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Replaced some overflowing muls with wrapping muls #40

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Replaced some overflowing muls with wrapping muls #40

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8b0fdd6
Replaced some overflowing muls with wrapping muls
NCGThompson Dec 3, 2023
a00d023
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NCGThompson Dec 3, 2023
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130 changes: 70 additions & 60 deletions src/int/api.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -532,11 +532,20 @@ impl I256 {
without modifying the original"]
#[inline]
pub fn checked_mul(self, rhs: Self) -> Option<Self> {
let (a, b) = self.overflowing_mul(rhs);
if b {
None
let abs_a = self.unsigned_abs();
let abs_b = rhs.unsigned_abs();
let res = abs_a.checked_mul(abs_b)?.as_i256();
let sign = self.is_negative() ^ rhs.is_negative();
if res.is_negative() {
if sign && res == Self::MIN {
Some(res)
} else {
None
}
} else if sign {
Some(-res)
} else {
Some(a)
Some(res)
}
}

Expand Down Expand Up @@ -745,25 +754,35 @@ impl I256 {
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub fn checked_pow(self, mut exp: u32) -> Option<Self> {
if exp == 0 {
return Some(Self::ONE);
pub fn checked_pow(self, exp: u32) -> Option<Self> {
if exp <= 1 {
if exp == 0 {
return Some(Self::ONE);
} else {
return Some(self);
}
}
let mut base = self;
let mut acc = Self::ONE;

while exp > 1 {
if (exp & 1) == 1 {
acc = acc.checked_mul(base)?;
}
exp /= 2;
base = base.checked_mul(base)?;
let mut acc;
let mut base = self.as_u256();

let exp_odd = exp & 1 == 1;
if *self.high() == -1 {
base = U256::new((-self.low()) as u128);
} else if *self.high() != 0 {
return None;
}
// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
acc.checked_mul(base)

acc = base.checked_pow(exp & !1)?.as_i256();
if acc.is_negative() {
return None;
}

if exp_odd {
acc = acc.checked_mul(self)?;
}

Some(acc)
}

/// Saturating integer addition. Computes `self + rhs`, saturating at the
Expand Down Expand Up @@ -1792,35 +1811,37 @@ impl I256 {
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub fn overflowing_pow(self, mut exp: u32) -> (Self, bool) {
if exp == 0 {
return (Self::ONE, false);
pub fn overflowing_pow(self, exp: u32) -> (Self, bool) {
if exp <= 1 {
if exp == 0 {
return (Self::ONE, false);
} else {
return (self, false);
}
}
let mut base = self;
let mut acc = Self::ONE;

let mut acc;
let mut overflown = false;
// Scratch space for storing results of overflowing_mul.
let mut r;


while exp > 1 {
if (exp & 1) == 1 {
r = acc.overflowing_mul(base);
acc = r.0;
overflown |= r.1;
}
exp /= 2;
r = base.overflowing_mul(base);
base = r.0;
let exp_odd = exp & 1 == 1;
overflown |= *self.high() != 0 && *self.high() != -1;
acc = self.wrapping_mul(self);
// If acc's sign bit is not set, the last operation did not overflow;
// if acc's sign bit is set, we will catch it later.

let r = acc.as_u256().overflowing_pow(exp / 2);
acc = r.0.as_i256();
overflown |= r.1 || acc.is_negative();

if exp_odd {
let r = acc.overflowing_mul(self);
acc = r.0;
overflown |= r.1;
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
r = acc.overflowing_mul(base);
r.1 |= overflown;
r
(acc, overflown)
}

/// Raises self to the power of `exp`, using exponentiation by squaring.
Expand All @@ -1837,26 +1858,15 @@ impl I256 {
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub fn pow(self, mut exp: u32) -> Self {
if exp == 0 {
return Self::ONE;
pub fn pow(self, exp: u32) -> Self {
#[cfg(debug_assertions)]
{
self.checked_pow(exp).unwrap()
}
let mut base = self;
let mut acc = Self::ONE;

while exp > 1 {
if (exp & 1) == 1 {
acc *= base;
}
exp /= 2;
base = base * base;
#[cfg(not(debug_assertions))]
{
self.wrapping_pow(exp)
}

// since exp!=0, finally the exp must be 1.
// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
acc * base
}

/// Calculates the quotient of Euclidean division of `self` by `rhs`.
Expand Down
45 changes: 18 additions & 27 deletions src/uint/api.rs
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -681,11 +681,15 @@ impl U256 {
let mut acc = U256::ONE;

while exp > 1 {
if *base.high() != 0 {
return None;
}
if (exp & 1) == 1 {
acc = acc.checked_mul(base)?;
debug_assert_eq!(*acc.high(), 0);
acc = Self::new(*acc.low()) * Self::new(*base.low());
}
exp /= 2;
base = base.checked_mul(base)?;
base = Self::new(*base.low()) * Self::new(*base.low());
}

// Deal with the final bit of the exponent separately, since
Expand Down Expand Up @@ -1466,18 +1470,16 @@ impl U256 {
let mut acc = U256::ONE;
let mut overflown = false;
// Scratch space for storing results of overflowing_mul.
let mut r;
let r;

while exp > 1 {
if (exp & 1) == 1 {
r = acc.overflowing_mul(base);
acc = r.0;
overflown |= r.1;
debug_assert!(overflown || acc <= base || acc <= 1);
acc = acc.wrapping_mul(base);
}
exp /= 2;
r = base.overflowing_mul(base);
base = r.0;
overflown |= r.1;
overflown |= *base.high() != 0;
base = base.wrapping_mul(base);
}

// Deal with the final bit of the exponent separately, since
Expand Down Expand Up @@ -1505,26 +1507,15 @@ impl U256 {
#[must_use = "this returns the result of the operation, \
without modifying the original"]
#[inline]
pub fn pow(self, mut exp: u32) -> Self {
let mut base = self;
let mut acc = U256::ONE;

while exp > 1 {
if (exp & 1) == 1 {
acc *= base;
}
exp /= 2;
base = base * base;
pub fn pow(self, exp: u32) -> Self {
#[cfg(debug_assertions)]
{
self.checked_pow(exp).unwrap()
}

// Deal with the final bit of the exponent separately, since
// squaring the base afterwards is not necessary and may cause a
// needless overflow.
if exp == 1 {
acc *= base;
#[cfg(not(debug_assertions))]
{
self.wrapping_pow(exp)
}

acc
}

/// Performs Euclidean division.
Expand Down