//! Checksum functions.

// Accumulator for CRC function.
fn crc_accum(a: u32, _: u32) -> u32
{
	if a & 1 == 1 {
		ISO_3309_POLYNOMIAL ^ a >> 1
	} else {
		a >> 1
	}
}

// Initializes a CRC array.
// FIXME: use const fn when stabilized
fn crc_init() -> [u32; 256]
{
	let mut t = [0; 256];
	for (n, v) in t.iter_mut().enumerate() {
		*v = (0..8).fold(n as u32, crc_accum);
	}
	t
}

/// Creates an ADLER32 of all bytes in `b`.
///
/// # Examples
///
/// ```
/// use maraiah::cksum::adler32;
///
/// assert_eq!(adler32(b"Lorem ipsum dolor sit amet"), 0x83D5_09C5);
/// ```
pub fn adler32(b: &[u8]) -> u32
{
	let mut x = 1;
	let mut y = 0;

	for &z in b {
		let z = u32::from(z);
		x = (x + z) % ADLER32_MODULO;
		y = (y + x) % ADLER32_MODULO;
	}

	(y << 16) | x
}

/// Creates a CRC-32 of all bytes in `b` with the starting sum `s`. The
/// polynomial used is the one from ISO-3309.
///
/// # Examples
///
/// ```
/// use maraiah::cksum::crc32;
///
/// assert_eq!(crc32(b"Lorem ipsum dolor sit amet", !0), 0x5F29_D461);
/// ```
pub fn crc32(b: &[u8], s: u32) -> u32
{
	let t = crc_init();
	!b.iter().fold(s, |a, &o| a >> 8 ^ t[usize::from(a as u8 ^ o)])
}

const ISO_3309_POLYNOMIAL: u32 = 0xEDB8_8320;
const ADLER32_MODULO:      u32 = 0xFFF1;

// EOF