Calculate a CRC

make-crc.reb

Rebol [
	Title: "Make CRC"
	Date: 25-May-2015
	Author: "Christopher Ross-Gill"
]

shift32: func [value [integer!] bits [integer!] /logical][
	apply :shift [to integer! #{00000000FFFFFFFF} and to binary! value bits logical]
]

make-crc: use [legacy-table crc-table crc][
	crc-table: new-line/all collect [
		repeat n 256 [
			crc: n - 1
			loop 8 [
				crc: either equal? 1 crc and 1 [
					-306674912 xor shift32/logical crc -1
				][
					shift32/logical crc -1
				]
			]
			keep crc
		]
	] false

	func [chk [binary!]][
		crc: -1

		foreach byte chk [
			byte: crc xor byte and 255 + 1
			crc: crc-table/:byte xor shift32/logical crc -8
		]

		remove/part to binary! crc xor -1 4
	]
]