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char to_digit: avoid unnecessary casts to u64

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Marcondiro 2024-12-31 15:57:46 +01:00
parent aea4e43703
commit aa685bccca
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1 changed files with 11 additions and 7 deletions
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18
library/core/src/char/methods.rs
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@ -394,17 +394,21 @@ impl char {
);
// check radix to remove letter handling code when radix is a known constant
let value = if self > '9' && radix > 10 {
// convert ASCII letters to lowercase
let lower = self as u32 | 0x20;
// convert an ASCII letter to the corresponding value,
// non-letters convert to values > 36
lower.wrapping_sub('a' as u32) as u64 + 10
// mask to convert ASCII letters to uppercase
const TO_UPPERCASE_MASK: u32 = !0b0010_0000;
// Converts an ASCII letter to its corresponding integer value:
// A-Z => 10-35, a-z => 10-35. Other characters produce values >= 36.
//
// Add Overflow Safety:
// By applying the mask after the subtraction, the first addendum is
// constrained such that it never exceeds u32::MAX - 0x20.
((self as u32).wrapping_sub('A' as u32) & TO_UPPERCASE_MASK) + 10
} else {
// convert digit to value, non-digits wrap to values > 36
(self as u32).wrapping_sub('0' as u32) as u64
(self as u32).wrapping_sub('0' as u32)
};
// FIXME(const-hack): once then_some is const fn, use it here
if value < radix as u64 { Some(value as u32) } else { None }
if value < radix { Some(value) } else { None }
}
/// Returns an iterator that yields the hexadecimal Unicode escape of a