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//! Morton undilation extracting odd bit positions from a [`U256`] into a `u128`.
use super::U256;
impl U256 {
/// Extracts bits at odd positions (1, 3, 5, ..., 255) and compacts them
/// into a `u128`.
///
/// Bit `2i+1` of the [`U256`] maps to bit `i` of the result. This is the
/// inverse of [`from_u128_dilated_odd`](U256::from_u128_dilated_odd):
/// `U256::from_u128_dilated_odd(v).undilate_odd() == v`.
///
/// Internally shifts each limb right by 1 to move odd positions into even
/// positions, then delegates to the even-position undilation cascade.
///
/// # Examples
///
/// ```
/// use cnfy_uint::u256::U256;
///
/// let v = U256::from_u128_dilated_odd(0xDEAD_BEEF);
/// assert_eq!(v.undilate_odd(), 0xDEAD_BEEF);
/// ```
#[inline]
pub const fn undilate_odd(&self) -> u128 {
let shifted = U256([
self.0[0] >> 1,
self.0[1] >> 1,
self.0[2] >> 1,
self.0[3] >> 1,
]);
shifted.undilate_even()
}
}
#[cfg(test)]
mod ai_tests {
use super::*;
/// Undilating zero produces zero.
#[test]
fn zero() {
assert_eq!(U256::ZERO.undilate_odd(), 0u128);
}
/// Round-trip: dilate odd then undilate odd recovers the original.
#[test]
fn round_trip_one() {
assert_eq!(U256::from_u128_dilated_odd(1).undilate_odd(), 1);
}
/// Round-trip with a value spanning all four quarters.
#[test]
fn round_trip_cross_quarter() {
let v: u128 = 0xDEAD_BEEF_CAFE_BABE_1234_5678_9ABC_DEF0;
assert_eq!(U256::from_u128_dilated_odd(v).undilate_odd(), v);
}
/// Round-trip with u128::MAX.
#[test]
fn round_trip_max() {
assert_eq!(
U256::from_u128_dilated_odd(u128::MAX).undilate_odd(),
u128::MAX,
);
}
/// Even-only dilated value undilates to zero from odd positions.
#[test]
fn even_input_gives_zero() {
let v = U256::from_u128_dilated_even(u128::MAX);
assert_eq!(v.undilate_odd(), 0);
}
/// Single high bit: bit 255 (odd) → bit 127.
#[test]
fn bit_255_to_127() {
let v = U256::from_u128_dilated_odd(1u128 << 127);
assert_eq!(v.undilate_odd(), 1u128 << 127);
}
}