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//! Miscelaneous functions.
use core::ops::Range;
/// Newtype wrapper to get around limitations in `const fn`s
pub(crate) struct Constructor<T>(#[allow(dead_code)] fn() -> T);
pub use crate::slice_cmp::{str_eq, u8_slice_eq};
#[doc(hidden)]
#[inline]
pub const fn saturate_range(s: &[u8], range: &Range<usize>) -> Range<usize> {
let len = s.len();
let end = min_usize(range.end, len);
min_usize(range.start, end)..end
}
#[doc(hidden)]
#[cfg(feature = "rust_1_64")]
#[repr(C)]
pub union Dereference<'a, T: ?Sized> {
pub ptr: *const T,
pub reff: &'a T,
}
////////////////////////////////////////////////////////////////////////////////
macro_rules! slice_up_to_len_alt_docs {
($item:item) => {
/// A const equivalent of `&slice[..len]`.
///
/// If `slice.len() < len`, this simply returns `slice` back.
///
/// # Runtime
///
/// If the "rust_1_64" feature is disabled,
/// thich takes linear time to remove the trailing elements,
/// proportional to `slice.len() - len`.
///
/// If the "rust_1_64" feature is enabled, it takes constant time to run.
///
/// # Example
///
/// ```rust
/// use const_format::utils::slice_up_to_len_alt;
///
/// const FIBB: &[u16] = &[3, 5, 8, 13, 21, 34, 55, 89];
///
/// const TWO: &[u16] = slice_up_to_len_alt(FIBB, 2);
/// const FOUR: &[u16] = slice_up_to_len_alt(FIBB, 4);
/// const ALL: &[u16] = slice_up_to_len_alt(FIBB, usize::MAX);
///
/// assert_eq!(TWO, &[3, 5]);
/// assert_eq!(FOUR, &[3, 5, 8, 13]);
/// assert_eq!(FIBB, ALL);
///
/// ```
$item
};
}
slice_up_to_len_alt_docs! {
#[cfg(feature = "rust_1_64")]
#[inline(always)]
pub const fn slice_up_to_len_alt<T>(slice: &[T], len: usize) -> &[T] {
slice_up_to_len(slice, len)
}
}
slice_up_to_len_alt_docs! {
#[cfg(not(feature = "rust_1_64"))]
pub const fn slice_up_to_len_alt<T>(slice: &[T], len: usize) -> &[T] {
let mut rem = slice.len().saturating_add(1).saturating_sub(len);
let mut ret = slice;
if rem == 0 {
return slice;
}
macro_rules! slice_up_to_len_alt_impl{
(
$( ($len:expr, [$($ignored:tt)*]) )*
)=>{
$(
while rem > $len {
if let [next @ .., $($ignored)* ] = ret {
ret = next;
rem -= $len;
}
}
)*
}
}
slice_up_to_len_alt_impl!{
(36, [_, _, _, _, _, _,_, _, _, _, _, _,_, _, _, _, _, _,_, _, _, _, _, _,_, _, _, _, _, _,_, _, _, _, _, _,])
(6, [_, _, _, _, _, _])
(1, [_])
}
ret
}
}
////////////////////////////////////////////////////////////////////////////////
macro_rules! slice_up_to_len_docs {
($item:item) => {
/// A conditionally-const equivalent of `&slice[..len]`.
///
/// If `slice.len() < len`, this simply returns `slice` back.
///
/// # Constness
///
/// This function takes constant time,
/// and in order to be `const fn` it requires the "rust_1_64"
/// feature to be enabled.
///
/// # Example
///
/// ```rust
/// use const_format::utils::slice_up_to_len_alt;
///
/// const FIBB: &[u16] = &[3, 5, 8, 13, 21, 34, 55, 89];
///
/// const TWO: &[u16] = slice_up_to_len_alt(FIBB, 2);
/// const FOUR: &[u16] = slice_up_to_len_alt(FIBB, 4);
/// const ALL: &[u16] = slice_up_to_len_alt(FIBB, usize::MAX);
///
/// assert_eq!(TWO, &[3, 5]);
/// assert_eq!(FOUR, &[3, 5, 8, 13]);
/// assert_eq!(FIBB, ALL);
///
/// ```
$item
};
}
slice_up_to_len_docs! {
#[cfg(feature = "rust_1_64")]
#[inline]
pub const fn slice_up_to_len<T>(slice: &[T], len: usize) -> &[T] {
if len > slice.len() {
return slice;
}
// Doing this to get a slice up to length at compile-time
unsafe {
let raw_slice = core::ptr::slice_from_raw_parts(slice.as_ptr(), len);
Dereference { ptr: raw_slice }.reff
}
}
}
slice_up_to_len_docs! {
#[cfg(not(feature = "rust_1_64"))]
#[inline]
pub fn slice_up_to_len<T>(slice: &[T], len: usize) -> &[T] {
if len > slice.len() {
return slice;
}
&slice[..len]
}
}
////////////////////////////////////////////////////////////////////////////////
pub(crate) const fn min_usize(l: usize, r: usize) -> usize {
if l < r {
l
} else {
r
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_slice_up_to_len_alt() {
let mut list = [0u16; 256];
(100..).zip(list.iter_mut()).for_each(|(i, m)| *m = i);
for i in 0..list.len() {
assert_eq!(slice_up_to_len_alt(&list, i), &list[..i]);
}
}
#[test]
fn slice_in_bounds() {
assert_eq!(slice_up_to_len(&[3, 5], 0), []);
assert_eq!(slice_up_to_len(&[3, 5], 1), [3]);
assert_eq!(slice_up_to_len(&[3, 5], 2), [3, 5]);
assert_eq!(slice_up_to_len(&[3, 5], 3), [3, 5]);
assert_eq!(slice_up_to_len(&[3, 5], 4), [3, 5]);
}
}