crcmod.fs

open System

let crcmod (a:int) (b:int) =
    if b = 0 then raise (new DivideByZeroException())
    let aa, ab = uint32(Math.Abs(a)), uint32(Math.Abs(b))
    if aa < ab then 0 else
        let shiftToTop v =
            let rec stt v n =
                if (v &&& 0x80000000u) <> 0u then n else stt (v <<< 1) (n + 1)
            stt v 0
        let s1, s2 = shiftToTop aa, shiftToTop ab
        let ab2, v = ab <<< (s2 - s1), 1u <<< (31 - s1)
        let rec mod' aa ab2 v =
            let aa = if (aa &&& v) <> 0u then aa ^^^ ab2 else aa
            if ab = ab2 then int(aa) else mod' aa (ab2 >>> 1) (v >>> 1)
        let aret = mod' aa ab2 v
        if Math.Sign(a) = Math.Sign(b) then aret else -aret

let r = new Random()
for i = 1 to 10 do
    let a, b = r.Next(), r.Next() &&& 0xffff
    printfn "%d / %d = %d ... %d(%d)" a b (a / b) (a % b) (crcmod a b)