fr, exp := math.Frexp(f)

v := int64(fr * (1 << 53))

e := exp - 53

buf := make([]byte, 1000)

n := copy(buf, strconv.FormatInt(v, 10))

//n := copy(buf, strconv.Itoa64(v))

for ; e > 0; e-- {

δ := 0

if buf[0] >= '5' {

δ = 1

}

x := byte(0)

for i := n - 1; i >= 0; i-- {

x += 2 * (buf[i] - '0')

x, buf[i+δ] = x/10, x%10+'0'

}

if δ == 1 {

buf[0] = '1'

n++

}

}

dp := n

for ; e < 0; e++ {

if buf[n-1]%2 != 0 {

buf[n] = '0'

n++

}

δ, x := 0, byte(0)

if buf[0] < '2' {

δ, x = 1, buf[0]-'0'

n--

dp--

}

for i := 0; i < n; i++ {

x = x*10 + buf[i+δ] - '0'

buf[i], x = x/2+'0', x%2

}

}

if prec > 0 {

if n > prec {

if buf[prec] > '5' || buf[prec] == '5' && (nonzero(buf[prec+1:n]) || buf[prec-1]%2 == 1) {

i := prec - 1

for i >= 0 && buf[i] == '9' {

buf[i] = '0'

i--

}

if i >= 0 {

buf[i]++

} else {

buf[0] = '1'

dp++

}

}

n = prec

}

for n < prec {

buf[n] = '0'

n++

}

}

return fmt.Sprintf("%c.%se%+d", buf[0], buf[1:n], dp-1)

for _, c := range buf {

if c != '0' {

return true

}

}

return false

fmt.Printf("%s\n", ftoa(math.Pi, 50)) // 50 digits of math.Pi (not 50 digits of π)

//fmt.Printf("%s\n", ftoa(math.Nextafter(0, 1), 0))

for _, tt := range ftoaTests {

if a := ftoa(tt.F, tt.N+1); a != tt.A {

fmt.Printf("ftoa(%g, %d) = %q, want %q\n", tt.F, tt.N+1, a, tt.A)

}

}