core/num/imp/dec2flt/
parse.rs

1//! Functions to parse floating-point numbers.
2
3use dec2flt::common::{ByteSlice, is_8digits};
4use dec2flt::decimal::Decimal;
5use dec2flt::float::RawFloat;
6
7use crate::num::imp::dec2flt;
8
9const MIN_19DIGIT_INT: u64 = 100_0000_0000_0000_0000;
10
11/// Parse 8 digits, loaded as bytes in little-endian order.
12///
13/// This uses the trick where every digit is in [0x030, 0x39],
14/// and therefore can be parsed in 3 multiplications, much
15/// faster than the normal 8.
16///
17/// This is based off the algorithm described in "Fast numeric string to
18/// int", available here: <https://johnnylee-sde.github.io/Fast-numeric-string-to-int/>.
19fn parse_8digits(mut v: u64) -> u64 {
20    const MASK: u64 = 0x0000_00FF_0000_00FF;
21    const MUL1: u64 = 0x000F_4240_0000_0064;
22    const MUL2: u64 = 0x0000_2710_0000_0001;
23    v -= 0x3030_3030_3030_3030;
24    v = (v * 10) + (v >> 8); // will not overflow, fits in 63 bits
25    let v1 = (v & MASK).wrapping_mul(MUL1);
26    let v2 = ((v >> 16) & MASK).wrapping_mul(MUL2);
27    ((v1.wrapping_add(v2) >> 32) as u32) as u64
28}
29
30/// Parse digits until a non-digit character is found.
31fn try_parse_digits(mut s: &[u8], mut x: u64) -> (&[u8], u64) {
32    // may cause overflows, to be handled later
33
34    while s.len() >= 8 {
35        let num = s.read_u64();
36        if is_8digits(num) {
37            x = x.wrapping_mul(1_0000_0000).wrapping_add(parse_8digits(num));
38            s = &s[8..];
39        } else {
40            break;
41        }
42    }
43
44    s = s.parse_digits(|digit| {
45        x = x.wrapping_mul(10).wrapping_add(digit as _);
46    });
47
48    (s, x)
49}
50
51/// Parse up to 19 digits (the max that can be stored in a 64-bit integer).
52fn try_parse_19digits(s_ref: &mut &[u8], x: &mut u64) {
53    let mut s = *s_ref;
54
55    while *x < MIN_19DIGIT_INT {
56        if let Some((c, s_next)) = s.split_first() {
57            let digit = c.wrapping_sub(b'0');
58
59            if digit < 10 {
60                *x = (*x * 10) + digit as u64; // no overflows here
61                s = s_next;
62            } else {
63                break;
64            }
65        } else {
66            break;
67        }
68    }
69
70    *s_ref = s;
71}
72
73/// Parse the scientific notation component of a float.
74fn parse_scientific(s_ref: &mut &[u8]) -> Option<i64> {
75    let mut exponent = 0i64;
76    let mut negative = false;
77
78    let mut s = *s_ref;
79
80    if let Some((&c, s_next)) = s.split_first() {
81        negative = c == b'-';
82        if c == b'-' || c == b'+' {
83            s = s_next;
84        }
85    }
86
87    if matches!(s.first(), Some(&x) if x.is_ascii_digit()) {
88        *s_ref = s.parse_digits(|digit| {
89            // no overflows here, saturate well before overflow
90            if exponent < 0x10000 {
91                exponent = 10 * exponent + digit as i64;
92            }
93        });
94        if negative { Some(-exponent) } else { Some(exponent) }
95    } else {
96        *s_ref = s;
97        None
98    }
99}
100
101/// Parse a partial, non-special floating point number.
102///
103/// This creates a representation of the float as the
104/// significant digits and the decimal exponent.
105fn parse_partial_number(mut s: &[u8]) -> Option<(Decimal, usize)> {
106    debug_assert!(!s.is_empty());
107
108    // parse initial digits before dot
109    let mut mantissa = 0_u64;
110    let start = s;
111    let tmp = try_parse_digits(s, mantissa);
112    s = tmp.0;
113    mantissa = tmp.1;
114    let mut n_digits = s.offset_from(start);
115
116    // handle dot with the following digits
117    let mut n_after_dot = 0;
118    let mut exponent = 0_i64;
119    let int_end = s;
120
121    if let Some((&b'.', s_next)) = s.split_first() {
122        s = s_next;
123        let before = s;
124        let tmp = try_parse_digits(s, mantissa);
125        s = tmp.0;
126        mantissa = tmp.1;
127        n_after_dot = s.offset_from(before);
128        exponent = -n_after_dot as i64;
129    }
130
131    n_digits += n_after_dot;
132    if n_digits == 0 {
133        return None;
134    }
135
136    // handle scientific format
137    let mut exp_number = 0_i64;
138    if let Some((&c, s_next)) = s.split_first() {
139        if c == b'e' || c == b'E' {
140            s = s_next;
141            // If None, we have no trailing digits after exponent, or an invalid float.
142            exp_number = parse_scientific(&mut s)?;
143            exponent += exp_number;
144        }
145    }
146
147    let len = s.offset_from(start) as _;
148
149    // handle uncommon case with many digits
150    if n_digits <= 19 {
151        return Some((Decimal { exponent, mantissa, negative: false, many_digits: false }, len));
152    }
153
154    n_digits -= 19;
155    let mut many_digits = false;
156    let mut p = start;
157    while let Some((&c, p_next)) = p.split_first() {
158        if c == b'.' || c == b'0' {
159            n_digits -= c.saturating_sub(b'0' - 1) as isize;
160            p = p_next;
161        } else {
162            break;
163        }
164    }
165    if n_digits > 0 {
166        // at this point we have more than 19 significant digits, let's try again
167        many_digits = true;
168        mantissa = 0;
169        let mut s = start;
170        try_parse_19digits(&mut s, &mut mantissa);
171        exponent = if mantissa >= MIN_19DIGIT_INT {
172            // big int
173            int_end.offset_from(s)
174        } else {
175            s = &s[1..];
176            let before = s;
177            try_parse_19digits(&mut s, &mut mantissa);
178            -s.offset_from(before)
179        } as i64;
180        // add back the explicit part
181        exponent += exp_number;
182    }
183
184    Some((Decimal { exponent, mantissa, negative: false, many_digits }, len))
185}
186
187/// Try to parse a non-special floating point number,
188/// as well as two slices with integer and fractional parts
189/// and the parsed exponent.
190pub fn parse_number(s: &[u8]) -> Option<Decimal> {
191    if let Some((float, rest)) = parse_partial_number(s) {
192        if rest == s.len() {
193            return Some(float);
194        }
195    }
196    None
197}
198
199/// Try to parse a special, non-finite float.
200pub(crate) fn parse_inf_nan<F: RawFloat>(s: &[u8], negative: bool) -> Option<F> {
201    // Since a valid string has at most the length 8, we can load
202    // all relevant characters into a u64 and work from there.
203    // This also generates much better code.
204
205    let mut register;
206    let len: usize;
207
208    // All valid strings are either of length 8 or 3.
209    if s.len() == 8 {
210        register = s.read_u64();
211        len = 8;
212    } else if s.len() == 3 {
213        let a = s[0] as u64;
214        let b = s[1] as u64;
215        let c = s[2] as u64;
216        register = (c << 16) | (b << 8) | a;
217        len = 3;
218    } else {
219        return None;
220    }
221
222    // Clear out the bits which turn ASCII uppercase characters into
223    // lowercase characters. The resulting string is all uppercase.
224    // What happens to other characters is irrelevant.
225    register &= 0xDFDFDFDFDFDFDFDF;
226
227    // u64 values corresponding to relevant cases
228    const INF_3: u64 = 0x464E49; // "INF"
229    const INF_8: u64 = 0x5954494E49464E49; // "INFINITY"
230    const NAN: u64 = 0x4E414E; // "NAN"
231
232    // Match register value to constant to parse string.
233    // Also match on the string length to catch edge cases
234    // like "inf\0\0\0\0\0".
235    let float = match (register, len) {
236        (INF_3, 3) => F::INFINITY,
237        (INF_8, 8) => F::INFINITY,
238        (NAN, 3) => F::NAN,
239        _ => return None,
240    };
241
242    if negative { Some(-float) } else { Some(float) }
243}
core/num/imp/dec2flt/parse.rs

core/num/imp/dec2flt/
parse.rs
