minimal_lexical/
bigint.rs

1//! A simple big-integer type for slow path algorithms.
2//!
3//! This includes minimal stackvector for use in big-integer arithmetic.
4
5#![doc(hidden)]
6
7#[cfg(feature = "alloc")]
8use crate::heapvec::HeapVec;
9use crate::num::Float;
10#[cfg(not(feature = "alloc"))]
11use crate::stackvec::StackVec;
12#[cfg(not(feature = "compact"))]
13use crate::table::{LARGE_POW5, LARGE_POW5_STEP};
14use core::{cmp, ops, ptr};
15
16/// Number of bits in a Bigint.
17///
18/// This needs to be at least the number of bits required to store
19/// a Bigint, which is `log2(radix**digits)`.
20/// ≅ 3600 for base-10, rounded-up.
21pub const BIGINT_BITS: usize = 4000;
22
23/// The number of limbs for the bigint.
24pub const BIGINT_LIMBS: usize = BIGINT_BITS / LIMB_BITS;
25
26#[cfg(feature = "alloc")]
27pub type VecType = HeapVec;
28
29#[cfg(not(feature = "alloc"))]
30pub type VecType = StackVec;
31
32/// Storage for a big integer type.
33///
34/// This is used for algorithms when we have a finite number of digits.
35/// Specifically, it stores all the significant digits scaled to the
36/// proper exponent, as an integral type, and then directly compares
37/// these digits.
38///
39/// This requires us to store the number of significant bits, plus the
40/// number of exponent bits (required) since we scale everything
41/// to the same exponent.
42#[derive(Clone, PartialEq, Eq)]
43pub struct Bigint {
44    /// Significant digits for the float, stored in a big integer in LE order.
45    ///
46    /// This is pretty much the same number of digits for any radix, since the
47    ///  significant digits balances out the zeros from the exponent:
48    ///     1. Decimal is 1091 digits, 767 mantissa digits + 324 exponent zeros.
49    ///     2. Base 6 is 1097 digits, or 680 mantissa digits + 417 exponent zeros.
50    ///     3. Base 36 is 1086 digits, or 877 mantissa digits + 209 exponent zeros.
51    ///
52    /// However, the number of bytes required is larger for large radixes:
53    /// for decimal, we need `log2(10**1091) ≅ 3600`, while for base 36
54    /// we need `log2(36**1086) ≅ 5600`. Since we use uninitialized data,
55    /// we avoid a major performance hit from the large buffer size.
56    pub data: VecType,
57}
58
59#[allow(clippy::new_without_default)]
60impl Bigint {
61    /// Construct a bigint representing 0.
62    #[inline(always)]
63    pub fn new() -> Self {
64        Self {
65            data: VecType::new(),
66        }
67    }
68
69    /// Construct a bigint from an integer.
70    #[inline(always)]
71    pub fn from_u64(value: u64) -> Self {
72        Self {
73            data: VecType::from_u64(value),
74        }
75    }
76
77    #[inline(always)]
78    pub fn hi64(&self) -> (u64, bool) {
79        self.data.hi64()
80    }
81
82    /// Multiply and assign as if by exponentiation by a power.
83    #[inline]
84    pub fn pow(&mut self, base: u32, exp: u32) -> Option<()> {
85        debug_assert!(base == 2 || base == 5 || base == 10);
86        if base % 5 == 0 {
87            pow(&mut self.data, exp)?;
88        }
89        if base % 2 == 0 {
90            shl(&mut self.data, exp as usize)?;
91        }
92        Some(())
93    }
94
95    /// Calculate the bit-length of the big-integer.
96    #[inline]
97    pub fn bit_length(&self) -> u32 {
98        bit_length(&self.data)
99    }
100}
101
102impl ops::MulAssign<&Bigint> for Bigint {
103    fn mul_assign(&mut self, rhs: &Bigint) {
104        self.data *= &rhs.data;
105    }
106}
107
108/// REVERSE VIEW
109
110/// Reverse, immutable view of a sequence.
111pub struct ReverseView<'a, T: 'a> {
112    inner: &'a [T],
113}
114
115impl<'a, T> ops::Index<usize> for ReverseView<'a, T> {
116    type Output = T;
117
118    #[inline]
119    fn index(&self, index: usize) -> &T {
120        let len = self.inner.len();
121        &(*self.inner)[len - index - 1]
122    }
123}
124
125/// Create a reverse view of the vector for indexing.
126#[inline]
127pub fn rview(x: &[Limb]) -> ReverseView<Limb> {
128    ReverseView {
129        inner: x,
130    }
131}
132
133// COMPARE
134// -------
135
136/// Compare `x` to `y`, in little-endian order.
137#[inline]
138pub fn compare(x: &[Limb], y: &[Limb]) -> cmp::Ordering {
139    match x.len().cmp(&y.len()) {
140        cmp::Ordering::Equal => {
141            let iter = x.iter().rev().zip(y.iter().rev());
142            for (&xi, yi) in iter {
143                match xi.cmp(yi) {
144                    cmp::Ordering::Equal => (),
145                    ord => return ord,
146                }
147            }
148            // Equal case.
149            cmp::Ordering::Equal
150        },
151        ord => ord,
152    }
153}
154
155// NORMALIZE
156// ---------
157
158/// Normalize the integer, so any leading zero values are removed.
159#[inline]
160pub fn normalize(x: &mut VecType) {
161    // We don't care if this wraps: the index is bounds-checked.
162    while let Some(&value) = x.get(x.len().wrapping_sub(1)) {
163        if value == 0 {
164            unsafe { x.set_len(x.len() - 1) };
165        } else {
166            break;
167        }
168    }
169}
170
171/// Get if the big integer is normalized.
172#[inline]
173#[allow(clippy::match_like_matches_macro)]
174pub fn is_normalized(x: &[Limb]) -> bool {
175    // We don't care if this wraps: the index is bounds-checked.
176    match x.get(x.len().wrapping_sub(1)) {
177        Some(&0) => false,
178        _ => true,
179    }
180}
181
182// FROM
183// ----
184
185/// Create StackVec from u64 value.
186#[inline(always)]
187#[allow(clippy::branches_sharing_code)]
188pub fn from_u64(x: u64) -> VecType {
189    let mut vec = VecType::new();
190    debug_assert!(vec.capacity() >= 2);
191    if LIMB_BITS == 32 {
192        vec.try_push(x as Limb).unwrap();
193        vec.try_push((x >> 32) as Limb).unwrap();
194    } else {
195        vec.try_push(x as Limb).unwrap();
196    }
197    vec.normalize();
198    vec
199}
200
201// HI
202// --
203
204/// Check if any of the remaining bits are non-zero.
205///
206/// # Safety
207///
208/// Safe as long as `rindex <= x.len()`.
209#[inline]
210pub fn nonzero(x: &[Limb], rindex: usize) -> bool {
211    debug_assert!(rindex <= x.len());
212
213    let len = x.len();
214    let slc = &x[..len - rindex];
215    slc.iter().rev().any(|&x| x != 0)
216}
217
218// These return the high X bits and if the bits were truncated.
219
220/// Shift 32-bit integer to high 64-bits.
221#[inline]
222pub fn u32_to_hi64_1(r0: u32) -> (u64, bool) {
223    u64_to_hi64_1(r0 as u64)
224}
225
226/// Shift 2 32-bit integers to high 64-bits.
227#[inline]
228pub fn u32_to_hi64_2(r0: u32, r1: u32) -> (u64, bool) {
229    let r0 = (r0 as u64) << 32;
230    let r1 = r1 as u64;
231    u64_to_hi64_1(r0 | r1)
232}
233
234/// Shift 3 32-bit integers to high 64-bits.
235#[inline]
236pub fn u32_to_hi64_3(r0: u32, r1: u32, r2: u32) -> (u64, bool) {
237    let r0 = r0 as u64;
238    let r1 = (r1 as u64) << 32;
239    let r2 = r2 as u64;
240    u64_to_hi64_2(r0, r1 | r2)
241}
242
243/// Shift 64-bit integer to high 64-bits.
244#[inline]
245pub fn u64_to_hi64_1(r0: u64) -> (u64, bool) {
246    let ls = r0.leading_zeros();
247    (r0 << ls, false)
248}
249
250/// Shift 2 64-bit integers to high 64-bits.
251#[inline]
252pub fn u64_to_hi64_2(r0: u64, r1: u64) -> (u64, bool) {
253    let ls = r0.leading_zeros();
254    let rs = 64 - ls;
255    let v = match ls {
256        0 => r0,
257        _ => (r0 << ls) | (r1 >> rs),
258    };
259    let n = r1 << ls != 0;
260    (v, n)
261}
262
263/// Extract the hi bits from the buffer.
264macro_rules! hi {
265    // # Safety
266    //
267    // Safe as long as the `stackvec.len() >= 1`.
268    (@1 $self:ident, $rview:ident, $t:ident, $fn:ident) => {{
269        $fn($rview[0] as $t)
270    }};
271
272    // # Safety
273    //
274    // Safe as long as the `stackvec.len() >= 2`.
275    (@2 $self:ident, $rview:ident, $t:ident, $fn:ident) => {{
276        let r0 = $rview[0] as $t;
277        let r1 = $rview[1] as $t;
278        $fn(r0, r1)
279    }};
280
281    // # Safety
282    //
283    // Safe as long as the `stackvec.len() >= 2`.
284    (@nonzero2 $self:ident, $rview:ident, $t:ident, $fn:ident) => {{
285        let (v, n) = hi!(@2 $self, $rview, $t, $fn);
286        (v, n || nonzero($self, 2 ))
287    }};
288
289    // # Safety
290    //
291    // Safe as long as the `stackvec.len() >= 3`.
292    (@3 $self:ident, $rview:ident, $t:ident, $fn:ident) => {{
293        let r0 = $rview[0] as $t;
294        let r1 = $rview[1] as $t;
295        let r2 = $rview[2] as $t;
296        $fn(r0, r1, r2)
297    }};
298
299    // # Safety
300    //
301    // Safe as long as the `stackvec.len() >= 3`.
302    (@nonzero3 $self:ident, $rview:ident, $t:ident, $fn:ident) => {{
303        let (v, n) = hi!(@3 $self, $rview, $t, $fn);
304        (v, n || nonzero($self, 3))
305    }};
306}
307
308/// Get the high 64 bits from the vector.
309#[inline(always)]
310pub fn hi64(x: &[Limb]) -> (u64, bool) {
311    let rslc = rview(x);
312    // SAFETY: the buffer must be at least length bytes long.
313    match x.len() {
314        0 => (0, false),
315        1 if LIMB_BITS == 32 => hi!(@1 x, rslc, u32, u32_to_hi64_1),
316        1 => hi!(@1 x, rslc, u64, u64_to_hi64_1),
317        2 if LIMB_BITS == 32 => hi!(@2 x, rslc, u32, u32_to_hi64_2),
318        2 => hi!(@2 x, rslc, u64, u64_to_hi64_2),
319        _ if LIMB_BITS == 32 => hi!(@nonzero3 x, rslc, u32, u32_to_hi64_3),
320        _ => hi!(@nonzero2 x, rslc, u64, u64_to_hi64_2),
321    }
322}
323
324// POWERS
325// ------
326
327/// MulAssign by a power of 5.
328///
329/// Theoretically...
330///
331/// Use an exponentiation by squaring method, since it reduces the time
332/// complexity of the multiplication to ~`O(log(n))` for the squaring,
333/// and `O(n*m)` for the result. Since `m` is typically a lower-order
334/// factor, this significantly reduces the number of multiplications
335/// we need to do. Iteratively multiplying by small powers follows
336/// the nth triangular number series, which scales as `O(p^2)`, but
337/// where `p` is `n+m`. In short, it scales very poorly.
338///
339/// Practically....
340///
341/// Exponentiation by Squaring:
342///     running 2 tests
343///     test bigcomp_f32_lexical ... bench:       1,018 ns/iter (+/- 78)
344///     test bigcomp_f64_lexical ... bench:       3,639 ns/iter (+/- 1,007)
345///
346/// Exponentiation by Iterative Small Powers:
347///     running 2 tests
348///     test bigcomp_f32_lexical ... bench:         518 ns/iter (+/- 31)
349///     test bigcomp_f64_lexical ... bench:         583 ns/iter (+/- 47)
350///
351/// Exponentiation by Iterative Large Powers (of 2):
352///     running 2 tests
353///     test bigcomp_f32_lexical ... bench:         671 ns/iter (+/- 31)
354///     test bigcomp_f64_lexical ... bench:       1,394 ns/iter (+/- 47)
355///
356/// The following benchmarks were run on `1 * 5^300`, using native `pow`,
357/// a version with only small powers, and one with pre-computed powers
358/// of `5^(3 * max_exp)`, rather than `5^(5 * max_exp)`.
359///
360/// However, using large powers is crucial for good performance for higher
361/// powers.
362///     pow/default             time:   [426.20 ns 427.96 ns 429.89 ns]
363///     pow/small               time:   [2.9270 us 2.9411 us 2.9565 us]
364///     pow/large:3             time:   [838.51 ns 842.21 ns 846.27 ns]
365///
366/// Even using worst-case scenarios, exponentiation by squaring is
367/// significantly slower for our workloads. Just multiply by small powers,
368/// in simple cases, and use precalculated large powers in other cases.
369///
370/// Furthermore, using sufficiently big large powers is also crucial for
371/// performance. This is a tradeoff of binary size and performance, and
372/// using a single value at ~`5^(5 * max_exp)` seems optimal.
373pub fn pow(x: &mut VecType, mut exp: u32) -> Option<()> {
374    // Minimize the number of iterations for large exponents: just
375    // do a few steps with a large powers.
376    #[cfg(not(feature = "compact"))]
377    {
378        while exp >= LARGE_POW5_STEP {
379            large_mul(x, &LARGE_POW5)?;
380            exp -= LARGE_POW5_STEP;
381        }
382    }
383
384    // Now use our pre-computed small powers iteratively.
385    // This is calculated as `⌊log(2^BITS - 1, 5)⌋`.
386    let small_step = if LIMB_BITS == 32 {
387        13
388    } else {
389        27
390    };
391    let max_native = (5 as Limb).pow(small_step);
392    while exp >= small_step {
393        small_mul(x, max_native)?;
394        exp -= small_step;
395    }
396    if exp != 0 {
397        // SAFETY: safe, since `exp < small_step`.
398        let small_power = unsafe { f64::int_pow_fast_path(exp as usize, 5) };
399        small_mul(x, small_power as Limb)?;
400    }
401    Some(())
402}
403
404// SCALAR
405// ------
406
407/// Add two small integers and return the resulting value and if overflow happens.
408#[inline(always)]
409pub fn scalar_add(x: Limb, y: Limb) -> (Limb, bool) {
410    x.overflowing_add(y)
411}
412
413/// Multiply two small integers (with carry) (and return the overflow contribution).
414///
415/// Returns the (low, high) components.
416#[inline(always)]
417pub fn scalar_mul(x: Limb, y: Limb, carry: Limb) -> (Limb, Limb) {
418    // Cannot overflow, as long as wide is 2x as wide. This is because
419    // the following is always true:
420    // `Wide::MAX - (Narrow::MAX * Narrow::MAX) >= Narrow::MAX`
421    let z: Wide = (x as Wide) * (y as Wide) + (carry as Wide);
422    (z as Limb, (z >> LIMB_BITS) as Limb)
423}
424
425// SMALL
426// -----
427
428/// Add small integer to bigint starting from offset.
429#[inline]
430pub fn small_add_from(x: &mut VecType, y: Limb, start: usize) -> Option<()> {
431    let mut index = start;
432    let mut carry = y;
433    while carry != 0 && index < x.len() {
434        let result = scalar_add(x[index], carry);
435        x[index] = result.0;
436        carry = result.1 as Limb;
437        index += 1;
438    }
439    // If we carried past all the elements, add to the end of the buffer.
440    if carry != 0 {
441        x.try_push(carry)?;
442    }
443    Some(())
444}
445
446/// Add small integer to bigint.
447#[inline(always)]
448pub fn small_add(x: &mut VecType, y: Limb) -> Option<()> {
449    small_add_from(x, y, 0)
450}
451
452/// Multiply bigint by small integer.
453#[inline]
454pub fn small_mul(x: &mut VecType, y: Limb) -> Option<()> {
455    let mut carry = 0;
456    for xi in x.iter_mut() {
457        let result = scalar_mul(*xi, y, carry);
458        *xi = result.0;
459        carry = result.1;
460    }
461    // If we carried past all the elements, add to the end of the buffer.
462    if carry != 0 {
463        x.try_push(carry)?;
464    }
465    Some(())
466}
467
468// LARGE
469// -----
470
471/// Add bigint to bigint starting from offset.
472pub fn large_add_from(x: &mut VecType, y: &[Limb], start: usize) -> Option<()> {
473    // The effective x buffer is from `xstart..x.len()`, so we need to treat
474    // that as the current range. If the effective y buffer is longer, need
475    // to resize to that, + the start index.
476    if y.len() > x.len().saturating_sub(start) {
477        // Ensure we panic if we can't extend the buffer.
478        // This avoids any unsafe behavior afterwards.
479        x.try_resize(y.len() + start, 0)?;
480    }
481
482    // Iteratively add elements from y to x.
483    let mut carry = false;
484    for (index, &yi) in y.iter().enumerate() {
485        // We panicked in `try_resize` if this wasn't true.
486        let xi = x.get_mut(start + index).unwrap();
487
488        // Only one op of the two ops can overflow, since we added at max
489        // Limb::max_value() + Limb::max_value(). Add the previous carry,
490        // and store the current carry for the next.
491        let result = scalar_add(*xi, yi);
492        *xi = result.0;
493        let mut tmp = result.1;
494        if carry {
495            let result = scalar_add(*xi, 1);
496            *xi = result.0;
497            tmp |= result.1;
498        }
499        carry = tmp;
500    }
501
502    // Handle overflow.
503    if carry {
504        small_add_from(x, 1, y.len() + start)?;
505    }
506    Some(())
507}
508
509/// Add bigint to bigint.
510#[inline(always)]
511pub fn large_add(x: &mut VecType, y: &[Limb]) -> Option<()> {
512    large_add_from(x, y, 0)
513}
514
515/// Grade-school multiplication algorithm.
516///
517/// Slow, naive algorithm, using limb-bit bases and just shifting left for
518/// each iteration. This could be optimized with numerous other algorithms,
519/// but it's extremely simple, and works in O(n*m) time, which is fine
520/// by me. Each iteration, of which there are `m` iterations, requires
521/// `n` multiplications, and `n` additions, or grade-school multiplication.
522///
523/// Don't use Karatsuba multiplication, since out implementation seems to
524/// be slower asymptotically, which is likely just due to the small sizes
525/// we deal with here. For example, running on the following data:
526///
527/// ```text
528/// const SMALL_X: &[u32] = &[
529///     766857581, 3588187092, 1583923090, 2204542082, 1564708913, 2695310100, 3676050286,
530///     1022770393, 468044626, 446028186
531/// ];
532/// const SMALL_Y: &[u32] = &[
533///     3945492125, 3250752032, 1282554898, 1708742809, 1131807209, 3171663979, 1353276095,
534///     1678845844, 2373924447, 3640713171
535/// ];
536/// const LARGE_X: &[u32] = &[
537///     3647536243, 2836434412, 2154401029, 1297917894, 137240595, 790694805, 2260404854,
538///     3872698172, 690585094, 99641546, 3510774932, 1672049983, 2313458559, 2017623719,
539///     638180197, 1140936565, 1787190494, 1797420655, 14113450, 2350476485, 3052941684,
540///     1993594787, 2901001571, 4156930025, 1248016552, 848099908, 2660577483, 4030871206,
541///     692169593, 2835966319, 1781364505, 4266390061, 1813581655, 4210899844, 2137005290,
542///     2346701569, 3715571980, 3386325356, 1251725092, 2267270902, 474686922, 2712200426,
543///     197581715, 3087636290, 1379224439, 1258285015, 3230794403, 2759309199, 1494932094,
544///     326310242
545/// ];
546/// const LARGE_Y: &[u32] = &[
547///     1574249566, 868970575, 76716509, 3198027972, 1541766986, 1095120699, 3891610505,
548///     2322545818, 1677345138, 865101357, 2650232883, 2831881215, 3985005565, 2294283760,
549///     3468161605, 393539559, 3665153349, 1494067812, 106699483, 2596454134, 797235106,
550///     705031740, 1209732933, 2732145769, 4122429072, 141002534, 790195010, 4014829800,
551///     1303930792, 3649568494, 308065964, 1233648836, 2807326116, 79326486, 1262500691,
552///     621809229, 2258109428, 3819258501, 171115668, 1139491184, 2979680603, 1333372297,
553///     1657496603, 2790845317, 4090236532, 4220374789, 601876604, 1828177209, 2372228171,
554///     2247372529
555/// ];
556/// ```
557///
558/// We get the following results:
559
560/// ```text
561/// mul/small:long          time:   [220.23 ns 221.47 ns 222.81 ns]
562/// Found 4 outliers among 100 measurements (4.00%)
563///   2 (2.00%) high mild
564///   2 (2.00%) high severe
565/// mul/small:karatsuba     time:   [233.88 ns 234.63 ns 235.44 ns]
566/// Found 11 outliers among 100 measurements (11.00%)
567///   8 (8.00%) high mild
568///   3 (3.00%) high severe
569/// mul/large:long          time:   [1.9365 us 1.9455 us 1.9558 us]
570/// Found 12 outliers among 100 measurements (12.00%)
571///   7 (7.00%) high mild
572///   5 (5.00%) high severe
573/// mul/large:karatsuba     time:   [4.4250 us 4.4515 us 4.4812 us]
574/// ```
575///
576/// In short, Karatsuba multiplication is never worthwhile for out use-case.
577pub fn long_mul(x: &[Limb], y: &[Limb]) -> Option<VecType> {
578    // Using the immutable value, multiply by all the scalars in y, using
579    // the algorithm defined above. Use a single buffer to avoid
580    // frequent reallocations. Handle the first case to avoid a redundant
581    // addition, since we know y.len() >= 1.
582    let mut z = VecType::try_from(x)?;
583    if !y.is_empty() {
584        let y0 = y[0];
585        small_mul(&mut z, y0)?;
586
587        for (index, &yi) in y.iter().enumerate().skip(1) {
588            if yi != 0 {
589                let mut zi = VecType::try_from(x)?;
590                small_mul(&mut zi, yi)?;
591                large_add_from(&mut z, &zi, index)?;
592            }
593        }
594    }
595
596    z.normalize();
597    Some(z)
598}
599
600/// Multiply bigint by bigint using grade-school multiplication algorithm.
601#[inline(always)]
602pub fn large_mul(x: &mut VecType, y: &[Limb]) -> Option<()> {
603    // Karatsuba multiplication never makes sense, so just use grade school
604    // multiplication.
605    if y.len() == 1 {
606        // SAFETY: safe since `y.len() == 1`.
607        small_mul(x, y[0])?;
608    } else {
609        *x = long_mul(y, x)?;
610    }
611    Some(())
612}
613
614// SHIFT
615// -----
616
617/// Shift-left `n` bits inside a buffer.
618#[inline]
619pub fn shl_bits(x: &mut VecType, n: usize) -> Option<()> {
620    debug_assert!(n != 0);
621
622    // Internally, for each item, we shift left by n, and add the previous
623    // right shifted limb-bits.
624    // For example, we transform (for u8) shifted left 2, to:
625    //      b10100100 b01000010
626    //      b10 b10010001 b00001000
627    debug_assert!(n < LIMB_BITS);
628    let rshift = LIMB_BITS - n;
629    let lshift = n;
630    let mut prev: Limb = 0;
631    for xi in x.iter_mut() {
632        let tmp = *xi;
633        *xi <<= lshift;
634        *xi |= prev >> rshift;
635        prev = tmp;
636    }
637
638    // Always push the carry, even if it creates a non-normal result.
639    let carry = prev >> rshift;
640    if carry != 0 {
641        x.try_push(carry)?;
642    }
643
644    Some(())
645}
646
647/// Shift-left `n` limbs inside a buffer.
648#[inline]
649pub fn shl_limbs(x: &mut VecType, n: usize) -> Option<()> {
650    debug_assert!(n != 0);
651    if n + x.len() > x.capacity() {
652        None
653    } else if !x.is_empty() {
654        let len = n + x.len();
655        // SAFE: since x is not empty, and `x.len() + n <= x.capacity()`.
656        unsafe {
657            // Move the elements.
658            let src = x.as_ptr();
659            let dst = x.as_mut_ptr().add(n);
660            ptr::copy(src, dst, x.len());
661            // Write our 0s.
662            ptr::write_bytes(x.as_mut_ptr(), 0, n);
663            x.set_len(len);
664        }
665        Some(())
666    } else {
667        Some(())
668    }
669}
670
671/// Shift-left buffer by n bits.
672#[inline]
673pub fn shl(x: &mut VecType, n: usize) -> Option<()> {
674    let rem = n % LIMB_BITS;
675    let div = n / LIMB_BITS;
676    if rem != 0 {
677        shl_bits(x, rem)?;
678    }
679    if div != 0 {
680        shl_limbs(x, div)?;
681    }
682    Some(())
683}
684
685/// Get number of leading zero bits in the storage.
686#[inline]
687pub fn leading_zeros(x: &[Limb]) -> u32 {
688    let length = x.len();
689    // wrapping_sub is fine, since it'll just return None.
690    if let Some(&value) = x.get(length.wrapping_sub(1)) {
691        value.leading_zeros()
692    } else {
693        0
694    }
695}
696
697/// Calculate the bit-length of the big-integer.
698#[inline]
699pub fn bit_length(x: &[Limb]) -> u32 {
700    let nlz = leading_zeros(x);
701    LIMB_BITS as u32 * x.len() as u32 - nlz
702}
703
704// LIMB
705// ----
706
707//  Type for a single limb of the big integer.
708//
709//  A limb is analogous to a digit in base10, except, it stores 32-bit
710//  or 64-bit numbers instead. We want types where 64-bit multiplication
711//  is well-supported by the architecture, rather than emulated in 3
712//  instructions. The quickest way to check this support is using a
713//  cross-compiler for numerous architectures, along with the following
714//  source file and command:
715//
716//  Compile with `gcc main.c -c -S -O3 -masm=intel`
717//
718//  And the source code is:
719//  ```text
720//  #include <stdint.h>
721//
722//  struct i128 {
723//      uint64_t hi;
724//      uint64_t lo;
725//  };
726//
727//  // Type your code here, or load an example.
728//  struct i128 square(uint64_t x, uint64_t y) {
729//      __int128 prod = (__int128)x * (__int128)y;
730//      struct i128 z;
731//      z.hi = (uint64_t)(prod >> 64);
732//      z.lo = (uint64_t)prod;
733//      return z;
734//  }
735//  ```
736//
737//  If the result contains `call __multi3`, then the multiplication
738//  is emulated by the compiler. Otherwise, it's natively supported.
739//
740//  This should be all-known 64-bit platforms supported by Rust.
741//      https://forge.rust-lang.org/platform-support.html
742//
743//  # Supported
744//
745//  Platforms where native 128-bit multiplication is explicitly supported:
746//      - x86_64 (Supported via `MUL`).
747//      - mips64 (Supported via `DMULTU`, which `HI` and `LO` can be read-from).
748//      - s390x (Supported via `MLGR`).
749//
750//  # Efficient
751//
752//  Platforms where native 64-bit multiplication is supported and
753//  you can extract hi-lo for 64-bit multiplications.
754//      - aarch64 (Requires `UMULH` and `MUL` to capture high and low bits).
755//      - powerpc64 (Requires `MULHDU` and `MULLD` to capture high and low bits).
756//      - riscv64 (Requires `MUL` and `MULH` to capture high and low bits).
757//
758//  # Unsupported
759//
760//  Platforms where native 128-bit multiplication is not supported,
761//  requiring software emulation.
762//      sparc64 (`UMUL` only supports double-word arguments).
763//      sparcv9 (Same as sparc64).
764//
765//  These tests are run via `xcross`, my own library for C cross-compiling,
766//  which supports numerous targets (far in excess of Rust's tier 1 support,
767//  or rust-embedded/cross's list). xcross may be found here:
768//      https://github.com/Alexhuszagh/xcross
769//
770//  To compile for the given target, run:
771//      `xcross gcc main.c -c -S -O3 --target $target`
772//
773//  All 32-bit architectures inherently do not have support. That means
774//  we can essentially look for 64-bit architectures that are not SPARC.
775
776#[cfg(all(target_pointer_width = "64", not(target_arch = "sparc")))]
777pub type Limb = u64;
778#[cfg(all(target_pointer_width = "64", not(target_arch = "sparc")))]
779pub type Wide = u128;
780#[cfg(all(target_pointer_width = "64", not(target_arch = "sparc")))]
781pub const LIMB_BITS: usize = 64;
782
783#[cfg(not(all(target_pointer_width = "64", not(target_arch = "sparc"))))]
784pub type Limb = u32;
785#[cfg(not(all(target_pointer_width = "64", not(target_arch = "sparc"))))]
786pub type Wide = u64;
787#[cfg(not(all(target_pointer_width = "64", not(target_arch = "sparc"))))]
788pub const LIMB_BITS: usize = 32;