extern crate byteorder;

use self::byteorder::{BigEndian, ByteOrder};

const GENERATOR_POLYNOMIAL: u16 = 0x8408;

fn checksum_bits(checksum: u16, bits_in_byte: i8) -> u16 {
    (0..bits_in_byte).fold(checksum, |acc, _| match acc & 1 {
        1 => (acc >> 1) ^ GENERATOR_POLYNOMIAL,
        _ => acc >> 1,
    })
}

pub fn checksum(bytes: Vec<&u8>) -> [u8; 2] {
    let initial_mask: u16 = 0xFFFF;
    let bits_in_byte = 8;
    let checksum: u16 = bytes.iter().fold(initial_mask, |acc, &item| {
        checksum_bits(acc ^ (*item as u16), bits_in_byte)
    }) ^ initial_mask;

    let mut buf = [0u8; 2];
    BigEndian::write_u16(&mut buf, checksum);

    buf
}

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

    #[test]
    fn creates_correct_checksum() {
        let mut vec: Vec<&u8> = Vec::new();

        for byte in "FZGX".as_bytes() {
            vec.push(byte);
        }
        let checksum = checksum(vec);

        assert_eq!(checksum, [0x84, 0xC9]);
    }
}