mirror of
https://github.com/superseriousbusiness/gotosocial.git
synced 2024-12-05 18:12:46 +00:00
217 lines
4 KiB
Go
217 lines
4 KiB
Go
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package bigfft
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import (
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"math/big"
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)
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// Arithmetic modulo 2^n+1.
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// A fermat of length w+1 represents a number modulo 2^(w*_W) + 1. The last
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// word is zero or one. A number has at most two representatives satisfying the
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// 0-1 last word constraint.
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type fermat nat
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func (n fermat) String() string { return nat(n).String() }
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func (z fermat) norm() {
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n := len(z) - 1
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c := z[n]
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if c == 0 {
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return
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}
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if z[0] >= c {
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z[n] = 0
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z[0] -= c
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return
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}
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// z[0] < z[n].
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subVW(z, z, c) // Substract c
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if c > 1 {
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z[n] -= c - 1
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c = 1
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}
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// Add back c.
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if z[n] == 1 {
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z[n] = 0
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return
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} else {
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addVW(z, z, 1)
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}
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}
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// Shift computes (x << k) mod (2^n+1).
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func (z fermat) Shift(x fermat, k int) {
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if len(z) != len(x) {
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panic("len(z) != len(x) in Shift")
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}
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n := len(x) - 1
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// Shift by n*_W is taking the opposite.
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k %= 2 * n * _W
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if k < 0 {
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k += 2 * n * _W
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}
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neg := false
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if k >= n*_W {
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k -= n * _W
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neg = true
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}
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kw, kb := k/_W, k%_W
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z[n] = 1 // Add (-1)
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if !neg {
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for i := 0; i < kw; i++ {
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z[i] = 0
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}
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// Shift left by kw words.
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// x = a·2^(n-k) + b
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// x<<k = (b<<k) - a
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copy(z[kw:], x[:n-kw])
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b := subVV(z[:kw+1], z[:kw+1], x[n-kw:])
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if z[kw+1] > 0 {
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z[kw+1] -= b
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} else {
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subVW(z[kw+1:], z[kw+1:], b)
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}
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} else {
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for i := kw + 1; i < n; i++ {
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z[i] = 0
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}
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// Shift left and negate, by kw words.
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copy(z[:kw+1], x[n-kw:n+1]) // z_low = x_high
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b := subVV(z[kw:n], z[kw:n], x[:n-kw]) // z_high -= x_low
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z[n] -= b
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}
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// Add back 1.
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if z[n] > 0 {
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z[n]--
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} else if z[0] < ^big.Word(0) {
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z[0]++
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} else {
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addVW(z, z, 1)
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}
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// Shift left by kb bits
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shlVU(z, z, uint(kb))
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z.norm()
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}
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// ShiftHalf shifts x by k/2 bits the left. Shifting by 1/2 bit
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// is multiplication by sqrt(2) mod 2^n+1 which is 2^(3n/4) - 2^(n/4).
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// A temporary buffer must be provided in tmp.
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func (z fermat) ShiftHalf(x fermat, k int, tmp fermat) {
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n := len(z) - 1
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if k%2 == 0 {
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z.Shift(x, k/2)
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return
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}
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u := (k - 1) / 2
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a := u + (3*_W/4)*n
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b := u + (_W/4)*n
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z.Shift(x, a)
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tmp.Shift(x, b)
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z.Sub(z, tmp)
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}
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// Add computes addition mod 2^n+1.
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func (z fermat) Add(x, y fermat) fermat {
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if len(z) != len(x) {
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panic("Add: len(z) != len(x)")
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}
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addVV(z, x, y) // there cannot be a carry here.
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z.norm()
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return z
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}
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// Sub computes substraction mod 2^n+1.
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func (z fermat) Sub(x, y fermat) fermat {
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if len(z) != len(x) {
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panic("Add: len(z) != len(x)")
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}
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n := len(y) - 1
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b := subVV(z[:n], x[:n], y[:n])
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b += y[n]
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// If b > 0, we need to subtract b<<n, which is the same as adding b.
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z[n] = x[n]
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if z[0] <= ^big.Word(0)-b {
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z[0] += b
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} else {
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addVW(z, z, b)
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}
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z.norm()
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return z
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}
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func (z fermat) Mul(x, y fermat) fermat {
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if len(x) != len(y) {
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panic("Mul: len(x) != len(y)")
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}
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n := len(x) - 1
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if n < 30 {
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z = z[:2*n+2]
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basicMul(z, x, y)
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z = z[:2*n+1]
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} else {
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var xi, yi, zi big.Int
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xi.SetBits(x)
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yi.SetBits(y)
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zi.SetBits(z)
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zb := zi.Mul(&xi, &yi).Bits()
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if len(zb) <= n {
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// Short product.
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copy(z, zb)
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for i := len(zb); i < len(z); i++ {
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z[i] = 0
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}
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return z
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}
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z = zb
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}
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// len(z) is at most 2n+1.
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if len(z) > 2*n+1 {
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panic("len(z) > 2n+1")
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}
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// We now have
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// z = z[:n] + 1<<(n*W) * z[n:2n+1]
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// which normalizes to:
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// z = z[:n] - z[n:2n] + z[2n]
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c1 := big.Word(0)
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if len(z) > 2*n {
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c1 = addVW(z[:n], z[:n], z[2*n])
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}
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c2 := big.Word(0)
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if len(z) >= 2*n {
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c2 = subVV(z[:n], z[:n], z[n:2*n])
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} else {
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m := len(z) - n
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c2 = subVV(z[:m], z[:m], z[n:])
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c2 = subVW(z[m:n], z[m:n], c2)
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}
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// Restore carries.
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// Substracting z[n] -= c2 is the same
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// as z[0] += c2
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z = z[:n+1]
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z[n] = c1
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c := addVW(z, z, c2)
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if c != 0 {
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panic("impossible")
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}
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z.norm()
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return z
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}
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// copied from math/big
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//
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// basicMul multiplies x and y and leaves the result in z.
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// The (non-normalized) result is placed in z[0 : len(x) + len(y)].
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func basicMul(z, x, y fermat) {
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// initialize z
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for i := 0; i < len(z); i++ {
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z[i] = 0
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}
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for i, d := range y {
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if d != 0 {
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z[len(x)+i] = addMulVVW(z[i:i+len(x)], x, d)
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}
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}
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}
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