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use std::hash::Hasher;
use std::io;

use byteorder::{BigEndian, ByteOrder, WriteBytesExt};
use sha1::{Digest, Sha1};

use crate::errors::Result;

/// Two octet checksum: sum of all octets mod 65535.
#[inline]
pub fn simple(actual: &[u8], data: &[u8]) -> Result<()> {
    // Then a two-octet checksum is appended, which is equal to the
    // sum of the preceding session key octets, not including the algorithm
    // identifier, modulo 65536.
    let expected_checksum = calculate_simple(data);

    ensure_eq!(
        &actual[..2],
        &expected_checksum.to_be_bytes()[..],
        "invalid simple checksum"
    );

    Ok(())
}

/// SHA1 checksum, first 20 octets.
#[inline]
pub fn sha1(hash: &[u8], data: &[u8]) -> Result<()> {
    ensure_eq!(hash, &calculate_sha1(data)[..], "invalid SHA1 checksum");

    Ok(())
}

#[inline]
pub fn simple_to_writer<W: io::Write>(data: &[u8], writer: &mut W) -> io::Result<()> {
    let mut hasher = SimpleChecksum::default();
    hasher.write(data);
    hasher.to_writer(writer)
}

#[inline]
pub fn calculate_simple(data: &[u8]) -> u16 {
    let mut hasher = SimpleChecksum::default();
    hasher.write(data);
    hasher.finish() as u16
}

#[derive(Debug, Default)]
pub struct SimpleChecksum(u16);

impl SimpleChecksum {
    #[inline]
    pub fn to_writer<W: io::Write>(&self, writer: &mut W) -> io::Result<()> {
        writer.write_u16::<BigEndian>(self.0)
    }

    #[inline]
    pub fn finalize(&self) -> [u8; 2] {
        let mut res = [0u8; 2];
        BigEndian::write_u16(&mut res[..], self.0);

        res
    }
}

impl io::Write for SimpleChecksum {
    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
        Hasher::write(self, buf);

        Ok(buf.len())
    }

    fn flush(&mut self) -> io::Result<()> {
        Ok(())
    }
}

impl Hasher for SimpleChecksum {
    #[inline]
    fn write(&mut self, buf: &[u8]) {
        let new_sum = buf.iter().map(|v| u32::from(*v)).sum::<u32>();
        self.0 = ((u32::from(self.0) + new_sum) & 0xffff) as u16;
    }

    #[inline]
    fn finish(&self) -> u64 {
        u64::from(self.0)
    }
}

#[inline]
pub fn calculate_sha1(data: &[u8]) -> Vec<u8> {
    Sha1::digest(data)[..20].to_vec()
}