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Alternative LUT rather than dividing.

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This commit is contained in:
Giles Cope 2021-03-04 07:04:21 +00:00
parent a69960a4ec
commit d07c43af31
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1 changed files with 34 additions and 1 deletions
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35
library/alloc/src/string.rs
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@ -2224,7 +2224,7 @@ impl ToString for char {
}
}
#[stable(feature = "u8_to_string_specialization", since="1.999.0")]
#[stable(feature = "u8_to_string_specialization", since = "1.999.0")]
impl ToString for u8 {
#[inline]
fn to_string(&self) -> String {
@ -2243,6 +2243,39 @@ impl ToString for u8 {
}
}
// 2 digit decimal look up table
static DEC_DIGITS_LUT: &[u8; 200] = b"0001020304050607080910111213141516171819\
2021222324252627282930313233343536373839\
4041424344454647484950515253545556575859\
6061626364656667686970717273747576777879\
8081828384858687888990919293949596979899";
#[stable(feature = "i8_to_string_specialization", since = "1.999.0")]
impl ToString for i8 {
#[inline]
fn to_string(&self) -> String {
let mut vec: Vec<u8> = if *self < 0 {
let mut v = Vec::with_capacity(4);
v.push(b'-');
v
} else {
Vec::with_capacity(3)
};
let mut n = self.abs();
if n >= 10 {
if n >= 100 {
n -= 100;
vec.push(b'1');
}
let nn = n * 2;
vec.extend_from_slice(&DEC_DIGITS_LUT[nn as usize..=nn as usize + 1]);
} else {
vec.push(b'0' + (n as u8));
}
unsafe { String::from_utf8_unchecked(vec) }
}
}
#[stable(feature = "str_to_string_specialization", since = "1.9.0")]
impl ToString for str {
#[inline]