itoa.go

package decimal

const smallsString = "00010203040506070809" +
	"10111213141516171819" +
	"20212223242526272829" +
	"30313233343536373839" +
	"40414243444546474849" +
	"50515253545556575859" +
	"60616263646566676869" +
	"70717273747576777879" +
	"80818283848586878889" +
	"90919293949596979899"

const host32bit = ^uint(0)>>32 == 0

// Adapted from standard library strconv/itoa.go.
func formatBits10(buf []byte, u uint64, w int) []byte {
	// Probably only needs 17, but let's play it safe.
	var a [32]byte
	i := len(a)

	if host32bit {
		// convert the lower digits using 32bit operations
		for u >= 1e9 {
			// Avoid using r = a%b in addition to q = a/b
			// since 64bit division and modulo operations
			// are calculated by runtime functions on 32bit machines.
			q := u / 1e9
			us := uint(u - q*1e9) // u % 1e9 fits into a uint
			for j := 4; j > 0; j-- {
				is := us % 100 * 2
				us /= 100
				i -= 2
				w -= 2
				a[i+1] = smallsString[is+1]
				a[i+0] = smallsString[is+0]
			}

			// us < 10, since it contains the last digit
			// from the initial 9-digit us.
			i--
			w--
			a[i] = smallsString[us*2+1]

			u = q
		}
		// u < 1e9
	}

	// u guaranteed to fit into a uint
	us := uint(u)
	for ; w > 0; w -= 2 {
		is := us % 100 * 2
		us /= 100
		i -= 2
		a[i+1] = smallsString[is+1]
		a[i+0] = smallsString[is+0]
	}

	if w < 0 {
		i++
	}

	return append(buf, a[i:]...)
}