fulmicoton commented on PR #116:
URL:
https://github.com/apache/datasketches-rust/pull/116#issuecomment-4237510940
To bullet proof this, should we introduce a Coupon struct?
```
/// A coupon encodes a (slot, value) pair derived from hashing an input.
///
/// Format: `[value (6 bits) << 26] | [slot (26 bits)]`
///
/// The slot identifies an HLL register (derived from the lower bits of the
hash),
/// and the value represents the number of leading zeros plus one (from the
upper bits).
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Coupon(u32);
impl Coupon {
/// Sentinel value indicating an empty coupon slot.
const EMPTY: Self = Coupon(0);
/// Create a coupon by hashing a value.
#[inline(always)]
pub fn from_hash(v: impl Hash) -> Self {
let mut hasher = MurmurHash3X64128::default();
v.hash(&mut hasher);
let (lo, hi) = hasher.finish128();
let addr26 = lo as u32 & KEY_MASK_26;
let lz = hi.leading_zeros();
let capped = lz.min(62);
let value = capped + 1;
Coupon((value << KEY_BITS_26) | addr26)
}
/// Pack slot number and value into a coupon.
///
/// Format: [value (6 bits) << 26] | [slot (26 bits)]
#[inline(always)]
fn pack(slot: u32, value: u8) -> Self {
Coupon(((value as u32) << KEY_BITS_26) | (slot & KEY_MASK_26))
}
/// Extract slot number (low 26 bits) from coupon.
#[inline(always)]
fn slot(self) -> u32 {
self.0 & KEY_MASK_26
}
/// Extract value (upper 6 bits) from coupon.
#[inline(always)]
fn value(self) -> u8 {
(self.0 >> KEY_BITS_26) as u8
}
}
```
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