perf: add repeat_slice_n_times to MutableBuffer

arrow-buffer/src/buffer/mutable.rs

@@ -222,6 +222,75 @@ impl MutableBuffer {
         }
     }
 
+    /// Adding to this mutable buffer `slice_to_repeat` repeated `repeat_count` times.
+    ///
+    /// # Example
+    ///
+    /// ## Repeat the same string bytes multiple times
+    /// ```
+    /// # use arrow_buffer::buffer::MutableBuffer;
+    /// let mut buffer = MutableBuffer::new(0);
+    /// let bytes_to_repeat = b"ab";
+    /// buffer.repeat_slice_n_times(bytes_to_repeat, 3);
+    /// assert_eq!(buffer.as_slice(), b"ababab");
+    /// ```
+    pub fn repeat_slice_n_times<T: ArrowNativeType>(
+        &mut self,
+        slice_to_repeat: &[T],
+        repeat_count: usize,
+    ) {
+        if repeat_count == 0 || slice_to_repeat.is_empty() {
+            return;
+        }
+
+        let bytes_to_repeat = size_of_val(slice_to_repeat);
+
+        // Ensure capacity
+        self.reserve(repeat_count * bytes_to_repeat);
+
+        // Save the length before we do all the copies to know where to start from
+        let length_before = self.len;
+
+        // Copy the initial slice once so we can use doubling strategy on it
+        self.extend_from_slice(slice_to_repeat);
+
+        // This tracks how much bytes we have added by repeating so far
+        let added_repeats_length = bytes_to_repeat;
+        assert_eq!(
+            self.len - length_before,
+            added_repeats_length,
+            "should copy exactly the same number of bytes"
+        );
+
+        // Number of times the slice was repeated
+        let mut already_repeated_times = 1;
+
+        // We will use doubling strategy to fill the buffer in log(repeat_count) steps
+        while already_repeated_times < repeat_count {
+            // How many slices can we copy in this iteration
+            // (either double what we have, or just the remaining ones)
+            let number_of_slices_to_copy =
+                already_repeated_times.min(repeat_count - already_repeated_times);
+            let number_of_bytes_to_copy = number_of_slices_to_copy * bytes_to_repeat;
+
+            unsafe {
+                // Get to the start of the data before we started copying anything
+                let src = self.data.as_ptr().add(length_before) as *const u8;
+
+                // Go to the current location to copy to (end of current data)
+                let dst = self.data.as_ptr().add(self.len);
+
+                // SAFETY: the pointers are not overlapping as there is `number_of_bytes_to_copy` or less between them
+                std::ptr::copy_nonoverlapping(src, dst, number_of_bytes_to_copy)
+            }
+
+            // Advance the length by the amount of data we just copied (doubled)
+            self.len += number_of_bytes_to_copy;
+
+            already_repeated_times += number_of_slices_to_copy;
+        }
+    }
+
     #[cold]
     fn reallocate(&mut self, capacity: usize) {
         let new_layout = Layout::from_size_align(capacity, self.layout.align()).unwrap();
@@ -1184,4 +1253,125 @@ mod tests {
             assert_eq!(pool.used(), 0);
         }
     }
+
+    fn create_expected_repeated_slice<T: ArrowNativeType>(
+        slice_to_repeat: &[T],
+        repeat_count: usize,
+    ) -> Buffer {
+        let mut expected = MutableBuffer::new(size_of_val(slice_to_repeat) * repeat_count);
+        for _ in 0..repeat_count {
+            // Not using push_slice_repeated as this is the function under test
+            expected.extend_from_slice(slice_to_repeat);
+        }
+        expected.into()
+    }
+
+    // Helper to test a specific repeat count with various slice sizes
+    fn test_repeat_count<T: ArrowNativeType + PartialEq + std::fmt::Debug>(
+        repeat_count: usize,
+        test_data: &[T],
+    ) {
+        let mut buffer = MutableBuffer::new(0);
+        buffer.repeat_slice_n_times(test_data, repeat_count);
+
+        let expected = create_expected_repeated_slice(test_data, repeat_count);
+        let result: Buffer = buffer.into();
+
+        assert_eq!(
+            result,
+            expected,
+            "Failed for repeat_count={}, slice_len={}",
+            repeat_count,
+            test_data.len()
+        );
+    }
+
+    #[test]
+    fn test_repeat_slice_count_edge_cases() {
+        // Empty slice
+        test_repeat_count(100, &[] as &[i32]);
+
+        // Zero repeats
+        test_repeat_count(0, &[1i32, 2, 3]);
+    }
+
+    #[test]
+    fn test_small_repeats_counts() {
+        // test any special implementation for small repeat counts
+        let data = &[1u8, 2, 3, 4, 5];
+
+        for _ in 1..=10 {
+            test_repeat_count(2, data);
+        }
+    }
+
+    #[test]
+    fn test_different_size_of_i32_repeat_slice() {
+        let data: &[i32] = &[1, 2, 3];
+        let data_with_single_item: &[i32] = &[42];
+
+        for data in &[data, data_with_single_item] {
+            for item in 1..=9 {
+                let base_repeat_count = 2_usize.pow(item);
+                test_repeat_count(base_repeat_count - 1, data);
+                test_repeat_count(base_repeat_count, data);
+                test_repeat_count(base_repeat_count + 1, data);
+            }
+        }
+    }
+
+    #[test]
+    fn test_different_size_of_u8_repeat_slice() {
+        let data: &[u8] = &[1, 2, 3];
+        let data_with_single_item: &[u8] = &[10];
+
+        for data in &[data, data_with_single_item] {
+            for item in 1..=9 {
+                let base_repeat_count = 2_usize.pow(item);
+                test_repeat_count(base_repeat_count - 1, data);
+                test_repeat_count(base_repeat_count, data);
+                test_repeat_count(base_repeat_count + 1, data);
+            }
+        }
+    }
+
+    #[test]
+    fn test_different_size_of_u16_repeat_slice() {
+        let data: &[u16] = &[1, 2, 3];
+        let data_with_single_item: &[u16] = &[10];
+
+        for data in &[data, data_with_single_item] {
+            for item in 1..=9 {
+                let base_repeat_count = 2_usize.pow(item);
+                test_repeat_count(base_repeat_count - 1, data);
+                test_repeat_count(base_repeat_count, data);
+                test_repeat_count(base_repeat_count + 1, data);
+            }
+        }
+    }
+
+    #[test]
+    fn test_various_slice_lengths() {
+        // Test different slice lengths with same repeat pattern
+        let repeat_count = 37; // Arbitrary non-power-of-2
+
+        // Single element
+        test_repeat_count(repeat_count, &[42i32]);
+
+        // Small slices
+        test_repeat_count(repeat_count, &[1i32, 2]);
+        test_repeat_count(repeat_count, &[1i32, 2, 3]);
+        test_repeat_count(repeat_count, &[1i32, 2, 3, 4]);
+        test_repeat_count(repeat_count, &[1i32, 2, 3, 4, 5]);
+
+        // Larger slices
+        let data_10: Vec<i32> = (0..10).collect();
+        test_repeat_count(repeat_count, &data_10);
+
+        let data_100: Vec<i32> = (0..100).collect();
+        test_repeat_count(repeat_count, &data_100);
+
+        let data_1000: Vec<i32> = (0..1000).collect();
+        test_repeat_count(repeat_count, &data_1000);
+    }
 }