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+/*!
+
+Floating-point number to decimal conversion routines.
+
+# Problem statement
+
+We are given the floating-point number `v = f * 2^e` with an integer `f`,
+and its bounds `minus` and `plus` such that any number between `v - minus` and
+`v + plus` will be rounded to `v`. For the simplicity we assume that
+this range is exclusive. Then we would like to get the unique decimal
+representation `V = 0.d[0..n-1] * 10^k` such that:
+
+- `d[0]` is non-zero.
+
+- It's correctly rounded when parsed back: `v - minus < V < v + plus`.
+ Furthermore it is shortest such one, i.e., there is no representation
+ with less than `n` digits that is correctly rounded.
+
+- It's closest to the original value: `abs(V - v) <= 10^(k-n) / 2`. Note that
+ there might be two representations satisfying this uniqueness requirement,
+ in which case some tie-breaking mechanism is used.
+
+We will call this mode of operation as to the *shortest* mode. This mode is used
+when there is no additional constraint, and can be thought as a "natural" mode
+as it matches the ordinary intuition (it at least prints `0.1f32` as "0.1").
+
+We have two more modes of operation closely related to each other. In these modes
+we are given either the number of significant digits `n` or the last-digit
+limitation `limit` (which determines the actual `n`), and we would like to get
+the representation `V = 0.d[0..n-1] * 10^k` such that:
+
+- `d[0]` is non-zero, unless `n` was zero in which case only `k` is returned.
+
+- It's closest to the original value: `abs(V - v) <= 10^(k-n) / 2`. Again,
+ there might be some tie-breaking mechanism.
+
+When `limit` is given but not `n`, we set `n` such that `k - n = limit`
+so that the last digit `d[n-1]` is scaled by `10^(k-n) = 10^limit`.
+If such `n` is negative, we clip it to zero so that we will only get `k`.
+We are also limited by the supplied buffer. This limitation is used to print
+the number up to given number of fractional digits without knowing
+the correct `k` beforehand.
+
+We will call the mode of operation requiring `n` as to the *exact* mode,
+and one requiring `limit` as to the *fixed* mode. The exact mode is a subset of
+the fixed mode: the sufficiently large last-digit limitation will eventually fill
+the supplied buffer and let the algorithm to return.
+
+# Implementation overview
+
+It is easy to get the floating point printing correct but slow (Russ Cox has
+[demonstrated](https://research.swtch.com/ftoa) how it's easy), or incorrect but
+fast (naïve division and modulo). But it is surprisingly hard to print
+floating point numbers correctly *and* efficiently.
+
+There are two classes of algorithms widely known to be correct.
+
+- The "Dragon" family of algorithm is first described by Guy L. Steele Jr. and
+ Jon L. White. They rely on the fixed-size big integer for their correctness.
+ A slight improvement was found later, which is posthumously described by
+ Robert G. Burger and R. Kent Dybvig. David Gay's `dtoa.c` routine is
+ a popular implementation of this strategy.
+
+- The "Grisu" family of algorithm is first described by Florian Loitsch.
+ They use very cheap integer-only procedure to determine the close-to-correct
+ representation which is at least guaranteed to be shortest. The variant,
+ Grisu3, actively detects if the resulting representation is incorrect.
+
+We implement both algorithms with necessary tweaks to suit our requirements.
+In particular, published literatures are short of the actual implementation
+difficulties like how to avoid arithmetic overflows. Each implementation,
+available in `strategy::dragon` and `strategy::grisu` respectively,
+extensively describes all necessary justifications and many proofs for them.
+(It is still difficult to follow though. You have been warned.)
+
+Both implementations expose two public functions:
+
+- `format_shortest(decoded, buf)`, which always needs at least
+ `MAX_SIG_DIGITS` digits of buffer. Implements the shortest mode.
+
+- `format_exact(decoded, buf, limit)`, which accepts as small as
+ one digit of buffer. Implements exact and fixed modes.
+
+They try to fill the `u8` buffer with digits and returns the number of digits
+written and the exponent `k`. They are total for all finite `f32` and `f64`
+inputs (Grisu internally falls back to Dragon if necessary).
+
+The rendered digits are formatted into the actual string form with
+four functions:
+
+- `to_shortest_str` prints the shortest representation, which can be padded by
+ zeroes to make *at least* given number of fractional digits.
+
+- `to_shortest_exp_str` prints the shortest representation, which can be
+ padded by zeroes when its exponent is in the specified ranges,
+ or can be printed in the exponential form such as `1.23e45`.
+
+- `to_exact_exp_str` prints the exact representation with given number of
+ digits in the exponential form.
+
+- `to_exact_fixed_str` prints the fixed representation with *exactly*
+ given number of fractional digits.
+
+They all return a slice of preallocated `Part` array, which corresponds to
+the individual part of strings: a fixed string, a part of rendered digits,
+a number of zeroes or a small (`u16`) number. The caller is expected to
+provide a large enough buffer and `Part` array, and to assemble the final
+string from resulting `Part`s itself.
+
+All algorithms and formatting functions are accompanied by extensive tests
+in `coretests::num::flt2dec` module. It also shows how to use individual
+functions.
+
+*/
+
+// while this is extensively documented, this is in principle private which is
+// only made public for testing. do not expose us.
+#![doc(hidden)]
+#![unstable(
+ feature = "flt2dec",
+ reason = "internal routines only exposed for testing",
+ issue = "none"
+)]
+
+pub use self::decoder::{decode, DecodableFloat, Decoded, FullDecoded};
+
+use super::fmt::{Formatted, Part};
+use crate::mem::MaybeUninit;
+
+pub mod decoder;
+pub mod estimator;
+
+/// Digit-generation algorithms.
+pub mod strategy {
+ pub mod dragon;
+ pub mod grisu;
+}
+
+/// The minimum size of buffer necessary for the shortest mode.
+///
+/// It is a bit non-trivial to derive, but this is one plus the maximal number of
+/// significant decimal digits from formatting algorithms with the shortest result.
+/// The exact formula is `ceil(# bits in mantissa * log_10 2 + 1)`.
+pub const MAX_SIG_DIGITS: usize = 17;
+
+/// When `d` contains decimal digits, increase the last digit and propagate carry.
+/// Returns a next digit when it causes the length to change.
+#[doc(hidden)]
+pub fn round_up(d: &mut [u8]) -> Option<u8> {
+ match d.iter().rposition(|&c| c != b'9') {
+ Some(i) => {
+ // d[i+1..n] is all nines
+ d[i] += 1;
+ for j in i + 1..d.len() {
+ d[j] = b'0';
+ }
+ None
+ }
+ None if d.len() > 0 => {
+ // 999..999 rounds to 1000..000 with an increased exponent
+ d[0] = b'1';
+ for j in 1..d.len() {
+ d[j] = b'0';
+ }
+ Some(b'0')
+ }
+ None => {
+ // an empty buffer rounds up (a bit strange but reasonable)
+ Some(b'1')
+ }
+ }
+}
+
+/// Formats given decimal digits `0.<...buf...> * 10^exp` into the decimal form
+/// with at least given number of fractional digits. The result is stored to
+/// the supplied parts array and a slice of written parts is returned.
+///
+/// `frac_digits` can be less than the number of actual fractional digits in `buf`;
+/// it will be ignored and full digits will be printed. It is only used to print
+/// additional zeroes after rendered digits. Thus `frac_digits` of 0 means that
+/// it will only print given digits and nothing else.
+fn digits_to_dec_str<'a>(
+ buf: &'a [u8],
+ exp: i16,
+ frac_digits: usize,
+ parts: &'a mut [MaybeUninit<Part<'a>>],
+) -> &'a [Part<'a>] {
+ assert!(!buf.is_empty());
+ assert!(buf[0] > b'0');
+ assert!(parts.len() >= 4);
+
+ // if there is the restriction on the last digit position, `buf` is assumed to be
+ // left-padded with the virtual zeroes. the number of virtual zeroes, `nzeroes`,
+ // equals to `max(0, exp + frac_digits - buf.len())`, so that the position of
+ // the last digit `exp - buf.len() - nzeroes` is no more than `-frac_digits`:
+ //
+ // |<-virtual->|
+ // |<---- buf ---->| zeroes | exp
+ // 0. 1 2 3 4 5 6 7 8 9 _ _ _ _ _ _ x 10
+ // | | |
+ // 10^exp 10^(exp-buf.len()) 10^(exp-buf.len()-nzeroes)
+ //
+ // `nzeroes` is individually calculated for each case in order to avoid overflow.
+
+ if exp <= 0 {
+ // the decimal point is before rendered digits: [0.][000...000][1234][____]
+ let minus_exp = -(exp as i32) as usize;
+ parts[0] = MaybeUninit::new(Part::Copy(b"0."));
+ parts[1] = MaybeUninit::new(Part::Zero(minus_exp));
+ parts[2] = MaybeUninit::new(Part::Copy(buf));
+ if frac_digits > buf.len() && frac_digits - buf.len() > minus_exp {
+ parts[3] = MaybeUninit::new(Part::Zero((frac_digits - buf.len()) - minus_exp));
+ // SAFETY: we just initialized the elements `..4`.
+ unsafe { MaybeUninit::slice_assume_init_ref(&parts[..4]) }
+ } else {
+ // SAFETY: we just initialized the elements `..3`.
+ unsafe { MaybeUninit::slice_assume_init_ref(&parts[..3]) }
+ }
+ } else {
+ let exp = exp as usize;
+ if exp < buf.len() {
+ // the decimal point is inside rendered digits: [12][.][34][____]
+ parts[0] = MaybeUninit::new(Part::Copy(&buf[..exp]));
+ parts[1] = MaybeUninit::new(Part::Copy(b"."));
+ parts[2] = MaybeUninit::new(Part::Copy(&buf[exp..]));
+ if frac_digits > buf.len() - exp {
+ parts[3] = MaybeUninit::new(Part::Zero(frac_digits - (buf.len() - exp)));
+ // SAFETY: we just initialized the elements `..4`.
+ unsafe { MaybeUninit::slice_assume_init_ref(&parts[..4]) }
+ } else {
+ // SAFETY: we just initialized the elements `..3`.
+ unsafe { MaybeUninit::slice_assume_init_ref(&parts[..3]) }
+ }
+ } else {
+ // the decimal point is after rendered digits: [1234][____0000] or [1234][__][.][__].
+ parts[0] = MaybeUninit::new(Part::Copy(buf));
+ parts[1] = MaybeUninit::new(Part::Zero(exp - buf.len()));
+ if frac_digits > 0 {
+ parts[2] = MaybeUninit::new(Part::Copy(b"."));
+ parts[3] = MaybeUninit::new(Part::Zero(frac_digits));
+ // SAFETY: we just initialized the elements `..4`.
+ unsafe { MaybeUninit::slice_assume_init_ref(&parts[..4]) }
+ } else {
+ // SAFETY: we just initialized the elements `..2`.
+ unsafe { MaybeUninit::slice_assume_init_ref(&parts[..2]) }
+ }
+ }
+ }
+}
+
+/// Formats the given decimal digits `0.<...buf...> * 10^exp` into the exponential
+/// form with at least the given number of significant digits. When `upper` is `true`,
+/// the exponent will be prefixed by `E`; otherwise that's `e`. The result is
+/// stored to the supplied parts array and a slice of written parts is returned.
+///
+/// `min_digits` can be less than the number of actual significant digits in `buf`;
+/// it will be ignored and full digits will be printed. It is only used to print
+/// additional zeroes after rendered digits. Thus, `min_digits == 0` means that
+/// it will only print the given digits and nothing else.
+fn digits_to_exp_str<'a>(
+ buf: &'a [u8],
+ exp: i16,
+ min_ndigits: usize,
+ upper: bool,
+ parts: &'a mut [MaybeUninit<Part<'a>>],
+) -> &'a [Part<'a>] {
+ assert!(!buf.is_empty());
+ assert!(buf[0] > b'0');
+ assert!(parts.len() >= 6);
+
+ let mut n = 0;
+
+ parts[n] = MaybeUninit::new(Part::Copy(&buf[..1]));
+ n += 1;
+
+ if buf.len() > 1 || min_ndigits > 1 {
+ parts[n] = MaybeUninit::new(Part::Copy(b"."));
+ parts[n + 1] = MaybeUninit::new(Part::Copy(&buf[1..]));
+ n += 2;
+ if min_ndigits > buf.len() {
+ parts[n] = MaybeUninit::new(Part::Zero(min_ndigits - buf.len()));
+ n += 1;
+ }
+ }
+
+ // 0.1234 x 10^exp = 1.234 x 10^(exp-1)
+ let exp = exp as i32 - 1; // avoid underflow when exp is i16::MIN
+ if exp < 0 {
+ parts[n] = MaybeUninit::new(Part::Copy(if upper { b"E-" } else { b"e-" }));
+ parts[n + 1] = MaybeUninit::new(Part::Num(-exp as u16));
+ } else {
+ parts[n] = MaybeUninit::new(Part::Copy(if upper { b"E" } else { b"e" }));
+ parts[n + 1] = MaybeUninit::new(Part::Num(exp as u16));
+ }
+ // SAFETY: we just initialized the elements `..n + 2`.
+ unsafe { MaybeUninit::slice_assume_init_ref(&parts[..n + 2]) }
+}
+
+/// Sign formatting options.
+#[derive(Copy, Clone, PartialEq, Eq, Debug)]
+pub enum Sign {
+ /// Prints `-` for any negative value.
+ Minus, // -inf -1 -0 0 1 inf nan
+ /// Prints `-` for any negative value, or `+` otherwise.
+ MinusPlus, // -inf -1 -0 +0 +1 +inf nan
+}
+
+/// Returns the static byte string corresponding to the sign to be formatted.
+/// It can be either `""`, `"+"` or `"-"`.
+fn determine_sign(sign: Sign, decoded: &FullDecoded, negative: bool) -> &'static str {
+ match (*decoded, sign) {
+ (FullDecoded::Nan, _) => "",
+ (_, Sign::Minus) => {
+ if negative {
+ "-"
+ } else {
+ ""
+ }
+ }
+ (_, Sign::MinusPlus) => {
+ if negative {
+ "-"
+ } else {
+ "+"
+ }
+ }
+ }
+}
+
+/// Formats the given floating point number into the decimal form with at least
+/// given number of fractional digits. The result is stored to the supplied parts
+/// array while utilizing given byte buffer as a scratch. `upper` is currently
+/// unused but left for the future decision to change the case of non-finite values,
+/// i.e., `inf` and `nan`. The first part to be rendered is always a `Part::Sign`
+/// (which can be an empty string if no sign is rendered).
+///
+/// `format_shortest` should be the underlying digit-generation function.
+/// It should return the part of the buffer that it initialized.
+/// You probably would want `strategy::grisu::format_shortest` for this.
+///
+/// `frac_digits` can be less than the number of actual fractional digits in `v`;
+/// it will be ignored and full digits will be printed. It is only used to print
+/// additional zeroes after rendered digits. Thus `frac_digits` of 0 means that
+/// it will only print given digits and nothing else.
+///
+/// The byte buffer should be at least `MAX_SIG_DIGITS` bytes long.
+/// There should be at least 4 parts available, due to the worst case like
+/// `[+][0.][0000][2][0000]` with `frac_digits = 10`.
+pub fn to_shortest_str<'a, T, F>(
+ mut format_shortest: F,
+ v: T,
+ sign: Sign,
+ frac_digits: usize,
+ buf: &'a mut [MaybeUninit<u8>],
+ parts: &'a mut [MaybeUninit<Part<'a>>],
+) -> Formatted<'a>
+where
+ T: DecodableFloat,
+ F: FnMut(&Decoded, &'a mut [MaybeUninit<u8>]) -> (&'a [u8], i16),
+{
+ assert!(parts.len() >= 4);
+ assert!(buf.len() >= MAX_SIG_DIGITS);
+
+ let (negative, full_decoded) = decode(v);
+ let sign = determine_sign(sign, &full_decoded, negative);
+ match full_decoded {
+ FullDecoded::Nan => {
+ parts[0] = MaybeUninit::new(Part::Copy(b"NaN"));
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Infinite => {
+ parts[0] = MaybeUninit::new(Part::Copy(b"inf"));
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Zero => {
+ if frac_digits > 0 {
+ // [0.][0000]
+ parts[0] = MaybeUninit::new(Part::Copy(b"0."));
+ parts[1] = MaybeUninit::new(Part::Zero(frac_digits));
+ Formatted {
+ sign,
+ // SAFETY: we just initialized the elements `..2`.
+ parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..2]) },
+ }
+ } else {
+ parts[0] = MaybeUninit::new(Part::Copy(b"0"));
+ Formatted {
+ sign,
+ // SAFETY: we just initialized the elements `..1`.
+ parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) },
+ }
+ }
+ }
+ FullDecoded::Finite(ref decoded) => {
+ let (buf, exp) = format_shortest(decoded, buf);
+ Formatted { sign, parts: digits_to_dec_str(buf, exp, frac_digits, parts) }
+ }
+ }
+}
+
+/// Formats the given floating point number into the decimal form or
+/// the exponential form, depending on the resulting exponent. The result is
+/// stored to the supplied parts array while utilizing given byte buffer
+/// as a scratch. `upper` is used to determine the case of non-finite values
+/// (`inf` and `nan`) or the case of the exponent prefix (`e` or `E`).
+/// The first part to be rendered is always a `Part::Sign` (which can be
+/// an empty string if no sign is rendered).
+///
+/// `format_shortest` should be the underlying digit-generation function.
+/// It should return the part of the buffer that it initialized.
+/// You probably would want `strategy::grisu::format_shortest` for this.
+///
+/// The `dec_bounds` is a tuple `(lo, hi)` such that the number is formatted
+/// as decimal only when `10^lo <= V < 10^hi`. Note that this is the *apparent* `V`
+/// instead of the actual `v`! Thus any printed exponent in the exponential form
+/// cannot be in this range, avoiding any confusion.
+///
+/// The byte buffer should be at least `MAX_SIG_DIGITS` bytes long.
+/// There should be at least 6 parts available, due to the worst case like
+/// `[+][1][.][2345][e][-][6]`.
+pub fn to_shortest_exp_str<'a, T, F>(
+ mut format_shortest: F,
+ v: T,
+ sign: Sign,
+ dec_bounds: (i16, i16),
+ upper: bool,
+ buf: &'a mut [MaybeUninit<u8>],
+ parts: &'a mut [MaybeUninit<Part<'a>>],
+) -> Formatted<'a>
+where
+ T: DecodableFloat,
+ F: FnMut(&Decoded, &'a mut [MaybeUninit<u8>]) -> (&'a [u8], i16),
+{
+ assert!(parts.len() >= 6);
+ assert!(buf.len() >= MAX_SIG_DIGITS);
+ assert!(dec_bounds.0 <= dec_bounds.1);
+
+ let (negative, full_decoded) = decode(v);
+ let sign = determine_sign(sign, &full_decoded, negative);
+ match full_decoded {
+ FullDecoded::Nan => {
+ parts[0] = MaybeUninit::new(Part::Copy(b"NaN"));
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Infinite => {
+ parts[0] = MaybeUninit::new(Part::Copy(b"inf"));
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Zero => {
+ parts[0] = if dec_bounds.0 <= 0 && 0 < dec_bounds.1 {
+ MaybeUninit::new(Part::Copy(b"0"))
+ } else {
+ MaybeUninit::new(Part::Copy(if upper { b"0E0" } else { b"0e0" }))
+ };
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Finite(ref decoded) => {
+ let (buf, exp) = format_shortest(decoded, buf);
+ let vis_exp = exp as i32 - 1;
+ let parts = if dec_bounds.0 as i32 <= vis_exp && vis_exp < dec_bounds.1 as i32 {
+ digits_to_dec_str(buf, exp, 0, parts)
+ } else {
+ digits_to_exp_str(buf, exp, 0, upper, parts)
+ };
+ Formatted { sign, parts }
+ }
+ }
+}
+
+/// Returns a rather crude approximation (upper bound) for the maximum buffer size
+/// calculated from the given decoded exponent.
+///
+/// The exact limit is:
+///
+/// - when `exp < 0`, the maximum length is `ceil(log_10 (5^-exp * (2^64 - 1)))`.
+/// - when `exp >= 0`, the maximum length is `ceil(log_10 (2^exp * (2^64 - 1)))`.
+///
+/// `ceil(log_10 (x^exp * (2^64 - 1)))` is less than `ceil(log_10 (2^64 - 1)) +
+/// ceil(exp * log_10 x)`, which is in turn less than `20 + (1 + exp * log_10 x)`.
+/// We use the facts that `log_10 2 < 5/16` and `log_10 5 < 12/16`, which is
+/// enough for our purposes.
+///
+/// Why do we need this? `format_exact` functions will fill the entire buffer
+/// unless limited by the last digit restriction, but it is possible that
+/// the number of digits requested is ridiculously large (say, 30,000 digits).
+/// The vast majority of buffer will be filled with zeroes, so we don't want to
+/// allocate all the buffer beforehand. Consequently, for any given arguments,
+/// 826 bytes of buffer should be sufficient for `f64`. Compare this with
+/// the actual number for the worst case: 770 bytes (when `exp = -1074`).
+fn estimate_max_buf_len(exp: i16) -> usize {
+ 21 + ((if exp < 0 { -12 } else { 5 } * exp as i32) as usize >> 4)
+}
+
+/// Formats given floating point number into the exponential form with
+/// exactly given number of significant digits. The result is stored to
+/// the supplied parts array while utilizing given byte buffer as a scratch.
+/// `upper` is used to determine the case of the exponent prefix (`e` or `E`).
+/// The first part to be rendered is always a `Part::Sign` (which can be
+/// an empty string if no sign is rendered).
+///
+/// `format_exact` should be the underlying digit-generation function.
+/// It should return the part of the buffer that it initialized.
+/// You probably would want `strategy::grisu::format_exact` for this.
+///
+/// The byte buffer should be at least `ndigits` bytes long unless `ndigits` is
+/// so large that only the fixed number of digits will be ever written.
+/// (The tipping point for `f64` is about 800, so 1000 bytes should be enough.)
+/// There should be at least 6 parts available, due to the worst case like
+/// `[+][1][.][2345][e][-][6]`.
+pub fn to_exact_exp_str<'a, T, F>(
+ mut format_exact: F,
+ v: T,
+ sign: Sign,
+ ndigits: usize,
+ upper: bool,
+ buf: &'a mut [MaybeUninit<u8>],
+ parts: &'a mut [MaybeUninit<Part<'a>>],
+) -> Formatted<'a>
+where
+ T: DecodableFloat,
+ F: FnMut(&Decoded, &'a mut [MaybeUninit<u8>], i16) -> (&'a [u8], i16),
+{
+ assert!(parts.len() >= 6);
+ assert!(ndigits > 0);
+
+ let (negative, full_decoded) = decode(v);
+ let sign = determine_sign(sign, &full_decoded, negative);
+ match full_decoded {
+ FullDecoded::Nan => {
+ parts[0] = MaybeUninit::new(Part::Copy(b"NaN"));
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Infinite => {
+ parts[0] = MaybeUninit::new(Part::Copy(b"inf"));
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Zero => {
+ if ndigits > 1 {
+ // [0.][0000][e0]
+ parts[0] = MaybeUninit::new(Part::Copy(b"0."));
+ parts[1] = MaybeUninit::new(Part::Zero(ndigits - 1));
+ parts[2] = MaybeUninit::new(Part::Copy(if upper { b"E0" } else { b"e0" }));
+ Formatted {
+ sign,
+ // SAFETY: we just initialized the elements `..3`.
+ parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..3]) },
+ }
+ } else {
+ parts[0] = MaybeUninit::new(Part::Copy(if upper { b"0E0" } else { b"0e0" }));
+ Formatted {
+ sign,
+ // SAFETY: we just initialized the elements `..1`.
+ parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) },
+ }
+ }
+ }
+ FullDecoded::Finite(ref decoded) => {
+ let maxlen = estimate_max_buf_len(decoded.exp);
+ assert!(buf.len() >= ndigits || buf.len() >= maxlen);
+
+ let trunc = if ndigits < maxlen { ndigits } else { maxlen };
+ let (buf, exp) = format_exact(decoded, &mut buf[..trunc], i16::MIN);
+ Formatted { sign, parts: digits_to_exp_str(buf, exp, ndigits, upper, parts) }
+ }
+ }
+}
+
+/// Formats given floating point number into the decimal form with exactly
+/// given number of fractional digits. The result is stored to the supplied parts
+/// array while utilizing given byte buffer as a scratch. `upper` is currently
+/// unused but left for the future decision to change the case of non-finite values,
+/// i.e., `inf` and `nan`. The first part to be rendered is always a `Part::Sign`
+/// (which can be an empty string if no sign is rendered).
+///
+/// `format_exact` should be the underlying digit-generation function.
+/// It should return the part of the buffer that it initialized.
+/// You probably would want `strategy::grisu::format_exact` for this.
+///
+/// The byte buffer should be enough for the output unless `frac_digits` is
+/// so large that only the fixed number of digits will be ever written.
+/// (The tipping point for `f64` is about 800, and 1000 bytes should be enough.)
+/// There should be at least 4 parts available, due to the worst case like
+/// `[+][0.][0000][2][0000]` with `frac_digits = 10`.
+pub fn to_exact_fixed_str<'a, T, F>(
+ mut format_exact: F,
+ v: T,
+ sign: Sign,
+ frac_digits: usize,
+ buf: &'a mut [MaybeUninit<u8>],
+ parts: &'a mut [MaybeUninit<Part<'a>>],
+) -> Formatted<'a>
+where
+ T: DecodableFloat,
+ F: FnMut(&Decoded, &'a mut [MaybeUninit<u8>], i16) -> (&'a [u8], i16),
+{
+ assert!(parts.len() >= 4);
+
+ let (negative, full_decoded) = decode(v);
+ let sign = determine_sign(sign, &full_decoded, negative);
+ match full_decoded {
+ FullDecoded::Nan => {
+ parts[0] = MaybeUninit::new(Part::Copy(b"NaN"));
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Infinite => {
+ parts[0] = MaybeUninit::new(Part::Copy(b"inf"));
+ // SAFETY: we just initialized the elements `..1`.
+ Formatted { sign, parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) } }
+ }
+ FullDecoded::Zero => {
+ if frac_digits > 0 {
+ // [0.][0000]
+ parts[0] = MaybeUninit::new(Part::Copy(b"0."));
+ parts[1] = MaybeUninit::new(Part::Zero(frac_digits));
+ Formatted {
+ sign,
+ // SAFETY: we just initialized the elements `..2`.
+ parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..2]) },
+ }
+ } else {
+ parts[0] = MaybeUninit::new(Part::Copy(b"0"));
+ Formatted {
+ sign,
+ // SAFETY: we just initialized the elements `..1`.
+ parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) },
+ }
+ }
+ }
+ FullDecoded::Finite(ref decoded) => {
+ let maxlen = estimate_max_buf_len(decoded.exp);
+ assert!(buf.len() >= maxlen);
+
+ // 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 };
+ let (buf, exp) = 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`.
+ debug_assert_eq!(buf.len(), 0);
+ if frac_digits > 0 {
+ // [0.][0000]
+ parts[0] = MaybeUninit::new(Part::Copy(b"0."));
+ parts[1] = MaybeUninit::new(Part::Zero(frac_digits));
+ Formatted {
+ sign,
+ // SAFETY: we just initialized the elements `..2`.
+ parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..2]) },
+ }
+ } else {
+ parts[0] = MaybeUninit::new(Part::Copy(b"0"));
+ Formatted {
+ sign,
+ // SAFETY: we just initialized the elements `..1`.
+ parts: unsafe { MaybeUninit::slice_assume_init_ref(&parts[..1]) },
+ }
+ }
+ } else {
+ Formatted { sign, parts: digits_to_dec_str(buf, exp, frac_digits, parts) }
+ }
+ }
+ }
+}
generated by cgit v1.2.3 (git 2.39.1) at 2024-07-10 06:57:31 +0000