// Same shape as lib/hash/crc32: per-byte inline polynomial shift, no
// precomputed table. Slower than Hare's table-driven path by ~8x per
// byte but produces identical answers for the documented polynomials.
//
// Polynomials are given in reversed form, matching Hare.

package crc64;

def ECMA: u64 = 0xC96C5795D7870F42u64;	// ECMA-182, xz-utils
def ISO:  u64 = 0xD800000000000000u64;	// ISO 3309 HDLC

// sum64 — fold `buf` under `poly` (reversed form). Initial cval is ~0u64.
export fn sum64(buf: []u8, poly: u64) u64 = {
	let c: u64 = 0xFFFFFFFFFFFFFFFFu64;
	let i: i32 = 0;
	for (i < buf.len) {
		let t: u64 = (c & 0xFFu64) ^ (buf[i]: u64);
		let z: i32 = 0;
		for (z < 8) {
			if ((t & 1u64) == 1u64) {
				t = (t >> 1u64) ^ poly;
			} else {
				t = t >> 1u64;
			};
			z += 1;
		};
		c = t ^ (c >> 8u64);
		i += 1;
	};
	return ~c;
};

export fn sum64ecma(buf: []u8) u64 = { return sum64(buf, ECMA); };
export fn sum64iso(buf: []u8) u64  = { return sum64(buf, ISO); };