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Port Duration::from_secs_f32|64 from std
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This ports the algorithm for constructing `Duration` from floating point
seconds from `std`. The new algorithm in `std` uses bit manipulation to
construct the `Duration` with the most precision possible. Additionally
this adds `saturating` and `checked` variants of those functions.

Original `std` PR: rust-lang/rust#90247

Co-authored-by: Ted Wollman <[email protected]>
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@CryZe @TheTedder
CryZe and TheTedder committed May 9, 2023
1 parent 22ae579 commit 97ac8d9
Showing 1 changed file with 278 additions and 22 deletions.
300 changes: 278 additions & 22 deletions time/src/duration.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,124 @@ impl fmt::Debug for Duration {
}
}

/// This is adapted from the `std` implementation, which uses mostly bit
/// operations to ensure the highest precision:
/// https://github.com/rust-lang/rust/blob/3a37c2f0523c87147b64f1b8099fc9df22e8c53e/library/core/src/time.rs#L1262-L1340
/// Changes from `std` are marked and explained below.
#[rustfmt::skip] // Skip `rustfmt` because it reformats the arguments of the macro weirdly.
macro_rules! try_from_secs {
(
secs = $secs: expr,
mantissa_bits = $mant_bits: literal,
exponent_bits = $exp_bits: literal,
offset = $offset: literal,
bits_ty = $bits_ty:ty,
bits_ty_signed = $bits_ty_signed:ty,
double_ty = $double_ty:ty,
float_ty = $float_ty:ty,
is_nan = $is_nan:expr,
is_overflow = $is_overflow:expr,
) => {{
'value: {
const MIN_EXP: i16 = 1 - (1i16 << $exp_bits) / 2;
const MANT_MASK: $bits_ty = (1 << $mant_bits) - 1;
const EXP_MASK: $bits_ty = (1 << $exp_bits) - 1;

// Change from std: No error check for negative values necessary.

let bits = $secs.to_bits();
let mant = (bits & MANT_MASK) | (MANT_MASK + 1);
let exp = ((bits >> $mant_bits) & EXP_MASK) as i16 + MIN_EXP;

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 {
// the input is less than 1 second
let t = <$double_ty>::from(mant) << ($offset + exp);
let nanos_offset = $mant_bits + $offset;
let nanos_tmp = u128::from(Nanosecond.per(Second)) * 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 precision 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 != Nanosecond.per(Second)) {
(0, nanos)
} else {
(1, 0)
}
} else if exp < $mant_bits {
let secs = u64::from(mant >> ($mant_bits - exp));
let t = <$double_ty>::from((mant << exp) & MANT_MASK);
let nanos_offset = $mant_bits;
let nanos_tmp = <$double_ty>::from(Nanosecond.per(Second)) * 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 precision 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 precision of the
// fractional part.
let nanos = nanos + add_ns as u32;
if ($mant_bits == 23) || (nanos != Nanosecond.per(Second)) {
(secs, nanos)
} else {
(secs + 1, 0)
}
} else if exp < 63 {
// Change from std: The exponent here is 63 instead of 64,
// because i64::MAX + 1 is 2^63.

// the input has no fractional part
let secs = u64::from(mant) << (exp - $mant_bits);
(secs, 0)
} else if bits == (i64::MIN as $float_ty).to_bits() {
// Change from std: Signed integers are asymmetrical in that
// iN::MIN is -iN::MAX - 1. So for example i8 covers the
// following numbers -128..=127. The check above (exp < 63)
// doesn't cover i64::MIN as that is -2^63, so we have this
// additional case to handle the asymmetry of iN::MIN.
break 'value Self::new_unchecked(i64::MIN, 0);
} else if $secs.is_nan() {
// Change from std: std doesn't differentiate between the error
// cases.
$is_nan
} else {
$is_overflow
};

// Change from std: All the code is mostly unmodified in that it
// simply calculates an unsigned integer. Here we extract the sign
// bit and assign it to the number. We basically manually do two's
// complement here, we could also use an if and just negate the
// numbers based on the sign, but it turns out to be quite a bit
// slower.
let mask = (bits as $bits_ty_signed) >> ($mant_bits + $exp_bits);
#[allow(trivial_numeric_casts)]
let secs_signed = ((secs as i64) ^ (mask as i64)) - (mask as i64);
#[allow(trivial_numeric_casts)]
let nanos_signed = ((nanos as i32) ^ (mask as i32)) - (mask as i32);
Self::new_unchecked(secs_signed, nanos_signed)
}
}};
}

impl Duration {
// region: constants
/// Equivalent to `0.seconds()`.
Expand Down Expand Up @@ -321,17 +439,18 @@ impl Duration {
/// assert_eq!(Duration::seconds_f64(-0.5), -0.5.seconds());
/// ```
pub fn seconds_f64(seconds: f64) -> Self {
if seconds > i64::MAX as f64 || seconds < i64::MIN as f64 {
crate::expect_failed("overflow constructing `time::Duration`");
}
if seconds.is_nan() {
crate::expect_failed("passed NaN to `time::Duration::seconds_f64`");
}
let seconds_truncated = seconds as i64;
// This only works because we handle the overflow condition above.
let nanoseconds =
((seconds - seconds_truncated as f64) * Nanosecond.per(Second) as f64) as i32;
Self::new_unchecked(seconds_truncated, nanoseconds)
try_from_secs!(
secs = seconds,
mantissa_bits = 52,
exponent_bits = 11,
offset = 44,
bits_ty = u64,
bits_ty_signed = i64,
double_ty = u128,
float_ty = f64,
is_nan = crate::expect_failed("passed NaN to `time::Duration::seconds_f64`"),
is_overflow = crate::expect_failed("overflow constructing `time::Duration`"),
)
}

/// Creates a new `Duration` from the specified number of seconds represented as `f32`.
Expand All @@ -342,17 +461,154 @@ impl Duration {
/// assert_eq!(Duration::seconds_f32(-0.5), (-0.5).seconds());
/// ```
pub fn seconds_f32(seconds: f32) -> Self {
if seconds > i64::MAX as f32 || seconds < i64::MIN as f32 {
crate::expect_failed("overflow constructing `time::Duration`");
}
if seconds.is_nan() {
crate::expect_failed("passed NaN to `time::Duration::seconds_f32`");
}
let seconds_truncated = seconds as i64;
// This only works because we handle the overflow condition above.
let nanoseconds =
((seconds - seconds_truncated as f32) * Nanosecond.per(Second) as f32) as i32;
Self::new_unchecked(seconds_truncated, nanoseconds)
try_from_secs!(
secs = seconds,
mantissa_bits = 23,
exponent_bits = 8,
offset = 41,
bits_ty = u32,
bits_ty_signed = i32,
double_ty = u64,
float_ty = f32,
is_nan = crate::expect_failed("passed NaN to `time::Duration::seconds_f32`"),
is_overflow = crate::expect_failed("overflow constructing `time::Duration`"),
)
}

/// Creates a new `Duration` from the specified number of seconds
/// represented as `f64`. Any values that are out of bounds are saturated at
/// the minimum or maximum respectively. `NaN` gets turned into a `Duration`
/// of 0 seconds.
///
/// ```rust
/// # use time::{Duration, ext::NumericalDuration};
/// assert_eq!(Duration::saturating_seconds_f64(0.5), 0.5.seconds());
/// assert_eq!(Duration::saturating_seconds_f64(-0.5), -0.5.seconds());
/// assert_eq!(
/// Duration::saturating_seconds_f64(f64::NAN),
/// Duration::new(0, 0),
/// );
/// assert_eq!(
/// Duration::saturating_seconds_f64(f64::NEG_INFINITY),
/// Duration::MIN,
/// );
/// assert_eq!(
/// Duration::saturating_seconds_f64(f64::INFINITY),
/// Duration::MAX,
/// );
/// ```
pub fn saturating_seconds_f64(seconds: f64) -> Self {
try_from_secs!(
secs = seconds,
mantissa_bits = 52,
exponent_bits = 11,
offset = 44,
bits_ty = u64,
bits_ty_signed = i64,
double_ty = u128,
float_ty = f64,
is_nan = (0, 0),
is_overflow = return if seconds < 0.0 {
Self::new_unchecked(i64::MIN, -999_999_999)
} else {
Self::new_unchecked(i64::MAX, 999_999_999)
},
)
}

/// Creates a new `Duration` from the specified number of seconds
/// represented as `f32`. Any values that are out of bounds are saturated at
/// the minimum or maximum respectively. `NaN` gets turned into a `Duration`
/// of 0 seconds.
///
/// ```rust
/// # use time::{Duration, ext::NumericalDuration};
/// assert_eq!(Duration::saturating_seconds_f32(0.5), 0.5.seconds());
/// assert_eq!(Duration::saturating_seconds_f32(-0.5), (-0.5).seconds());
/// assert_eq!(
/// Duration::saturating_seconds_f32(f32::NAN),
/// Duration::new(0, 0),
/// );
/// assert_eq!(
/// Duration::saturating_seconds_f32(f32::NEG_INFINITY),
/// Duration::MIN,
/// );
/// assert_eq!(
/// Duration::saturating_seconds_f32(f32::INFINITY),
/// Duration::MAX,
/// );
/// ```
pub fn saturating_seconds_f32(seconds: f32) -> Self {
try_from_secs!(
secs = seconds,
mantissa_bits = 23,
exponent_bits = 8,
offset = 41,
bits_ty = u32,
bits_ty_signed = i32,
double_ty = u64,
float_ty = f32,
is_nan = (0, 0),
is_overflow = return if seconds < 0.0 {
Self::new_unchecked(i64::MIN, -999_999_999)
} else {
Self::new_unchecked(i64::MAX, 999_999_999)
},
)
}

/// Creates a new `Duration` from the specified number of seconds
/// represented as `f64`. Returns `None` if the `Duration` can't be
/// represented.
///
/// ```rust
/// # use time::{Duration, ext::NumericalDuration};
/// assert_eq!(Duration::checked_seconds_f64(0.5), Some(0.5.seconds()));
/// assert_eq!(Duration::checked_seconds_f64(-0.5), Some(-0.5.seconds()));
/// assert_eq!(Duration::checked_seconds_f64(f64::NAN), None);
/// assert_eq!(Duration::checked_seconds_f64(f64::NEG_INFINITY), None);
/// assert_eq!(Duration::checked_seconds_f64(f64::INFINITY), None);
/// ```
pub fn checked_seconds_f64(seconds: f64) -> Option<Self> {
Some(try_from_secs!(
secs = seconds,
mantissa_bits = 52,
exponent_bits = 11,
offset = 44,
bits_ty = u64,
bits_ty_signed = i64,
double_ty = u128,
float_ty = f64,
is_nan = return None,
is_overflow = return None,
))
}

/// Creates a new `Duration` from the specified number of seconds
/// represented as `f32`. Returns `None` if the `Duration` can't be
/// represented.
///
/// ```rust
/// # use time::{Duration, ext::NumericalDuration};
/// assert_eq!(Duration::checked_seconds_f32(0.5), Some(0.5.seconds()));
/// assert_eq!(Duration::checked_seconds_f32(-0.5), Some(-0.5.seconds()));
/// assert_eq!(Duration::checked_seconds_f32(f32::NAN), None);
/// assert_eq!(Duration::checked_seconds_f32(f32::NEG_INFINITY), None);
/// assert_eq!(Duration::checked_seconds_f32(f32::INFINITY), None);
/// ```
pub fn checked_seconds_f32(seconds: f32) -> Option<Self> {
Some(try_from_secs!(
secs = seconds,
mantissa_bits = 23,
exponent_bits = 8,
offset = 41,
bits_ty = u32,
bits_ty_signed = i32,
double_ty = u64,
float_ty = f32,
is_nan = return None,
is_overflow = return None,
))
}

/// Create a new `Duration` with the given number of milliseconds.
Expand Down

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