// hash/crc16 — CRC-16 checksum. Pure ww.
//
// Inline polynomial-shift per byte (no precomputed tables). Slower
// than a table-driven CRC by ~8x per byte but matches the
// table-driven answer bit-for-bit.
//
// Polynomials are given in reversed form, matching Hare.

package crc16;

def CCITT: u16    = 0x8408u16;	// X.25, Bluetooth, XMODEM
def CMDA2000: u16 = 0xE613u16;	// CDMA2000 infra
def DECT: u16     = 0x91A0u16;	// DECT cordless
def ANSI: u16     = 0xA001u16;	// Modbus, USB, ANSI X3.28

// sum16 — fold `buf` under `poly` and return ~cval. Initial value is
// ~0u16, matching the streaming CRC-16 contract for a single
// write-then-sum. Per byte: XOR low byte of cval with msg byte to form
// an 8-bit index, run 8 polynomial shifts on that index, XOR the
// result with the high byte of cval shifted down.
export fn sum16(buf: []u8, poly: u16) u16 = {
	let c: u16 = 0xFFFFu16;
	let i: i32 = 0;
	for (i < buf.len) {
		let t: u16 = (c & 0xFFu16) ^ (buf[i]: u16);
		let z: i32 = 0;
		for (z < 8) {
			if ((t & 1u16) == 1u16) {
				t = (t >> 1u16) ^ poly;
			} else {
				t = t >> 1u16;
			};
			z += 1;
		};
		c = t ^ (c >> 8u16);
		i += 1;
	};
	return ~c;
};

export fn sum16ccitt(buf: []u8) u16    = { return sum16(buf, CCITT); };
export fn sum16cmda2000(buf: []u8) u16 = { return sum16(buf, CMDA2000); };
export fn sum16dect(buf: []u8) u16     = { return sum16(buf, DECT); };
export fn sum16ansi(buf: []u8) u16     = { return sum16(buf, ANSI); };