Daily bump.

[pf3gnuchains/gcc-fork.git] / libgo / go / big / 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.

// Package big implements multi-precision arithmetic (big numbers).
// The following numeric types are supported:
//
//      - Int   signed integers
//      - Rat   rational numbers
//
// All methods on Int take the result as the receiver; if it is one
// of the operands it may be overwritten (and its memory reused).
// To enable chaining of operations, the result is also returned.
//
package big

// This file contains operations on unsigned multi-precision integers.
// These are the building blocks for the operations on signed integers
// and rationals.

import (
        "errors"
        "io"
        "rand"
)

// 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

var (
        natOne = nat{1}
        natTwo = nat{2}
        natTen = nat{10}
)

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[0:n] // reuse z
        }
        // 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.make(0)
        }
        z = z.make(1)
        z[0] = x
        return z
}

func (z nat) setUint64(x uint64) nat {
        // single-digit values
        if w := Word(x); uint64(w) == x {
                return z.setWord(w)
        }

        // compute number of words n required to represent x
        n := 0
        for t := x; t > 0; t >>= _W {
                n++
        }

        // split x into n words
        z = z.make(n)
        for i := range z {
                z[i] = Word(x & _M)
                x >>= _W
        }

        return z
}

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

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

        switch {
        case m < n:
                return z.add(y, x)
        case m == 0:
                // n == 0 because m >= n; result is 0
                return z.make(0)
        case n == 0:
                // result is x
                return z.set(x)
        }
        // m > 0

        z = z.make(m + 1)
        c := addVV(z[0:n], x, y)
        if m > n {
                c = addVW(z[n:m], x[n:], c)
        }
        z[m] = c

        return z.norm()
}

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.make(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)
                }
        }
}

// 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 int = 32 // computed by calibrate.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                    + z1 + z0
        //      = (x1-x0)*(y0 - y1)             + z1 + z0
        //      = x1*y0 - x1*y1 - x0*y0 + x0*y1 + z1 + z0
        //      = x1*y0 -    z1 -    z0 + x0*y1 + z1 + 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 recursing)
        r := z[n*4:]
        copy(r, z)

        // 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 returns true if x and y share the same base array.
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*(1<<(_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 <= karatsubaThreshold and an i >= 0. Thus, the
// result is the largest number that can be divided repeatedly by 2 before
// becoming about the value of karatsubaThreshold.
func karatsubaLen(n int) int {
        i := uint(0)
        for n > karatsubaThreshold {
                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.make(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 || n < 2 {
                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 = x1*b + x0
        //   y = y1*b + y0  (and k <= len(y), which implies k <= len(x))
        //   b = 1<<(_W*k)  ("base" of digits xi, yi)
        //
        k := karatsubaLen(n)
        // 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, upper portion is garbage

        // If x1 and/or y1 are not 0, add missing terms to z explicitly:
        //
        //     m+n       2*k       0
        //   z = [   ...   | x0*y0 ]
        //     +   [ x1*y1 ]
        //     +   [ x1*y0 ]
        //     +   [ x0*y1 ]
        //
        if k < n || m != n {
                x1 := x[k:] // x1 is normalized because x is
                y1 := y[k:] // y1 is normalized because y is
                var t nat
                t = t.mul(x1, y1)
                copy(z[2*k:], t)
                z[2*k+len(t):].clear() // upper portion of z is garbage
                t = t.mul(x1, y0.norm())
                addAt(z, t, k)
                t = t.mul(x0.norm(), y1)
                addAt(z, t, k)
        }

        return z.norm()
}

// mulRange computes the product of all the unsigned integers in the
// range [a, b] inclusively. If a > b (empty range), the result is 1.
func (z nat) mulRange(a, b uint64) nat {
        switch {
        case a == 0:
                // cut long ranges short (optimization)
                return z.setUint64(0)
        case a > b:
                return z.setUint64(1)
        case a == b:
                return z.setUint64(a)
        case a+1 == b:
                return z.mul(nat{}.setUint64(a), nat{}.setUint64(b))
        }
        m := (a + b) / 2
        return z.mul(nat{}.mulRange(a, m), nat{}.mulRange(m+1, b))
}

// q = (x-r)/y, with 0 <= r < y
func (z nat) divW(x nat, y Word) (q nat, r Word) {
        m := len(x)
        switch {
        case y == 0:
                panic("division by zero")
        case y == 1:
                q = z.set(x) // result is x
                return
        case m == 0:
                q = z.make(0) // result is 0
                return
        }
        // m > 0
        z = z.make(m)
        r = divWVW(z, 0, x, y)
        q = z.norm()
        return
}

func (z nat) div(z2, u, v nat) (q, r nat) {
        if len(v) == 0 {
                panic("division by zero")
        }

        if u.cmp(v) < 0 {
                q = z.make(0)
                r = z2.set(u)
                return
        }

        if len(v) == 1 {
                var rprime Word
                q, rprime = z.divW(u, v[0])
                if rprime > 0 {
                        r = z2.make(1)
                        r[0] = rprime
                } else {
                        r = z2.make(0)
                }
                return
        }

        q, r = z.divLarge(z2, u, v)
        return
}

// q = (uIn-r)/v, with 0 <= r < y
// Uses z as storage for q, and u as storage for r if possible.
// See Knuth, Volume 2, section 4.3.1, Algorithm D.
// Preconditions:
//    len(v) >= 2
//    len(uIn) >= len(v)
func (z nat) divLarge(u, uIn, v nat) (q, r nat) {
        n := len(v)
        m := len(uIn) - n

        // 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 := leadingZeros(v[n-1])
        if shift > 0 {
                // do not modify v, it may be used by another goroutine simultaneously
                v1 := make(nat, n)
                shlVU(v1, v, shift)
                v = v1
        }
        u[len(uIn)] = shlVU(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()
        shrVU(u, u, 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
}

// MaxBase is the largest number base accepted for string conversions.
const MaxBase = 'z' - 'a' + 10 + 1 // = hexValue('z') + 1

func hexValue(ch rune) Word {
        d := MaxBase + 1 // illegal base
        switch {
        case '0' <= ch && ch <= '9':
                d = int(ch - '0')
        case 'a' <= ch && ch <= 'z':
                d = int(ch - 'a' + 10)
        case 'A' <= ch && ch <= 'Z':
                d = int(ch - 'A' + 10)
        }
        return Word(d)
}

// scan sets z to the natural number corresponding to the longest possible prefix
// read from r representing an unsigned integer in a given conversion base.
// It returns z, the actual conversion base used, and an error, if any. In the
// error case, the value of z is undefined. The syntax follows the syntax of
// unsigned integer literals in Go.
//
// The base argument must be 0 or a value from 2 through MaxBase. If the base
// 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(r io.RuneScanner, base int) (nat, int, error) {
        // reject illegal bases
        if base < 0 || base == 1 || MaxBase < base {
                return z, 0, errors.New("illegal number base")
        }

        // one char look-ahead
        ch, _, err := r.ReadRune()
        if err != nil {
                return z, 0, err
        }

        // determine base if necessary
        b := Word(base)
        if base == 0 {
                b = 10
                if ch == '0' {
                        switch ch, _, err = r.ReadRune(); err {
                        case nil:
                                b = 8
                                switch ch {
                                case 'x', 'X':
                                        b = 16
                                case 'b', 'B':
                                        b = 2
                                }
                                if b == 2 || b == 16 {
                                        if ch, _, err = r.ReadRune(); err != nil {
                                                return z, 0, err
                                        }
                                }
                        case io.EOF:
                                return z.make(0), 10, nil
                        default:
                                return z, 10, err
                        }
                }
        }

        // convert string
        // - group as many digits d as possible together into a "super-digit" dd with "super-base" bb
        // - only when bb does not fit into a word anymore, do a full number mulAddWW using bb and dd
        z = z.make(0)
        bb := Word(1)
        dd := Word(0)
        for max := _M / b; ; {
                d := hexValue(ch)
                if d >= b {
                        r.UnreadRune() // ch does not belong to number anymore
                        break
                }

                if bb <= max {
                        bb *= b
                        dd = dd*b + d
                } else {
                        // bb * b would overflow
                        z = z.mulAddWW(z, bb, dd)
                        bb = b
                        dd = d
                }

                if ch, _, err = r.ReadRune(); err != nil {
                        if err != io.EOF {
                                return z, int(b), err
                        }
                        break
                }
        }

        switch {
        case bb > 1:
                // there was at least one mantissa digit
                z = z.mulAddWW(z, bb, dd)
        case base == 0 && b == 8:
                // there was only the octal prefix 0 (possibly followed by digits > 7);
                // return base 10, not 8
                return z, 10, nil
        case base != 0 || b != 8:
                // there was neither a mantissa digit nor the octal prefix 0
                return z, int(b), errors.New("syntax error scanning number")
        }

        return z.norm(), int(b), nil
}

// Character sets for string conversion.
const (
        lowercaseDigits = "0123456789abcdefghijklmnopqrstuvwxyz"
        uppercaseDigits = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"
)

// decimalString returns a decimal representation of x.
// It calls x.string with the charset "0123456789".
func (x nat) decimalString() string {
        return x.string(lowercaseDigits[0:10])
}

// string converts x to a string using digits from a charset; a digit with
// value d is represented by charset[d]. The conversion base is determined
// by len(charset), which must be >= 2.
func (x nat) string(charset string) string {
        b := Word(len(charset))

        // special cases
        switch {
        case b < 2 || b > 256:
                panic("illegal base")
        case len(x) == 0:
                return string(charset[0])
        }

        // allocate buffer for conversion
        i := x.bitLen()/log2(b) + 1 // +1: round up
        s := make([]byte, i)

        // special case: power of two bases can avoid divisions completely
        if b == b&-b {
                // shift is base-b digit size in bits
                shift := uint(trailingZeroBits(b)) // shift > 0 because b >= 2
                mask := Word(1)<<shift - 1
                w := x[0]
                nbits := uint(_W) // number of unprocessed bits in w

                // convert less-significant words
                for k := 1; k < len(x); k++ {
                        // convert full digits
                        for nbits >= shift {
                                i--
                                s[i] = charset[w&mask]
                                w >>= shift
                                nbits -= shift
                        }

                        // convert any partial leading digit and advance to next word
                        if nbits == 0 {
                                // no partial digit remaining, just advance
                                w = x[k]
                                nbits = _W
                        } else {
                                // partial digit in current (k-1) and next (k) word
                                w |= x[k] << nbits
                                i--
                                s[i] = charset[w&mask]

                                // advance
                                w = x[k] >> (shift - nbits)
                                nbits = _W - (shift - nbits)
                        }
                }

                // convert digits of most-significant word (omit leading zeros)
                for nbits >= 0 && w != 0 {
                        i--
                        s[i] = charset[w&mask]
                        w >>= shift
                        nbits -= shift
                }

                return string(s[i:])
        }

        // general case: extract groups of digits by multiprecision division

        // maximize ndigits where b**ndigits < 2^_W; bb (big base) is b**ndigits
        bb := Word(1)
        ndigits := 0
        for max := Word(_M / b); bb <= max; bb *= b {
                ndigits++
        }

        // preserve x, create local copy for use in repeated divisions
        q := nat{}.set(x)
        var r Word

        // convert
        if b == 10 { // hard-coding for 10 here speeds this up by 1.25x
                for len(q) > 0 {
                        // extract least significant, base bb "digit"
                        q, r = q.divW(q, bb) // N.B. >82% of time is here. Optimize divW
                        if len(q) == 0 {
                                // skip leading zeros in most-significant group of digits
                                for j := 0; j < ndigits && r != 0; j++ {
                                        i--
                                        s[i] = charset[r%10]
                                        r /= 10
                                }
                        } else {
                                for j := 0; j < ndigits; j++ {
                                        i--
                                        s[i] = charset[r%10]
                                        r /= 10
                                }
                        }
                }
        } else {
                for len(q) > 0 {
                        // extract least significant group of digits
                        q, r = q.divW(q, bb) // N.B. >82% of time is here. Optimize divW
                        if len(q) == 0 {
                                // skip leading zeros in most-significant group of digits
                                for j := 0; j < ndigits && r != 0; j++ {
                                        i--
                                        s[i] = charset[r%b]
                                        r /= b
                                }
                        } else {
                                for j := 0; j < ndigits; j++ {
                                        i--
                                        s[i] = charset[r%b]
                                        r /= b
                                }
                        }
                }
        }

        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. Multiplying 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] = shlVU(z[n-m:n], x, 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)
        shrVU(z, x[m-n:], s%_W)

        return z.norm()
}

func (z nat) setBit(x nat, i uint, b uint) nat {
        j := int(i / _W)
        m := Word(1) << (i % _W)
        n := len(x)
        switch b {
        case 0:
                z = z.make(n)
                copy(z, x)
                if j >= n {
                        // no need to grow
                        return z
                }
                z[j] &^= m
                return z.norm()
        case 1:
                if j >= n {
                        n = j + 1
                }
                z = z.make(n)
                copy(z, x)
                z[j] |= m
                // no need to normalize
                return z
        }
        panic("set bit is not 0 or 1")
}

func (z nat) bit(i uint) uint {
        j := int(i / _W)
        if j >= len(z) {
                return 0
        }
        return uint(z[j] >> (i % _W) & 1)
}

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 with x = q * 1<<k and q is odd, or q and k are 0.
func (x nat) powersOfTwoDecompose() (q nat, k int) {
        if len(x) == 0 {
                return x, 0
        }

        // One of the words must be non-zero by definition,
        // so this loop will terminate with i < len(x), and
        // i is the number of 0 words.
        i := 0
        for x[i] == 0 {
                i++
        }
        n := trailingZeroBits(x[i]) // x[i] != 0

        q = make(nat, len(x)-i)
        shrVU(q, x[i:], uint(n))

        q = q.norm()
        k = i*_W + n
        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{}.sub(n, natOne)
        // 1<<k * q = nm1;
        q, k := nm1.powersOfTwoDecompose()

        nm3 := nat{}.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 := 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()
}