libgo/go/math/big/natconv.go - gofrontend - Git at Google

 // Copyright 2015 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 nat-to-string conversion functions.

 package big

 import (
 	"errors"
 	"fmt"
 	"io"
 	"math"
 	"math/bits"
 	"sync"
 )

 const digits = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

 // Note: MaxBase = len(digits), but it must remain an untyped rune constant
 //       for API compatibility.

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

 // maxPow returns (b**n, n) such that b**n is the largest power b**n <= _M.
 // For instance maxPow(10) == (1e19, 19) for 19 decimal digits in a 64bit Word.
 // In other words, at most n digits in base b fit into a Word.
 // TODO(gri) replace this with a table, generated at build time.
 func maxPow(b Word) (p Word, n int) {
 	p, n = b, 1 // assuming b <= _M
 	for max := _M / b; p <= max; {
 		// p == b**n && p <= max
 		p *= b
 		n++
 	}
 	// p == b**n && p <= _M
 	return
 }

 // pow returns x**n for n > 0, and 1 otherwise.
 func pow(x Word, n int) (p Word) {
 	// n == sum of bi * 2**i, for 0 <= i < imax, and bi is 0 or 1
 	// thus x**n == product of x**(2**i) for all i where bi == 1
 	// (Russian Peasant Method for exponentiation)
 	p = 1
 	for n > 0 {
 		if n&1 != 0 {
 			p *= x
 		}
 		x *= x
 		n >>= 1
 	}
 	return
 }

 // scan errors
 var (
 	errNoDigits = errors.New("number has no digits")
 	errInvalSep = errors.New("'_' must separate successive digits")
 )

 // scan scans the number corresponding to the longest possible prefix
 // from r representing an unsigned number in a given conversion base.
 // scan returns the corresponding natural number res, the actual base b,
 // a digit count, and a read or syntax error err, if any.
 //
 // For base 0, an underscore character ``_'' may appear between a base
 // prefix and an adjacent digit, and between successive digits; such
 // underscores do not change the value of the number, or the returned
 // digit count. Incorrect placement of underscores is reported as an
 // error if there are no other errors. If base != 0, underscores are
 // not recognized and thus terminate scanning like any other character
 // that is not a valid radix point or digit.
 //
 //     number    = mantissa | prefix pmantissa .
 //     prefix    = "0" [ "b" | "B" | "o" | "O" | "x" | "X" ] .
 //     mantissa  = digits "." [ digits ] | digits | "." digits .
 //     pmantissa = [ "_" ] digits "." [ digits ] | [ "_" ] digits | "." digits .
 //     digits    = digit { [ "_" ] digit } .
 //     digit     = "0" ... "9" | "a" ... "z" | "A" ... "Z" .
 //
 // Unless fracOk is set, the base argument must be 0 or a value between
 // 2 and MaxBase. If fracOk is set, the base argument must be one of
 // 0, 2, 8, 10, or 16. Providing an invalid base argument leads to a run-
 // time panic.
 //
 // For base 0, the number prefix determines the actual base: A prefix of
 // ``0b'' or ``0B'' selects base 2, ``0o'' or ``0O'' selects base 8, and
 // ``0x'' or ``0X'' selects base 16. If fracOk is false, a ``0'' prefix
 // (immediately followed by digits) selects base 8 as well. Otherwise,
 // the selected base is 10 and no prefix is accepted.
 //
 // If fracOk is set, a period followed by a fractional part is permitted.
 // The result value is computed as if there were no period present; and
 // the count value is used to determine the fractional part.
 //
 // For bases <= 36, lower and upper case letters are considered the same:
 // The letters 'a' to 'z' and 'A' to 'Z' represent digit values 10 to 35.
 // For bases > 36, the upper case letters 'A' to 'Z' represent the digit
 // values 36 to 61.
 //
 // A result digit count > 0 corresponds to the number of (non-prefix) digits
 // parsed. A digit count <= 0 indicates the presence of a period (if fracOk
 // is set, only), and -count is the number of fractional digits found.
 // In this case, the actual value of the scanned number is res * b**count.
 //
 func (z nat) scan(r io.ByteScanner, base int, fracOk bool) (res nat, b, count int, err error) {
 	// reject invalid bases
 	baseOk := base == 0 ||
 		!fracOk && 2 <= base && base <= MaxBase ||
 		fracOk && (base == 2 || base == 8 || base == 10 || base == 16)
 	if !baseOk {
 		panic(fmt.Sprintf("invalid number base %d", base))
 	}

 	// prev encodes the previously seen char: it is one
 	// of '_', '0' (a digit), or '.' (anything else). A
 	// valid separator '_' may only occur after a digit
 	// and if base == 0.
 	prev := '.'
 	invalSep := false

 	// one char look-ahead
 	ch, err := r.ReadByte()

 	// determine actual base
 	b, prefix := base, 0
 	if base == 0 {
 		// actual base is 10 unless there's a base prefix
 		b = 10
 		if err == nil && ch == '0' {
 			prev = '0'
 			count = 1
 			ch, err = r.ReadByte()
 			if err == nil {
 				// possibly one of 0b, 0B, 0o, 0O, 0x, 0X
 				switch ch {
 				case 'b', 'B':
 					b, prefix = 2, 'b'
 				case 'o', 'O':
 					b, prefix = 8, 'o'
 				case 'x', 'X':
 					b, prefix = 16, 'x'
 				default:
 					if !fracOk {
 						b, prefix = 8, '0'
 					}
 				}
 				if prefix != 0 {
 					count = 0 // prefix is not counted
 					if prefix != '0' {
 						ch, err = r.ReadByte()
 					}
 				}
 			}
 		}
 	}

 	// convert string
 	// Algorithm: Collect digits in groups of at most n digits in di
 	// and then use mulAddWW for every such group to add them to the
 	// result.
 	z = z[:0]
 	b1 := Word(b)
 	bn, n := maxPow(b1) // at most n digits in base b1 fit into Word
 	di := Word(0)       // 0 <= di < b1**i < bn
 	i := 0              // 0 <= i < n
 	dp := -1            // position of decimal point
 	for err == nil {
 		if ch == '.' && fracOk {
 			fracOk = false
 			if prev == '_' {
 				invalSep = true
 			}
 			prev = '.'
 			dp = count
 		} else if ch == '_' && base == 0 {
 			if prev != '0' {
 				invalSep = true
 			}
 			prev = '_'
 		} else {
 			// convert rune into digit value d1
 			var d1 Word
 			switch {
 			case '0' <= ch && ch <= '9':
 				d1 = Word(ch - '0')
 			case 'a' <= ch && ch <= 'z':
 				d1 = Word(ch - 'a' + 10)
 			case 'A' <= ch && ch <= 'Z':
 				if b <= maxBaseSmall {
 					d1 = Word(ch - 'A' + 10)
 				} else {
 					d1 = Word(ch - 'A' + maxBaseSmall)
 				}
 			default:
 				d1 = MaxBase + 1
 			}
 			if d1 >= b1 {
 				r.UnreadByte() // ch does not belong to number anymore
 				break
 			}
 			prev = '0'
 			count++

 			// collect d1 in di
 			di = di*b1 + d1
 			i++

 			// if di is "full", add it to the result
 			if i == n {
 				z = z.mulAddWW(z, bn, di)
 				di = 0
 				i = 0
 			}
 		}

 		ch, err = r.ReadByte()
 	}

 	if err == io.EOF {
 		err = nil
 	}

 	// other errors take precedence over invalid separators
 	if err == nil && (invalSep || prev == '_') {
 		err = errInvalSep
 	}

 	if count == 0 {
 		// no digits found
 		if prefix == '0' {
 			// there was only the octal prefix 0 (possibly followed by separators and digits > 7);
 			// interpret as decimal 0
 			return z[:0], 10, 1, err
 		}
 		err = errNoDigits // fall through; result will be 0
 	}

 	// add remaining digits to result
 	if i > 0 {
 		z = z.mulAddWW(z, pow(b1, i), di)
 	}
 	res = z.norm()

 	// adjust count for fraction, if any
 	if dp >= 0 {
 		// 0 <= dp <= count
 		count = dp - count
 	}

 	return
 }

 // utoa converts x to an ASCII representation in the given base;
 // base must be between 2 and MaxBase, inclusive.
 func (x nat) utoa(base int) []byte {
 	return x.itoa(false, base)
 }

 // itoa is like utoa but it prepends a '-' if neg && x != 0.
 func (x nat) itoa(neg bool, base int) []byte {
 	if base < 2 || base > MaxBase {
 		panic("invalid base")
 	}

 	// x == 0
 	if len(x) == 0 {
 		return []byte("0")
 	}
 	// len(x) > 0

 	// allocate buffer for conversion
 	i := int(float64(x.bitLen())/math.Log2(float64(base))) + 1 // off by 1 at most
 	if neg {
 		i++
 	}
 	s := make([]byte, i)

 	// convert power of two and non power of two bases separately
 	if b := Word(base); b == b&-b {
 		// shift is base b digit size in bits
 		shift := uint(bits.TrailingZeros(uint(b))) // shift > 0 because b >= 2
 		mask := Word(1<<shift - 1)
 		w := x[0]         // current word
 		nbits := uint(_W) // number of unprocessed bits in w

 		// convert less-significant words (include leading zeros)
 		for k := 1; k < len(x); k++ {
 			// convert full digits
 			for nbits >= shift {
 				i--
 				s[i] = digits[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 word w (== x[k-1]) and next word x[k]
 				w |= x[k] << nbits
 				i--
 				s[i] = digits[w&mask]

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

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

 	} else {
 		bb, ndigits := maxPow(b)

 		// construct table of successive squares of bb*leafSize to use in subdivisions
 		// result (table != nil) <=> (len(x) > leafSize > 0)
 		table := divisors(len(x), b, ndigits, bb)

 		// preserve x, create local copy for use by convertWords
 		q := nat(nil).set(x)

 		// convert q to string s in base b
 		q.convertWords(s, b, ndigits, bb, table)

 		// strip leading zeros
 		// (x != 0; thus s must contain at least one non-zero digit
 		// and the loop will terminate)
 		i = 0
 		for s[i] == '0' {
 			i++
 		}
 	}

 	if neg {
 		i--
 		s[i] = '-'
 	}

 	return s[i:]
 }

 // Convert words of q to base b digits in s. If q is large, it is recursively "split in half"
 // by nat/nat division using tabulated divisors. Otherwise, it is converted iteratively using
 // repeated nat/Word division.
 //
 // The iterative method processes n Words by n divW() calls, each of which visits every Word in the
 // incrementally shortened q for a total of n + (n-1) + (n-2) ... + 2 + 1, or n(n+1)/2 divW()'s.
 // Recursive conversion divides q by its approximate square root, yielding two parts, each half
 // the size of q. Using the iterative method on both halves means 2 * (n/2)(n/2 + 1)/2 divW()'s
 // plus the expensive long div(). Asymptotically, the ratio is favorable at 1/2 the divW()'s, and
 // is made better by splitting the subblocks recursively. Best is to split blocks until one more
 // split would take longer (because of the nat/nat div()) than the twice as many divW()'s of the
 // iterative approach. This threshold is represented by leafSize. Benchmarking of leafSize in the
 // range 2..64 shows that values of 8 and 16 work well, with a 4x speedup at medium lengths and
 // ~30x for 20000 digits. Use nat_test.go's BenchmarkLeafSize tests to optimize leafSize for
 // specific hardware.
 //
 func (q nat) convertWords(s []byte, b Word, ndigits int, bb Word, table []divisor) {
 	// split larger blocks recursively
 	if table != nil {
 		// len(q) > leafSize > 0
 		var r nat
 		index := len(table) - 1
 		for len(q) > leafSize {
 			// find divisor close to sqrt(q) if possible, but in any case < q
 			maxLength := q.bitLen()     // ~= log2 q, or at of least largest possible q of this bit length
 			minLength := maxLength >> 1 // ~= log2 sqrt(q)
 			for index > 0 && table[index-1].nbits > minLength {
 				index-- // desired
 			}
 			if table[index].nbits >= maxLength && table[index].bbb.cmp(q) >= 0 {
 				index--
 				if index < 0 {
 					panic("internal inconsistency")
 				}
 			}

 			// split q into the two digit number (q'*bbb + r) to form independent subblocks
 			q, r = q.div(r, q, table[index].bbb)

 			// convert subblocks and collect results in s[:h] and s[h:]
 			h := len(s) - table[index].ndigits
 			r.convertWords(s[h:], b, ndigits, bb, table[0:index])
 			s = s[:h] // == q.convertWords(s, b, ndigits, bb, table[0:index+1])
 		}
 	}

 	// having split any large blocks now process the remaining (small) block iteratively
 	i := len(s)
 	var r Word
 	if b == 10 {
 		// hard-coding for 10 here speeds this up by 1.25x (allows for / and % by constants)
 		for len(q) > 0 {
 			// extract least significant, base bb "digit"
 			q, r = q.divW(q, bb)
 			for j := 0; j < ndigits && i > 0; j++ {
 				i--
 				// avoid % computation since r%10 == r - int(r/10)*10;
 				// this appears to be faster for BenchmarkString10000Base10
 				// and smaller strings (but a bit slower for larger ones)
 				t := r / 10
 				s[i] = '0' + byte(r-t*10)
 				r = t
 			}
 		}
 	} else {
 		for len(q) > 0 {
 			// extract least significant, base bb "digit"
 			q, r = q.divW(q, bb)
 			for j := 0; j < ndigits && i > 0; j++ {
 				i--
 				s[i] = digits[r%b]
 				r /= b
 			}
 		}
 	}

 	// prepend high-order zeros
 	for i > 0 { // while need more leading zeros
 		i--
 		s[i] = '0'
 	}
 }

 // Split blocks greater than leafSize Words (or set to 0 to disable recursive conversion)
 // Benchmark and configure leafSize using: go test -bench="Leaf"
 //   8 and 16 effective on 3.0 GHz Xeon "Clovertown" CPU (128 byte cache lines)
 //   8 and 16 effective on 2.66 GHz Core 2 Duo "Penryn" CPU
 var leafSize int = 8 // number of Word-size binary values treat as a monolithic block

 type divisor struct {
 	bbb     nat // divisor
 	nbits   int // bit length of divisor (discounting leading zeros) ~= log2(bbb)
 	ndigits int // digit length of divisor in terms of output base digits
 }

 var cacheBase10 struct {
 	sync.Mutex
 	table [64]divisor // cached divisors for base 10
 }

 // expWW computes x**y
 func (z nat) expWW(x, y Word) nat {
 	return z.expNN(nat(nil).setWord(x), nat(nil).setWord(y), nil)
 }

 // construct table of powers of bb*leafSize to use in subdivisions
 func divisors(m int, b Word, ndigits int, bb Word) []divisor {
 	// only compute table when recursive conversion is enabled and x is large
 	if leafSize == 0 || m <= leafSize {
 		return nil
 	}

 	// determine k where (bb**leafSize)**(2**k) >= sqrt(x)
 	k := 1
 	for words := leafSize; words < m>>1 && k < len(cacheBase10.table); words <<= 1 {
 		k++
 	}

 	// reuse and extend existing table of divisors or create new table as appropriate
 	var table []divisor // for b == 10, table overlaps with cacheBase10.table
 	if b == 10 {
 		cacheBase10.Lock()
 		table = cacheBase10.table[0:k] // reuse old table for this conversion
 	} else {
 		table = make([]divisor, k) // create new table for this conversion
 	}

 	// extend table
 	if table[k-1].ndigits == 0 {
 		// add new entries as needed
 		var larger nat
 		for i := 0; i < k; i++ {
 			if table[i].ndigits == 0 {
 				if i == 0 {
 					table[0].bbb = nat(nil).expWW(bb, Word(leafSize))
 					table[0].ndigits = ndigits * leafSize
 				} else {
 					table[i].bbb = nat(nil).sqr(table[i-1].bbb)
 					table[i].ndigits = 2 * table[i-1].ndigits
 				}

 				// optimization: exploit aggregated extra bits in macro blocks
 				larger = nat(nil).set(table[i].bbb)
 				for mulAddVWW(larger, larger, b, 0) == 0 {
 					table[i].bbb = table[i].bbb.set(larger)
 					table[i].ndigits++
 				}

 				table[i].nbits = table[i].bbb.bitLen()
 			}
 		}
 	}

 	if b == 10 {
 		cacheBase10.Unlock()
 	}

 	return table
 }
	// Copyright 2015 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 nat-to-string conversion functions.

	package big

	import (
	"errors"
	"fmt"
	"io"
	"math"
	"math/bits"
	"sync"
	)

	const digits = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"

	// Note: MaxBase = len(digits), but it must remain an untyped rune constant
	// for API compatibility.

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

	// maxPow returns (bn, n) such that bn is the largest power b**n <= _M.
	// For instance maxPow(10) == (1e19, 19) for 19 decimal digits in a 64bit Word.
	// In other words, at most n digits in base b fit into a Word.
	// TODO(gri) replace this with a table, generated at build time.
	func maxPow(b Word) (p Word, n int) {
	p, n = b, 1 // assuming b <= _M
	for max := _M / b; p <= max; {
	// p == b**n && p <= max
	p *= b
	n++
	}
	// p == b**n && p <= _M
	return
	}

	// pow returns x**n for n > 0, and 1 otherwise.
	func pow(x Word, n int) (p Word) {
	// n == sum of bi * 2**i, for 0 <= i < imax, and bi is 0 or 1
	// thus xn == product of x(2**i) for all i where bi == 1
	// (Russian Peasant Method for exponentiation)
	p = 1
	for n > 0 {
	if n&1 != 0 {
	p *= x
	}
	x *= x
	n >>= 1
	}
	return
	}

	// scan errors
	var (
	errNoDigits = errors.New("number has no digits")
	errInvalSep = errors.New("'_' must separate successive digits")
	)

	// scan scans the number corresponding to the longest possible prefix
	// from r representing an unsigned number in a given conversion base.
	// scan returns the corresponding natural number res, the actual base b,
	// a digit count, and a read or syntax error err, if any.
	//
	// For base 0, an underscore character ``_'' may appear between a base
	// prefix and an adjacent digit, and between successive digits; such
	// underscores do not change the value of the number, or the returned
	// digit count. Incorrect placement of underscores is reported as an
	// error if there are no other errors. If base != 0, underscores are
	// not recognized and thus terminate scanning like any other character
	// that is not a valid radix point or digit.
	//
	// number = mantissa \| prefix pmantissa .
	// prefix = "0" [ "b" \| "B" \| "o" \| "O" \| "x" \| "X" ] .
	// mantissa = digits "." [ digits ] \| digits \| "." digits .
	// pmantissa = [ "_" ] digits "." [ digits ] \| [ "_" ] digits \| "." digits .
	// digits = digit { [ "_" ] digit } .
	// digit = "0" ... "9" \| "a" ... "z" \| "A" ... "Z" .
	//
	// Unless fracOk is set, the base argument must be 0 or a value between
	// 2 and MaxBase. If fracOk is set, the base argument must be one of
	// 0, 2, 8, 10, or 16. Providing an invalid base argument leads to a run-
	// time panic.
	//
	// For base 0, the number prefix determines the actual base: A prefix of
	// ``0b'' or ``0B'' selects base 2, ``0o'' or ``0O'' selects base 8, and
	// ``0x'' or ``0X'' selects base 16. If fracOk is false, a ``0'' prefix
	// (immediately followed by digits) selects base 8 as well. Otherwise,
	// the selected base is 10 and no prefix is accepted.
	//
	// If fracOk is set, a period followed by a fractional part is permitted.
	// The result value is computed as if there were no period present; and
	// the count value is used to determine the fractional part.
	//
	// For bases <= 36, lower and upper case letters are considered the same:
	// The letters 'a' to 'z' and 'A' to 'Z' represent digit values 10 to 35.
	// For bases > 36, the upper case letters 'A' to 'Z' represent the digit
	// values 36 to 61.
	//
	// A result digit count > 0 corresponds to the number of (non-prefix) digits
	// parsed. A digit count <= 0 indicates the presence of a period (if fracOk
	// is set, only), and -count is the number of fractional digits found.
	// In this case, the actual value of the scanned number is res * b**count.
	//
	func (z nat) scan(r io.ByteScanner, base int, fracOk bool) (res nat, b, count int, err error) {
	// reject invalid bases
	baseOk := base == 0 \|\|
	!fracOk && 2 <= base && base <= MaxBase \|\|
	fracOk && (base == 2 \|\| base == 8 \|\| base == 10 \|\| base == 16)
	if !baseOk {
	panic(fmt.Sprintf("invalid number base %d", base))
	}

	// prev encodes the previously seen char: it is one
	// of '_', '0' (a digit), or '.' (anything else). A
	// valid separator '_' may only occur after a digit
	// and if base == 0.
	prev := '.'
	invalSep := false

	// one char look-ahead
	ch, err := r.ReadByte()

	// determine actual base
	b, prefix := base, 0
	if base == 0 {
	// actual base is 10 unless there's a base prefix
	b = 10
	if err == nil && ch == '0' {
	prev = '0'
	count = 1
	ch, err = r.ReadByte()
	if err == nil {
	// possibly one of 0b, 0B, 0o, 0O, 0x, 0X
	switch ch {
	case 'b', 'B':
	b, prefix = 2, 'b'
	case 'o', 'O':
	b, prefix = 8, 'o'
	case 'x', 'X':
	b, prefix = 16, 'x'
	default:
	if !fracOk {
	b, prefix = 8, '0'
	}
	}
	if prefix != 0 {
	count = 0 // prefix is not counted
	if prefix != '0' {
	ch, err = r.ReadByte()
	}
	}
	}
	}
	}

	// convert string
	// Algorithm: Collect digits in groups of at most n digits in di
	// and then use mulAddWW for every such group to add them to the
	// result.
	z = z[:0]
	b1 := Word(b)
	bn, n := maxPow(b1) // at most n digits in base b1 fit into Word
	di := Word(0) // 0 <= di < b1**i < bn
	i := 0 // 0 <= i < n
	dp := -1 // position of decimal point
	for err == nil {
	if ch == '.' && fracOk {
	fracOk = false
	if prev == '_' {
	invalSep = true
	}
	prev = '.'
	dp = count
	} else if ch == '_' && base == 0 {
	if prev != '0' {
	invalSep = true
	}
	prev = '_'
	} else {
	// convert rune into digit value d1
	var d1 Word
	switch {
	case '0' <= ch && ch <= '9':
	d1 = Word(ch - '0')
	case 'a' <= ch && ch <= 'z':
	d1 = Word(ch - 'a' + 10)
	case 'A' <= ch && ch <= 'Z':
	if b <= maxBaseSmall {
	d1 = Word(ch - 'A' + 10)
	} else {
	d1 = Word(ch - 'A' + maxBaseSmall)
	}
	default:
	d1 = MaxBase + 1
	}
	if d1 >= b1 {
	r.UnreadByte() // ch does not belong to number anymore
	break
	}
	prev = '0'
	count++

	// collect d1 in di
	di = di*b1 + d1
	i++

	// if di is "full", add it to the result
	if i == n {
	z = z.mulAddWW(z, bn, di)
	di = 0
	i = 0
	}
	}

	ch, err = r.ReadByte()
	}

	if err == io.EOF {
	err = nil
	}

	// other errors take precedence over invalid separators
	if err == nil && (invalSep \|\| prev == '_') {
	err = errInvalSep
	}

	if count == 0 {
	// no digits found
	if prefix == '0' {
	// there was only the octal prefix 0 (possibly followed by separators and digits > 7);
	// interpret as decimal 0
	return z[:0], 10, 1, err
	}
	err = errNoDigits // fall through; result will be 0
	}

	// add remaining digits to result
	if i > 0 {
	z = z.mulAddWW(z, pow(b1, i), di)
	}
	res = z.norm()

	// adjust count for fraction, if any
	if dp >= 0 {
	// 0 <= dp <= count
	count = dp - count
	}

	return
	}

	// utoa converts x to an ASCII representation in the given base;
	// base must be between 2 and MaxBase, inclusive.
	func (x nat) utoa(base int) []byte {
	return x.itoa(false, base)
	}

	// itoa is like utoa but it prepends a '-' if neg && x != 0.
	func (x nat) itoa(neg bool, base int) []byte {
	if base < 2 \|\| base > MaxBase {
	panic("invalid base")
	}

	// x == 0
	if len(x) == 0 {
	return []byte("0")
	}
	// len(x) > 0

	// allocate buffer for conversion
	i := int(float64(x.bitLen())/math.Log2(float64(base))) + 1 // off by 1 at most
	if neg {
	i++
	}
	s := make([]byte, i)

	// convert power of two and non power of two bases separately
	if b := Word(base); b == b&-b {
	// shift is base b digit size in bits
	shift := uint(bits.TrailingZeros(uint(b))) // shift > 0 because b >= 2
	mask := Word(1<<shift - 1)
	w := x[0] // current word
	nbits := uint(_W) // number of unprocessed bits in w

	// convert less-significant words (include leading zeros)
	for k := 1; k < len(x); k++ {
	// convert full digits
	for nbits >= shift {
	i--
	s[i] = digits[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 word w (== x[k-1]) and next word x[k]
	w \|= x[k] << nbits
	i--
	s[i] = digits[w&mask]

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

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

	} else {
	bb, ndigits := maxPow(b)

	// construct table of successive squares of bb*leafSize to use in subdivisions
	// result (table != nil) <=> (len(x) > leafSize > 0)
	table := divisors(len(x), b, ndigits, bb)

	// preserve x, create local copy for use by convertWords
	q := nat(nil).set(x)

	// convert q to string s in base b
	q.convertWords(s, b, ndigits, bb, table)

	// strip leading zeros
	// (x != 0; thus s must contain at least one non-zero digit
	// and the loop will terminate)
	i = 0
	for s[i] == '0' {
	i++
	}
	}

	if neg {
	i--
	s[i] = '-'
	}

	return s[i:]
	}

	// Convert words of q to base b digits in s. If q is large, it is recursively "split in half"
	// by nat/nat division using tabulated divisors. Otherwise, it is converted iteratively using
	// repeated nat/Word division.
	//
	// The iterative method processes n Words by n divW() calls, each of which visits every Word in the
	// incrementally shortened q for a total of n + (n-1) + (n-2) ... + 2 + 1, or n(n+1)/2 divW()'s.
	// Recursive conversion divides q by its approximate square root, yielding two parts, each half
	// the size of q. Using the iterative method on both halves means 2 * (n/2)(n/2 + 1)/2 divW()'s
	// plus the expensive long div(). Asymptotically, the ratio is favorable at 1/2 the divW()'s, and
	// is made better by splitting the subblocks recursively. Best is to split blocks until one more
	// split would take longer (because of the nat/nat div()) than the twice as many divW()'s of the
	// iterative approach. This threshold is represented by leafSize. Benchmarking of leafSize in the
	// range 2..64 shows that values of 8 and 16 work well, with a 4x speedup at medium lengths and
	// ~30x for 20000 digits. Use nat_test.go's BenchmarkLeafSize tests to optimize leafSize for
	// specific hardware.
	//
	func (q nat) convertWords(s []byte, b Word, ndigits int, bb Word, table []divisor) {
	// split larger blocks recursively
	if table != nil {
	// len(q) > leafSize > 0
	var r nat
	index := len(table) - 1
	for len(q) > leafSize {
	// find divisor close to sqrt(q) if possible, but in any case < q
	maxLength := q.bitLen() // ~= log2 q, or at of least largest possible q of this bit length
	minLength := maxLength >> 1 // ~= log2 sqrt(q)
	for index > 0 && table[index-1].nbits > minLength {
	index-- // desired
	}
	if table[index].nbits >= maxLength && table[index].bbb.cmp(q) >= 0 {
	index--
	if index < 0 {
	panic("internal inconsistency")
	}
	}

	// split q into the two digit number (q'*bbb + r) to form independent subblocks
	q, r = q.div(r, q, table[index].bbb)

	// convert subblocks and collect results in s[:h] and s[h:]
	h := len(s) - table[index].ndigits
	r.convertWords(s[h:], b, ndigits, bb, table[0:index])
	s = s[:h] // == q.convertWords(s, b, ndigits, bb, table[0:index+1])
	}
	}

	// having split any large blocks now process the remaining (small) block iteratively
	i := len(s)
	var r Word
	if b == 10 {
	// hard-coding for 10 here speeds this up by 1.25x (allows for / and % by constants)
	for len(q) > 0 {
	// extract least significant, base bb "digit"
	q, r = q.divW(q, bb)
	for j := 0; j < ndigits && i > 0; j++ {
	i--
	// avoid % computation since r%10 == r - int(r/10)*10;
	// this appears to be faster for BenchmarkString10000Base10
	// and smaller strings (but a bit slower for larger ones)
	t := r / 10
	s[i] = '0' + byte(r-t*10)
	r = t
	}
	}
	} else {
	for len(q) > 0 {
	// extract least significant, base bb "digit"
	q, r = q.divW(q, bb)
	for j := 0; j < ndigits && i > 0; j++ {
	i--
	s[i] = digits[r%b]
	r /= b
	}
	}
	}

	// prepend high-order zeros
	for i > 0 { // while need more leading zeros
	i--
	s[i] = '0'
	}
	}

	// Split blocks greater than leafSize Words (or set to 0 to disable recursive conversion)
	// Benchmark and configure leafSize using: go test -bench="Leaf"
	// 8 and 16 effective on 3.0 GHz Xeon "Clovertown" CPU (128 byte cache lines)
	// 8 and 16 effective on 2.66 GHz Core 2 Duo "Penryn" CPU
	var leafSize int = 8 // number of Word-size binary values treat as a monolithic block

	type divisor struct {
	bbb nat // divisor
	nbits int // bit length of divisor (discounting leading zeros) ~= log2(bbb)
	ndigits int // digit length of divisor in terms of output base digits
	}

	var cacheBase10 struct {
	sync.Mutex
	table [64]divisor // cached divisors for base 10
	}

	// expWW computes x**y
	func (z nat) expWW(x, y Word) nat {
	return z.expNN(nat(nil).setWord(x), nat(nil).setWord(y), nil)
	}

	// construct table of powers of bb*leafSize to use in subdivisions
	func divisors(m int, b Word, ndigits int, bb Word) []divisor {
	// only compute table when recursive conversion is enabled and x is large
	if leafSize == 0 \|\| m <= leafSize {
	return nil
	}

	// determine k where (bbleafSize)(2**k) >= sqrt(x)
	k := 1
	for words := leafSize; words < m>>1 && k < len(cacheBase10.table); words <<= 1 {
	k++
	}

	// reuse and extend existing table of divisors or create new table as appropriate
	var table []divisor // for b == 10, table overlaps with cacheBase10.table
	if b == 10 {
	cacheBase10.Lock()
	table = cacheBase10.table[0:k] // reuse old table for this conversion
	} else {
	table = make([]divisor, k) // create new table for this conversion
	}

	// extend table
	if table[k-1].ndigits == 0 {
	// add new entries as needed
	var larger nat
	for i := 0; i < k; i++ {
	if table[i].ndigits == 0 {
	if i == 0 {
	table[0].bbb = nat(nil).expWW(bb, Word(leafSize))
	table[0].ndigits = ndigits * leafSize
	} else {
	table[i].bbb = nat(nil).sqr(table[i-1].bbb)
	table[i].ndigits = 2 * table[i-1].ndigits
	}

	// optimization: exploit aggregated extra bits in macro blocks
	larger = nat(nil).set(table[i].bbb)
	for mulAddVWW(larger, larger, b, 0) == 0 {
	table[i].bbb = table[i].bbb.set(larger)
	table[i].ndigits++
	}

	table[i].nbits = table[i].bbb.bitLen()
	}
	}
	}

	if b == 10 {
	cacheBase10.Unlock()
	}

	return table
	}