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    use bytes::{BufMut, BytesMut};
    use http::Version;
    use std::{mem, ptr, slice};

    const DEC_DIGITS_LUT: &[u8] = b"0001020304050607080910111213141516171819\
          2021222324252627282930313233343536373839\
          4041424344454647484950515253545556575859\
          6061626364656667686970717273747576777879\
          8081828384858687888990919293949596979899";

    pub(crate) fn write_status_line(version: Version, mut n: u16, bytes: &mut BytesMut) {
        let mut buf: [u8; 13] = [
            b'H', b'T', b'T', b'P', b'/', b'1', b'.', b'1', b' ', b' ', b' ', b' ', b' ',
        ];
        match version {
            Version::HTTP_2 => buf[5] = b'2',
            Version::HTTP_10 => buf[7] = b'0',
            Version::HTTP_09 => {
                buf[5] = b'0';
                buf[7] = b'9';
            }
            _ => (),
        }

        let mut curr: isize = 12;
        let buf_ptr = buf.as_mut_ptr();
        let lut_ptr = DEC_DIGITS_LUT.as_ptr();
        let four = n > 999;

        unsafe {
            // decode 2 more chars, if > 2 chars
            let d1 = (n % 100) << 1;
            n /= 100;
            curr -= 2;
            ptr::copy_nonoverlapping(lut_ptr.offset(d1 as isize), buf_ptr.offset(curr), 2);

            // decode last 1 or 2 chars
            if n < 10 {
                curr -= 1;
                *buf_ptr.offset(curr) = (n as u8) + b'0';
            } else {
                let d1 = n << 1;
                curr -= 2;
                ptr::copy_nonoverlapping(
                    lut_ptr.offset(d1 as isize),
                    buf_ptr.offset(curr),
                    2,
                );
            }
        }

        bytes.put_slice(&buf);
        if four {
            bytes.put(b' ');
        }
    }

    /// NOTE: bytes object has to contain enough space
    pub fn write_content_length(mut n: usize, bytes: &mut BytesMut) {
        if n < 10 {
            let mut buf: [u8; 21] = [
                b'\r', b'\n', b'c', b'o', b'n', b't', b'e', b'n', b't', b'-', b'l', b'e',
                b'n', b'g', b't', b'h', b':', b' ', b'0', b'\r', b'\n',
            ];
            buf[18] = (n as u8) + b'0';
            bytes.put_slice(&buf);
        } else if n < 100 {
            let mut buf: [u8; 22] = [
                b'\r', b'\n', b'c', b'o', b'n', b't', b'e', b'n', b't', b'-', b'l', b'e',
                b'n', b'g', b't', b'h', b':', b' ', b'0', b'0', b'\r', b'\n',
            ];
            let d1 = n << 1;
            unsafe {
                ptr::copy_nonoverlapping(
                    DEC_DIGITS_LUT.as_ptr().offset(d1 as isize),
                    buf.as_mut_ptr().offset(18),
                    2,
                );
            }
            bytes.put_slice(&buf);
        } else if n < 1000 {
            let mut buf: [u8; 23] = [
                b'\r', b'\n', b'c', b'o', b'n', b't', b'e', b'n', b't', b'-', b'l', b'e',
                b'n', b'g', b't', b'h', b':', b' ', b'0', b'0', b'0', b'\r', b'\n',
            ];
            // decode 2 more chars, if > 2 chars
            let d1 = (n % 100) << 1;
            n /= 100;
            unsafe {
                ptr::copy_nonoverlapping(
                    DEC_DIGITS_LUT.as_ptr().offset(d1 as isize),
                    buf.as_mut_ptr().offset(19),
                    2,
                )
            };

            // decode last 1
            buf[18] = (n as u8) + b'0';

            bytes.put_slice(&buf);
        } else {
            bytes.put_slice(b"\r\ncontent-length: ");
            convert_usize(n, bytes);
        }
    }

    pub(crate) fn convert_usize(mut n: usize, bytes: &mut BytesMut) {
        unsafe {
            let mut curr: isize = 39;
            let mut buf: [u8; 41] = mem::uninitialized();
            buf[39] = b'\r';
            buf[40] = b'\n';
            let buf_ptr = buf.as_mut_ptr();
            let lut_ptr = DEC_DIGITS_LUT.as_ptr();

            // eagerly decode 4 characters at a time
            while n >= 10_000 {
                let rem = (n % 10_000) as isize;
                n /= 10_000;

                let d1 = (rem / 100) << 1;
                let d2 = (rem % 100) << 1;
                curr -= 4;
                ptr::copy_nonoverlapping(lut_ptr.offset(d1), buf_ptr.offset(curr), 2);
                ptr::copy_nonoverlapping(lut_ptr.offset(d2), buf_ptr.offset(curr + 2), 2);
            }

            // if we reach here numbers are <= 9999, so at most 4 chars long
            let mut n = n as isize; // possibly reduce 64bit math

            // decode 2 more chars, if > 2 chars
            if n >= 100 {
                let d1 = (n % 100) << 1;
                n /= 100;
                curr -= 2;
                ptr::copy_nonoverlapping(lut_ptr.offset(d1), buf_ptr.offset(curr), 2);
            }

            // decode last 1 or 2 chars
            if n < 10 {
                curr -= 1;
                *buf_ptr.offset(curr) = (n as u8) + b'0';
            } else {
                let d1 = n << 1;
                curr -= 2;
                ptr::copy_nonoverlapping(lut_ptr.offset(d1), buf_ptr.offset(curr), 2);
            }

            bytes.extend_from_slice(slice::from_raw_parts(
                buf_ptr.offset(curr),
                41 - curr as usize,
            ));
        }
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn test_write_content_length() {
            let mut bytes = BytesMut::new();
            bytes.reserve(50);
            write_content_length(0, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 0\r\n"[..]);
            bytes.reserve(50);
            write_content_length(9, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 9\r\n"[..]);
            bytes.reserve(50);
            write_content_length(10, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 10\r\n"[..]);
            bytes.reserve(50);
            write_content_length(99, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 99\r\n"[..]);
            bytes.reserve(50);
            write_content_length(100, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 100\r\n"[..]);
            bytes.reserve(50);
            write_content_length(101, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 101\r\n"[..]);
            bytes.reserve(50);
            write_content_length(998, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 998\r\n"[..]);
            bytes.reserve(50);
            write_content_length(1000, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 1000\r\n"[..]);
            bytes.reserve(50);
            write_content_length(1001, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 1001\r\n"[..]);
            bytes.reserve(50);
            write_content_length(5909, &mut bytes);
            assert_eq!(bytes.take().freeze(), b"\r\ncontent-length: 5909\r\n"[..]);
        }
    }

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