Use sensible maths for from_rational (#13660)

* Use sensible maths for from_rational * Fixes * Fixes * More fixes * Remove debugging * Add fuzzer tests Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * Prevent panics Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * docs Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * Clean up old code Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * Test all rounding modes of from_rational Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * Clean up code Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * Revert "Prevent panics" This reverts commit 7e88ac76138a1b590e68b68318505b69efe1e1f6. * fix imports Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * cleanup Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * Fuzz test multiply_rational Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * Fix import Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * fmt Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> * Return None in multiply_rational on zero div Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> --------- Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io> Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
2026-04-28 01:38:04 +00:00 · 2023-03-24 14:48:37 +00:00
parent 40e1704e1c
commit 2c77f8f4ca
12 changed files with 372 additions and 227 deletions
@@ -284,6 +284,54 @@ impl PartialEq for Rational128 {
 	}
 }

+pub trait MultiplyRational: Sized {
+	fn multiply_rational(self, n: Self, d: Self, r: Rounding) -> Option<Self>;
+}
+
+macro_rules! impl_rrm {
+	($ulow:ty, $uhi:ty) => {
+		impl MultiplyRational for $ulow {
+			fn multiply_rational(self, n: Self, d: Self, r: Rounding) -> Option<Self> {
+				if d.is_zero() {
+					return None
+				}
+
+				let sn = (self as $uhi) * (n as $uhi);
+				let mut result = sn / (d as $uhi);
+				let remainder = (sn % (d as $uhi)) as $ulow;
+				if match r {
+					Rounding::Up => remainder > 0,
+					// cannot be `(d + 1) / 2` since `d` might be `max_value` and overflow.
+					Rounding::NearestPrefUp => remainder >= d / 2 + d % 2,
+					Rounding::NearestPrefDown => remainder > d / 2,
+					Rounding::Down => false,
+				} {
+					result = match result.checked_add(1) {
+						Some(v) => v,
+						None => return None,
+					};
+				}
+				if result > (<$ulow>::max_value() as $uhi) {
+					None
+				} else {
+					Some(result as $ulow)
+				}
+			}
+		}
+	};
+}
+
+impl_rrm!(u8, u16);
+impl_rrm!(u16, u32);
+impl_rrm!(u32, u64);
+impl_rrm!(u64, u128);
+
+impl MultiplyRational for u128 {
+	fn multiply_rational(self, n: Self, d: Self, r: Rounding) -> Option<Self> {
+		crate::helpers_128bit::multiply_by_rational_with_rounding(self, n, d, r)
+	}
+}
+
 #[cfg(test)]
 mod tests {
 	use super::{helpers_128bit::*, *};