1 //! Validating and decomposing a decimal string of the form:
3 //! `(digits | digits? '.'? digits?) (('e' | 'E') ('+' | '-')? digits)?`
5 //! In other words, standard floating-point syntax, with two exceptions: No sign, and no
6 //! handling of "inf" and "NaN". These are handled by the driver function (super::dec2flt).
8 //! Although recognizing valid inputs is relatively easy, this module also has to reject the
9 //! countless invalid variations, never panic, and perform numerous checks that the other
10 //! modules rely on to not panic (or overflow) in turn.
11 //! To make matters worse, all that happens in a single pass over the input.
12 //! So, be careful when modifying anything, and double-check with the other modules.
13 use self::ParseResult
::{Invalid, ShortcutToInf, ShortcutToZero, Valid}
;
22 #[derive(Debug, PartialEq, Eq)]
23 /// The interesting parts of a decimal string.
24 pub struct Decimal
<'a
> {
25 pub integral
: &'a
[u8],
26 pub fractional
: &'a
[u8],
27 /// The decimal exponent, guaranteed to have fewer than 18 decimal digits.
31 impl<'a
> Decimal
<'a
> {
32 pub fn new(integral
: &'a
[u8], fractional
: &'a
[u8], exp
: i64) -> Decimal
<'a
> {
33 Decimal { integral, fractional, exp }
37 #[derive(Debug, PartialEq, Eq)]
38 pub enum ParseResult
<'a
> {
45 /// Checks if the input string is a valid floating point number and if so, locate the integral
46 /// part, the fractional part, and the exponent in it. Does not handle signs.
47 pub fn parse_decimal(s
: &str) -> ParseResult
<'_
> {
53 let (integral
, s
) = eat_digits(s
);
56 None
=> Valid(Decimal
::new(integral
, b
"", 0)),
57 Some(&b'e'
) | Some(&b'E'
) => {
58 if integral
.is_empty() {
59 return Invalid
; // No digits before 'e'
62 parse_exp(integral
, b
"", &s
[1..])
65 let (fractional
, s
) = eat_digits(&s
[1..]);
66 if integral
.is_empty() && fractional
.is_empty() {
67 // We require at least a single digit before or after the point.
72 None
=> Valid(Decimal
::new(integral
, fractional
, 0)),
73 Some(&b'e'
) | Some(&b'E'
) => parse_exp(integral
, fractional
, &s
[1..]),
74 _
=> Invalid
, // Trailing junk after fractional part
77 _
=> Invalid
, // Trailing junk after first digit string
81 /// Carves off decimal digits up to the first non-digit character.
82 fn eat_digits(s
: &[u8]) -> (&[u8], &[u8]) {
84 while i
< s
.len() && b'
0'
<= s
[i
] && s
[i
] <= b'
9'
{
90 /// Exponent extraction and error checking.
91 fn parse_exp
<'a
>(integral
: &'a
[u8], fractional
: &'a
[u8], rest
: &'a
[u8]) -> ParseResult
<'a
> {
92 let (sign
, rest
) = match rest
.first() {
93 Some(&b'
-'
) => (Sign
::Negative
, &rest
[1..]),
94 Some(&b'
+'
) => (Sign
::Positive
, &rest
[1..]),
95 _
=> (Sign
::Positive
, rest
),
97 let (mut number
, trailing
) = eat_digits(rest
);
98 if !trailing
.is_empty() {
99 return Invalid
; // Trailing junk after exponent
101 if number
.is_empty() {
102 return Invalid
; // Empty exponent
104 // At this point, we certainly have a valid string of digits. It may be too long to put into
105 // an `i64`, but if it's that huge, the input is certainly zero or infinity. Since each zero
106 // in the decimal digits only adjusts the exponent by +/- 1, at exp = 10^18 the input would
107 // have to be 17 exabyte (!) of zeros to get even remotely close to being finite.
108 // This is not exactly a use case we need to cater to.
109 while number
.first() == Some(&b'
0'
) {
110 number
= &number
[1..];
112 if number
.len() >= 18 {
114 Sign
::Positive
=> ShortcutToInf
,
115 Sign
::Negative
=> ShortcutToZero
,
118 let abs_exp
= num
::from_str_unchecked(number
);
120 Sign
::Positive
=> abs_exp
as i64,
121 Sign
::Negative
=> -(abs_exp
as i64),
123 Valid(Decimal
::new(integral
, fractional
, e
))