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scalar.rs
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scalar.rs
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// Licensed to the Apache Software Foundation (ASF) under one
// or more contributor license agreements. See the NOTICE file
// distributed with this work for additional information
// regarding copyright ownership. The ASF licenses this file
// to you under the Apache License, Version 2.0 (the
// "License"); you may not use this file except in compliance
// with the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing,
// software distributed under the License is distributed on an
// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
// KIND, either express or implied. See the License for the
// specific language governing permissions and limitations
// under the License.
use crate::buffer::Buffer;
use crate::native::ArrowNativeType;
use std::ops::Deref;
/// Provides a safe API for interpreting a [`Buffer`] as a slice of [`ArrowNativeType`]
///
/// # Safety
///
/// All [`ArrowNativeType`] are valid for all possible backing byte representations, and as
/// a result they are "trivially safely transmutable".
#[derive(Debug)]
pub struct ScalarBuffer<T: ArrowNativeType> {
#[allow(unused)]
buffer: Buffer,
// Borrows from `buffer` and is valid for the lifetime of `buffer`
ptr: *const T,
// The length of this slice
len: usize,
}
impl<T: ArrowNativeType> ScalarBuffer<T> {
/// Create a new [`ScalarBuffer`] from a [`Buffer`], and an `offset`
/// and `length` in units of `T`
///
/// # Panics
///
/// This method will panic if
///
/// * `offset` or `len` would result in overflow
/// * `buffer` is not aligned to a multiple of `std::mem::size_of::<T>`
/// * `bytes` is not large enough for the requested slice
pub fn new(buffer: Buffer, offset: usize, len: usize) -> Self {
let size = std::mem::size_of::<T>();
let offset_len = offset.checked_add(len).expect("length overflow");
let start_bytes = offset.checked_mul(size).expect("start bytes overflow");
let end_bytes = offset_len.checked_mul(size).expect("end bytes overflow");
let bytes = &buffer.as_slice()[start_bytes..end_bytes];
// SAFETY: all byte sequences correspond to a valid instance of T
let (prefix, offsets, suffix) = unsafe { bytes.align_to::<T>() };
assert!(
prefix.is_empty() && suffix.is_empty(),
"buffer is not aligned to {} byte boundary",
size
);
let ptr = offsets.as_ptr();
Self { buffer, ptr, len }
}
}
impl<T: ArrowNativeType> Deref for ScalarBuffer<T> {
type Target = [T];
fn deref(&self) -> &Self::Target {
// SAFETY: Bounds checked in constructor and ptr is valid for the lifetime of self
unsafe { std::slice::from_raw_parts(self.ptr, self.len) }
}
}
impl<T: ArrowNativeType> AsRef<[T]> for ScalarBuffer<T> {
fn as_ref(&self) -> &[T] {
self
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_basic() {
let expected = [0_i32, 1, 2];
let buffer = Buffer::from_iter(expected.iter().cloned());
let typed = ScalarBuffer::<i32>::new(buffer.clone(), 0, 3);
assert_eq!(*typed, expected);
let typed = ScalarBuffer::<i32>::new(buffer.clone(), 1, 2);
assert_eq!(*typed, expected[1..]);
let typed = ScalarBuffer::<i32>::new(buffer.clone(), 1, 0);
assert!(typed.is_empty());
let typed = ScalarBuffer::<i32>::new(buffer, 3, 0);
assert!(typed.is_empty());
}
#[test]
#[should_panic(expected = "buffer is not aligned to 4 byte boundary")]
fn test_unaligned() {
let expected = [0_i32, 1, 2];
let buffer = Buffer::from_iter(expected.iter().cloned());
let buffer = buffer.slice(1);
ScalarBuffer::<i32>::new(buffer, 0, 2);
}
#[test]
#[should_panic(expected = "range end index 16 out of range for slice of length 12")]
fn test_length_out_of_bounds() {
let buffer = Buffer::from_iter([0_i32, 1, 2]);
ScalarBuffer::<i32>::new(buffer, 1, 3);
}
#[test]
#[should_panic(expected = "range end index 16 out of range for slice of length 12")]
fn test_offset_out_of_bounds() {
let buffer = Buffer::from_iter([0_i32, 1, 2]);
ScalarBuffer::<i32>::new(buffer, 4, 0);
}
#[test]
#[should_panic(expected = "length overflow")]
fn test_length_overflow() {
let buffer = Buffer::from_iter([0_i32, 1, 2]);
ScalarBuffer::<i32>::new(buffer, usize::MAX, 1);
}
#[test]
#[should_panic(expected = "start bytes overflow")]
fn test_start_overflow() {
let buffer = Buffer::from_iter([0_i32, 1, 2]);
ScalarBuffer::<i32>::new(buffer, usize::MAX / 4 + 1, 0);
}
#[test]
#[should_panic(expected = "end bytes overflow")]
fn test_end_overflow() {
let buffer = Buffer::from_iter([0_i32, 1, 2]);
ScalarBuffer::<i32>::new(buffer, 0, usize::MAX / 4 + 1);
}
}