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doc: expand description for f32 and f64 associated constants #104153

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Oct 5, 2023
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doc: expand description for f32 and f64 associated constants #104153

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42 changes: 40 additions & 2 deletions library/core/src/num/f32.rs
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Expand Up @@ -357,38 +357,76 @@ impl f32 {
pub const MANTISSA_DIGITS: u32 = 24;

/// Approximate number of significant digits in base 10.
///
/// This is the maximum <i>x</i> such that any decimal number with <i>x</i>
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/// significant digits can be converted to `f32` and back without loss.
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I admit I've not paged discussion on #89238 into cache, but since that annotates this with "A minimum number of significant digits in base 10." it feels somewhat surprising to have "maximum" here.

cc @workingjubilee

(Perhaps the language is actually equivalent, I would need to actually thoroughly understand what the significance is to make a call there).

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If I understand correctly, f32::DIGITS is Rust's version of C's FLT_DIG, and f64::DIGITS is Rust's version of DBL_DIG. The working draft N3054 of the C standard describes those like this (with radix b = 2 in our case):

number of decimal digits, q, such that any floating-point number with q decimal digits can be rounded into a floating-point number with p radix b digits and back again without change to the q decimal digits,

image

I wrote "maximum" there because if f32::DIGITS is 6, then 6 is the maximum number of decimal digits that can be converted to f32 losslessly. A 7-digit decimal converted to f32 and back can be altered by the conversions.

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An example for f32 where a 7-digit number has a lossy conversion:

let s1 = "8.589973e9";
let float: f32 = s1.parse().unwrap();
let s2 = format!("{float:.6e}");
assert_eq!(s1, s2);

panics with

assertion failed: `(left == right)`
  left: `"8.589973e9"`,
 right: `"8.589974e9"`

///
/// Equal to floor(log<sub>10</sub>&nbsp;2<sup>[`MANTISSA_DIGITS`]&nbsp;&minus;&nbsp;1</sup>).
///
/// [`MANTISSA_DIGITS`]: f32::MANTISSA_DIGITS
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const DIGITS: u32 = 6;

/// [Machine epsilon] value for `f32`.
///
/// This is the difference between `1.0` and the next larger representable number.
///
/// Equal to 2<sup>1&nbsp;&minus;&nbsp;[`MANTISSA_DIGITS`]</sup>.
///
/// [Machine epsilon]: https://en.wikipedia.org/wiki/Machine_epsilon
/// [`MANTISSA_DIGITS`]: f32::MANTISSA_DIGITS
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const EPSILON: f32 = 1.19209290e-07_f32;

/// Smallest finite `f32` value.
///
/// Equal to &minus;[`MAX`].
///
/// [`MAX`]: f32::MAX
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MIN: f32 = -3.40282347e+38_f32;
/// Smallest positive normal `f32` value.
///
/// Equal to 2<sup>[`MIN_EXP`]&nbsp;&minus;&nbsp;1</sup>.
///
/// [`MIN_EXP`]: f32::MIN_EXP
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MIN_POSITIVE: f32 = 1.17549435e-38_f32;
/// Largest finite `f32` value.
///
/// Equal to
/// (1&nbsp;&minus;&nbsp;2<sup>&minus;[`MANTISSA_DIGITS`]</sup>)&nbsp;2<sup>[`MAX_EXP`]</sup>.
///
/// [`MANTISSA_DIGITS`]: f32::MANTISSA_DIGITS
/// [`MAX_EXP`]: f32::MAX_EXP
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MAX: f32 = 3.40282347e+38_f32;

/// One greater than the minimum possible normal power of 2 exponent.
///
/// If <i>x</i>&nbsp;=&nbsp;`MIN_EXP`, then normal numbers
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/// ≥&nbsp;0.5&nbsp;×&nbsp;2<sup><i>x</i></sup>.
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MIN_EXP: i32 = -125;
/// Maximum possible power of 2 exponent.
///
/// If <i>x</i>&nbsp;=&nbsp;`MAX_EXP`, then normal numbers
/// &lt;&nbsp;1&nbsp;×&nbsp;2<sup><i>x</i></sup>.
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MAX_EXP: i32 = 128;

/// Minimum possible normal power of 10 exponent.
/// Minimum <i>x</i> for which 10<sup><i>x</i></sup> is normal.
///
/// Equal to ceil(log<sub>10</sub>&nbsp;[`MIN_POSITIVE`]).
///
/// [`MIN_POSITIVE`]: f32::MIN_POSITIVE
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MIN_10_EXP: i32 = -37;
/// Maximum possible power of 10 exponent.
/// Maximum <i>x</i> for which 10<sup><i>x</i></sup> is normal.
///
/// Equal to floor(log<sub>10</sub>&nbsp;[`MAX`]).
///
/// [`MAX`]: f32::MAX
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MAX_10_EXP: i32 = 38;

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42 changes: 40 additions & 2 deletions library/core/src/num/f64.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -356,38 +356,76 @@ impl f64 {
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MANTISSA_DIGITS: u32 = 53;
/// Approximate number of significant digits in base 10.
///
/// This is the maximum <i>x</i> such that any decimal number with <i>x</i>
/// significant digits can be converted to `f64` and back without loss.
///
/// Equal to floor(log<sub>10</sub>&nbsp;2<sup>[`MANTISSA_DIGITS`]&nbsp;&minus;&nbsp;1</sup>).
///
/// [`MANTISSA_DIGITS`]: f64::MANTISSA_DIGITS
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const DIGITS: u32 = 15;

/// [Machine epsilon] value for `f64`.
///
/// This is the difference between `1.0` and the next larger representable number.
///
/// Equal to 2<sup>1&nbsp;&minus;&nbsp;[`MANTISSA_DIGITS`]</sup>.
///
/// [Machine epsilon]: https://en.wikipedia.org/wiki/Machine_epsilon
/// [`MANTISSA_DIGITS`]: f64::MANTISSA_DIGITS
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const EPSILON: f64 = 2.2204460492503131e-16_f64;

/// Smallest finite `f64` value.
///
/// Equal to &minus;[`MAX`].
///
/// [`MAX`]: f64::MAX
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MIN: f64 = -1.7976931348623157e+308_f64;
/// Smallest positive normal `f64` value.
///
/// Equal to 2<sup>[`MIN_EXP`]&nbsp;&minus;&nbsp;1</sup>.
///
/// [`MIN_EXP`]: f64::MIN_EXP
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MIN_POSITIVE: f64 = 2.2250738585072014e-308_f64;
/// Largest finite `f64` value.
///
/// Equal to
/// (1&nbsp;&minus;&nbsp;2<sup>&minus;[`MANTISSA_DIGITS`]</sup>)&nbsp;2<sup>[`MAX_EXP`]</sup>.
///
/// [`MANTISSA_DIGITS`]: f64::MANTISSA_DIGITS
/// [`MAX_EXP`]: f64::MAX_EXP
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MAX: f64 = 1.7976931348623157e+308_f64;

/// One greater than the minimum possible normal power of 2 exponent.
///
/// If <i>x</i>&nbsp;=&nbsp;`MIN_EXP`, then normal numbers
/// ≥&nbsp;0.5&nbsp;×&nbsp;2<sup><i>x</i></sup>.
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MIN_EXP: i32 = -1021;
/// Maximum possible power of 2 exponent.
///
/// If <i>x</i>&nbsp;=&nbsp;`MAX_EXP`, then normal numbers
/// &lt;&nbsp;1&nbsp;×&nbsp;2<sup><i>x</i></sup>.
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MAX_EXP: i32 = 1024;

/// Minimum possible normal power of 10 exponent.
/// Minimum <i>x</i> for which 10<sup><i>x</i></sup> is normal.
///
/// Equal to ceil(log<sub>10</sub>&nbsp;[`MIN_POSITIVE`]).
///
/// [`MIN_POSITIVE`]: f64::MIN_POSITIVE
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MIN_10_EXP: i32 = -307;
/// Maximum possible power of 10 exponent.
/// Maximum <i>x</i> for which 10<sup><i>x</i></sup> is normal.
///
/// Equal to floor(log<sub>10</sub>&nbsp;[`MAX`]).
///
/// [`MAX`]: f64::MAX
#[stable(feature = "assoc_int_consts", since = "1.43.0")]
pub const MAX_10_EXP: i32 = 308;

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