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Merged
merged 18 commits into from
Nov 9, 2025
Merged

Improve performance of encoding #64

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870590e
Use lookup table for decoding
taiki-e Jul 18, 2021
3b75b32
Inline val function
taiki-e Jul 18, 2021
ff1d115
Use chunks_exact instead of chunks
taiki-e Jul 18, 2021
4fd0ed8
Use decode_to_slice in Vec::from_hex
taiki-e Jul 18, 2021
3e2aa25
Inline decode_to_slice function
taiki-e Jul 18, 2021
f3251bd
Fix build errors
taiki-e Jul 18, 2021
97abf03
Adjust #[inline] on encoding functions
taiki-e Aug 28, 2021
0129182
Use encode_to_slice in encode
taiki-e Aug 28, 2021
19b333d
Use chunks_exact_mut instead of generate_iter
taiki-e Aug 28, 2021
8b9307b
Add encode_to_slice_upper to improve performance of encode_upper
taiki-e Aug 28, 2021
e981d8b
Merge branch 'main' into pref
KokaKiwi Nov 8, 2025
5c23286
Merge branch 'pref' of github.com:taiki-e/rust-hex into taiki-e-pref
KokaKiwi Nov 8, 2025
ded6948
refactor: improve code style and clarity in src/lib.rs
KokaKiwi Nov 9, 2025
0bf4b24
Apply suggestions from code review
KokaKiwi Nov 9, 2025
c965048
Merge branch 'pref' of github.com:taiki-e/rust-hex into taiki-e-pref
KokaKiwi Nov 9, 2025
76f37c7
Merge pull request #62 from taiki-e/pref
KokaKiwi Nov 9, 2025
986b288
Merge branch 'perf-encode' of github.com:taiki-e/rust-hex into taiki-…
KokaKiwi Nov 9, 2025
e134cd6
chore: fixup warning
KokaKiwi Nov 9, 2025
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38 changes: 8 additions & 30 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -102,6 +102,7 @@ impl<'a> BytesToHexChars<'a> {
impl<'a> Iterator for BytesToHexChars<'a> {
type Item = char;

#[inline]
fn next(&mut self) -> Option<Self::Item> {
match self.next.take() {
Some(current) => Some(current),
Expand Down Expand Up @@ -129,7 +130,6 @@ impl<'a> iter::ExactSizeIterator for BytesToHexChars<'a> {
}
}

#[inline]
fn encode_to_iter<T: iter::FromIterator<char>>(table: &'static [u8; 16], source: &[u8]) -> T {
BytesToHexChars::new(source, table).collect()
}
Expand Down Expand Up @@ -257,7 +257,10 @@ from_hex_array_impl! {
#[must_use]
#[cfg(feature = "alloc")]
pub fn encode<T: AsRef<[u8]>>(data: T) -> String {
data.encode_hex()
let data = data.as_ref();
let mut out = vec![0; data.len() * 2];
encode_to_slice(data, &mut out).unwrap();
String::from_utf8(out).unwrap()
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@taiki-e taiki-e Aug 28, 2021

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When using from_utf8_unckecked here, it can improve performance by about 8%. (It is safe because we emit only hex characters.)
However, I didn't apply that change because I don't know the policy regarding the unsafe code in this crate.
If It is okay with using unsafe code, I'll add that change.

hex_encode              time:   [9.4555 us 9.4828 us 9.5106 us]                        
                        change: [-9.2447% -8.3200% -7.4582%] (p = 0.00 < 0.05)
                        Performance has improved.

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@cgwalters cgwalters Jan 8, 2022

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Some overlap with #66 here I think. In particular I suspect the combination of ExactSizeIterator will allow us to collect into a String efficiently without indirecting through Vec.

}

/// Encodes `data` as hex string using uppercase characters.
Expand Down Expand Up @@ -326,17 +329,6 @@ pub fn decode_to_slice<T: AsRef<[u8]>>(data: T, out: &mut [u8]) -> Result<(), Fr
Ok(())
}

// generates an iterator like this
// (0, 1)
// (2, 3)
// (4, 5)
// (6, 7)
// ...
#[inline]
fn generate_iter(len: usize) -> impl Iterator<Item = (usize, usize)> {
(0..len).step_by(2).zip((0..len).skip(1).step_by(2))
}

// the inverse of `val`.
#[inline]
#[must_use]
Expand Down Expand Up @@ -385,14 +377,10 @@ pub fn encode_to_slice<T: AsRef<[u8]>>(input: T, output: &mut [u8]) -> Result<()
return Err(FromHexError::InvalidStringLength);
}

for (byte, (i, j)) in input
.as_ref()
.iter()
.zip(generate_iter(input.as_ref().len() * 2))
{
for (byte, output) in input.as_ref().iter().zip(output.chunks_exact_mut(2)) {
let (high, low) = byte2hex(*byte, HEX_CHARS_LOWER);
output[i] = high;
output[j] = low;
output[0] = high;
output[1] = low;
}

Ok(())
Expand All @@ -403,18 +391,8 @@ mod test {
use super::*;
#[cfg(feature = "alloc")]
use alloc::string::ToString;
#[cfg(feature = "alloc")]
use alloc::vec;
use pretty_assertions::assert_eq;

#[test]
#[cfg(feature = "alloc")]
fn test_gen_iter() {
let result = vec![(0, 1), (2, 3)];

assert_eq!(generate_iter(5).collect::<Vec<_>>(), result);
}

#[test]
fn test_encode_to_slice() {
let mut output_1 = [0; 4 * 2];
Expand Down