github.com/ader1990/go@v0.0.0-20140630135419-8c24447fa791/src/pkg/strconv/itoa.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 strconv

// FormatUint returns the string representation of i in the given base,
// for 2 <= base <= 36. The result uses the lower-case letters 'a' to 'z'
// for digit values >= 10.
func FormatUint(i uint64, base int) string {
	_, s := formatBits(nil, i, base, false, false)
	return s
}

// FormatInt returns the string representation of i in the given base,
// for 2 <= base <= 36. The result uses the lower-case letters 'a' to 'z'
// for digit values >= 10.
func FormatInt(i int64, base int) string {
	_, s := formatBits(nil, uint64(i), base, i < 0, false)
	return s
}

// Itoa is shorthand for FormatInt(i, 10).
func Itoa(i int) string {
	return FormatInt(int64(i), 10)
}

// AppendInt appends the string form of the integer i,
// as generated by FormatInt, to dst and returns the extended buffer.
func AppendInt(dst []byte, i int64, base int) []byte {
	dst, _ = formatBits(dst, uint64(i), base, i < 0, true)
	return dst
}

// AppendUint appends the string form of the unsigned integer i,
// as generated by FormatUint, to dst and returns the extended buffer.
func AppendUint(dst []byte, i uint64, base int) []byte {
	dst, _ = formatBits(dst, i, base, false, true)
	return dst
}

const (
	digits   = "0123456789abcdefghijklmnopqrstuvwxyz"
	digits01 = "0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789"
	digits10 = "0000000000111111111122222222223333333333444444444455555555556666666666777777777788888888889999999999"
)

var shifts = [len(digits) + 1]uint{
	1 << 1: 1,
	1 << 2: 2,
	1 << 3: 3,
	1 << 4: 4,
	1 << 5: 5,
}

// formatBits computes the string representation of u in the given base.
// If neg is set, u is treated as negative int64 value. If append_ is
// set, the string is appended to dst and the resulting byte slice is
// returned as the first result value; otherwise the string is returned
// as the second result value.
//
func formatBits(dst []byte, u uint64, base int, neg, append_ bool) (d []byte, s string) {
	if base < 2 || base > len(digits) {
		panic("strconv: illegal AppendInt/FormatInt base")
	}
	// 2 <= base && base <= len(digits)

	var a [64 + 1]byte // +1 for sign of 64bit value in base 2
	i := len(a)

	if neg {
		u = -u
	}

	// convert bits
	if base == 10 {
		// common case: use constants for / and % because
		// the compiler can optimize it into a multiply+shift,
		// and unroll loop
		for u >= 100 {
			i -= 2
			q := u / 100
			j := uintptr(u - q*100)
			a[i+1] = digits01[j]
			a[i+0] = digits10[j]
			u = q
		}
		if u >= 10 {
			i--
			q := u / 10
			a[i] = digits[uintptr(u-q*10)]
			u = q
		}

	} else if s := shifts[base]; s > 0 {
		// base is power of 2: use shifts and masks instead of / and %
		b := uint64(base)
		m := uintptr(b) - 1 // == 1<<s - 1
		for u >= b {
			i--
			a[i] = digits[uintptr(u)&m]
			u >>= s
		}

	} else {
		// general case
		b := uint64(base)
		for u >= b {
			i--
			a[i] = digits[uintptr(u%b)]
			u /= b
		}
	}

	// u < base
	i--
	a[i] = digits[uintptr(u)]

	// add sign, if any
	if neg {
		i--
		a[i] = '-'
	}

	if append_ {
		d = append(dst, a[i:]...)
		return
	}
	s = string(a[i:])
	return
}