sudachi/util/cow_array.rs
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/*
* Copyright (c) 2021-2024 Works Applications Co., Ltd.
*
* Licensed 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 std::array::TryFromSliceError;
use std::convert::TryInto;
use std::ops::Deref;
pub trait ReadLE {
fn from_le_bytes(bytes: &[u8]) -> Result<Self, TryFromSliceError>
where
Self: Sized;
}
impl ReadLE for i16 {
fn from_le_bytes(bytes: &[u8]) -> Result<Self, TryFromSliceError> {
bytes.try_into().map(Self::from_le_bytes)
}
}
impl ReadLE for u32 {
fn from_le_bytes(bytes: &[u8]) -> Result<Self, TryFromSliceError>
where
Self: Sized,
{
bytes.try_into().map(Self::from_le_bytes)
}
}
/// Copy-on-write array.
///
/// Is used for storing performance critical dictionary parts.
/// `slice` is always valid, `storage` is used in owned mode.
/// Unfortunately, `Cow<&[T]>` does not equal to `&[T]` in assembly:
/// See: https://rust.godbolt.org/z/r4a9efjqh
///
/// It implements Deref for `&[T]`, so it can be used as slice.
pub struct CowArray<'a, T> {
slice: &'a [T],
storage: Option<Vec<T>>,
}
impl<T: ReadLE + Clone> CowArray<'static, T> {
/// Creates from the owned data
pub fn from_owned<D: Into<Vec<T>>>(data: D) -> Self {
let data = data.into();
let slice1: &[T] = &data;
let slice: &'static [T] = unsafe { std::mem::transmute(slice1) };
Self {
storage: Some(data),
slice,
}
}
}
impl<'a, T: ReadLE + Clone> CowArray<'a, T> {
/// Create the CowArray from bytes, reinterpreting bytes as T.
///
/// Original data may or not be aligned.
/// In non-aligned case, it makes a copy of the original data.
pub fn from_bytes(data: &'a [u8], offset: usize, size: usize) -> Self {
let align = std::mem::align_of::<T>();
let real_size = size * std::mem::size_of::<T>();
let real_slice = &data[offset..offset + real_size];
let ptr = real_slice.as_ptr() as *const T;
if is_aligned(ptr as usize, align) {
// SAFETY: ptr is aligned and trait bounds are ensuring so the type is sane
let reslice = unsafe { std::slice::from_raw_parts(ptr, size) };
Self {
slice: reslice,
storage: None,
}
} else {
let data = copy_of_bytes::<T>(real_slice);
let slice_1: &[T] = data.as_slice();
// we need transmute to make correct lifetime
// slice will always point to vector contents and it is impossible to have
// self-referential types in Rust yet
let slice: &'a [T] = unsafe { std::mem::transmute(slice_1) };
Self {
storage: Some(data),
slice,
}
}
}
/// Updates the value of the array
///
/// Copies the data array if needed and updates it in place
/// Current implementation is not super for Rust because it
/// violates borrowing rules, but
/// 1. this object does not expose any references outside
/// 2. usage of data still follows the pattern 1-mut xor many-read
pub fn set(&mut self, offset: usize, value: T) {
if self.storage.is_none() {
self.storage = Some(self.slice.to_vec());
//refresh slice
let slice: &[T] = self.storage.as_ref().unwrap().as_slice();
self.slice = unsafe { std::mem::transmute(slice) };
}
if let Some(s) = self.storage.as_mut() {
s[offset] = value;
}
}
}
impl<'a, T> Deref for CowArray<'a, T> {
type Target = [T];
fn deref(&self) -> &Self::Target {
self.slice
}
}
fn is_aligned(offset: usize, alignment: usize) -> bool {
debug_assert!(alignment.is_power_of_two());
offset % alignment == 0
}
fn copy_of_bytes<T: ReadLE>(data: &[u8]) -> Vec<T> {
let size_t = std::mem::size_of::<T>();
assert_eq!(data.len() % size_t, 0);
let nelems = data.len() / size_t;
let mut result = Vec::with_capacity(nelems);
for i in (0..data.len()).step_by(size_t) {
let sl = &data[i..i + size_t];
result.push(T::from_le_bytes(sl).unwrap());
}
result
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn aligned_1() {
assert!(is_aligned(0, 1));
assert!(is_aligned(1, 1));
assert!(is_aligned(2, 1));
assert!(is_aligned(3, 1));
assert!(is_aligned(4, 1));
assert!(is_aligned(5, 1));
assert!(is_aligned(6, 1));
assert!(is_aligned(7, 1));
assert!(is_aligned(8, 1));
}
#[test]
fn aligned_2() {
assert!(is_aligned(0, 2));
assert!(!is_aligned(1, 2));
assert!(is_aligned(2, 2));
assert!(!is_aligned(3, 2));
assert!(is_aligned(4, 2));
assert!(!is_aligned(5, 2));
assert!(is_aligned(6, 2));
assert!(!is_aligned(7, 2));
assert!(is_aligned(8, 2));
}
#[test]
fn aligned_4() {
assert!(is_aligned(0, 4));
assert!(!is_aligned(1, 4));
assert!(!is_aligned(2, 4));
assert!(!is_aligned(3, 4));
assert!(is_aligned(4, 4));
assert!(!is_aligned(5, 4));
assert!(!is_aligned(6, 4));
assert!(!is_aligned(7, 4));
assert!(is_aligned(8, 4));
}
}