nat.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This file implements unsigned multi-precision integers (natural
// numbers). They are the building blocks for the implementation
// of signed integers, rationals, and floating-point numbers.
//
// Caution: This implementation relies on the function "alias"
//          which assumes that (nat) slice capacities are never
//          changed (no 3-operand slice expressions). If that
//          changes, alias needs to be updated for correctness.

package multiexp

import (
	"math/big"
	"sync"
)

type Word uint

// An unsigned integer x of the form
//
//	x = x[n-1]*_B^(n-1) + x[n-2]*_B^(n-2) + ... + x[1]*_B + x[0]
//
// with 0 <= x[i] < _B and 0 <= i < n is stored in a slice of length n,
// with the digits x[i] as the slice elements.
//
// A number is normalized if the slice contains no leading 0 digits.
// During arithmetic operations, denormalized values may occur but are
// always normalized before returning the final result. The normalized
// representation of 0 is the empty or nil slice (length = 0).
type nat []Word

func newNat(n *big.Int) nat {
	if n.Sign() < 0 {
		panic("multiexp: negative number")
	}
	if n.BitLen() == 0 {
		return nil
	}
	// n is positive and non-zero
	zBits := n.Bits()
	z := make(nat, len(zBits))
	for i, d := range zBits {
		z[i] = Word(d)
	}
	return z
}

func (z nat) intBits() []big.Word {
	if len(z) == 0 {
		return nil
	}
	// z is positive and non-zero
	zBits := make([]big.Word, len(z))
	for i, d := range z {
		zBits[i] = big.Word(d)
	}
	return zBits
}

func (z nat) clear() {
	for i := range z {
		z[i] = 0
	}
}

func (z nat) norm() nat {
	i := len(z)
	for i > 0 && z[i-1] == 0 {
		i--
	}
	return z[0:i]
}

func (z nat) make(n int) nat {
	if n <= cap(z) {
		return z[:n] // reuse z
	}
	if n == 1 {
		// Most nats start small and stay that way; don't over-allocate.
		return make(nat, 1)
	}
	// Choosing a good value for e has significant performance impact
	// because it increases the chance that a value can be reused.
	const e = 4 // extra capacity
	return make(nat, n, n+e)
}

func (z nat) setWord(x Word) nat {
	if x == 0 {
		return z[:0]
	}
	z = z.make(1)
	z[0] = x
	return z
}

func (z nat) set(x nat) nat {
	z = z.make(len(x))
	copy(z, x)
	return z
}

func (z nat) sub(x, y nat) nat {
	m := len(x)
	n := len(y)

	switch {
	case m < n:
		panic("underflow")
	case m == 0:
		// n == 0 because m >= n; result is 0
		return z[:0]
	case n == 0:
		// result is x
		return z.set(x)
	}
	// m > 0

	z = z.make(m)
	c := subVV(z[0:n], x, y)
	if m > n {
		c = subVW(z[n:], x[n:], c)
	}
	if c != 0 {
		panic("underflow")
	}

	return z.norm()
}

func (x nat) cmp(y nat) (r int) {
	m := len(x)
	n := len(y)
	if m != n || m == 0 {
		switch {
		case m < n:
			r = -1
		case m > n:
			r = 1
		}
		return
	}

	i := m - 1
	for i > 0 && x[i] == y[i] {
		i--
	}

	switch {
	case x[i] < y[i]:
		r = -1
	case x[i] > y[i]:
		r = 1
	}
	return
}

func (z nat) mulAddWW(x nat, y, r Word) nat {
	m := len(x)
	if m == 0 || y == 0 {
		return z.setWord(r) // result is r
	}
	// m > 0

	z = z.make(m + 1)
	z[m] = mulAddVWW(z[0:m], x, y, r)

	return z.norm()
}

// basicMul multiplies x and y and leaves the result in z.
// The (non-normalized) result is placed in z[0 : len(x) + len(y)].
func basicMul(z, x, y nat) {
	z[0 : len(x)+len(y)].clear() // initialize z
	for i, d := range y {
		if d != 0 {
			z[len(x)+i] = addMulVVW(z[i:i+len(x)], x, d)
		}
	}
}

// montgomery computes z mod m = x*y*2**(-n*_W) mod m,
// assuming k = -1/m mod 2**_W.
// z is used for storing the result which is returned;
// z must not alias x, y or m.
// See Gueron, "Efficient Software Implementations of Modular Exponentiation".
// https://eprint.iacr.org/2011/239.pdf
// In the terminology of that paper, this is an "Almost Montgomery Multiplication":
// x and y are required to satisfy 0 <= z < 2**(n*_W) and then the result
// z is guaranteed to satisfy 0 <= z < 2**(n*_W), but it may not be < m.
func (z nat) montgomery(x, y, m nat, k Word, n int) nat {
	// This code assumes x, y, m are all the same length, n.
	// (required by addMulVVW and the for loop).
	// It also assumes that x, y are already reduced mod m,
	// or else the result will not be properly reduced.
	if len(x) != n || len(y) != n || len(m) != n {
		panic("math/big: mismatched montgomery number lengths")
	}
	z = z.make(n * 2)
	z.clear()
	var c Word
	for i := 0; i < n; i++ {
		d := y[i]
		c2 := addMulVVW(z[i:n+i], x, d)
		t := z[i] * k
		c3 := addMulVVW(z[i:n+i], m, t)
		cx := c + c2
		cy := cx + c3
		z[n+i] = cy
		if cx < c2 || cy < c3 {
			c = 1
		} else {
			c = 0
		}
	}
	if c != 0 {
		subVV(z[:n], z[n:], m)
	} else {
		copy(z[:n], z[n:])
	}
	return z[:n]
}

// Fast version of z[0:n+n>>1].add(z[0:n+n>>1], x[0:n]) w/o bounds checks.
// Factored out for readability - do not use outside karatsuba.
func karatsubaAdd(z, x nat, n int) {
	if c := addVV(z[0:n], z, x); c != 0 {
		addVW(z[n:n+n>>1], z[n:], c)
	}
}

// Like karatsubaAdd, but does subtract.
func karatsubaSub(z, x nat, n int) {
	if c := subVV(z[0:n], z, x); c != 0 {
		subVW(z[n:n+n>>1], z[n:], c)
	}
}

// Operands that are shorter than karatsubaThreshold are multiplied using
// "grade school" multiplication; for longer operands the Karatsuba algorithm
// is used.
var karatsubaThreshold = 40 // computed by calibrate_test.go

// karatsuba multiplies x and y and leaves the result in z.
// Both x and y must have the same length n and n must be a
// power of 2. The result vector z must have len(z) >= 6*n.
// The (non-normalized) result is placed in z[0 : 2*n].
func karatsuba(z, x, y nat) {
	n := len(y)

	// Switch to basic multiplication if numbers are odd or small.
	// (n is always even if karatsubaThreshold is even, but be
	// conservative)
	if n&1 != 0 || n < karatsubaThreshold || n < 2 {
		basicMul(z, x, y)
		return
	}
	// n&1 == 0 && n >= karatsubaThreshold && n >= 2

	// Karatsuba multiplication is based on the observation that
	// for two numbers x and y with:
	//
	//   x = x1*b + x0
	//   y = y1*b + y0
	//
	// the product x*y can be obtained with 3 products z2, z1, z0
	// instead of 4:
	//
	//   x*y = x1*y1*b*b + (x1*y0 + x0*y1)*b + x0*y0
	//       =    z2*b*b +              z1*b +    z0
	//
	// with:
	//
	//   xd = x1 - x0
	//   yd = y0 - y1
	//
	//   z1 =      xd*yd                    + z2 + z0
	//      = (x1-x0)*(y0 - y1)             + z2 + z0
	//      = x1*y0 - x1*y1 - x0*y0 + x0*y1 + z2 + z0
	//      = x1*y0 -    z2 -    z0 + x0*y1 + z2 + z0
	//      = x1*y0                 + x0*y1

	// split x, y into "digits"
	n2 := n >> 1              // n2 >= 1
	x1, x0 := x[n2:], x[0:n2] // x = x1*b + y0
	y1, y0 := y[n2:], y[0:n2] // y = y1*b + y0

	// z is used for the result and temporary storage:
	//
	//   6*n     5*n     4*n     3*n     2*n     1*n     0*n
	// z = [z2 copy|z0 copy| xd*yd | yd:xd | x1*y1 | x0*y0 ]
	//
	// For each recursive call of karatsuba, an unused slice of
	// z is passed in that has (at least) half the length of the
	// caller's z.

	// compute z0 and z2 with the result "in place" in z
	karatsuba(z, x0, y0)     // z0 = x0*y0
	karatsuba(z[n:], x1, y1) // z2 = x1*y1

	// compute xd (or the negative value if underflow occurs)
	s := 1 // sign of product xd*yd
	xd := z[2*n : 2*n+n2]
	if subVV(xd, x1, x0) != 0 { // x1-x0
		s = -s
		subVV(xd, x0, x1) // x0-x1
	}

	// compute yd (or the negative value if underflow occurs)
	yd := z[2*n+n2 : 3*n]
	if subVV(yd, y0, y1) != 0 { // y0-y1
		s = -s
		subVV(yd, y1, y0) // y1-y0
	}

	// p = (x1-x0)*(y0-y1) == x1*y0 - x1*y1 - x0*y0 + x0*y1 for s > 0
	// p = (x0-x1)*(y0-y1) == x0*y0 - x0*y1 - x1*y0 + x1*y1 for s < 0
	p := z[n*3:]
	karatsuba(p, xd, yd)

	// save original z2:z0
	// (ok to use upper half of z since we're done recurring)
	r := z[n*4:]
	copy(r, z[:n*2])

	// add up all partial products
	//
	//   2*n     n     0
	// z = [ z2  | z0  ]
	//   +    [ z0  ]
	//   +    [ z2  ]
	//   +    [  p  ]
	//
	karatsubaAdd(z[n2:], r, n)
	karatsubaAdd(z[n2:], r[n:], n)
	if s > 0 {
		karatsubaAdd(z[n2:], p, n)
	} else {
		karatsubaSub(z[n2:], p, n)
	}
}

// alias reports whether x and y share the same base array.
//
// Note: alias assumes that the capacity of underlying arrays
// is never changed for nat values; i.e. that there are
// no 3-operand slice expressions in this code (or worse,
// reflect-based operations to the same effect).
func alias(x, y nat) bool {
	return cap(x) > 0 && cap(y) > 0 && &x[0:cap(x)][cap(x)-1] == &y[0:cap(y)][cap(y)-1]
}

// addAt implements z += x<<(_W*i); z must be long enough.
// (we don't use nat.add because we need z to stay the same
// slice, and we don't need to normalize z after each addition)
func addAt(z, x nat, i int) {
	if n := len(x); n > 0 {
		if c := addVV(z[i:i+n], z[i:], x); c != 0 {
			j := i + n
			if j < len(z) {
				addVW(z[j:], z[j:], c)
			}
		}
	}
}

func max(x, y int) int {
	if x > y {
		return x
	}
	return y
}

// karatsubaLen computes an approximation to the maximum k <= n such that
// k = p<<i for a number p <= threshold and an i >= 0. Thus, the
// result is the largest number that can be divided repeatedly by 2 before
// becoming about the value of threshold.
func karatsubaLen(n, threshold int) int {
	i := uint(0)
	for n > threshold {
		n >>= 1
		i++
	}
	return n << i
}

func (z nat) mul(x, y nat) nat {
	m := len(x)
	n := len(y)

	switch {
	case m < n:
		return z.mul(y, x)
	case m == 0 || n == 0:
		return z[:0]
	case n == 1:
		return z.mulAddWW(x, y[0], 0)
	}
	// m >= n > 1

	// determine if z can be reused
	if alias(z, x) || alias(z, y) {
		z = nil // z is an alias for x or y - cannot reuse
	}

	// use basic multiplication if the numbers are small
	if n < karatsubaThreshold {
		z = z.make(m + n)
		basicMul(z, x, y)
		return z.norm()
	}
	// m >= n && n >= karatsubaThreshold && n >= 2

	// determine Karatsuba length k such that
	//
	//   x = xh*b + x0  (0 <= x0 < b)
	//   y = yh*b + y0  (0 <= y0 < b)
	//   b = 1<<(_W*k)  ("base" of digits xi, yi)
	//
	k := karatsubaLen(n, karatsubaThreshold)
	// k <= n

	// multiply x0 and y0 via Karatsuba
	x0 := x[0:k]              // x0 is not normalized
	y0 := y[0:k]              // y0 is not normalized
	z = z.make(max(6*k, m+n)) // enough space for karatsuba of x0*y0 and full result of x*y
	karatsuba(z, x0, y0)
	z = z[0 : m+n]  // z has final length but may be incomplete
	z[2*k:].clear() // upper portion of z is garbage (and 2*k <= m+n since k <= n <= m)

	// If xh != 0 or yh != 0, add the missing terms to z. For
	//
	//   xh = xi*b^i + ... + x2*b^2 + x1*b (0 <= xi < b)
	//   yh =                         y1*b (0 <= y1 < b)
	//
	// the missing terms are
	//
	//   x0*y1*b and xi*y0*b^i, xi*y1*b^(i+1) for i > 0
	//
	// since all the yi for i > 1 are 0 by choice of k: If any of them
	// were > 0, then yh >= b^2 and thus y >= b^2. Then k' = k*2 would
	// be a larger valid threshold contradicting the assumption about k.
	//
	if k < n || m != n {
		tp := getNat(3 * k)
		t := *tp

		// add x0*y1*b
		x0 := x0.norm()
		y1 := y[k:]       // y1 is normalized because y is
		t = t.mul(x0, y1) // update t so we don't lose t's underlying array
		addAt(z, t, k)

		// add xi*y0<<i, xi*y1*b<<(i+k)
		y0 := y0.norm()
		for i := k; i < len(x); i += k {
			xi := x[i:]
			if len(xi) > k {
				xi = xi[:k]
			}
			xi = xi.norm()
			t = t.mul(xi, y0)
			addAt(z, t, i)
			t = t.mul(xi, y1)
			addAt(z, t, i+k)
		}

		putNat(tp)
	}

	return z.norm()
}

// getNat returns a *nat of len n. The contents may not be zero.
// The pool holds *nat to avoid allocation when converting to interface{}.
func getNat(n int) *nat {
	var z *nat
	if v := natPool.Get(); v != nil {
		z = v.(*nat)
	}
	if z == nil {
		z = new(nat)
	}
	*z = z.make(n)
	if n > 0 {
		(*z)[0] = 0xfedcb // break code expecting zero
	}
	return z
}

func putNat(x *nat) {
	natPool.Put(x)
}

var natPool sync.Pool

func same(x, y nat) bool {
	return len(x) == len(y) && len(x) > 0 && &x[0] == &y[0]
}

// z = x << s
func (z nat) shl(x nat, s uint) nat {
	if s == 0 {
		if same(z, x) {
			return z
		}
		if !alias(z, x) {
			return z.set(x)
		}
	}

	m := len(x)
	if m == 0 {
		return z[:0]
	}
	// m > 0

	n := m + int(s/_W)
	z = z.make(n + 1)
	z[n] = shlVU(z[n-m:n], x, s%_W)
	z[0 : n-m].clear()

	return z.norm()
}