1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
//! Checked subtraction for [`U512`].
use super::U512;
impl U512 {
/// Checked 512-bit subtraction, returning `None` on underflow.
///
/// Computes `self - other` via [`overflowing_sub`](U512::overflowing_sub).
/// Returns `Some(result)` if `self >= other`, or `None` if the
/// subtraction would underflow.
///
/// # Examples
///
/// ```
/// use cnfy_uint::u512::U512;
///
/// let a = U512::from_be_limbs([0, 0, 0, 0, 0, 0, 0, 10]);
/// let b = U512::from_be_limbs([0, 0, 0, 0, 0, 0, 0, 3]);
/// assert_eq!(a.checked_sub(&b), Some(U512::from_be_limbs([0, 0, 0, 0, 0, 0, 0, 7])));
/// assert_eq!(U512::ZERO.checked_sub(&U512::ONE), None);
/// ```
#[inline]
pub const fn checked_sub(&self, other: &U512) -> Option<U512> {
let (result, underflow) = self.overflowing_sub(other);
if underflow {
None
} else {
Some(result)
}
}
}
#[cfg(test)]
mod ai_tests {
use super::*;
/// Small values subtract without underflow.
#[test]
fn small_sub() {
let a = U512::from_be_limbs([0, 0, 0, 0, 0, 0, 0, 10]);
let b = U512::from_be_limbs([0, 0, 0, 0, 0, 0, 0, 3]);
assert_eq!(
a.checked_sub(&b),
Some(U512::from_be_limbs([0, 0, 0, 0, 0, 0, 0, 7])),
);
}
/// 0 - 1 underflows to None.
#[test]
fn zero_minus_one() {
assert_eq!(U512::ZERO.checked_sub(&U512::ONE), None);
}
/// Self minus self is zero.
#[test]
fn self_cancellation() {
let a = U512::from_be_limbs([0x1234, 0x5678, 0x9ABC, 0xDEF0, 1, 2, 3, 4]);
assert_eq!(a.checked_sub(&a), Some(U512::ZERO));
}
/// Subtracting zero is identity.
#[test]
fn subtractive_identity() {
let a = U512::from_be_limbs([0x1234, 0x5678, 0x9ABC, 0xDEF0, 1, 2, 3, 4]);
assert_eq!(a.checked_sub(&U512::ZERO), Some(a));
}
/// Larger minus smaller succeeds with borrow.
#[test]
fn borrow_succeeds() {
let a = U512::from_be_limbs([1, 0, 0, 0, 0, 0, 0, 0]);
let b = U512::ONE;
assert_eq!(
a.checked_sub(&b),
Some(U512::from_be_limbs([0, u64::MAX, u64::MAX, u64::MAX, u64::MAX, u64::MAX, u64::MAX, u64::MAX])),
);
}
/// Smaller minus larger underflows.
#[test]
fn underflow() {
let a = U512::from_be_limbs([0, 0, 0, 0, 0, 0, 0, 5]);
let b = U512::from_be_limbs([0, 0, 0, 0, 0, 0, 0, 10]);
assert_eq!(a.checked_sub(&b), None);
}
}