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