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//! This crate provides low-level functions to convert floating point //! numbers (`f32` and `f64`) to decimal strings and vice versa. //! //! Implementing accurate float-string conversion is non-trivial (see //! for example "Printing floating-point numbers: An always correct //! method" by Marc Andrysco, Ranjit Jhala and Sorin Lerner). The Rust //! Standard Library internally uses complex algorithms (found in //! `libcore/num`) to fulfill this purpose, but they are not exposed //! and can only be used through `FromStr::from_str`, `ToString::to_string` //! or `Display::fmt`. //! //! These functions impose a format on the string representation //! of floating point numbers, which may not always be suitable for you. //! For example, you may want to write `1.2 * 10^4` instead of `1.2e4`. //! //! This crate exposes the core algorithms, allowing you to implement //! your custom float-string conversions without worrying about complex //! mathematical part. //! //! The functionality of this crate is provided through the `FloatExt` //! trait, which is implemented for `f32` and `f64`. //! //! # Minimum Rust version //! //! The minimum Rust version required by this crate is 1.34. //! //! # Example (float to string) //! //! ``` //! // Let's say we want to convert this value to a string //! // in exponential form. //! let value = 1.25e20; //! //! // Using the standard library, you can use `format!("{:e}")`: //! assert_eq!(format!("{:e}", value), "1.25e20"); //! // It also allows tu use a capital 'E': //! assert_eq!(format!("{:E}", value), "1.25E20"); //! // or to include an explicit '+': //! assert_eq!(format!("{:+e}", value), "+1.25e20"); //! // or to used a fixed number of digits: //! assert_eq!(format!("{:.04e}", value), "1.2500e20"); //! //! // However, besides those options, `format!` imposes the //! // format of the string representation of the floating point //! // number (using `.` as decimal separator and `e` or `E` as //! // exponential separator). //! //! // This crate provides low-level functions to conver floating point //! // numbers to strings without an imposed format. //! use flt2dec2flt::FloatExt as _; //! //! // The `FloatExt::preformat_*` functions pre-converts the floating //! // point numbers into string, providing a decomposed intermediate //! // result. //! //! let mut buf = [0; flt2dec2flt::PREFORMAT_SHORTEST_BUF_LEN]; //! // You could also use `f32::preformat_shortest(value, &mut buf)` //! let preformatted = value.preformat_shortest(&mut buf); //! // `false` means the the number is positive, `b"125"` are the //! // significant digits and `21` is the exponent (such as //! // `1.25e20 == 0.125e21`) //! assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"125", 21)); //! //! // From this decomposed form, you can now build your custom string //! // representation of the floating point number. //! ``` //! //! # Example (string to float) //! //! ``` //! use std::str::FromStr as _; //! // Let's say you want to convert a string to a floating //! // point number. //! //! // Using the standard library, you can use `FromStr::from_str`: //! assert_eq!(f32::from_str("1.25e20").unwrap(), 1.25e20); //! //! // However, this function imposes the format of the input string. //! //! // This crate provides functions to convert a pre-parsed string to //! // a floating. //! use flt2dec2flt::FloatExt as _; //! //! // You have to implement your pre-parsing based on your string format. //! // So, `1.25e20` (or `1.25*10^20`) would be pre-parsed as: //! let preparsed = flt2dec2flt::PreParsed { //! // positive //! sign: false, //! // digits of the integer part //! int_digits: b"1", //! // digits of the fractional part //! frac_digits: b"25", //! // exponent //! exp: 20, //! }; //! // Which can be converted to a floating point number: //! assert_eq!(f32::from_preparsed(preparsed).unwrap(), 1.25e20); //! ``` #![deny( rust_2018_idioms, trivial_casts, trivial_numeric_casts, unreachable_pub, unused_must_use, unused_qualifications )] #![forbid(unsafe_code)] #![no_std] #[cfg(test)] extern crate std; #[rustfmt::skip] #[allow(clippy::all, trivial_numeric_casts, unreachable_pub, unused_qualifications)] mod core_num; #[cfg(test)] mod tests; mod sealed { pub trait Sealed {} } /// Minimum buffer size that has to be passed to `FloatExt::preformat_shortest`. pub const PREFORMAT_SHORTEST_BUF_LEN: usize = core_num::flt2dec::MAX_SIG_DIGITS; /// Minimum base buffer size that has to be passed to `FloatExt::preformat_exact_fixed`. // See comment in `core_num::flt2dec::estimate_max_buf_len` for the origin of the value pub const PREFORMAT_EXACT_FIXED_BASE_BUF_LEN: usize = 826; /// Represents a pre-formatted floating point number. /// /// Returned by `flt2dec2flt::f{32,64}::format_{shortest,exact_fixed,exact_exp}`. #[derive(Copy, Clone, Debug, PartialEq, Eq)] pub enum PreFormatted<'a> { /// The number is NaN. NaN, /// The is infinity. The boolean specifies the sign. Inf(bool), /// The number is (after a possible rounding made by the representation) absolute /// zero. The boolean specifies the sign. Zero(bool), /// The number is finite. The boolean specifies the sign, the slice /// specifies the mantissa digits and the integer specifies the /// exponent. /// /// The represented value is `sign 0.mant * 10 ^ exp` Finite(bool, &'a [u8], i16), } /// A pre-parsed decimal floating point number. /// /// The represented value is `sign int_digits.frac_digits * 10 ^ exp`. /// /// Passed to `flt2dec2flt::f{32,64}::from_preparsed`. #[derive(Copy, Clone, Debug, PartialEq, Eq)] pub struct PreParsed<'a> { pub sign: bool, pub int_digits: &'a [u8], pub frac_digits: &'a [u8], pub exp: i16, } /// This trait is used to extend `f32` and `f64`. /// /// Provides low-level methods to convert floating point numbers /// to decimal strings and vice versa. pub trait FloatExt: sealed::Sealed + Sized { /// Pre-formats `self` with the lowest lowest number of significant /// digits without lossing precision. /// /// `buf` must be at least `flt2dec2flt::PREFORMAT_SHORTEST_BUF_LEN` long. /// /// # Example /// /// ``` /// use flt2dec2flt::FloatExt as _; /// /// let mut buf = [0; flt2dec2flt::PREFORMAT_SHORTEST_BUF_LEN]; /// /// let preformatted = f32::preformat_shortest(12.34, &mut buf); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"1234", 2)); /// /// let preformatted = f32::preformat_shortest(0.00401, &mut buf); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"401", -2)); /// /// let preformatted = f32::preformat_shortest(330.0, &mut buf); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"33", 3)); /// /// let preformatted = f32::preformat_shortest(4.58e31, &mut buf); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"458", 32)); /// /// let preformatted = f32::preformat_shortest(4.58e-31, &mut buf); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"458", -30)); /// ``` fn preformat_shortest(self, buf: &mut [u8]) -> PreFormatted<'_>; /// Pre-formats a `f32` with an exact number of significant digits. /// /// `buf` must be at least `num_digits` long. /// /// # Example /// /// ``` /// use flt2dec2flt::FloatExt as _; /// /// let mut buf = [0; 10]; /// /// let preformatted = f32::preformat_exact_exp(200.0, &mut buf, 2); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"20", 3)); /// /// let preformatted = f32::preformat_exact_exp(0.012, &mut buf, 3); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"120", -1)); /// /// let preformatted = f32::preformat_exact_exp(12.34, &mut buf, 5); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"12340", 2)); /// /// let preformatted = f32::preformat_exact_exp(12.3456, &mut buf, 5); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"12346", 2)); /// /// let preformatted = f32::preformat_exact_exp(4.0, &mut buf, 10); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"4000000000", 1)); /// ``` fn preformat_exact_exp(self, buf: &mut [u8], num_digits: usize) -> PreFormatted<'_>; /// Pre-formats a `f32` with an exact number of fractional digits. /// /// `buf` must be at least `flt2dec2flt::PREFORMAT_EXACT_FIXED_BASE_BUF_LEN + num_frac_digits`. /// /// ``` /// use flt2dec2flt::FloatExt as _; /// /// let mut buf = [0; flt2dec2flt::PREFORMAT_EXACT_FIXED_BASE_BUF_LEN + 10]; /// /// let preformatted = f32::preformat_exact_fixed(12.34, &mut buf, 4); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"123400", 2)); /// /// let preformatted = f32::preformat_exact_fixed(12.3456, &mut buf, 2); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"1235", 2)); /// /// let preformatted = f32::preformat_exact_fixed(200.0, &mut buf, 2); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"20000", 3)); /// /// // Note that leading zeros count as digits but are omitted. /// let preformatted = f32::preformat_exact_fixed(0.03, &mut buf, 3); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Finite(false, b"30", -1)); /// /// let preformatted = f32::preformat_exact_fixed(0.3e-4, &mut buf, 2); /// assert_eq!(preformatted, flt2dec2flt::PreFormatted::Zero(false)); /// ``` fn preformat_exact_fixed(self, buf: &mut [u8], num_frac_digits: usize) -> PreFormatted<'_>; /// Creates a floating point number from a pre-parsed decimal /// floating point number (see `PreParsed`). /// /// # Example /// /// ``` /// use flt2dec2flt::FloatExt as _; /// /// let v = f32::from_preparsed(flt2dec2flt::PreParsed { /// sign: false, /// int_digits: b"12", /// frac_digits: b"34", /// exp: 0, /// }); /// assert!((v.unwrap() - 12.34).abs() < 1e-9); /// /// let v = f32::from_preparsed(flt2dec2flt::PreParsed { /// sign: false, /// int_digits: b"0", /// frac_digits: b"41", /// exp: -4, /// }); /// assert!((v.unwrap() - 0.41e-4).abs() < 1e-12); /// ``` fn from_preparsed(preparsed: PreParsed<'_>) -> Option<Self>; } mod generic { use crate::core_num::flt2dec::decoder::DecodableFloat; use crate::{core_num, PreFormatted, PreParsed}; pub(crate) fn preformat_shortest<T: DecodableFloat>(v: T, buf: &mut [u8]) -> PreFormatted<'_> { let (sign, full_decoded) = core_num::flt2dec::decoder::decode(v); match full_decoded { core_num::flt2dec::decoder::FullDecoded::Nan => PreFormatted::NaN, core_num::flt2dec::decoder::FullDecoded::Infinite => PreFormatted::Inf(sign), core_num::flt2dec::decoder::FullDecoded::Zero => PreFormatted::Zero(sign), core_num::flt2dec::decoder::FullDecoded::Finite(ref decoded) => { let (digits, exp) = core_num::flt2dec::strategy::grisu::format_shortest(decoded, buf); PreFormatted::Finite(sign, &buf[..digits], exp) } } } pub(crate) fn preformat_exact_exp<T: DecodableFloat>( v: T, buf: &mut [u8], ndigits: usize, ) -> PreFormatted<'_> { let (sign, full_decoded) = core_num::flt2dec::decoder::decode(v); match full_decoded { core_num::flt2dec::decoder::FullDecoded::Nan => PreFormatted::NaN, core_num::flt2dec::decoder::FullDecoded::Infinite => PreFormatted::Inf(sign), core_num::flt2dec::decoder::FullDecoded::Zero => PreFormatted::Zero(sign), core_num::flt2dec::decoder::FullDecoded::Finite(ref decoded) => { // Similar as done in `core::num::flt2dec::to_exact_exp_str` let maxlen = core_num::flt2dec::estimate_max_buf_len(decoded.exp); let trunc = if ndigits < maxlen { ndigits } else { maxlen }; let (digits, exp) = core_num::flt2dec::strategy::grisu::format_exact( decoded, &mut buf[..trunc], i16::min_value(), ); for chr in buf[digits..ndigits].iter_mut() { *chr = b'0'; } PreFormatted::Finite(sign, &buf[..ndigits], exp) } } } pub(crate) fn preformat_exact_fixed<T: DecodableFloat>( v: T, buf: &mut [u8], frac_digits: usize, ) -> PreFormatted<'_> { let (sign, full_decoded) = core_num::flt2dec::decoder::decode(v); match full_decoded { core_num::flt2dec::decoder::FullDecoded::Nan => PreFormatted::NaN, core_num::flt2dec::decoder::FullDecoded::Infinite => PreFormatted::Inf(sign), core_num::flt2dec::decoder::FullDecoded::Zero => PreFormatted::Zero(sign), core_num::flt2dec::decoder::FullDecoded::Finite(ref decoded) => { // Similar as done in `core::num::flt2dec::to_exact_fixed_str` let maxlen = core_num::flt2dec::estimate_max_buf_len(decoded.exp); // it *is* possible that `frac_digits` is ridiculously large. // `format_exact` will end rendering digits much earlier in this case, // because we are strictly limited by `maxlen`. let limit = if frac_digits < 0x8000 { -(frac_digits as i16) } else { i16::min_value() }; let (len, exp) = core_num::flt2dec::strategy::grisu::format_exact( decoded, &mut buf[..maxlen], limit, ); if exp <= limit { // the restriction couldn't been met, so this should render like zero no matter // `exp` was. this does not include the case that the restriction has been met // only after the final rounding-up; it's a regular case with `exp = limit + 1`. PreFormatted::Zero(sign) } else { let ndigits = if exp > 0 { let ndigits = frac_digits + exp as usize; for c in buf[len..ndigits].iter_mut() { *c = b'0'; } ndigits } else { len }; PreFormatted::Finite(sign, &buf[..ndigits], exp) } } } } pub(crate) fn from_preparsed<T: core_num::dec2flt::rawfp::RawFloat>( preparsed: PreParsed<'_>, ) -> Option<T> { // `core_num::dec2flt` does not handle cases where `exp` has // more than 18 digits, but it cannot be the case here because // we use `i16`. let parsed = core_num::dec2flt::parse::Decimal::new( preparsed.int_digits, preparsed.frac_digits, i64::from(preparsed.exp), ); let v = core_num::dec2flt::convert::<T>(parsed).ok()?; if preparsed.sign { Some(-v) } else { Some(v) } } } impl sealed::Sealed for f32 {} impl sealed::Sealed for f64 {} impl FloatExt for f32 { fn preformat_shortest(self, buf: &mut [u8]) -> PreFormatted<'_> { generic::preformat_shortest(self, buf) } fn preformat_exact_exp(self, buf: &mut [u8], num_digits: usize) -> PreFormatted<'_> { generic::preformat_exact_exp(self, buf, num_digits) } fn preformat_exact_fixed(self, buf: &mut [u8], num_frac_digits: usize) -> PreFormatted<'_> { generic::preformat_exact_fixed(self, buf, num_frac_digits) } fn from_preparsed(preparsed: PreParsed<'_>) -> Option<Self> { generic::from_preparsed(preparsed) } } impl FloatExt for f64 { fn preformat_shortest(self, buf: &mut [u8]) -> PreFormatted<'_> { generic::preformat_shortest(self, buf) } fn preformat_exact_exp(self, buf: &mut [u8], num_digits: usize) -> PreFormatted<'_> { generic::preformat_exact_exp(self, buf, num_digits) } fn preformat_exact_fixed(self, buf: &mut [u8], num_frac_digits: usize) -> PreFormatted<'_> { generic::preformat_exact_fixed(self, buf, num_frac_digits) } fn from_preparsed(preparsed: PreParsed<'_>) -> Option<Self> { generic::from_preparsed(preparsed) } }