⚠️ Warning: This is a draft ⚠️

This means it might contain formatting issues, incorrect code, conceptual problems, or other severe issues.

If you want to help to improve and eventually enable this page, please fork RosettaGit's repository and open a merge request on GitHub.

An implementation of one of Peter Luschny's [http://www.luschny.de/math/factorial/FastFactorialFunctions.htm fast factorial algorithms]. Fast as this algorithm is, I believe there is room to speed it up more with parallelization and attention to cache effects. The Go library has a nice Karatsuba multiplier but it is yet single threaded.

```package main

import (
"math/big"
"fmt"
"time"
)

func main() {
const max = 1e6
var s sieve
s.sieve(max)

// some trivial cases
b := big.NewInt(1)
tf(&s, 0, b)
tf(&s, 1, b)
tf(&s, 3, b.SetInt64(6))
tf(&s, 10, b.SetInt64(3628800))

// 20 is the first number that exercises the split factorial code
tf(&s, 20, b.SetInt64(2432902008176640000))

// 65 is the first number that exercises the split odd swing code
b.SetString("8247650592082470666723170306785496252186258551345437492922123134388955774976000000000000000", 10)
tf(&s, 65, b)

// 402 is the first number that exercises the split primorial code
b.SetString("10322493151921465164081017511444523549144957788957729070658850054871632028467255601190963314928373192348001901396930189622367360453148777593779130493841936873495349332423413459470518031076600468677681086479354644916620480632630350145970538235260826120203515476630017152557002993632050731959317164706296917171625287200618560036028326143938282329483693985566225033103398611546364400484246579470387915281737632989645795534475998050620039413447425490893877731061666015468384131920640823824733578473025588407103553854530737735183050931478983505845362197959913863770041359352031682005647007823330600995250982455385703739491695583970372977196372367980241040180516191489137558020294105537577853569647066137370488100581103217089054291400441697731894590238418118698720784367447615471616000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)
tf(&s, 402, b)

df(&s, 800)
df(&s, 1e5)
df(&s, max)
}

func tf(s *sieve, n uint, answer *big.Int) {
f := s.factorial(n)
fmt.Printf("%d! pass.\n", n)
} else {
fmt.Printf("%d! fail.\nExpected %s\nFound    %s\n",
}
}

func df(s *sieve, n uint) {
start := time.Now()
a := s.factorial(n)
stop := time.Now()
fmt.Printf("n = %d  -> factorial %v\n", n, stop.Sub(start))

dtrunc := int64(float64(a.BitLen())*.30103) - 10
var first, rest big.Int
rest.Exp(first.SetInt64(10), rest.SetInt64(dtrunc), nil)
first.Quo(a, &rest)
fstr := first.String()
fmt.Printf("%d! begins %s... and has %d digits.\n",
n, fstr, int64(len(fstr))+dtrunc)
}

func (s *sieve) factorial(n uint) *big.Int {
if n < 20 {
var r big.Int
return r.MulRange(1, int64(n))
}

if int64(n) > s.limit {
return nil
}

r := s.factorialS(n)
return r.Lsh(r, n-bitCount32(uint32(n)))
}

func (s *sieve) factorialS(n uint) (swing *big.Int) {
if n < 2 {
return big.NewInt(1)
}

f2 := s.factorialS(n / 2) // recurse
f2.Mul(f2, f2)            // square

if n < uint(len(smallOddSwing)) {
swing = big.NewInt(smallOddSwing[n])
} else {
swing = s.oddSwing(n)
}

return swing.Mul(swing, f2)
}

func (s *sieve) oddSwing(k uint) *big.Int {
if k < uint(len(smallOddSwing)) {
return big.NewInt(smallOddSwing[k])
}

factors := make([]int64, k/2)
rootK := int64(floorSqrt(uint64(k)))
var i int

s.iterateFunc(3, rootK, func(p int64) bool {
q := int64(k) / p
for q > 0 {
if q&1 == 1 {
factors[i] = p
i++
}
q /= p
}
return false
})

s.iterateFunc(rootK+1, int64(k/3), func(p int64) bool {
if (int64(k) / p & 1) == 1 {
factors[i] = p
i++
}
return false
})

r := product(factors[0:i])
return r.Mul(r, s.primorial(int64(k/2+1), int64(k)))
}

func (s *sieve) primorial(m, n int64) *big.Int {
if m > n {
return big.NewInt(1)
}

if n-m < 200 {
var r, r2 big.Int
r.SetInt64(1)
s.iterateFunc(m, n, func(p int64) bool {
r.Mul(&r, r2.SetInt64(p))
return false
})
return &r
}

h := (m + n) / 2
r := s.primorial(m, h)
return r.Mul(r, s.primorial(h+1, n))
}

type sieve struct {
limit       int64
isComposite []uint64
}

func (s *sieve) iterateFunc(min, max int64, visitor func(prime int64) (terminate bool)) (ok bool) {

if max > s.limit {
return false // Max larger than sieve
}
if min < 2 {
min = 2
}
if min > max {
return true
}
if min == 2 && max >= 2 {
if visitor(2) {
return true
}
}
if min <= 3 && max >= 3 {
if visitor(3) {
return true
}
}

absPos := (min+(min+1)%2)/3 - 1
index := absPos / bitsPerInt
bitPos := absPos % bitsPerInt
prime := 5 + 3*(bitsPerInt*index+bitPos) - bitPos&1
inc := bitPos&1*2 + 2

for prime <= max {
bitField := s.isComposite[index] >> uint64(bitPos)
index++
for ; bitPos < bitsPerInt; bitPos++ {
if bitField&1 == 0 {
if visitor(prime) {
return true
}
}
prime += inc
if prime > max {
return true
}
inc = 6 - inc
bitField >>= 1
}
bitPos = 0
}
return true
}

// constants dependent on the word size of sieve.isComposite.
const (
bitsPerInt = 64
log2Int    = 6
)

func (s *sieve) sieve(n int64) {
if n <= 0 {
*s = sieve{}
}

s.limit = n
s.isComposite = make([]uint64, n/(3*bitsPerInt)+1)

var (
d1, d2, p1, p2, s1, s2 uint64 = 8, 8, 3, 7, 7, 3
l, c, max              uint64 = 0, 1, uint64(n) / 3
toggle                 bool
)

for s1 < max {
if (s.isComposite[l>>log2Int] & (1 << (l & mask))) == 0 {
inc := p1 + p2
for c = s1; c < max; c += inc {
s.isComposite[c>>log2Int] |= 1 << (c & mask)
}
for c = s1 + s2; c < max; c += inc {
s.isComposite[c>>log2Int] |= 1 << (c & mask)
}
}

l++
if toggle {
toggle = false
s1 += d1
d2 += 8
p1 += 2
p2 += 6
s2 = p1
} else {
toggle = true
s1 += d2
d1 += 16
p1 += 2
p2 += 2
s2 = p2
}
}
return
}

var smallOddSwing []int64 = []int64{1, 1, 1, 3, 3, 15, 5,
35, 35, 315, 63, 693, 231, 3003, 429, 6435, 6435,
109395, 12155, 230945, 46189, 969969, 88179, 2028117, 676039,
16900975, 1300075, 35102025, 5014575, 145422675, 9694845,
300540195, 300540195, 9917826435, 583401555, 20419054425,
2268783825, 83945001525, 4418157975, 172308161025,
34461632205, 1412926920405, 67282234305, 2893136075115,
263012370465, 11835556670925, 514589420475, 24185702762325,
8061900920775, 395033145117975, 15801325804719,
805867616040669, 61989816618513, 3285460280781189,
121683714103007, 6692604275665385, 956086325095055,
54496920530418135, 1879204156221315, 110873045217057585,
7391536347803839, 450883717216034179, 14544636039226909,
916312070471295267, 916312070471295267}

func bitCount32(w uint32) uint {
const (
ff    = 1<<32 - 1
)
w -= w >> 1 & mask1
w = (w + w>>4) & maskf
return uint(w * maskp >> 24)
}

func floorSqrt(n uint64) (a uint64) {
for b := n; ; {
a := b
b = (n/a + a) / 2
if b >= a {
return a
}
}
return 0
}

func product(seq []int64) *big.Int {
if len(seq) <= 20 {
var b big.Int
sprod := big.NewInt(seq[0])
for _, s := range seq[1:] {
b.SetInt64(s)
sprod.Mul(sprod, &b)
}
return sprod
}

halfLen := len(seq) / 2
lprod := product(seq[0:halfLen])
rprod := product(seq[halfLen:])
return lprod.Mul(lprod, rprod)
}
```

Output: I did 800 because a few others had computed it. Answers come pretty fast up to 10^5 but slow down after that. Times shown are for computing the factorial only. Producing the printable base 10 representation takes longer and isn't fun to wait for.

```
0! pass.
1! pass.
3! pass.
10! pass.
20! pass.
65! pass.
402! pass.
n = 800  -> factorial 318us
800! begins 7710530113... and has 1977 digits.
n = 100000  -> factorial 508.322ms
100000! begins 28242294079... and has 456574 digits.
n = 1000000  -> factorial 3m33.118625s
1000000! begins 8263931688... and has 5565709 digits.

```