https://c9x.me/notes/2019-01-15.html

    uint32_t mulinv(uint32_t a) {
        uint32_t b = a;   /* 1/a mod 2² */
        b *= 2 - a*b;     /* 1/a mod 2⁴ */
        b *= 2 - a*b;     /* 1/a mod 2⁸ */
        b *= 2 - a*b;     /* 1/a mod 2¹⁶ */
        b *= 2 - a*b;     /* 1/a mod 2³² */
        return b;
    }

In Joy:


    b *= 2 - a*b

    b = b * (2 - a*b)



    a 2 a b * - b *
    a 2 a*b   - b *
    a 2-(a*b)   b *
    a b*(2-(a*b))

    a b over over
    a b a b [* 2 swap -] dip *
    a b a * 2 swap - b *
    a b*a   2 swap - b *
    a 2 b*a        - b *
    a 2-b*a          b *
    a (2-b*a)*b

    G == over over [* 2 swap -] dip *
    mulinv == dup 5 [G] times popd

Can compile G (mulinv must wait on times.)

    ?- gronk("fn", `over over [* 2 swap -] dip *`).

    def fn(stack, expression, dictionary):
        (i1, (i2, stack)) = stack
        i3 = i1 * i2
        i4 = 2 - i3
        i5 = i4 * i1
        return (i5, (i2, stack)), expression, dictionary



Using Unary

    a b [F] dupdip *
    a b F b *
    a b * 2 swap - b *


    G == [* 2 swap -] dupdip *
    mulinv == dup 5 [[G] unary] times popd

Bleah.