1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
//! Parses hexadecimal float literals.
//! There are two functions `parse_hexf32` and `parse_hexf64` provided for each type.
//!
//! ```rust
//! use hexf_parse::*;
//! assert_eq!(parse_hexf32("0x1.99999ap-4", false), Ok(0.1f32));
//! assert_eq!(parse_hexf64("0x1.999999999999ap-4", false), Ok(0.1f64));
//! ```
//!
//! An additional `bool` parameter can be set to true if you want to allow underscores.
//!
//! ```rust
//! use hexf_parse::*;
//! assert!(parse_hexf64("0x0.1_7p8", false).is_err());
//! assert_eq!(parse_hexf64("0x0.1_7p8", true), Ok(23.0f64));
//! ```
//!
//! The error is reported via an opaque `ParseHexfError` type.

use std::{f32, f64, fmt, isize, str};

/// An opaque error type from `parse_hexf32` and `parse_hexf64`.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ParseHexfError {
    kind: ParseHexfErrorKind,
}

#[derive(Debug, Clone, PartialEq, Eq)]
enum ParseHexfErrorKind {
    Empty,
    Invalid,
    Inexact,
}

const EMPTY: ParseHexfError = ParseHexfError {
    kind: ParseHexfErrorKind::Empty,
};
const INVALID: ParseHexfError = ParseHexfError {
    kind: ParseHexfErrorKind::Invalid,
};
const INEXACT: ParseHexfError = ParseHexfError {
    kind: ParseHexfErrorKind::Inexact,
};

impl ParseHexfError {
    fn text(&self) -> &'static str {
        match self.kind {
            ParseHexfErrorKind::Empty => "cannot parse float from empty string",
            ParseHexfErrorKind::Invalid => "invalid hexadecimal float literal",
            ParseHexfErrorKind::Inexact => "cannot exactly represent float in target type",
        }
    }
}

impl fmt::Display for ParseHexfError {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        fmt::Display::fmt(self.text(), f)
    }
}

impl std::error::Error for ParseHexfError {
    fn description(&self) -> &'static str {
        self.text()
    }
}

fn parse(s: &[u8], allow_underscore: bool) -> Result<(bool, u64, isize), ParseHexfError> {
    // ^[+-]?
    let (s, negative) = match s.split_first() {
        Some((&b'+', s)) => (s, false),
        Some((&b'-', s)) => (s, true),
        Some(_) => (s, false),
        None => return Err(EMPTY),
    };

    // 0[xX]
    if !(s.starts_with(b"0x") || s.starts_with(b"0X")) {
        return Err(INVALID);
    }

    // ([0-9a-fA-F][0-9a-fA-F_]*)?
    let mut s = &s[2..];
    let mut acc = 0; // the accumulated mantissa
    let mut digit_seen = false;
    loop {
        let (s_, digit) = match s.split_first() {
            Some((&c @ b'0'..=b'9', s)) => (s, c - b'0'),
            Some((&c @ b'a'..=b'f', s)) => (s, c - b'a' + 10),
            Some((&c @ b'A'..=b'F', s)) => (s, c - b'A' + 10),
            Some((&b'_', s_)) if allow_underscore && digit_seen => {
                s = s_;
                continue;
            }
            _ => break,
        };

        s = s_;
        digit_seen = true;

        // if `acc << 4` fails, mantissa definitely exceeds 64 bits so we should bail out
        if acc >> 60 != 0 {
            return Err(INEXACT);
        }
        acc = acc << 4 | digit as u64;
    }

    // (\.[0-9a-fA-F][0-9a-fA-F_]*)?
    // we want to ignore trailing zeroes but shifting at each digit will overflow first.
    // therefore we separately count the number of zeroes and flush it on non-zero digits.
    let mut nfracs = 0isize; // this is suboptimal but also practical, see below
    let mut nzeroes = 0isize;
    let mut frac_digit_seen = false;
    if s.starts_with(b".") {
        s = &s[1..];
        loop {
            let (s_, digit) = match s.split_first() {
                Some((&c @ b'0'..=b'9', s)) => (s, c - b'0'),
                Some((&c @ b'a'..=b'f', s)) => (s, c - b'a' + 10),
                Some((&c @ b'A'..=b'F', s)) => (s, c - b'A' + 10),
                Some((&b'_', s_)) if allow_underscore && frac_digit_seen => {
                    s = s_;
                    continue;
                }
                _ => break,
            };

            s = s_;
            frac_digit_seen = true;

            if digit == 0 {
                nzeroes = nzeroes.checked_add(1).ok_or(INEXACT)?;
            } else {
                // flush nzeroes
                let nnewdigits = nzeroes.checked_add(1).ok_or(INEXACT)?;
                nfracs = nfracs.checked_add(nnewdigits).ok_or(INEXACT)?;
                nzeroes = 0;

                // if the accumulator is non-zero, the shift cannot exceed 64
                // (therefore the number of new digits cannot exceed 16).
                // this will catch e.g. `0.40000....00001` with sufficiently many zeroes
                if acc != 0 {
                    if nnewdigits >= 16 || acc >> (64 - nnewdigits * 4) != 0 {
                        return Err(INEXACT);
                    }
                    acc = acc << (nnewdigits * 4);
                }
                acc |= digit as u64;
            }
        }
    }

    // at least one digit should be present
    if !(digit_seen || frac_digit_seen) {
        return Err(INVALID);
    }

    // [pP]
    let s = match s.split_first() {
        Some((&b'P', s)) | Some((&b'p', s)) => s,
        _ => return Err(INVALID),
    };

    // [+-]?
    let (mut s, negative_exponent) = match s.split_first() {
        Some((&b'+', s)) => (s, false),
        Some((&b'-', s)) => (s, true),
        Some(_) => (s, false),
        None => return Err(INVALID),
    };

    // [0-9_]*[0-9][0-9_]*$
    let mut digit_seen = false;
    let mut exponent = 0isize; // this is suboptimal but also practical, see below
    loop {
        let (s_, digit) = match s.split_first() {
            Some((&c @ b'0'..=b'9', s)) => (s, c - b'0'),
            Some((&b'_', s_)) if allow_underscore => {
                s = s_;
                continue;
            }
            None if digit_seen => break,
            // no more bytes expected, and at least one exponent digit should be present
            _ => return Err(INVALID),
        };

        s = s_;
        digit_seen = true;

        // if we have no non-zero digits at this point, ignore the exponent :-)
        if acc != 0 {
            exponent = exponent
                .checked_mul(10)
                .and_then(|v| v.checked_add(digit as isize))
                .ok_or(INEXACT)?;
        }
    }
    if negative_exponent {
        exponent = -exponent;
    }

    if acc == 0 {
        // ignore the exponent as above
        Ok((negative, 0, 0))
    } else {
        // the exponent should be biased by (nfracs * 4) to match with the mantissa read.
        // we still miss valid inputs like `0.0000...0001pX` where the input is filling
        // at least 1/4 of the total addressable memory, but I dare not handle them!
        let exponent = nfracs
            .checked_mul(4)
            .and_then(|v| exponent.checked_sub(v))
            .ok_or(INEXACT)?;
        Ok((negative, acc, exponent))
    }
}

#[test]
fn test_parse() {
    assert_eq!(parse(b"", false), Err(EMPTY));
    assert_eq!(parse(b" ", false), Err(INVALID));
    assert_eq!(parse(b"3.14", false), Err(INVALID));
    assert_eq!(parse(b"0x3.14", false), Err(INVALID));
    assert_eq!(parse(b"0x3.14fp+3", false), Ok((false, 0x314f, 3 - 12)));
    assert_eq!(parse(b" 0x3.14p+3", false), Err(INVALID));
    assert_eq!(parse(b"0x3.14p+3 ", false), Err(INVALID));
    assert_eq!(parse(b"+0x3.14fp+3", false), Ok((false, 0x314f, 3 - 12)));
    assert_eq!(parse(b"-0x3.14fp+3", false), Ok((true, 0x314f, 3 - 12)));
    assert_eq!(parse(b"0xAbC.p1", false), Ok((false, 0xabc, 1)));
    assert_eq!(parse(b"0x0.7p1", false), Ok((false, 0x7, 1 - 4)));
    assert_eq!(parse(b"0x.dEfP-1", false), Ok((false, 0xdef, -1 - 12)));
    assert_eq!(parse(b"0x.p1", false), Err(INVALID));
    assert_eq!(parse(b"0x.P1", false), Err(INVALID));
    assert_eq!(parse(b"0xp1", false), Err(INVALID));
    assert_eq!(parse(b"0xP1", false), Err(INVALID));
    assert_eq!(parse(b"0x0p", false), Err(INVALID));
    assert_eq!(parse(b"0xp", false), Err(INVALID));
    assert_eq!(parse(b"0x.p", false), Err(INVALID));
    assert_eq!(parse(b"0x0p1", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x0P1", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x0.p1", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x0.P1", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x0.0p1", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x0.0P1", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x.0p1", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x.0P1", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x0p0", false), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x0.p999999999", false), Ok((false, 0, 0)));
    assert_eq!(
        parse(b"0x0.p99999999999999999999999999999", false),
        Ok((false, 0, 0))
    );
    assert_eq!(
        parse(b"0x0.p-99999999999999999999999999999", false),
        Ok((false, 0, 0))
    );
    assert_eq!(
        parse(b"0x1.p99999999999999999999999999999", false),
        Err(INEXACT)
    );
    assert_eq!(
        parse(b"0x1.p-99999999999999999999999999999", false),
        Err(INEXACT)
    );
    assert_eq!(
        parse(b"0x4.00000000000000000000p55", false),
        Ok((false, 4, 55))
    );
    assert_eq!(
        parse(b"0x4.00001000000000000000p55", false),
        Ok((false, 0x400001, 55 - 20))
    );
    assert_eq!(parse(b"0x4.00000000000000000001p55", false), Err(INEXACT));

    // underscore insertion
    assert_eq!(
        parse(b"-0x3____.1_4___p+___5___", true),
        Ok((true, 0x314, 5 - 8))
    );
    assert_eq!(parse(b"-_0x3.14p+5", true), Err(INVALID));
    assert_eq!(parse(b"_0x3.14p+5", true), Err(INVALID));
    assert_eq!(parse(b"0x_3.14p+5", true), Err(INVALID));
    assert_eq!(parse(b"0x3._14p+5", true), Err(INVALID));
    assert_eq!(parse(b"0x3.14p_+5", true), Err(INVALID));
    assert_eq!(parse(b"-0x____.1_4___p+___5___", true), Err(INVALID));
    assert_eq!(parse(b"-0x3____.____p+___5___", true), Err(INVALID));
    assert_eq!(parse(b"-0x3____.1_4___p+______", true), Err(INVALID));
    assert_eq!(parse(b"0x_p0", false), Err(INVALID));
    assert_eq!(parse(b"0x_0p0", true), Err(INVALID));
    assert_eq!(parse(b"0x_p0", true), Err(INVALID));
    assert_eq!(parse(b"0x._p0", true), Err(INVALID));
    assert_eq!(parse(b"0x._0p0", true), Err(INVALID));
    assert_eq!(parse(b"0x0._0p0", true), Err(INVALID));
    assert_eq!(parse(b"0x0_p0", true), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x.0_p0", true), Ok((false, 0, 0)));
    assert_eq!(parse(b"0x0.0_p0", true), Ok((false, 0, 0)));

    // issues
    // #11 (https://github.com/lifthrasiir/hexf/issues/11)
    assert_eq!(parse(b"0x1p-149", false), parse(b"0x1.0p-149", false));
}

macro_rules! define_convert {
    ($name:ident => $f:ident) => {
        fn $name(negative: bool, mantissa: u64, exponent: isize) -> Result<$f, ParseHexfError> {
            // guard the exponent with the definitely safe range (we will exactly bound it later)
            if exponent < -0xffff || exponent > 0xffff {
                return Err(INEXACT);
            }

            // strip the trailing zeroes in mantissa and adjust exponent.
            // we do this because a unit in the least significant bit of mantissa is
            // always safe to represent while one in the most significant bit isn't.
            let trailing = mantissa.trailing_zeros() & 63; // guard mantissa=0 case
            let mantissa = mantissa >> trailing;
            let exponent = exponent + trailing as isize;

            // normalize the exponent that the number is (1.xxxx * 2^normalexp),
            // and check for the mantissa and exponent ranges
            let leading = mantissa.leading_zeros();
            let normalexp = exponent + (63 - leading as isize);
            let mantissasize = if normalexp < $f::MIN_EXP as isize - $f::MANTISSA_DIGITS as isize {
                // the number is smaller than the minimal denormal number
                return Err(INEXACT);
            } else if normalexp < ($f::MIN_EXP - 1) as isize {
                // the number is denormal, the # of bits in the mantissa is:
                // - minimum (1) at MIN_EXP - MANTISSA_DIGITS
                // - maximum (MANTISSA_DIGITS - 1) at MIN_EXP - 2
                $f::MANTISSA_DIGITS as isize - $f::MIN_EXP as isize + normalexp + 1
            } else if normalexp < $f::MAX_EXP as isize {
                // the number is normal, the # of bits in the mantissa is fixed
                $f::MANTISSA_DIGITS as isize
            } else {
                // the number is larger than the maximal denormal number
                // ($f::MAX_EXP denotes NaN and infinities here)
                return Err(INEXACT);
            };

            if mantissa >> mantissasize == 0 {
                let mut mantissa = mantissa as $f;
                if negative {
                    mantissa = -mantissa;
                }
                // yes, powi somehow does not work!
                Ok(mantissa * (2.0 as $f).powf(exponent as $f))
            } else {
                Err(INEXACT)
            }
        }
    };
}

define_convert!(convert_hexf32 => f32);
define_convert!(convert_hexf64 => f64);

#[test]
fn test_convert_hexf32() {
    assert_eq!(convert_hexf32(false, 0, 0), Ok(0.0));
    assert_eq!(convert_hexf32(false, 1, 0), Ok(1.0));
    assert_eq!(convert_hexf32(false, 10, 0), Ok(10.0));
    assert_eq!(convert_hexf32(false, 10, 1), Ok(20.0));
    assert_eq!(convert_hexf32(false, 10, -1), Ok(5.0));
    assert_eq!(convert_hexf32(true, 0, 0), Ok(-0.0));
    assert_eq!(convert_hexf32(true, 1, 0), Ok(-1.0));

    // negative zeroes
    assert_eq!(convert_hexf32(false, 0, 0).unwrap().signum(), 1.0);
    assert_eq!(convert_hexf32(true, 0, 0).unwrap().signum(), -1.0);

    // normal truncation
    assert_eq!(
        convert_hexf32(false, 0x0000_0000_00ff_ffff, 0),
        Ok(16777215.0)
    );
    assert_eq!(
        convert_hexf32(false, 0x0000_0000_01ff_ffff, 0),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf32(false, 0xffff_ff00_0000_0000, -40),
        Ok(16777215.0)
    );
    assert_eq!(
        convert_hexf32(false, 0xffff_ff80_0000_0000, -40),
        Err(INEXACT)
    );

    // denormal truncation
    assert!(convert_hexf32(false, 0x0000_0000_007f_ffff, -149).is_ok());
    assert!(convert_hexf32(false, 0x0000_0000_00ff_ffff, -150).is_err());
    assert!(convert_hexf32(false, 0x0000_0000_00ff_fffe, -150).is_ok());
    assert!(convert_hexf32(false, 0xffff_ff00_0000_0000, -190).is_err());
    assert!(convert_hexf32(false, 0xffff_fe00_0000_0000, -190).is_ok());

    // minimum
    assert!(convert_hexf32(false, 0x0000_0000_0000_0001, -149).is_ok());
    assert!(convert_hexf32(false, 0x0000_0000_0000_0001, -150).is_err());
    assert!(convert_hexf32(false, 0x0000_0000_0000_0002, -150).is_ok());
    assert!(convert_hexf32(false, 0x0000_0000_0000_0002, -151).is_err());
    assert!(convert_hexf32(false, 0x0000_0000_0000_0003, -150).is_err());
    assert!(convert_hexf32(false, 0x0000_0000_0000_0003, -151).is_err());
    assert!(convert_hexf32(false, 0x8000_0000_0000_0000, -212).is_ok());
    assert!(convert_hexf32(false, 0x8000_0000_0000_0000, -213).is_err());

    // maximum
    assert_eq!(
        convert_hexf32(false, 0x0000_0000_00ff_ffff, 104),
        Ok(f32::MAX)
    );
    assert_eq!(
        convert_hexf32(false, 0x0000_0000_01ff_ffff, 104),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf32(false, 0x0000_0000_01ff_fffe, 104),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf32(false, 0x0000_0000_0000_0001, 128),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf32(false, 0x8000_0000_0000_0000, 65),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf32(false, 0xffff_ff00_0000_0000, 64),
        Ok(f32::MAX)
    );
    assert_eq!(
        convert_hexf32(false, 0xffff_ff80_0000_0000, 64),
        Err(INEXACT)
    );
}

#[test]
fn test_convert_hexf64() {
    assert_eq!(convert_hexf64(false, 0, 0), Ok(0.0));
    assert_eq!(convert_hexf64(false, 1, 0), Ok(1.0));
    assert_eq!(convert_hexf64(false, 10, 0), Ok(10.0));
    assert_eq!(convert_hexf64(false, 10, 1), Ok(20.0));
    assert_eq!(convert_hexf64(false, 10, -1), Ok(5.0));
    assert_eq!(convert_hexf64(true, 0, 0), Ok(-0.0));
    assert_eq!(convert_hexf64(true, 1, 0), Ok(-1.0));

    // negative zeroes
    assert_eq!(convert_hexf64(false, 0, 0).unwrap().signum(), 1.0);
    assert_eq!(convert_hexf64(true, 0, 0).unwrap().signum(), -1.0);

    // normal truncation
    assert_eq!(
        convert_hexf64(false, 0x001f_ffff_ffff_ffff, 0),
        Ok(9007199254740991.0)
    );
    assert_eq!(
        convert_hexf64(false, 0x003f_ffff_ffff_ffff, 0),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf64(false, 0xffff_ffff_ffff_f800, -11),
        Ok(9007199254740991.0)
    );
    assert_eq!(
        convert_hexf64(false, 0xffff_ffff_ffff_fc00, -11),
        Err(INEXACT)
    );

    // denormal truncation
    assert!(convert_hexf64(false, 0x000f_ffff_ffff_ffff, -1074).is_ok());
    assert!(convert_hexf64(false, 0x001f_ffff_ffff_ffff, -1075).is_err());
    assert!(convert_hexf64(false, 0x001f_ffff_ffff_fffe, -1075).is_ok());
    assert!(convert_hexf64(false, 0xffff_ffff_ffff_f800, -1086).is_err());
    assert!(convert_hexf64(false, 0xffff_ffff_ffff_f000, -1086).is_ok());

    // minimum
    assert!(convert_hexf64(false, 0x0000_0000_0000_0001, -1074).is_ok());
    assert!(convert_hexf64(false, 0x0000_0000_0000_0001, -1075).is_err());
    assert!(convert_hexf64(false, 0x0000_0000_0000_0002, -1075).is_ok());
    assert!(convert_hexf64(false, 0x0000_0000_0000_0002, -1076).is_err());
    assert!(convert_hexf64(false, 0x0000_0000_0000_0003, -1075).is_err());
    assert!(convert_hexf64(false, 0x0000_0000_0000_0003, -1076).is_err());
    assert!(convert_hexf64(false, 0x8000_0000_0000_0000, -1137).is_ok());
    assert!(convert_hexf64(false, 0x8000_0000_0000_0000, -1138).is_err());

    // maximum
    assert_eq!(
        convert_hexf64(false, 0x001f_ffff_ffff_ffff, 971),
        Ok(f64::MAX)
    );
    assert_eq!(
        convert_hexf64(false, 0x003f_ffff_ffff_ffff, 971),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf64(false, 0x003f_ffff_ffff_fffe, 971),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf32(false, 0x0000_0000_0000_0001, 1024),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf32(false, 0x8000_0000_0000_0000, 961),
        Err(INEXACT)
    );
    assert_eq!(
        convert_hexf64(false, 0xffff_ffff_ffff_f800, 960),
        Ok(f64::MAX)
    );
    assert_eq!(
        convert_hexf64(false, 0xffff_ffff_ffff_fc00, 960),
        Err(INEXACT)
    );
}

/// Tries to parse a hexadecimal float literal to `f32`.
/// The underscore is allowed only when `allow_underscore` is true.
pub fn parse_hexf32(s: &str, allow_underscore: bool) -> Result<f32, ParseHexfError> {
    let (negative, mantissa, exponent) = parse(s.as_bytes(), allow_underscore)?;
    convert_hexf32(negative, mantissa, exponent)
}

/// Tries to parse a hexadecimal float literal to `f64`.
/// The underscore is allowed only when `allow_underscore` is true.
pub fn parse_hexf64(s: &str, allow_underscore: bool) -> Result<f64, ParseHexfError> {
    let (negative, mantissa, exponent) = parse(s.as_bytes(), allow_underscore)?;
    convert_hexf64(negative, mantissa, exponent)
}

#[test]
fn test_parse_hexf() {
    // issues
    // #6 (https://github.com/lifthrasiir/hexf/issues/6)
    assert!(parse_hexf64("0x.000000000000000000102", false).is_err());
}