1115 lines
22 KiB
Go
1115 lines
22 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// This file contains operations on unsigned multi-precision integers.
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// These are the building blocks for the operations on signed integers
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// and rationals.
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// This package implements multi-precision arithmetic (big numbers).
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// The following numeric types are supported:
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//
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// - Int signed integers
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// - Rat rational numbers
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//
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// All methods on Int take the result as the receiver; if it is one
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// of the operands it may be overwritten (and its memory reused).
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// To enable chaining of operations, the result is also returned.
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//
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package big
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import "rand"
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// An unsigned integer x of the form
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//
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// x = x[n-1]*_B^(n-1) + x[n-2]*_B^(n-2) + ... + x[1]*_B + x[0]
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//
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// with 0 <= x[i] < _B and 0 <= i < n is stored in a slice of length n,
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// with the digits x[i] as the slice elements.
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//
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// A number is normalized if the slice contains no leading 0 digits.
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// During arithmetic operations, denormalized values may occur but are
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// always normalized before returning the final result. The normalized
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// representation of 0 is the empty or nil slice (length = 0).
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type nat []Word
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var (
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natOne = nat{1}
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natTwo = nat{2}
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natTen = nat{10}
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)
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func (z nat) clear() {
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for i := range z {
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z[i] = 0
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}
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}
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func (z nat) norm() nat {
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i := len(z)
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for i > 0 && z[i-1] == 0 {
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i--
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}
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return z[0:i]
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}
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func (z nat) make(n int) nat {
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if n <= cap(z) {
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return z[0:n] // reuse z
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}
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// Choosing a good value for e has significant performance impact
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// because it increases the chance that a value can be reused.
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const e = 4 // extra capacity
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return make(nat, n, n+e)
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}
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func (z nat) setWord(x Word) nat {
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if x == 0 {
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return z.make(0)
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}
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z = z.make(1)
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z[0] = x
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return z
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}
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func (z nat) setUint64(x uint64) nat {
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// single-digit values
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if w := Word(x); uint64(w) == x {
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return z.setWord(w)
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}
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// compute number of words n required to represent x
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n := 0
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for t := x; t > 0; t >>= _W {
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n++
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}
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// split x into n words
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z = z.make(n)
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for i := range z {
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z[i] = Word(x & _M)
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x >>= _W
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}
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return z
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}
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func (z nat) set(x nat) nat {
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z = z.make(len(x))
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copy(z, x)
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return z
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}
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func (z nat) add(x, y nat) nat {
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m := len(x)
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n := len(y)
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switch {
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case m < n:
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return z.add(y, x)
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case m == 0:
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// n == 0 because m >= n; result is 0
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return z.make(0)
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case n == 0:
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// result is x
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return z.set(x)
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}
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// m > 0
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z = z.make(m + 1)
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c := addVV(z[0:n], x, y)
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if m > n {
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c = addVW(z[n:m], x[n:], c)
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}
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z[m] = c
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return z.norm()
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}
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func (z nat) sub(x, y nat) nat {
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m := len(x)
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n := len(y)
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switch {
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case m < n:
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panic("underflow")
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case m == 0:
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// n == 0 because m >= n; result is 0
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return z.make(0)
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case n == 0:
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// result is x
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return z.set(x)
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}
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// m > 0
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z = z.make(m)
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c := subVV(z[0:n], x, y)
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if m > n {
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c = subVW(z[n:], x[n:], c)
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}
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if c != 0 {
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panic("underflow")
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}
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return z.norm()
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}
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func (x nat) cmp(y nat) (r int) {
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m := len(x)
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n := len(y)
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if m != n || m == 0 {
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switch {
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case m < n:
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r = -1
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case m > n:
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r = 1
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}
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return
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}
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i := m - 1
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for i > 0 && x[i] == y[i] {
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i--
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}
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switch {
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case x[i] < y[i]:
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r = -1
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case x[i] > y[i]:
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r = 1
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}
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return
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}
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func (z nat) mulAddWW(x nat, y, r Word) nat {
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m := len(x)
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if m == 0 || y == 0 {
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return z.setWord(r) // result is r
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}
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// m > 0
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z = z.make(m + 1)
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z[m] = mulAddVWW(z[0:m], x, y, r)
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return z.norm()
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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 nat) {
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z[0 : len(x)+len(y)].clear() // initialize z
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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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// Fast version of z[0:n+n>>1].add(z[0:n+n>>1], x[0:n]) w/o bounds checks.
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// Factored out for readability - do not use outside karatsuba.
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func karatsubaAdd(z, x nat, n int) {
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if c := addVV(z[0:n], z, x); c != 0 {
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addVW(z[n:n+n>>1], z[n:], c)
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}
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}
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// Like karatsubaAdd, but does subtract.
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func karatsubaSub(z, x nat, n int) {
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if c := subVV(z[0:n], z, x); c != 0 {
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subVW(z[n:n+n>>1], z[n:], c)
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}
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}
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// Operands that are shorter than karatsubaThreshold are multiplied using
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// "grade school" multiplication; for longer operands the Karatsuba algorithm
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// is used.
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var karatsubaThreshold int = 32 // computed by calibrate.go
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// karatsuba multiplies x and y and leaves the result in z.
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// Both x and y must have the same length n and n must be a
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// power of 2. The result vector z must have len(z) >= 6*n.
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// The (non-normalized) result is placed in z[0 : 2*n].
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func karatsuba(z, x, y nat) {
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n := len(y)
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// Switch to basic multiplication if numbers are odd or small.
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// (n is always even if karatsubaThreshold is even, but be
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// conservative)
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if n&1 != 0 || n < karatsubaThreshold || n < 2 {
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basicMul(z, x, y)
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return
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}
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// n&1 == 0 && n >= karatsubaThreshold && n >= 2
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// Karatsuba multiplication is based on the observation that
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// for two numbers x and y with:
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//
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// x = x1*b + x0
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// y = y1*b + y0
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//
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// the product x*y can be obtained with 3 products z2, z1, z0
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// instead of 4:
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//
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// x*y = x1*y1*b*b + (x1*y0 + x0*y1)*b + x0*y0
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// = z2*b*b + z1*b + z0
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//
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// with:
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//
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// xd = x1 - x0
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// yd = y0 - y1
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//
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// z1 = xd*yd + z1 + z0
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// = (x1-x0)*(y0 - y1) + z1 + z0
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// = x1*y0 - x1*y1 - x0*y0 + x0*y1 + z1 + z0
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// = x1*y0 - z1 - z0 + x0*y1 + z1 + z0
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// = x1*y0 + x0*y1
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// split x, y into "digits"
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n2 := n >> 1 // n2 >= 1
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x1, x0 := x[n2:], x[0:n2] // x = x1*b + y0
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y1, y0 := y[n2:], y[0:n2] // y = y1*b + y0
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// z is used for the result and temporary storage:
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//
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// 6*n 5*n 4*n 3*n 2*n 1*n 0*n
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// z = [z2 copy|z0 copy| xd*yd | yd:xd | x1*y1 | x0*y0 ]
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//
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// For each recursive call of karatsuba, an unused slice of
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// z is passed in that has (at least) half the length of the
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// caller's z.
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// compute z0 and z2 with the result "in place" in z
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karatsuba(z, x0, y0) // z0 = x0*y0
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karatsuba(z[n:], x1, y1) // z2 = x1*y1
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// compute xd (or the negative value if underflow occurs)
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s := 1 // sign of product xd*yd
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xd := z[2*n : 2*n+n2]
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if subVV(xd, x1, x0) != 0 { // x1-x0
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s = -s
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subVV(xd, x0, x1) // x0-x1
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}
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// compute yd (or the negative value if underflow occurs)
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yd := z[2*n+n2 : 3*n]
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if subVV(yd, y0, y1) != 0 { // y0-y1
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s = -s
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subVV(yd, y1, y0) // y1-y0
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}
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// p = (x1-x0)*(y0-y1) == x1*y0 - x1*y1 - x0*y0 + x0*y1 for s > 0
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// p = (x0-x1)*(y0-y1) == x0*y0 - x0*y1 - x1*y0 + x1*y1 for s < 0
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p := z[n*3:]
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karatsuba(p, xd, yd)
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// save original z2:z0
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// (ok to use upper half of z since we're done recursing)
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r := z[n*4:]
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copy(r, z)
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// add up all partial products
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//
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// 2*n n 0
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// z = [ z2 | z0 ]
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// + [ z0 ]
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// + [ z2 ]
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// + [ p ]
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//
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karatsubaAdd(z[n2:], r, n)
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karatsubaAdd(z[n2:], r[n:], n)
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if s > 0 {
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karatsubaAdd(z[n2:], p, n)
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} else {
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karatsubaSub(z[n2:], p, n)
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}
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}
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// alias returns true if x and y share the same base array.
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func alias(x, y nat) bool {
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return cap(x) > 0 && cap(y) > 0 && &x[0:cap(x)][cap(x)-1] == &y[0:cap(y)][cap(y)-1]
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}
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// addAt implements z += x*(1<<(_W*i)); z must be long enough.
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// (we don't use nat.add because we need z to stay the same
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// slice, and we don't need to normalize z after each addition)
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func addAt(z, x nat, i int) {
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if n := len(x); n > 0 {
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if c := addVV(z[i:i+n], z[i:], x); c != 0 {
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j := i + n
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if j < len(z) {
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addVW(z[j:], z[j:], c)
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}
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}
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}
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}
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func max(x, y int) int {
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if x > y {
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return x
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}
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return y
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}
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// karatsubaLen computes an approximation to the maximum k <= n such that
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// k = p<<i for a number p <= karatsubaThreshold and an i >= 0. Thus, the
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// result is the largest number that can be divided repeatedly by 2 before
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// becoming about the value of karatsubaThreshold.
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func karatsubaLen(n int) int {
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i := uint(0)
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for n > karatsubaThreshold {
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n >>= 1
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i++
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}
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return n << i
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}
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func (z nat) mul(x, y nat) nat {
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m := len(x)
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n := len(y)
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switch {
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case m < n:
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return z.mul(y, x)
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case m == 0 || n == 0:
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return z.make(0)
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case n == 1:
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return z.mulAddWW(x, y[0], 0)
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}
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// m >= n > 1
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// determine if z can be reused
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if alias(z, x) || alias(z, y) {
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z = nil // z is an alias for x or y - cannot reuse
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}
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// use basic multiplication if the numbers are small
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if n < karatsubaThreshold || n < 2 {
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z = z.make(m + n)
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basicMul(z, x, y)
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return z.norm()
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}
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// m >= n && n >= karatsubaThreshold && n >= 2
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// determine Karatsuba length k such that
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//
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// x = x1*b + x0
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// y = y1*b + y0 (and k <= len(y), which implies k <= len(x))
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// b = 1<<(_W*k) ("base" of digits xi, yi)
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//
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k := karatsubaLen(n)
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// k <= n
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// multiply x0 and y0 via Karatsuba
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x0 := x[0:k] // x0 is not normalized
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y0 := y[0:k] // y0 is not normalized
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z = z.make(max(6*k, m+n)) // enough space for karatsuba of x0*y0 and full result of x*y
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karatsuba(z, x0, y0)
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z = z[0 : m+n] // z has final length but may be incomplete, upper portion is garbage
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// If x1 and/or y1 are not 0, add missing terms to z explicitly:
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//
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// m+n 2*k 0
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// z = [ ... | x0*y0 ]
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// + [ x1*y1 ]
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// + [ x1*y0 ]
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// + [ x0*y1 ]
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//
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if k < n || m != n {
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x1 := x[k:] // x1 is normalized because x is
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y1 := y[k:] // y1 is normalized because y is
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var t nat
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t = t.mul(x1, y1)
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copy(z[2*k:], t)
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z[2*k+len(t):].clear() // upper portion of z is garbage
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t = t.mul(x1, y0.norm())
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addAt(z, t, k)
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t = t.mul(x0.norm(), y1)
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addAt(z, t, k)
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}
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return z.norm()
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}
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// mulRange computes the product of all the unsigned integers in the
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// range [a, b] inclusively. If a > b (empty range), the result is 1.
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func (z nat) mulRange(a, b uint64) nat {
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switch {
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case a == 0:
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// cut long ranges short (optimization)
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return z.setUint64(0)
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case a > b:
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return z.setUint64(1)
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case a == b:
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return z.setUint64(a)
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case a+1 == b:
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return z.mul(nat(nil).setUint64(a), nat(nil).setUint64(b))
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}
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m := (a + b) / 2
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return z.mul(nat(nil).mulRange(a, m), nat(nil).mulRange(m+1, b))
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}
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// q = (x-r)/y, with 0 <= r < y
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func (z nat) divW(x nat, y Word) (q nat, r Word) {
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m := len(x)
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switch {
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case y == 0:
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panic("division by zero")
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case y == 1:
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q = z.set(x) // result is x
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return
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case m == 0:
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q = z.make(0) // result is 0
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return
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}
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// m > 0
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z = z.make(m)
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r = divWVW(z, 0, x, y)
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q = z.norm()
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return
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}
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func (z nat) div(z2, u, v nat) (q, r nat) {
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if len(v) == 0 {
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panic("division by zero")
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}
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if u.cmp(v) < 0 {
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q = z.make(0)
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r = z2.set(u)
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return
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}
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if len(v) == 1 {
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var rprime Word
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q, rprime = z.divW(u, v[0])
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if rprime > 0 {
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r = z2.make(1)
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r[0] = rprime
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} else {
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r = z2.make(0)
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}
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return
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}
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q, r = z.divLarge(z2, u, v)
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return
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}
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// q = (uIn-r)/v, with 0 <= r < y
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// Uses z as storage for q, and u as storage for r if possible.
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// See Knuth, Volume 2, section 4.3.1, Algorithm D.
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// Preconditions:
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// len(v) >= 2
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// len(uIn) >= len(v)
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func (z nat) divLarge(u, uIn, v nat) (q, r nat) {
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n := len(v)
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m := len(uIn) - n
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// determine if z can be reused
|
|
// TODO(gri) should find a better solution - this if statement
|
|
// is very costly (see e.g. time pidigits -s -n 10000)
|
|
if alias(z, uIn) || alias(z, v) {
|
|
z = nil // z is an alias for uIn or v - cannot reuse
|
|
}
|
|
q = z.make(m + 1)
|
|
|
|
qhatv := make(nat, n+1)
|
|
if alias(u, uIn) || alias(u, v) {
|
|
u = nil // u is an alias for uIn or v - cannot reuse
|
|
}
|
|
u = u.make(len(uIn) + 1)
|
|
u.clear()
|
|
|
|
// D1.
|
|
shift := Word(leadingZeros(v[n-1]))
|
|
shlVW(v, v, shift)
|
|
u[len(uIn)] = shlVW(u[0:len(uIn)], uIn, shift)
|
|
|
|
// D2.
|
|
for j := m; j >= 0; j-- {
|
|
// D3.
|
|
qhat := Word(_M)
|
|
if u[j+n] != v[n-1] {
|
|
var rhat Word
|
|
qhat, rhat = divWW(u[j+n], u[j+n-1], v[n-1])
|
|
|
|
// x1 | x2 = q̂v_{n-2}
|
|
x1, x2 := mulWW(qhat, v[n-2])
|
|
// test if q̂v_{n-2} > br̂ + u_{j+n-2}
|
|
for greaterThan(x1, x2, rhat, u[j+n-2]) {
|
|
qhat--
|
|
prevRhat := rhat
|
|
rhat += v[n-1]
|
|
// v[n-1] >= 0, so this tests for overflow.
|
|
if rhat < prevRhat {
|
|
break
|
|
}
|
|
x1, x2 = mulWW(qhat, v[n-2])
|
|
}
|
|
}
|
|
|
|
// D4.
|
|
qhatv[n] = mulAddVWW(qhatv[0:n], v, qhat, 0)
|
|
|
|
c := subVV(u[j:j+len(qhatv)], u[j:], qhatv)
|
|
if c != 0 {
|
|
c := addVV(u[j:j+n], u[j:], v)
|
|
u[j+n] += c
|
|
qhat--
|
|
}
|
|
|
|
q[j] = qhat
|
|
}
|
|
|
|
q = q.norm()
|
|
shrVW(u, u, shift)
|
|
shrVW(v, v, shift)
|
|
r = u.norm()
|
|
|
|
return q, r
|
|
}
|
|
|
|
|
|
// Length of x in bits. x must be normalized.
|
|
func (x nat) bitLen() int {
|
|
if i := len(x) - 1; i >= 0 {
|
|
return i*_W + bitLen(x[i])
|
|
}
|
|
return 0
|
|
}
|
|
|
|
|
|
func hexValue(ch byte) int {
|
|
var d byte
|
|
switch {
|
|
case '0' <= ch && ch <= '9':
|
|
d = ch - '0'
|
|
case 'a' <= ch && ch <= 'f':
|
|
d = ch - 'a' + 10
|
|
case 'A' <= ch && ch <= 'F':
|
|
d = ch - 'A' + 10
|
|
default:
|
|
return -1
|
|
}
|
|
return int(d)
|
|
}
|
|
|
|
|
|
// scan returns the natural number corresponding to the
|
|
// longest possible prefix of s representing a natural number in a
|
|
// given conversion base, the actual conversion base used, and the
|
|
// prefix length. The syntax of natural numbers follows the syntax
|
|
// of unsigned integer literals in Go.
|
|
//
|
|
// If the base argument is 0, the string prefix determines the actual
|
|
// conversion base. A prefix of ``0x'' or ``0X'' selects base 16; the
|
|
// ``0'' prefix selects base 8, and a ``0b'' or ``0B'' prefix selects
|
|
// base 2. Otherwise the selected base is 10.
|
|
//
|
|
func (z nat) scan(s string, base int) (nat, int, int) {
|
|
// determine base if necessary
|
|
i, n := 0, len(s)
|
|
if base == 0 {
|
|
base = 10
|
|
if n > 0 && s[0] == '0' {
|
|
base, i = 8, 1
|
|
if n > 1 {
|
|
switch s[1] {
|
|
case 'x', 'X':
|
|
base, i = 16, 2
|
|
case 'b', 'B':
|
|
base, i = 2, 2
|
|
}
|
|
}
|
|
}
|
|
}
|
|
|
|
// reject illegal bases or strings consisting only of prefix
|
|
if base < 2 || 16 < base || (base != 8 && i >= n) {
|
|
return z, 0, 0
|
|
}
|
|
|
|
// convert string
|
|
z = z.make(0)
|
|
for ; i < n; i++ {
|
|
d := hexValue(s[i])
|
|
if 0 <= d && d < base {
|
|
z = z.mulAddWW(z, Word(base), Word(d))
|
|
} else {
|
|
break
|
|
}
|
|
}
|
|
|
|
return z.norm(), base, i
|
|
}
|
|
|
|
|
|
// string converts x to a string for a given base, with 2 <= base <= 16.
|
|
// TODO(gri) in the style of the other routines, perhaps this should take
|
|
// a []byte buffer and return it
|
|
func (x nat) string(base int) string {
|
|
if base < 2 || 16 < base {
|
|
panic("illegal base")
|
|
}
|
|
|
|
if len(x) == 0 {
|
|
return "0"
|
|
}
|
|
|
|
// allocate buffer for conversion
|
|
i := x.bitLen()/log2(Word(base)) + 1 // +1: round up
|
|
s := make([]byte, i)
|
|
|
|
// don't destroy x
|
|
q := nat(nil).set(x)
|
|
|
|
// convert
|
|
for len(q) > 0 {
|
|
i--
|
|
var r Word
|
|
q, r = q.divW(q, Word(base))
|
|
s[i] = "0123456789abcdef"[r]
|
|
}
|
|
|
|
return string(s[i:])
|
|
}
|
|
|
|
|
|
const deBruijn32 = 0x077CB531
|
|
|
|
var deBruijn32Lookup = []byte{
|
|
0, 1, 28, 2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4, 8,
|
|
31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6, 11, 5, 10, 9,
|
|
}
|
|
|
|
const deBruijn64 = 0x03f79d71b4ca8b09
|
|
|
|
var deBruijn64Lookup = []byte{
|
|
0, 1, 56, 2, 57, 49, 28, 3, 61, 58, 42, 50, 38, 29, 17, 4,
|
|
62, 47, 59, 36, 45, 43, 51, 22, 53, 39, 33, 30, 24, 18, 12, 5,
|
|
63, 55, 48, 27, 60, 41, 37, 16, 46, 35, 44, 21, 52, 32, 23, 11,
|
|
54, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19, 9, 13, 8, 7, 6,
|
|
}
|
|
|
|
// trailingZeroBits returns the number of consecutive zero bits on the right
|
|
// side of the given Word.
|
|
// See Knuth, volume 4, section 7.3.1
|
|
func trailingZeroBits(x Word) int {
|
|
// x & -x leaves only the right-most bit set in the word. Let k be the
|
|
// index of that bit. Since only a single bit is set, the value is two
|
|
// to the power of k. Multipling by a power of two is equivalent to
|
|
// left shifting, in this case by k bits. The de Bruijn constant is
|
|
// such that all six bit, consecutive substrings are distinct.
|
|
// Therefore, if we have a left shifted version of this constant we can
|
|
// find by how many bits it was shifted by looking at which six bit
|
|
// substring ended up at the top of the word.
|
|
switch _W {
|
|
case 32:
|
|
return int(deBruijn32Lookup[((x&-x)*deBruijn32)>>27])
|
|
case 64:
|
|
return int(deBruijn64Lookup[((x&-x)*(deBruijn64&_M))>>58])
|
|
default:
|
|
panic("Unknown word size")
|
|
}
|
|
|
|
return 0
|
|
}
|
|
|
|
|
|
// z = x << s
|
|
func (z nat) shl(x nat, s uint) nat {
|
|
m := len(x)
|
|
if m == 0 {
|
|
return z.make(0)
|
|
}
|
|
// m > 0
|
|
|
|
n := m + int(s/_W)
|
|
z = z.make(n + 1)
|
|
z[n] = shlVW(z[n-m:n], x, Word(s%_W))
|
|
z[0 : n-m].clear()
|
|
|
|
return z.norm()
|
|
}
|
|
|
|
|
|
// z = x >> s
|
|
func (z nat) shr(x nat, s uint) nat {
|
|
m := len(x)
|
|
n := m - int(s/_W)
|
|
if n <= 0 {
|
|
return z.make(0)
|
|
}
|
|
// n > 0
|
|
|
|
z = z.make(n)
|
|
shrVW(z, x[m-n:], Word(s%_W))
|
|
|
|
return z.norm()
|
|
}
|
|
|
|
|
|
func (z nat) and(x, y nat) nat {
|
|
m := len(x)
|
|
n := len(y)
|
|
if m > n {
|
|
m = n
|
|
}
|
|
// m <= n
|
|
|
|
z = z.make(m)
|
|
for i := 0; i < m; i++ {
|
|
z[i] = x[i] & y[i]
|
|
}
|
|
|
|
return z.norm()
|
|
}
|
|
|
|
|
|
func (z nat) andNot(x, y nat) nat {
|
|
m := len(x)
|
|
n := len(y)
|
|
if n > m {
|
|
n = m
|
|
}
|
|
// m >= n
|
|
|
|
z = z.make(m)
|
|
for i := 0; i < n; i++ {
|
|
z[i] = x[i] &^ y[i]
|
|
}
|
|
copy(z[n:m], x[n:m])
|
|
|
|
return z.norm()
|
|
}
|
|
|
|
|
|
func (z nat) or(x, y nat) nat {
|
|
m := len(x)
|
|
n := len(y)
|
|
s := x
|
|
if m < n {
|
|
n, m = m, n
|
|
s = y
|
|
}
|
|
// m >= n
|
|
|
|
z = z.make(m)
|
|
for i := 0; i < n; i++ {
|
|
z[i] = x[i] | y[i]
|
|
}
|
|
copy(z[n:m], s[n:m])
|
|
|
|
return z.norm()
|
|
}
|
|
|
|
|
|
func (z nat) xor(x, y nat) nat {
|
|
m := len(x)
|
|
n := len(y)
|
|
s := x
|
|
if m < n {
|
|
n, m = m, n
|
|
s = y
|
|
}
|
|
// m >= n
|
|
|
|
z = z.make(m)
|
|
for i := 0; i < n; i++ {
|
|
z[i] = x[i] ^ y[i]
|
|
}
|
|
copy(z[n:m], s[n:m])
|
|
|
|
return z.norm()
|
|
}
|
|
|
|
|
|
// greaterThan returns true iff (x1<<_W + x2) > (y1<<_W + y2)
|
|
func greaterThan(x1, x2, y1, y2 Word) bool { return x1 > y1 || x1 == y1 && x2 > y2 }
|
|
|
|
|
|
// modW returns x % d.
|
|
func (x nat) modW(d Word) (r Word) {
|
|
// TODO(agl): we don't actually need to store the q value.
|
|
var q nat
|
|
q = q.make(len(x))
|
|
return divWVW(q, 0, x, d)
|
|
}
|
|
|
|
|
|
// powersOfTwoDecompose finds q and k such that q * 1<<k = n and q is odd.
|
|
func (n nat) powersOfTwoDecompose() (q nat, k Word) {
|
|
if len(n) == 0 {
|
|
return n, 0
|
|
}
|
|
|
|
zeroWords := 0
|
|
for n[zeroWords] == 0 {
|
|
zeroWords++
|
|
}
|
|
// One of the words must be non-zero by invariant, therefore
|
|
// zeroWords < len(n).
|
|
x := trailingZeroBits(n[zeroWords])
|
|
|
|
q = q.make(len(n) - zeroWords)
|
|
shrVW(q, n[zeroWords:], Word(x))
|
|
q = q.norm()
|
|
|
|
k = Word(_W*zeroWords + x)
|
|
return
|
|
}
|
|
|
|
|
|
// random creates a random integer in [0..limit), using the space in z if
|
|
// possible. n is the bit length of limit.
|
|
func (z nat) random(rand *rand.Rand, limit nat, n int) nat {
|
|
bitLengthOfMSW := uint(n % _W)
|
|
if bitLengthOfMSW == 0 {
|
|
bitLengthOfMSW = _W
|
|
}
|
|
mask := Word((1 << bitLengthOfMSW) - 1)
|
|
z = z.make(len(limit))
|
|
|
|
for {
|
|
for i := range z {
|
|
switch _W {
|
|
case 32:
|
|
z[i] = Word(rand.Uint32())
|
|
case 64:
|
|
z[i] = Word(rand.Uint32()) | Word(rand.Uint32())<<32
|
|
}
|
|
}
|
|
|
|
z[len(limit)-1] &= mask
|
|
|
|
if z.cmp(limit) < 0 {
|
|
break
|
|
}
|
|
}
|
|
|
|
return z.norm()
|
|
}
|
|
|
|
|
|
// If m != nil, expNN calculates x**y mod m. Otherwise it calculates x**y. It
|
|
// reuses the storage of z if possible.
|
|
func (z nat) expNN(x, y, m nat) nat {
|
|
if alias(z, x) || alias(z, y) {
|
|
// We cannot allow in place modification of x or y.
|
|
z = nil
|
|
}
|
|
|
|
if len(y) == 0 {
|
|
z = z.make(1)
|
|
z[0] = 1
|
|
return z
|
|
}
|
|
|
|
if m != nil {
|
|
// We likely end up being as long as the modulus.
|
|
z = z.make(len(m))
|
|
}
|
|
z = z.set(x)
|
|
v := y[len(y)-1]
|
|
// It's invalid for the most significant word to be zero, therefore we
|
|
// will find a one bit.
|
|
shift := leadingZeros(v) + 1
|
|
v <<= shift
|
|
var q nat
|
|
|
|
const mask = 1 << (_W - 1)
|
|
|
|
// We walk through the bits of the exponent one by one. Each time we
|
|
// see a bit, we square, thus doubling the power. If the bit is a one,
|
|
// we also multiply by x, thus adding one to the power.
|
|
|
|
w := _W - int(shift)
|
|
for j := 0; j < w; j++ {
|
|
z = z.mul(z, z)
|
|
|
|
if v&mask != 0 {
|
|
z = z.mul(z, x)
|
|
}
|
|
|
|
if m != nil {
|
|
q, z = q.div(z, z, m)
|
|
}
|
|
|
|
v <<= 1
|
|
}
|
|
|
|
for i := len(y) - 2; i >= 0; i-- {
|
|
v = y[i]
|
|
|
|
for j := 0; j < _W; j++ {
|
|
z = z.mul(z, z)
|
|
|
|
if v&mask != 0 {
|
|
z = z.mul(z, x)
|
|
}
|
|
|
|
if m != nil {
|
|
q, z = q.div(z, z, m)
|
|
}
|
|
|
|
v <<= 1
|
|
}
|
|
}
|
|
|
|
return z
|
|
}
|
|
|
|
|
|
// probablyPrime performs reps Miller-Rabin tests to check whether n is prime.
|
|
// If it returns true, n is prime with probability 1 - 1/4^reps.
|
|
// If it returns false, n is not prime.
|
|
func (n nat) probablyPrime(reps int) bool {
|
|
if len(n) == 0 {
|
|
return false
|
|
}
|
|
|
|
if len(n) == 1 {
|
|
if n[0] < 2 {
|
|
return false
|
|
}
|
|
|
|
if n[0]%2 == 0 {
|
|
return n[0] == 2
|
|
}
|
|
|
|
// We have to exclude these cases because we reject all
|
|
// multiples of these numbers below.
|
|
switch n[0] {
|
|
case 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53:
|
|
return true
|
|
}
|
|
}
|
|
|
|
const primesProduct32 = 0xC0CFD797 // Π {p ∈ primes, 2 < p <= 29}
|
|
const primesProduct64 = 0xE221F97C30E94E1D // Π {p ∈ primes, 2 < p <= 53}
|
|
|
|
var r Word
|
|
switch _W {
|
|
case 32:
|
|
r = n.modW(primesProduct32)
|
|
case 64:
|
|
r = n.modW(primesProduct64 & _M)
|
|
default:
|
|
panic("Unknown word size")
|
|
}
|
|
|
|
if r%3 == 0 || r%5 == 0 || r%7 == 0 || r%11 == 0 ||
|
|
r%13 == 0 || r%17 == 0 || r%19 == 0 || r%23 == 0 || r%29 == 0 {
|
|
return false
|
|
}
|
|
|
|
if _W == 64 && (r%31 == 0 || r%37 == 0 || r%41 == 0 ||
|
|
r%43 == 0 || r%47 == 0 || r%53 == 0) {
|
|
return false
|
|
}
|
|
|
|
nm1 := nat(nil).sub(n, natOne)
|
|
// 1<<k * q = nm1;
|
|
q, k := nm1.powersOfTwoDecompose()
|
|
|
|
nm3 := nat(nil).sub(nm1, natTwo)
|
|
rand := rand.New(rand.NewSource(int64(n[0])))
|
|
|
|
var x, y, quotient nat
|
|
nm3Len := nm3.bitLen()
|
|
|
|
NextRandom:
|
|
for i := 0; i < reps; i++ {
|
|
x = x.random(rand, nm3, nm3Len)
|
|
x = x.add(x, natTwo)
|
|
y = y.expNN(x, q, n)
|
|
if y.cmp(natOne) == 0 || y.cmp(nm1) == 0 {
|
|
continue
|
|
}
|
|
for j := Word(1); j < k; j++ {
|
|
y = y.mul(y, y)
|
|
quotient, y = quotient.div(y, y, n)
|
|
if y.cmp(nm1) == 0 {
|
|
continue NextRandom
|
|
}
|
|
if y.cmp(natOne) == 0 {
|
|
return false
|
|
}
|
|
}
|
|
return false
|
|
}
|
|
|
|
return true
|
|
}
|
|
|
|
|
|
// bytes writes the value of z into buf using big-endian encoding.
|
|
// len(buf) must be >= len(z)*_S. The value of z is encoded in the
|
|
// slice buf[i:]. The number i of unused bytes at the beginning of
|
|
// buf is returned as result.
|
|
func (z nat) bytes(buf []byte) (i int) {
|
|
i = len(buf)
|
|
for _, d := range z {
|
|
for j := 0; j < _S; j++ {
|
|
i--
|
|
buf[i] = byte(d)
|
|
d >>= 8
|
|
}
|
|
}
|
|
|
|
for i < len(buf) && buf[i] == 0 {
|
|
i++
|
|
}
|
|
|
|
return
|
|
}
|
|
|
|
|
|
// setBytes interprets buf as the bytes of a big-endian unsigned
|
|
// integer, sets z to that value, and returns z.
|
|
func (z nat) setBytes(buf []byte) nat {
|
|
z = z.make((len(buf) + _S - 1) / _S)
|
|
|
|
k := 0
|
|
s := uint(0)
|
|
var d Word
|
|
for i := len(buf); i > 0; i-- {
|
|
d |= Word(buf[i-1]) << s
|
|
if s += 8; s == _S*8 {
|
|
z[k] = d
|
|
k++
|
|
s = 0
|
|
d = 0
|
|
}
|
|
}
|
|
if k < len(z) {
|
|
z[k] = d
|
|
}
|
|
|
|
return z.norm()
|
|
}
|