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Fix mixed arithmetic rounding #151

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Merged
terror merged 5 commits into from
Aug 3, 2026
Merged

Fix mixed arithmetic rounding #151

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622d42c
Fix mixed arithmetic rounding
terror Aug 3, 2026
56a1309
Inline approximate rounding
terror Aug 3, 2026
c9350d1
Separate test closures
terror Aug 3, 2026
ab90c60
Merge remote-tracking branch 'origin/master' into fix/mixed-arithmeti…
terror Aug 3, 2026
11f9329
Refine mixed arithmetic handling
terror Aug 3, 2026
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168 changes: 140 additions & 28 deletions src/number.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -22,17 +22,27 @@ impl Number {

#[must_use]
pub fn add(&self, rhs: &Self, config: Config) -> Self {
if let (Self::Exact(lhs), Self::Exact(rhs)) = (self, rhs) {
Self::Exact((lhs + rhs).complete())
} else {
Self::Approx(
match (self, rhs) {
(Self::Approx(lhs), Self::Approx(rhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
&self.to_float(config) + &rhs.to_float(config),
lhs + rhs,
config.rounding_mode,
)
.0,
)
),
(Self::Approx(lhs), Self::Exact(rhs))
| (Self::Exact(rhs), Self::Approx(lhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
lhs + rhs,
config.rounding_mode,
)
.0,
),
(Self::Exact(lhs), Self::Exact(rhs)) => {
Self::Exact((lhs + rhs).complete())
}
}
}

Expand Down Expand Up @@ -125,17 +135,36 @@ impl Number {
) -> std::result::Result<Self, Error> {
if rhs.is_zero() {
Err(Error::DivisionByZero)
} else if let (Self::Exact(lhs), Self::Exact(rhs)) = (self, rhs) {
Ok(Self::Exact((lhs / rhs).complete()))
} else {
Ok(Self::Approx(
Float::with_val_round(
config.precision(),
&self.to_float(config) / &rhs.to_float(config),
config.rounding_mode,
)
.0,
))
Ok(match (self, rhs) {
(Self::Approx(lhs), Self::Approx(rhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
lhs / rhs,
config.rounding_mode,
)
.0,
),
(Self::Approx(lhs), Self::Exact(rhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
lhs / rhs,
config.rounding_mode,
)
.0,
),
(Self::Exact(lhs), Self::Approx(rhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
lhs / rhs,
config.rounding_mode,
)
.0,
),
(Self::Exact(lhs), Self::Exact(rhs)) => {
Self::Exact((lhs / rhs).complete())
}
})
}
}

Expand Down Expand Up @@ -192,17 +221,27 @@ impl Number {

#[must_use]
pub fn mul(&self, rhs: &Self, config: Config) -> Self {
if let (Self::Exact(lhs), Self::Exact(rhs)) = (self, rhs) {
Self::Exact((lhs * rhs).complete())
} else {
Self::Approx(
match (self, rhs) {
(Self::Approx(lhs), Self::Approx(rhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
&self.to_float(config) * &rhs.to_float(config),
lhs * rhs,
config.rounding_mode,
)
.0,
)
),
(Self::Approx(lhs), Self::Exact(rhs))
| (Self::Exact(rhs), Self::Approx(lhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
lhs * rhs,
config.rounding_mode,
)
.0,
),
(Self::Exact(lhs), Self::Exact(rhs)) => {
Self::Exact((lhs * rhs).complete())
}
}
}

Expand Down Expand Up @@ -373,17 +412,34 @@ impl Number {

#[must_use]
pub fn sub(&self, rhs: &Self, config: Config) -> Self {
if let (Self::Exact(lhs), Self::Exact(rhs)) = (self, rhs) {
Self::Exact((lhs - rhs).complete())
} else {
Self::Approx(
match (self, rhs) {
(Self::Approx(lhs), Self::Approx(rhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
&self.to_float(config) - &rhs.to_float(config),
lhs - rhs,
config.rounding_mode,
)
.0,
)
),
(Self::Approx(lhs), Self::Exact(rhs)) => Self::Approx(
Float::with_val_round(
config.precision(),
lhs - rhs,
config.rounding_mode,
)
.0,
),
(Self::Exact(lhs), Self::Approx(rhs)) => Self::Approx(
-Float::with_val_round(
config.precision(),
rhs - lhs,
config.rounding_mode.reverse(),
)
.0,
),
(Self::Exact(lhs), Self::Exact(rhs)) => {
Self::Exact((lhs - rhs).complete())
}
}
}

Expand Down Expand Up @@ -731,6 +787,62 @@ mod tests {
assert!(greater > approx);
}

#[test]
fn mixed_exact_approx_operations_round_once() {
#[track_caller]
fn case(actual: &Number, expected: f64) {
let expected = Number::Approx(Float::with_val(2, expected));
assert_eq!(actual, &expected);
}

let config = Config {
precision: 2,
..Config::default()
};

let approx = |value| Number::Approx(Float::with_val(2, value));

let exact = |numerator, denominator| {
Number::Exact(Rational::from((numerator, denominator)))
};

case(&approx(1.0).add(&exact(13, 50), config), 1.5);
case(&exact(13, 50).add(&approx(1.0), config), 1.5);

case(&approx(1.0).sub(&exact(9, 25), config), 0.75);
case(&exact(9, 25).sub(&approx(1.0), config), -0.75);

case(&approx(1.5).mul(&exact(21, 50), config), 0.75);
case(&exact(21, 50).mul(&approx(1.5), config), 0.75);

case(&approx(1.0).div(&exact(13, 8), config).unwrap(), 0.5);
case(&exact(9, 10).div(&approx(1.5), config).unwrap(), 0.5);
}

#[test]
fn mixed_exact_approx_subtraction_signed_zero() {
#[track_caller]
fn case(rounding_mode: Round, negative: bool) {
let config = Config {
precision: 2,
rounding_mode,
..Config::default()
};

let Number::Approx(result) =
Number::from(1_i64).sub(&Number::Approx(Float::with_val(2, 1)), config)
else {
panic!("expected approximate number");
};

assert!(result.is_zero());
assert_eq!(result.is_sign_negative(), negative);
}

case(Round::Down, true);
case(Round::Up, false);
}

#[test]
fn undefined_exact_arithmetic_returns_error() {
let zero = Number::from(0_i64);
Expand Down
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