Square Spiral Example Joy Code¶

Here is the example of Joy code from the README file:

[[[abs]ii <=][[<>][pop !-]||]&&][[!-][[++]][[--]]ifte dip][[pop !-][--][++]ifte]ifte

It might seem unreadable but with a little familiarity it becomes just as legible as any other notation. Some layout helps:

[   [[abs] ii <=]
    [
        [<>] [pop !-] ||
    ] &&
]
[[    !-] [[++]] [[--]] ifte dip]
[[pop !-]  [--]   [++]  ifte    ]
ifte

This function accepts two integers on the stack and increments or decrements one of them such that the new pair of numbers is the next coordinate pair in a square spiral (like the kind used to construct an Ulam Spiral).

Original Form¶

It's adapted from the original code on StackOverflow:

If all you're trying to do is generate the first N points in the spiral (without the original problem's constraint of masking to an N x M region), the code becomes very simple:

void spiral(const int N)
{
    int x = 0;
    int y = 0;
    for(int i = 0; i < N; ++i)
    {
        cout << x << '\t' << y << '\n';
        if(abs(x) <= abs(y) && (x != y || x >= 0))
            x += ((y >= 0) ? 1 : -1);
        else
            y += ((x >= 0) ? -1 : 1);
    }
}

Translation to Joy¶

I'm going to make a function that take two ints (x and y) and generates the next pair, we'll turn it into a generator later using the x combinator.

First Boolean Predicate¶

We need a function that computes abs(x) <= abs(y), we can use ii to apply abs to both values and then compare them with <=:

[abs] ii <=

Short-Circuiting Boolean Combinators¶

I've defined two short-circuiting Boolean combinators && and || that each accept two quoted predicate programs, run the first, and conditionally run the second only if required (to compute the final Boolean value). They run their predicate arguments nullary.

Translating the Conditionals¶

Given those, we can define x != y || x >= 0 as:

_a == [!=] [pop 0 >=] ||

And (abs(x) <= abs(y) && (x != y || x >= 0)) as:

_b == [_p] [_a] &&

It's a little rough, but, as I say, with a little familiarity it becomes legible.

The Increment / Decrement Branches¶

Turning to the branches of the main if statement:

x += ((y >= 0) ? 1 : -1);

Rewrite as a hybrid (pseudo-code) ifte expression:

[y >= 0] [x += 1] [X -= 1] ifte

Change each C phrase to Joy code:

[0 >=] [[++] dip] [[--] dip] ifte

Factor out the dip from each branch:

[0 >=] [[++]] [[--]] ifte dip

Similar logic applies to the other branch:

y += ((x >= 0) ? -1 : 1);

[x >= 0] [y -= 1] [y += 1] ifte

[pop 0 >=] [--] [++] ifte

Putting the Pieces Together¶

We can assemble the three functions we just defined in quotes and give them them to the ifte combinator. With some arrangement to show off the symmetry of the two branches, we have:

[[[abs] ii <=] [[<>] [pop !-] ||] &&]
[[    !-] [[++]] [[--]] ifte dip]
[[pop !-]  [--]   [++]  ifte    ]
ifte

As I was writing this up I realized that, since the && combinator doesn't consume the stack (below its quoted args), I can unquote the predicate, swap the branches, and use the branch combinator instead of ifte:

[[abs] ii <=] [[<>] [pop !-] ||] &&
[[pop !-]  [--]   [++]  ifte    ]
[[    !-] [[++]] [[--]] ifte dip]
branch

Let's try it out:

Turning it into a Generator with x¶

It can be used with the x combinator to make a kind of generator for spiral square coordinates.

We can use codireco to make a generator

codireco == cons dip rest cons

It will look like this:

[value [F] codireco]

Here's a trace of how it works:

But first we have to change the spiral_next function to work on a quoted pair of integers, and leave a copy of the pair on the stack. From:

   y x spiral_next
---------------------
        y' x'

to:

   [x y] [spiral_next] infra
-------------------------------
           [x' y']

So our generator is:

[[x y] [dup [spiral_next] infra] codireco]

Or rather:

[[0 0] [dup [spiral_next] infra] codireco]

There is a function make_generator that will build the generator for us out of the value and stepper function:

   [0 0] [dup [spiral_next] infra] make_generator
----------------------------------------------------
     [[0 0] [dup [spiral_next] infra] codireco]

Here it is in action:

Four x combinators, four pairs of coordinates.

Or you can leave out dup and let the value stay in the generator until you want it:

Conclusion¶

So that's an example of Joy code. It's a straightforward translation of the original. It's a little long for a single definition, you might break it up like so:

_spn_Pa == [abs] ii <=
_spn_Pb == [!=] [pop 0 >=] ||
_spn_P  == [_spn_Pa] [_spn_Pb] &&

_spn_T == [    !-] [[++]] [[--]] ifte dip
_spn_E == [pop !-]  [--]   [++]  ifte

spiral_next == _spn_P [_spn_E] [_spn_T] branch

This way it's easy to see that the function is a branch with two quasi-symmetrical paths.

We then used this function to make a simple generator of coordinate pairs, where the next pair in the series can be generated at any time by using the x combinator on the generator (which is just a quoted expression containing a copy of the current pair and the "stepper function" to generate the next pair from that.)