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fix confusion about parts required for float formatting #41859

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merged 1 commit into from
May 15, 2017
Merged

fix confusion about parts required for float formatting #41859

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8 changes: 4 additions & 4 deletions src/libcore/fmt/float.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -21,7 +21,7 @@ fn float_to_decimal_common_exact<T>(fmt: &mut Formatter, num: &T,
{
unsafe {
let mut buf: [u8; 1024] = mem::uninitialized(); // enough for f32 and f64
let mut parts: [flt2dec::Part; 5] = mem::uninitialized();
let mut parts: [flt2dec::Part; 4] = mem::uninitialized();
let formatted = flt2dec::to_exact_fixed_str(flt2dec::strategy::grisu::format_exact,
*num, sign, precision,
false, &mut buf, &mut parts);
Expand All @@ -39,7 +39,7 @@ fn float_to_decimal_common_shortest<T>(fmt: &mut Formatter,
unsafe {
// enough for f32 and f64
let mut buf: [u8; flt2dec::MAX_SIG_DIGITS] = mem::uninitialized();
let mut parts: [flt2dec::Part; 5] = mem::uninitialized();
let mut parts: [flt2dec::Part; 4] = mem::uninitialized();
let formatted = flt2dec::to_shortest_str(flt2dec::strategy::grisu::format_shortest,
*num, sign, 0, false, &mut buf, &mut parts);
fmt.pad_formatted_parts(&formatted)
Expand Down Expand Up @@ -75,7 +75,7 @@ fn float_to_exponential_common_exact<T>(fmt: &mut Formatter, num: &T,
{
unsafe {
let mut buf: [u8; 1024] = mem::uninitialized(); // enough for f32 and f64
let mut parts: [flt2dec::Part; 7] = mem::uninitialized();
let mut parts: [flt2dec::Part; 6] = mem::uninitialized();
let formatted = flt2dec::to_exact_exp_str(flt2dec::strategy::grisu::format_exact,
*num, sign, precision,
upper, &mut buf, &mut parts);
Expand All @@ -94,7 +94,7 @@ fn float_to_exponential_common_shortest<T>(fmt: &mut Formatter,
unsafe {
// enough for f32 and f64
let mut buf: [u8; flt2dec::MAX_SIG_DIGITS] = mem::uninitialized();
let mut parts: [flt2dec::Part; 7] = mem::uninitialized();
let mut parts: [flt2dec::Part; 6] = mem::uninitialized();
let formatted = flt2dec::to_shortest_exp_str(flt2dec::strategy::grisu::format_shortest,
*num, sign, (0, 0), upper,
&mut buf, &mut parts);
Expand Down
16 changes: 8 additions & 8 deletions src/libcore/num/flt2dec/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -410,8 +410,8 @@ fn determine_sign(sign: Sign, decoded: &FullDecoded, negative: bool) -> &'static
/// 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 5 parts available, due to the worst case like
/// `[+][0.][0000][45][0000]` with `frac_digits = 10`.
/// There should be at least 4 parts available, due to the worst case like
/// `[+][0.][0000][2][0000]` with `frac_digits = 10`.
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??? I still count 5 parts +, 0., 0000, 2 and 0000.

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It's unfortunate that the [] notation is being used in two ways here: to denote a Part and to--I think--indicate the optional quality of the sign of the formatted number. Since the [+] is being used in the latter sense, it does not count as a Part that would be stored in the parts array.

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OK, perhaps the + should use a different bracket or no brackets, or just omit the + altogether. Also, why the 45 is changed to 2? That doesn't seem to make any difference.

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My understanding of the use of the numbers here was that they were denoting the index of the associated Part in the parts array. So the 45 shouldn't be understood as the number "45", but parts[4] and parts[5]. (Which is not correct in the first place, I believe.) You can see this notation on display a little more clearly in to_shortest_exp_str, where we have [+][1][.][2345][e][-][6] as a result of this patch, though note that 1-based indexing is being used here rather than 0-based... I changed 45 to 2 following said understanding, as the non-zero numbers there would be parts[2].

I agree that this is not the best notation for explaining things! Perhaps a separate pull request would be appropriate to make the documentation a little clearer?

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… I think you are overthinking about this 😄 When the documentation was originally written, the + sign is indeed a part, so [+][1][.][2345][e][-][67] had 7 parts because there are 7 []s, not due to 1234567. Later the + sign is no longer a part but the documentation failed to catch up until this PR.

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Ah, indeed! :)

pub fn to_shortest_str<'a, T, F>(mut format_shortest: F, v: T,
sign: Sign, frac_digits: usize, _upper: bool,
buf: &'a mut [u8], parts: &'a mut [Part<'a>]) -> Formatted<'a>
Expand Down Expand Up @@ -465,8 +465,8 @@ pub fn to_shortest_str<'a, T, F>(mut format_shortest: F, v: T,
/// 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 7 parts available, due to the worst case like
/// `[+][1][.][2345][e][-][67]`.
/// 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 [u8], parts: &'a mut [Part<'a>]) -> Formatted<'a>
Expand Down Expand Up @@ -544,8 +544,8 @@ fn estimate_max_buf_len(exp: i16) -> usize {
/// 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 7 parts available, due to the worst case like
/// `[+][1][.][2345][e][-][67]`.
/// 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 [u8], parts: &'a mut [Part<'a>]) -> Formatted<'a>
Expand Down Expand Up @@ -600,8 +600,8 @@ pub fn to_exact_exp_str<'a, T, F>(mut format_exact: F, v: T,
/// 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 5 parts available, due to the worst case like
/// `[+][0.][0000][45][0000]` with `frac_digits = 10`.
/// 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, _upper: bool,
buf: &'a mut [u8], parts: &'a mut [Part<'a>]) -> Formatted<'a>
Expand Down