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Introduce checked_div for PerThings and checked_rational_mul_correction #14673

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Introduce checked_div for PerThings and checked_rational_mul_correction #14673

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9c1f226
introduce checked_div for PerThings and checked_rational_mul_correction
Gioyik Jul 28, 2023
53ea375
".git/.scripts/commands/fmt/fmt.sh"
Aug 2, 2023
ffdaf16
remove unchecked definition and add a few tests
Gioyik Aug 2, 2023
53438b0
fix comments
Gioyik Aug 2, 2023
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48 changes: 45 additions & 3 deletions primitives/arithmetic/src/helpers_128bit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -75,6 +75,7 @@ mod double128 {
}

/// Returns 2^128 / a
/// `checked_div()` could be used here but is not possible to `unwrap()` inside a `const fn`
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@ggwpez ggwpez Aug 18, 2023

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I think you can use a match or if in const functions.

But for this function i would probably make more sense to add a checked_div128 function instead of changing it.

const fn div128(a: u128) -> u128 {
(neg128(a) / a).wrapping_add(1)
}
Expand Down Expand Up @@ -182,6 +183,19 @@ mod double128 {
}
}

pub const fn checked_multiply_by_rational_with_rounding(
a: u128,
b: u128,
c: u128,
r: Rounding,
) -> Result<Option<u128>, &'static str> {
if c == 0 {
return Err("Division by zero")
}

Ok(multiply_by_rational_with_rounding(a, b, c, r))
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@ggwpez ggwpez Aug 18, 2023

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As a general consideration of design:
I think its easier to put the actual logic inside the checked_ function such that the unchecked variant is just checked_function().expect() or checked_function.unwrap_or(whatever) since otherwise you have the case like now, where the checked variant does not have any logic and can therefore not ensure that it wont ever panic.

}

/// Returns `a * b / c` (wrapping to 128 bits) or `None` in the case of
/// overflow.
pub const fn multiply_by_rational_with_rounding(
Expand All @@ -191,9 +205,6 @@ pub const fn multiply_by_rational_with_rounding(
r: Rounding,
) -> Option<u128> {
use double128::Double128;
if c == 0 {
return None
}
Comment on lines -194 to -196
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@ggwpez ggwpez Aug 18, 2023

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This is actually making it less secure, which is why i would rather put the logic in the checked variant (basically same comment as above).

let (result, remainder) = Double128::product_of(a, b).div(c);
let mut result: u128 = match result.try_into_u128() {
Ok(v) => v,
Expand Down Expand Up @@ -246,6 +257,7 @@ mod tests {
use super::*;
use codec::{Decode, Encode};
use multiply_by_rational_with_rounding as mulrat;
use checked_multiply_by_rational_with_rounding as checked_mulrat;
use Rounding::*;

const MAX: u128 = u128::max_value();
Expand Down Expand Up @@ -278,6 +290,36 @@ mod tests {
assert_eq!(mulrat(1, MAX / 2 + 1, MAX, NearestPrefUp), Some(1));
}

#[test]
fn rational_checked_multiply_basic_rounding_works() {
assert_eq!(checked_mulrat(1, 1, 1, Up), Ok(Some(1)));
assert_eq!(checked_mulrat(3, 1, 3, Up), Ok(Some(1)));
assert_eq!(checked_mulrat(1, 1, 3, Up), Ok(Some(1)));
assert_eq!(checked_mulrat(1, 2, 3, Down), Ok(Some(0)));
assert_eq!(checked_mulrat(1, 1, 3, NearestPrefDown), Ok(Some(0)));
assert_eq!(checked_mulrat(1, 1, 2, NearestPrefDown), Ok(Some(0)));
assert_eq!(checked_mulrat(1, 2, 3, NearestPrefDown), Ok(Some(1)));
assert_eq!(checked_mulrat(1, 1, 3, NearestPrefUp), Ok(Some(0)));
assert_eq!(checked_mulrat(1, 1, 2, NearestPrefUp), Ok(Some(1)));
assert_eq!(checked_mulrat(1, 2, 3, NearestPrefUp), Ok(Some(1)));
assert_eq!(checked_mulrat(3, 1, 0, Up), Err("Division by zero"));
}

#[test]
fn rational_checked_multiply_big_number_works() {
assert_eq!(checked_mulrat(MAX, MAX - 1, MAX, Down), Ok(Some(MAX - 1)));
assert_eq!(checked_mulrat(MAX, 1, MAX, Down), Ok(Some(1)));
assert_eq!(checked_mulrat(MAX, MAX - 1, MAX, Up), Ok(Some(MAX - 1)));
assert_eq!(checked_mulrat(MAX, 1, MAX, Up), Ok(Some(1)));
assert_eq!(checked_mulrat(1, MAX - 1, MAX, Down), Ok(Some(0)));
assert_eq!(checked_mulrat(1, 1, MAX, Up), Ok(Some(1)));
assert_eq!(checked_mulrat(1, MAX / 2, MAX, NearestPrefDown), Ok(Some(0)));
assert_eq!(checked_mulrat(1, MAX / 2 + 1, MAX, NearestPrefDown), Ok(Some(1)));
assert_eq!(checked_mulrat(1, MAX / 2, MAX, NearestPrefUp), Ok(Some(0)));
assert_eq!(checked_mulrat(1, MAX / 2 + 1, MAX, NearestPrefUp), Ok(Some(1)));
assert_eq!(checked_mulrat(1, MAX / 2 + 1, 0, NearestPrefUp), Err("Division by zero"));
}

#[test]
fn sqrt_works() {
for i in 0..100_000u32 {
Expand Down
131 changes: 128 additions & 3 deletions primitives/arithmetic/src/per_things.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -19,8 +19,8 @@
use serde::{Deserialize, Serialize};

use crate::traits::{
BaseArithmetic, Bounded, CheckedAdd, CheckedMul, CheckedSub, One, SaturatedConversion,
Saturating, UniqueSaturatedInto, Unsigned, Zero,
BaseArithmetic, Bounded, CheckedAdd, CheckedDiv, CheckedMul, CheckedSub, One,
SaturatedConversion, Saturating, UniqueSaturatedInto, Unsigned, Zero,
};
use codec::{CompactAs, Encode};
use num_traits::{Pow, SaturatingAdd, SaturatingSub};
Expand Down Expand Up @@ -496,11 +496,34 @@ where
(x / maximum) * part_n + c
}

/// Checked compute of the error due to integer division in the expression `x / denom * numer`.
fn checked_rational_mul_correction<N, P>(
x: N,
numer: P::Inner,
denom: P::Inner,
rounding: Rounding,
) -> Result<N, &'static str>
where
N: MultiplyArg + UniqueSaturatedInto<P::Inner>,
P: PerThing,
P::Inner: Into<N>,
{
if P::Inner::is_zero(&denom) {
return Err("Division by zero")
}
Ok(rational_mul_correction::<N, P>(x, numer, denom, rounding))
Comment on lines +511 to +514
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@ggwpez ggwpez Aug 18, 2023

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Now you have the implication that the underlying function can only error iff denom is null, which is an implementation detail. Again i would rather move the logic into the checked variant and then call it from the unchecked one.

}

/// Compute the error due to integer division in the expression `x / denom * numer`.
///
/// Take the remainder of `x / denom` and multiply by `numer / denom`. The result can be added
/// to `x / denom * numer` for an accurate result.
fn rational_mul_correction<N, P>(x: N, numer: P::Inner, denom: P::Inner, rounding: Rounding) -> N
fn rational_mul_correction<N, P>(
x: N,
numer: P::Inner,
denom: P::Inner,
rounding: Rounding,
) -> N
where
N: MultiplyArg + UniqueSaturatedInto<P::Inner>,
P: PerThing,
Expand Down Expand Up @@ -1021,6 +1044,21 @@ macro_rules! implement_per_thing {
}
}

/// # Note
/// I am not sure if this is the best way to div safely for PerThings
impl CheckedDiv for $name {
#[inline]
fn checked_div(&self, rhs: &Self) -> Option<Self> {
let q = rhs.0;
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@ggwpez ggwpez Aug 18, 2023

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I think there should be an rhs.is_zero()


if q.is_zero() {
return None
}

Some(*self / *rhs)
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@ggwpez ggwpez Aug 18, 2023

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probably want to use the deconstruct() function here to get the inner value.

}
}

impl $crate::traits::Zero for $name {
fn zero() -> Self {
Self::zero()
Expand Down Expand Up @@ -1544,6 +1582,77 @@ macro_rules! implement_per_thing {
}
}

#[test]
fn checked_rational_mul_correction_works() {
assert_eq!(
super::checked_rational_mul_correction::<$type, $name>(
<$type>::max_value(),
<$type>::max_value(),
<$type>::max_value(),
super::Rounding::NearestPrefDown,
),
Ok(0),
);
assert_eq!(
super::checked_rational_mul_correction::<$type, $name>(
<$type>::max_value() - 1,
<$type>::max_value(),
<$type>::max_value(),
super::Rounding::NearestPrefDown,
),
Ok(<$type>::max_value() - 1),
);
assert_eq!(
super::checked_rational_mul_correction::<$upper_type, $name>(
((<$type>::max_value() - 1) as $upper_type).pow(2),
<$type>::max_value(),
<$type>::max_value(),
super::Rounding::NearestPrefDown,
),
Ok(1),
);
// ((max^2 - 1) % max) * max / max == max - 1
assert_eq!(
super::checked_rational_mul_correction::<$upper_type, $name>(
(<$type>::max_value() as $upper_type).pow(2) - 1,
<$type>::max_value(),
<$type>::max_value(),
super::Rounding::NearestPrefDown,
),
Ok(<$upper_type>::from((<$type>::max_value() - 1))),
);
// (max % 2) * max / 2 == max / 2
assert_eq!(
super::checked_rational_mul_correction::<$upper_type, $name>(
(<$type>::max_value() as $upper_type).pow(2),
<$type>::max_value(),
2 as $type,
super::Rounding::NearestPrefDown,
),
Ok(<$type>::max_value() as $upper_type / 2),
);
// ((max^2 - 1) % max) * 2 / max == 2 (rounded up)
assert_eq!(
super::checked_rational_mul_correction::<$upper_type, $name>(
(<$type>::max_value() as $upper_type).pow(2) - 1,
2 as $type,
<$type>::max_value(),
super::Rounding::NearestPrefDown,
),
Ok(2),
);
// ((max^2 - 1) % max) * 2 / max == 1 (rounded down)
assert_eq!(
super::checked_rational_mul_correction::<$upper_type, $name>(
(<$type>::max_value() as $upper_type).pow(2) - 1,
2 as $type,
<$type>::max_value(),
super::Rounding::Down,
),
Ok(1),
);
}

#[test]
fn rational_mul_correction_works() {
assert_eq!(
Expand Down Expand Up @@ -1737,6 +1846,22 @@ macro_rules! implement_per_thing {
Some($name::from_percent(0))
);
}

#[test]
fn test_basic_checked_div() {
assert_eq!(
$name::from_parts($max).checked_div(&$name::from_parts($max)),
Some($name::from_percent(100))
);
assert_eq!(
$name::from_percent(100).checked_div(&$name::from_percent(100)),
Some($name::from_percent(100))
);
assert_eq!(
$name::from_percent(0).checked_div(&$name::from_percent(0)),
None
);
}
}
};
}
Expand Down