1 // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Validating and decomposing a decimal string of the form:
13 //! `(digits | digits? '.'? digits?) (('e' | 'E') ('+' | '-')? digits)?`
15 //! In other words, standard floating-point syntax, with two exceptions: No sign, and no
16 //! handling of "inf" and "NaN". These are handled by the driver function (super::dec2flt).
18 //! Although recognizing valid inputs is relatively easy, this module also has to reject the
19 //! countless invalid variations, never panic, and perform numerous checks that the other
20 //! modules rely on to not panic (or overflow) in turn.
21 //! To make matters worse, all that happens in a single pass over the input.
22 //! So, be careful when modifying anything, and double-check with the other modules.
24 use self::ParseResult::{Valid, ShortcutToInf, ShortcutToZero, Invalid};
32 #[derive(Debug, PartialEq, Eq)]
33 /// The interesting parts of a decimal string.
34 pub struct Decimal<'a> {
35 pub integral: &'a [u8],
36 pub fractional: &'a [u8],
37 /// The decimal exponent, guaranteed to have fewer than 18 decimal digits.
41 impl<'a> Decimal<'a> {
42 pub fn new(integral: &'a [u8], fractional: &'a [u8], exp: i64) -> Decimal<'a> {
43 Decimal { integral, fractional, exp }
47 #[derive(Debug, PartialEq, Eq)]
48 pub enum ParseResult<'a> {
55 /// Check if the input string is a valid floating point number and if so, locate the integral
56 /// part, the fractional part, and the exponent in it. Does not handle signs.
57 pub fn parse_decimal(s: &str) -> ParseResult {
63 let (integral, s) = eat_digits(s);
66 None => Valid(Decimal::new(integral, b"", 0)),
67 Some(&b'e') | Some(&b'E') => {
68 if integral.is_empty() {
69 return Invalid; // No digits before 'e'
72 parse_exp(integral, b"", &s[1..])
75 let (fractional, s) = eat_digits(&s[1..]);
76 if integral.is_empty() && fractional.is_empty() {
77 // We require at least a single digit before or after the point.
82 None => Valid(Decimal::new(integral, fractional, 0)),
83 Some(&b'e') | Some(&b'E') => parse_exp(integral, fractional, &s[1..]),
84 _ => Invalid, // Trailing junk after fractional part
87 _ => Invalid, // Trailing junk after first digit string
91 /// Carve off decimal digits up to the first non-digit character.
92 fn eat_digits(s: &[u8]) -> (&[u8], &[u8]) {
94 while i < s.len() && b'0' <= s[i] && s[i] <= b'9' {
100 /// Exponent extraction and error checking.
101 fn parse_exp<'a>(integral: &'a [u8], fractional: &'a [u8], rest: &'a [u8]) -> ParseResult<'a> {
102 let (sign, rest) = match rest.first() {
103 Some(&b'-') => (Sign::Negative, &rest[1..]),
104 Some(&b'+') => (Sign::Positive, &rest[1..]),
105 _ => (Sign::Positive, rest),
107 let (mut number, trailing) = eat_digits(rest);
108 if !trailing.is_empty() {
109 return Invalid; // Trailing junk after exponent
111 if number.is_empty() {
112 return Invalid; // Empty exponent
114 // At this point, we certainly have a valid string of digits. It may be too long to put into
115 // an `i64`, but if it's that huge, the input is certainly zero or infinity. Since each zero
116 // in the decimal digits only adjusts the exponent by +/- 1, at exp = 10^18 the input would
117 // have to be 17 exabyte (!) of zeros to get even remotely close to being finite.
118 // This is not exactly a use case we need to cater to.
119 while number.first() == Some(&b'0') {
120 number = &number[1..];
122 if number.len() >= 18 {
124 Sign::Positive => ShortcutToInf,
125 Sign::Negative => ShortcutToZero,
128 let abs_exp = num::from_str_unchecked(number);
130 Sign::Positive => abs_exp as i64,
131 Sign::Negative => -(abs_exp as i64),
133 Valid(Decimal::new(integral, fractional, e))