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Optimize char_try_from_u32 #94112

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merged 2 commits into from
Feb 20, 2022
Merged

Optimize char_try_from_u32 #94112

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0f14bea
Optimize char_try_from_u32
digama0 Feb 18, 2022
7c3ebec
fix
digama0 Feb 18, 2022
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17 changes: 14 additions & 3 deletions library/core/src/char/convert.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,8 +6,6 @@ use crate::fmt;
use crate::mem::transmute;
use crate::str::FromStr;

use super::MAX;

/// Converts a `u32` to a `char`.
///
/// Note that all [`char`]s are valid [`u32`]s, and can be cast to one with
Expand Down Expand Up @@ -271,7 +269,20 @@ impl FromStr for char {

#[inline]
const fn char_try_from_u32(i: u32) -> Result<char, CharTryFromError> {
if (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF) {
// This is an optimized version of the check
// (i > MAX as u32) || (i >= 0xD800 && i <= 0xDFFF),
// which can also be written as
// i >= 0x110000 || (i >= 0xD800 && i < 0xE000).
//
// The XOR with 0xD800 permutes the ranges such that 0xD800..0xE000 is
// mapped to 0x0000..0x0800, while keeping all the high bits outside 0xFFFF the same.
// In particular, numbers >= 0x110000 stay in this range.
//
// Subtracting 0x800 causes 0x0000..0x0800 to wrap, meaning that a single
// unsigned comparison against 0x110000 - 0x800 will detect both the wrapped
// surrogate range as well as the numbers originally larger than 0x110000.
//
if (i ^ 0xD800).wrapping_sub(0x800) >= 0x110000 - 0x800 {
Err(CharTryFromError(()))
} else {
// SAFETY: checked that it's a legal unicode value
Expand Down