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Use rounding in float to Duration conversion methods #96051

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merged 6 commits into from
May 27, 2022
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Merged

Use rounding in float to Duration conversion methods #96051

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65c75b2
Use rounding in float to Duration conversion methods
newpavlov Apr 14, 2022
c2d8445
implement tie to even
newpavlov May 25, 2022
26f8594
tweak doctests
newpavlov May 25, 2022
06af3a6
add debug asserts
newpavlov May 27, 2022
38609cd
fix nanos overflow for f64
newpavlov May 27, 2022
6495963
fmt
newpavlov May 27, 2022
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136 changes: 109 additions & 27 deletions library/core/src/time.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -730,9 +730,9 @@ impl Duration {
/// // subnormal float
/// let res = Duration::from_secs_f64(f64::from_bits(1));
/// assert_eq!(res, Duration::new(0, 0));
/// // conversion uses truncation, not rounding
/// // conversion uses rounding
/// let res = Duration::from_secs_f64(0.999e-9);
/// assert_eq!(res, Duration::new(0, 0));
/// assert_eq!(res, Duration::new(0, 1));
/// ```
#[stable(feature = "duration_float", since = "1.38.0")]
#[must_use]
Expand Down Expand Up @@ -760,17 +760,17 @@ impl Duration {
/// let res = Duration::from_secs_f32(1e-20);
/// assert_eq!(res, Duration::new(0, 0));
/// let res = Duration::from_secs_f32(4.2e-7);
/// assert_eq!(res, Duration::new(0, 419));
/// assert_eq!(res, Duration::new(0, 420));
/// let res = Duration::from_secs_f32(2.7);
/// assert_eq!(res, Duration::new(2, 700_000_047));
/// assert_eq!(res, Duration::new(2, 700_000_048));
/// let res = Duration::from_secs_f32(3e10);
/// assert_eq!(res, Duration::new(30_000_001_024, 0));
/// // subnormal float
/// let res = Duration::from_secs_f32(f32::from_bits(1));
/// assert_eq!(res, Duration::new(0, 0));
/// // conversion uses truncation, not rounding
/// // conversion uses rounding
/// let res = Duration::from_secs_f32(0.999e-9);
/// assert_eq!(res, Duration::new(0, 0));
/// assert_eq!(res, Duration::new(0, 1));
/// ```
#[stable(feature = "duration_float", since = "1.38.0")]
#[must_use]
Expand Down Expand Up @@ -815,7 +815,7 @@ impl Duration {
/// use std::time::Duration;
///
/// let dur = Duration::new(2, 700_000_000);
/// assert_eq!(dur.mul_f32(3.14), Duration::new(8, 478_000_640));
/// assert_eq!(dur.mul_f32(3.14), Duration::new(8, 478_000_641));
/// assert_eq!(dur.mul_f32(3.14e5), Duration::new(847800, 0));
/// ```
#[stable(feature = "duration_float", since = "1.38.0")]
Expand All @@ -838,8 +838,7 @@ impl Duration {
///
/// let dur = Duration::new(2, 700_000_000);
/// assert_eq!(dur.div_f64(3.14), Duration::new(0, 859_872_611));
/// // note that truncation is used, not rounding
/// assert_eq!(dur.div_f64(3.14e5), Duration::new(0, 8_598));
/// assert_eq!(dur.div_f64(3.14e5), Duration::new(0, 8_599));
/// ```
#[stable(feature = "duration_float", since = "1.38.0")]
#[must_use = "this returns the result of the operation, \
Expand All @@ -862,9 +861,8 @@ impl Duration {
/// let dur = Duration::new(2, 700_000_000);
/// // note that due to rounding errors result is slightly
/// // different from 0.859_872_611
/// assert_eq!(dur.div_f32(3.14), Duration::new(0, 859_872_579));
/// // note that truncation is used, not rounding
/// assert_eq!(dur.div_f32(3.14e5), Duration::new(0, 8_598));
/// assert_eq!(dur.div_f32(3.14), Duration::new(0, 859_872_580));
/// assert_eq!(dur.div_f32(3.14e5), Duration::new(0, 8_599));
/// ```
#[stable(feature = "duration_float", since = "1.38.0")]
#[must_use = "this returns the result of the operation, \
Expand Down Expand Up @@ -1272,19 +1270,53 @@ macro_rules! try_from_secs {
let mant = (bits & MANT_MASK) | (MANT_MASK + 1);
let exp = ((bits >> $mant_bits) & EXP_MASK) as i16 + MIN_EXP;

let (secs, nanos) = if exp < -30 {
// the input represents less than 1ns.
let (secs, nanos) = if exp < -31 {
// the input represents less than 1ns and can not be rounded to it
(0u64, 0u32)
} else if exp < 0 {
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// the input is less than 1 second
let t = <$double_ty>::from(mant) << ($offset + exp);
let nanos = (u128::from(NANOS_PER_SEC) * u128::from(t)) >> ($mant_bits + $offset);
(0, nanos as u32)
let nanos_offset = $mant_bits + $offset;
let nanos_tmp = u128::from(NANOS_PER_SEC) * u128::from(t);
let nanos = (nanos_tmp >> nanos_offset) as u32;

let rem_mask = (1 << nanos_offset) - 1;
let rem_msb_mask = 1 << (nanos_offset - 1);
let rem = nanos_tmp & rem_mask;
let is_tie = rem == rem_msb_mask;
let is_even = (nanos & 1) == 0;
let rem_msb = nanos_tmp & rem_msb_mask == 0;
let add_ns = !(rem_msb || (is_even && is_tie));

// f32 does not have enough presicion to trigger the second branch
// since it can not represent numbers between 0.999_999_940_395 and 1.0.
let nanos = nanos + add_ns as u32;
if ($mant_bits == 23) || (nanos != NANOS_PER_SEC) { (0, nanos) } else { (1, 0) }
} else if exp < $mant_bits {
let secs = mant >> ($mant_bits - exp);
let secs = u64::from(mant >> ($mant_bits - exp));
let t = <$double_ty>::from((mant << exp) & MANT_MASK);
let nanos = (<$double_ty>::from(NANOS_PER_SEC) * t) >> $mant_bits;
(u64::from(secs), nanos as u32)
let nanos_offset = $mant_bits;
let nanos_tmp = <$double_ty>::from(NANOS_PER_SEC) * t;
let nanos = (nanos_tmp >> nanos_offset) as u32;

let rem_mask = (1 << nanos_offset) - 1;
let rem_msb_mask = 1 << (nanos_offset - 1);
let rem = nanos_tmp & rem_mask;
let is_tie = rem == rem_msb_mask;
let is_even = (nanos & 1) == 0;
let rem_msb = nanos_tmp & rem_msb_mask == 0;
let add_ns = !(rem_msb || (is_even && is_tie));

// f32 does not have enough presicion to trigger the second branch.
// For example, it can not represent numbers between 1.999_999_880...
// and 2.0. Bigger values result in even smaller presicion of the
// fractional part.
let nanos = nanos + add_ns as u32;
if ($mant_bits == 23) || (nanos != NANOS_PER_SEC) {
(secs, nanos)
} else {
(secs + 1, 0)
}
} else if exp < 64 {
// the input has no fractional part
let secs = u64::from(mant) << (exp - $mant_bits);
Expand Down Expand Up @@ -1315,24 +1347,45 @@ impl Duration {
/// let res = Duration::try_from_secs_f32(1e-20);
/// assert_eq!(res, Ok(Duration::new(0, 0)));
/// let res = Duration::try_from_secs_f32(4.2e-7);
/// assert_eq!(res, Ok(Duration::new(0, 419)));
/// assert_eq!(res, Ok(Duration::new(0, 420)));
/// let res = Duration::try_from_secs_f32(2.7);
/// assert_eq!(res, Ok(Duration::new(2, 700_000_047)));
/// assert_eq!(res, Ok(Duration::new(2, 700_000_048)));
/// let res = Duration::try_from_secs_f32(3e10);
/// assert_eq!(res, Ok(Duration::new(30_000_001_024, 0)));
/// // subnormal float:
/// let res = Duration::try_from_secs_f32(f32::from_bits(1));
/// assert_eq!(res, Ok(Duration::new(0, 0)));
/// // conversion uses truncation, not rounding
/// let res = Duration::try_from_secs_f32(0.999e-9);
/// assert_eq!(res, Ok(Duration::new(0, 0)));
///
/// let res = Duration::try_from_secs_f32(-5.0);
/// assert!(res.is_err());
/// let res = Duration::try_from_secs_f32(f32::NAN);
/// assert!(res.is_err());
/// let res = Duration::try_from_secs_f32(2e19);
/// assert!(res.is_err());
///
/// // the conversion uses rounding with tie resolution to even
/// let res = Duration::try_from_secs_f32(0.999e-9);
/// assert_eq!(res, Ok(Duration::new(0, 1)));
///
/// // this float represents exactly 976562.5e-9
/// let val = f32::from_bits(0x3A80_0000);
/// let res = Duration::try_from_secs_f32(val);
/// assert_eq!(res, Ok(Duration::new(0, 976_562)));
///
/// // this float represents exactly 2929687.5e-9
/// let val = f32::from_bits(0x3B40_0000);
/// let res = Duration::try_from_secs_f32(val);
/// assert_eq!(res, Ok(Duration::new(0, 2_929_688)));
///
/// // this float represents exactly 1.000_976_562_5
/// let val = f32::from_bits(0x3F802000);
/// let res = Duration::try_from_secs_f32(val);
/// assert_eq!(res, Ok(Duration::new(1, 976_562)));
///
/// // this float represents exactly 1.002_929_687_5
/// let val = f32::from_bits(0x3F806000);
/// let res = Duration::try_from_secs_f32(val);
/// assert_eq!(res, Ok(Duration::new(1, 2_929_688)));
/// ```
#[unstable(feature = "duration_checked_float", issue = "83400")]
#[inline]
Expand Down Expand Up @@ -1372,16 +1425,45 @@ impl Duration {
/// // subnormal float
/// let res = Duration::try_from_secs_f64(f64::from_bits(1));
/// assert_eq!(res, Ok(Duration::new(0, 0)));
/// // conversion uses truncation, not rounding
/// let res = Duration::try_from_secs_f32(0.999e-9);
/// assert_eq!(res, Ok(Duration::new(0, 0)));
///
/// let res = Duration::try_from_secs_f64(-5.0);
/// assert!(res.is_err());
/// let res = Duration::try_from_secs_f64(f64::NAN);
/// assert!(res.is_err());
/// let res = Duration::try_from_secs_f64(2e19);
/// assert!(res.is_err());
///
/// // the conversion uses rounding with tie resolution to even
/// let res = Duration::try_from_secs_f64(0.999e-9);
/// assert_eq!(res, Ok(Duration::new(0, 1)));
/// let res = Duration::try_from_secs_f64(0.999_999_999_499);
/// assert_eq!(res, Ok(Duration::new(0, 999_999_999)));
/// let res = Duration::try_from_secs_f64(0.999_999_999_501);
/// assert_eq!(res, Ok(Duration::new(1, 0)));
/// let res = Duration::try_from_secs_f64(42.999_999_999_499);
/// assert_eq!(res, Ok(Duration::new(42, 999_999_999)));
/// let res = Duration::try_from_secs_f64(42.999_999_999_501);
/// assert_eq!(res, Ok(Duration::new(43, 0)));
///
/// // this float represents exactly 976562.5e-9
/// let val = f64::from_bits(0x3F50_0000_0000_0000);
/// let res = Duration::try_from_secs_f64(val);
/// assert_eq!(res, Ok(Duration::new(0, 976_562)));
///
/// // this float represents exactly 2929687.5e-9
/// let val = f64::from_bits(0x3F68_0000_0000_0000);
/// let res = Duration::try_from_secs_f64(val);
/// assert_eq!(res, Ok(Duration::new(0, 2_929_688)));
///
/// // this float represents exactly 1.000_976_562_5
/// let val = f64::from_bits(0x3FF0_0400_0000_0000);
/// let res = Duration::try_from_secs_f64(val);
/// assert_eq!(res, Ok(Duration::new(1, 976_562)));
///
/// // this float represents exactly 1.002_929_687_5
/// let val = f64::from_bits(0x3_FF00_C000_0000_000);
/// let res = Duration::try_from_secs_f64(val);
/// assert_eq!(res, Ok(Duration::new(1, 2_929_688)));
/// ```
#[unstable(feature = "duration_checked_float", issue = "83400")]
#[inline]
Expand Down