// trailingZeros counts the number of trailing zeros in the
// big.Int value. It first attempts to use an unsigned integer
// representation of the big.Int to compute this because it is
// roughly 8x faster. If this unsigned integer would overflow,
// it falls back to formatting the big.Int itself.
func trailingZeros(bi *big.Int, tmp []byte) int {
if bi.BitLen() <= 64 {
i := bi.Uint64()
bs := strconv.AppendUint(tmp, i, 10)
return trailingZerosFromBytes(bs)
}
bs := bi.Append(tmp, 10)
return trailingZerosFromBytes(bs)
}
func main() {
factorial := new(big.Int).MulRange(1, 100)
bytes := factorial.Append([]byte{}, 10)
sum := 0
for b := range bytes {
sum += int(bytes[b] - '0')
}
fmt.Println("sum:", sum)
}
// numDigits returns the number of decimal digits that make up
// big.Int value. The function first attempts to look this digit
// count up in the digitsLookupTable. If the value is not there,
// it defaults to constructing a string value for the big.Int and
// using this to determine the number of digits. If a string value
// is constructed, it will be returned so it can be used again.
func numDigits(bi *big.Int, tmp []byte) (int, []byte) {
if val, ok := lookupBits(bi.BitLen()); ok {
if bi.Cmp(&val.border) < 0 {
return val.digits, nil
}
return val.digits + 1, nil
}
bs := bi.Append(tmp, 10)
return len(bs), bs
}