jiff/util/round/mode.rs
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use crate::{
util::{
rangeint::{RFrom, RInto},
t::{NoUnits, NoUnits128, C, C128},
},
Unit,
};
/// The mode for dealing with the remainder when rounding datetimes or spans.
///
/// This is used in APIs like [`Span::round`](crate::Span::round) for rounding
/// spans, and APIs like [`Zoned::round`](crate::Zoned::round) for rounding
/// datetimes.
///
/// In the documentation for each variant, we refer to concepts like the
/// "smallest" unit and the "rounding increment." These are best described
/// in the documentation for what you're rounding. For example,
/// [`SpanRound::smallest`](crate::SpanRound::smallest)
/// and [`SpanRound::increment`](crate::SpanRound::increment).
///
/// # Example
///
/// This shows how to round a span with a different rounding mode than the
/// default:
///
/// ```
/// use jiff::{RoundMode, SpanRound, ToSpan, Unit};
///
/// // The default rounds like how you were taught in school:
/// assert_eq!(
/// 1.hour().minutes(59).round(Unit::Hour)?,
/// 2.hours().fieldwise(),
/// );
/// // But we can change the mode, e.g., truncation:
/// let options = SpanRound::new().smallest(Unit::Hour).mode(RoundMode::Trunc);
/// assert_eq!(
/// 1.hour().minutes(59).round(options)?,
/// 1.hour().fieldwise(),
/// );
///
/// # Ok::<(), Box<dyn std::error::Error>>(())
/// ```
#[non_exhaustive]
#[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
pub enum RoundMode {
/// Rounds toward positive infinity.
///
/// For negative spans and datetimes, this option will make the value
/// smaller, which could be unexpected. To round away from zero, use
/// `Expand`.
Ceil,
/// Rounds toward negative infinity.
///
/// This mode acts like `Trunc` for positive spans and datetimes, but
/// for negative values it will make the value larger, which could be
/// unexpected. To round towards zero, use `Trunc`.
Floor,
/// Rounds away from zero like `Ceil` for positive spans and datetimes,
/// and like `Floor` for negative spans and datetimes.
Expand,
/// Rounds toward zero, chopping off any fractional part of a unit.
///
/// This is the default when rounding spans returned from
/// datetime arithmetic. (But it is not the default for
/// [`Span::round`](crate::Span::round).)
Trunc,
/// Rounds to the nearest allowed value like `HalfExpand`, but when there
/// is a tie, round towards positive infinity like `Ceil`.
HalfCeil,
/// Rounds to the nearest allowed value like `HalfExpand`, but when there
/// is a tie, round towards negative infinity like `Floor`.
HalfFloor,
/// Rounds to the nearest value allowed by the rounding increment and the
/// smallest unit. When there is a tie, round away from zero like `Ceil`
/// for positive spans and datetimes and like `Floor` for negative spans
/// and datetimes.
///
/// This corresponds to how rounding is often taught in school.
///
/// This is the default for rounding spans and datetimes.
HalfExpand,
/// Rounds to the nearest allowed value like `HalfExpand`, but when there
/// is a tie, round towards zero like `Trunc`.
HalfTrunc,
/// Rounds to the nearest allowed value like `HalfExpand`, but when there
/// is a tie, round towards the value that is an even multiple of the
/// rounding increment. For example, with a rounding increment of `3`,
/// the number `10` would round up to `12` instead of down to `9`, because
/// `12` is an even multiple of `3`, where as `9` is is an odd multiple.
HalfEven,
}
impl RoundMode {
/// Given a `quantity` in nanoseconds and an `increment` in units of
/// `unit`, this rounds it according to this mode and returns the result
/// in nanoseconds.
pub(crate) fn round_by_unit_in_nanoseconds(
self,
quantity: impl RInto<NoUnits128>,
unit: Unit,
increment: impl RInto<NoUnits128>,
) -> NoUnits128 {
let quantity = quantity.rinto();
let increment = unit.nanoseconds() * increment.rinto();
let rounded = self.round(quantity, increment);
rounded
}
fn round(
self,
quantity: impl RInto<NoUnits128>,
increment: impl RInto<NoUnits128>,
) -> NoUnits128 {
// ref: https://tc39.es/proposal-temporal/#sec-temporal-roundnumbertoincrement
fn inner(
mode: RoundMode,
quantity: NoUnits128,
increment: NoUnits128,
) -> NoUnits128 {
let mut quotient = quantity.div_ceil(increment);
let remainder = quantity.rem_ceil(increment);
if remainder == 0 {
return quantity;
}
let sign = if remainder < 0 { C128(-1) } else { C128(1) };
let tiebreaker = (remainder * C128(2)).abs();
let tie = tiebreaker == increment;
let expand_is_nearer = tiebreaker > increment;
// ref: https://tc39.es/proposal-temporal/#sec-temporal-roundnumbertoincrement
match mode {
RoundMode::Ceil => {
if sign > 0 {
quotient += sign;
}
}
RoundMode::Floor => {
if sign < 0 {
quotient += sign;
}
}
RoundMode::Expand => {
quotient += sign;
}
RoundMode::Trunc => {}
RoundMode::HalfCeil => {
if expand_is_nearer || (tie && sign > 0) {
quotient += sign;
}
}
RoundMode::HalfFloor => {
if expand_is_nearer || (tie && sign < 0) {
quotient += sign;
}
}
RoundMode::HalfExpand => {
if expand_is_nearer || tie {
quotient += sign;
}
}
RoundMode::HalfTrunc => {
if expand_is_nearer {
quotient += sign;
}
}
RoundMode::HalfEven => {
if expand_is_nearer || (tie && quotient % C(2) == 1) {
quotient += sign;
}
}
}
// We use saturating arithmetic here because this can overflow
// when `quantity` is the max value. Since we're rounding, we just
// refuse to go over the maximum. I'm not 100% convinced this is
// correct, but I think the only alternative is to return an error,
// and I'm not sure that's ideal either.
quotient.saturating_mul(increment)
}
inner(self, quantity.rinto(), increment.rinto())
}
pub(crate) fn round_float(
self,
quantity: f64,
increment: NoUnits128,
) -> NoUnits128 {
#[cfg(not(feature = "std"))]
use crate::util::libm::Float;
let quotient = quantity / (increment.get() as f64);
let rounded = match self {
RoundMode::Ceil => quotient.ceil(),
RoundMode::Floor => quotient.floor(),
RoundMode::Expand => {
if quotient < 0.0 {
quotient.floor()
} else {
quotient.ceil()
}
}
RoundMode::Trunc => quotient.trunc(),
RoundMode::HalfCeil => {
if quotient % 1.0 == 0.5 {
quotient.ceil()
} else {
quotient.round()
}
}
RoundMode::HalfFloor => {
if quotient % 1.0 == 0.5 {
quotient.floor()
} else {
quotient.round()
}
}
RoundMode::HalfExpand => {
quotient.signum() * quotient.abs().round()
}
RoundMode::HalfTrunc => {
if quotient % 1.0 == 0.5 {
quotient.trunc()
} else {
quotient.round()
}
}
RoundMode::HalfEven => {
if quotient % 1.0 == 0.5 {
quotient.trunc() + (quotient % 2.0)
} else {
quotient.round()
}
}
};
let rounded = NoUnits::new(rounded as i64).unwrap();
NoUnits128::rfrom(rounded.saturating_mul(increment))
}
}
#[cfg(test)]
mod tests {
use super::*;
// Some ad hoc tests I wrote while writing the rounding increment code.
#[test]
fn round_to_increment_half_expand_ad_hoc() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
i64::from(RoundMode::HalfExpand.round(quantity, increment))
};
assert_eq!(26, round(20, 13));
assert_eq!(0, round(29, 60));
assert_eq!(60, round(30, 60));
assert_eq!(60, round(31, 60));
assert_eq!(0, round(3, 7));
assert_eq!(7, round(4, 7));
}
// The Temporal tests are inspired by the table from here:
// https://tc39.es/proposal-temporal/#sec-temporal-roundnumbertoincrement
//
// The main difference is that our rounding function specifically does not
// use floating point, so we tweak the values a bit.
#[test]
fn round_to_increment_temporal_table_ceil() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::Ceil.round(quantity, increment).into()
};
assert_eq!(-10, round(-15, 10));
assert_eq!(0, round(-5, 10));
assert_eq!(10, round(4, 10));
assert_eq!(10, round(5, 10));
assert_eq!(10, round(6, 10));
assert_eq!(20, round(15, 10));
}
#[test]
fn round_to_increment_temporal_table_floor() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::Floor.round(quantity, increment).into()
};
assert_eq!(-20, round(-15, 10));
assert_eq!(-10, round(-5, 10));
assert_eq!(0, round(4, 10));
assert_eq!(0, round(5, 10));
assert_eq!(0, round(6, 10));
assert_eq!(10, round(15, 10));
}
#[test]
fn round_to_increment_temporal_table_expand() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::Expand.round(quantity, increment).into()
};
assert_eq!(-20, round(-15, 10));
assert_eq!(-10, round(-5, 10));
assert_eq!(10, round(4, 10));
assert_eq!(10, round(5, 10));
assert_eq!(10, round(6, 10));
assert_eq!(20, round(15, 10));
}
#[test]
fn round_to_increment_temporal_table_trunc() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::Trunc.round(quantity, increment).into()
};
assert_eq!(-10, round(-15, 10));
assert_eq!(0, round(-5, 10));
assert_eq!(0, round(4, 10));
assert_eq!(0, round(5, 10));
assert_eq!(0, round(6, 10));
assert_eq!(10, round(15, 10));
}
#[test]
fn round_to_increment_temporal_table_half_ceil() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::HalfCeil.round(quantity, increment).into()
};
assert_eq!(-10, round(-15, 10));
assert_eq!(0, round(-5, 10));
assert_eq!(0, round(4, 10));
assert_eq!(10, round(5, 10));
assert_eq!(10, round(6, 10));
assert_eq!(20, round(15, 10));
}
#[test]
fn round_to_increment_temporal_table_half_floor() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::HalfFloor.round(quantity, increment).into()
};
assert_eq!(-20, round(-15, 10));
assert_eq!(-10, round(-5, 10));
assert_eq!(0, round(4, 10));
assert_eq!(0, round(5, 10));
assert_eq!(10, round(6, 10));
assert_eq!(10, round(15, 10));
}
#[test]
fn round_to_increment_temporal_table_half_expand() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::HalfExpand.round(quantity, increment).into()
};
assert_eq!(-20, round(-15, 10));
assert_eq!(-10, round(-5, 10));
assert_eq!(0, round(4, 10));
assert_eq!(10, round(5, 10));
assert_eq!(10, round(6, 10));
assert_eq!(20, round(15, 10));
}
#[test]
fn round_to_increment_temporal_table_half_trunc() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::HalfTrunc.round(quantity, increment).into()
};
assert_eq!(-10, round(-15, 10));
assert_eq!(0, round(-5, 10));
assert_eq!(0, round(4, 10));
assert_eq!(0, round(5, 10));
assert_eq!(10, round(6, 10));
assert_eq!(10, round(15, 10));
}
#[test]
fn round_to_increment_temporal_table_half_even() {
let round = |quantity: i64, increment: i64| -> i64 {
let quantity = NoUnits::new(quantity).unwrap();
let increment = NoUnits::new(increment).unwrap();
RoundMode::HalfEven.round(quantity, increment).into()
};
assert_eq!(-20, round(-15, 10));
assert_eq!(0, round(-5, 10));
assert_eq!(0, round(4, 10));
assert_eq!(0, round(5, 10));
assert_eq!(10, round(6, 10));
assert_eq!(20, round(15, 10));
}
}