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const-oid: eliminate split_high_bits side effects #1597

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Nov 2, 2024
+17 −9
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const-oid: eliminate split_high_bits side effects #1597

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26 changes: 17 additions & 9 deletions const-oid/src/encoder.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -126,21 +126,29 @@ const fn base128_len(arc: Arc) -> usize {
/// Split the highest 7-bits of an [`Arc`] from the rest of an arc.
///
/// Returns: `(hi, lo)`
// TODO(tarcieri): always use checked arithmetic
#[allow(clippy::arithmetic_side_effects)]
#[inline]
const fn split_high_bits(arc: Arc) -> (u8, Arc) {
if arc < 0x80 {
return (arc as u8, 0);
}

let hi_bit = 32 - arc.leading_zeros();
let hi_bit = match 32u32.checked_sub(arc.leading_zeros()) {
Some(bit) => bit,
None => unreachable!(),
};

let hi_bit_mod7 = hi_bit % 7;
let upper_bit_pos = hi_bit
- if hi_bit > 0 && hi_bit_mod7 == 0 {
7
} else {
hi_bit_mod7
};
let upper_bit_offset = if hi_bit > 0 && hi_bit_mod7 == 0 {
7
} else {
hi_bit_mod7
};

let upper_bit_pos = match hi_bit.checked_sub(upper_bit_offset) {
Some(bit) => bit,
None => unreachable!(),
};

let upper_bits = arc >> upper_bit_pos;
let lower_bits = arc ^ (upper_bits << upper_bit_pos);
(upper_bits as u8, lower_bits)
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