialEq for ArrayData {

Review comment:
       One Idea would be to use the equality code from this PR, but sprinkle it with `debug_assert_eq`, so that during debug, it is more clear where the difference is. An alternative is to use `assert_eq`, so that the two arrays are shown side-by-side (this only works for small arrays, though).




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