Thun/docs/Square_Spiral.ipynb

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{
"cells": [
{
"cell_type": "code",
"execution_count": 1,
"metadata": {},
"outputs": [],
"source": [
"from notebook_preamble import J, V, define"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Square Spiral Example Joy Code"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"\n",
"Here is the example of Joy code from the `README` file:\n",
"\n",
" [[[abs]ii <=][[<>][pop !-]||]&&][[!-][[++]][[--]]ifte dip][[pop !-][--][++]ifte]ifte\n",
"\n",
"It might seem unreadable but with a little familiarity it becomes just as\n",
"legible as any other notation. Some layout helps:\n",
"\n",
" [ [[abs] ii <=]\n",
" [\n",
" [<>] [pop !-] ||\n",
" ] &&\n",
" ]\n",
" [[ !-] [[++]] [[--]] ifte dip]\n",
" [[pop !-] [--] [++] ifte ]\n",
" ifte\n",
"\n",
"This function accepts two integers on the stack and increments or\n",
"decrements one of them such that the new pair of numbers is the next\n",
"coordinate pair in a square spiral (like the kind used to construct an\n",
"Ulam Spiral). \n",
"\n"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Original Form\n",
"\n",
"It's adapted from [the original code on StackOverflow](https://stackoverflow.com/questions/398299/looping-in-a-spiral/31864777#31864777):\n",
"\n",
"\n",
"> If all you're trying to do is generate the first N points in the spiral\n",
"> (without the original problem's constraint of masking to an N x M\n",
"> region), the code becomes very simple:\n",
"\n",
" void spiral(const int N)\n",
" {\n",
" int x = 0;\n",
" int y = 0;\n",
" for(int i = 0; i < N; ++i)\n",
" {\n",
" cout << x << '\\t' << y << '\\n';\n",
" if(abs(x) <= abs(y) && (x != y || x >= 0))\n",
" x += ((y >= 0) ? 1 : -1);\n",
" else\n",
" y += ((x >= 0) ? -1 : 1);\n",
" }\n",
" }"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Translation to Joy\n",
"\n",
"I'm going to make a function that take two ints (`x` and `y`) and\n",
"generates the next pair, we'll turn it into a generator later using the\n",
"`x` combinator."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### First Boolean Predicate\n",
"\n",
"We need a function that computes `abs(x) <= abs(y)`, we can use `ii` to\n",
"apply `abs` to both values and then compare them\n",
"with `<=`:\n",
"\n",
" [abs] ii <=\n",
"\n",
"I've defined two short-circuiting Boolean combinators `&&` and `||` that\n",
"each accept two quoted predicate programs, run the first, and\n",
"conditionally run the second only if required (to compute the final\n",
"Boolean value). They run their predicate arguments `nullary`."
]
},
{
"cell_type": "code",
"execution_count": 2,
"metadata": {},
"outputs": [],
"source": [
"define('&& [nullary] cons [nullary [0]] dip branch')\n",
"define('|| [nullary] cons [nullary] dip [1] branch')"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Given those, we can define `x != y || x >= 0` as:\n",
"\n",
" [<>] [pop 0 >=] ||\n",
"\n",
"And `(abs(x) <= abs(y) && (x != y || x >= 0))` as:\n",
"\n",
" [[abs] ii <=] [[<>] [pop 0 >=] ||] &&\n",
"\n",
"It's a little rough, but, as I say, with a little familiarity it becomes\n",
"legible."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### The Increment / Decrement Branches\n",
"\n",
"Turning to the branches of the main `if` statement:\n",
"\n",
" x += ((y >= 0) ? 1 : -1);\n",
"\n",
"Rewrite as a hybrid (pseudo-code) `ifte` expression:\n",
"\n",
" [y >= 0] [x += 1] [X -= 1] ifte\n",
"\n",
"Change each C phrase to Joy code:\n",
"\n",
" [0 >=] [[++] dip] [[--] dip] ifte\n",
"\n",
"Factor out the dip from each branch:\n",
"\n",
" [0 >=] [[++]] [[--]] ifte dip\n",
"\n",
"Similar logic applies to the other branch:\n",
"\n",
" y += ((x >= 0) ? -1 : 1);\n",
"\n",
" [x >= 0] [y -= 1] [y += 1] ifte\n",
"\n",
" [pop 0 >=] [--] [++] ifte"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"### \"Not Negative\""
]
},
{
"cell_type": "code",
"execution_count": 3,
"metadata": {},
"outputs": [],
"source": [
"define('!- 0 >=')"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Putting the Pieces Together\n",
"\n",
"We can assemble the three functions we just defined in quotes and give\n",
"them them to the `ifte` combinator. With some arrangement to show off\n",
"the symmetry of the two branches, we have:\n",
"\n",
" [[[abs] ii <=] [[<>] [pop !-] ||] &&]\n",
" [[ !-] [[++]] [[--]] ifte dip]\n",
" [[pop !-] [--] [++] ifte ]\n",
" ifte\n",
"\n",
"As I was writing this up I realized that, since the `&&` combinator\n",
"doesn't consume the stack (below its quoted args), I can unquote the\n",
"predicate, swap the branches, and use the `branch` combinator instead of\n",
"`ifte`:\n",
"\n",
" [[abs] ii <=] [[<>] [pop !-] ||] &&\n",
" [[pop !-] [--] [++] ifte ]\n",
" [[ !-] [[++]] [[--]] ifte dip]\n",
" branch"
]
},
{
"cell_type": "code",
"execution_count": 4,
"metadata": {},
"outputs": [],
"source": [
"define('spiral_next [[[abs] ii <=] [[<>] [pop !-] ||] &&] [[!-] [[++]] [[--]] ifte dip] [[pop !-] [--] [++] ifte] ifte')"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Let's try it out:"
]
},
{
"cell_type": "code",
"execution_count": 5,
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"1 0\n"
]
}
],
"source": [
"J('0 0 spiral_next')"
]
},
{
"cell_type": "code",
"execution_count": 6,
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"1 -1\n"
]
}
],
"source": [
"J('1 0 spiral_next')"
]
},
{
"cell_type": "code",
"execution_count": 7,
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"0 -1\n"
]
}
],
"source": [
"J('1 -1 spiral_next')"
]
},
{
"cell_type": "code",
"execution_count": 8,
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"-1 -1\n"
]
}
],
"source": [
"J('0 -1 spiral_next')"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Turning it into a Generator with `x`\n",
"\n",
"It can be used with the x combinator to make a kind of generator for\n",
"spiral square coordinates.\n",
"\n",
"\n",
"We can use `codireco` to make a generator\n",
"\n",
" codireco ::= cons dip rest cons\n",
"\n",
"It will look like this:\n",
"\n",
" [value [F] codireco]\n",
"\n",
"Here's a trace of how it works:\n",
"\n",
" [0 [dup ++] codireco] . x\n",
" [0 [dup ++] codireco] . 0 [dup ++] codireco\n",
" [0 [dup ++] codireco] 0 . [dup ++] codireco\n",
" [0 [dup ++] codireco] 0 [dup ++] . codireco\n",
" [0 [dup ++] codireco] 0 [dup ++] . cons dip rest cons\n",
" [0 [dup ++] codireco] [0 dup ++] . dip rest cons\n",
" . 0 dup ++ [0 [dup ++] codireco] rest cons\n",
" 0 . dup ++ [0 [dup ++] codireco] rest cons\n",
" 0 0 . ++ [0 [dup ++] codireco] rest cons\n",
" 0 1 . [0 [dup ++] codireco] rest cons\n",
" 0 1 [0 [dup ++] codireco] . rest cons\n",
" 0 1 [[dup ++] codireco] . cons\n",
" 0 [1 [dup ++] codireco] . "
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"But first we have to change the `spiral_next` function to work on a\n",
"quoted pair of integers, and leave a copy of the pair on the stack.\n",
"From:\n",
"\n",
" y x spiral_next\n",
" ---------------------\n",
" y' x'\n",
"\n",
"to:\n",
"\n",
" [x y] [spiral_next] infra\n",
" -------------------------------\n",
" [x' y']"
]
},
{
"cell_type": "code",
"execution_count": 9,
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"[0 1]\n"
]
}
],
"source": [
"J('[0 0] [spiral_next] infra')"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"So our generator is:\n",
"\n",
" [[x y] [dup [spiral_next] infra] codireco]\n",
"\n",
"Or rather:\n",
"\n",
" [[0 0] [dup [spiral_next] infra] codireco]\n",
"\n",
"There is a function `make_generator` that will build the generator for us\n",
"out of the value and stepper function:\n",
"\n",
" [0 0] [dup [spiral_next] infra] make_generator\n",
" ----------------------------------------------------\n",
" [[0 0] [dup [spiral_next] infra] codireco]"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Here it is in action:"
]
},
{
"cell_type": "code",
"execution_count": 10,
"metadata": {},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"[0 0] [0 1] [-1 1] [-1 0]\n"
]
}
],
"source": [
"J('[0 0] [dup [spiral_next] infra] make_generator x x x x pop')"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Four `x` combinators, four pairs of coordinates."
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Conclusion\n",
"\n",
"So that's an example of Joy code. It's a straightforward translation of\n",
"the original. It's a little long for a single definition, you might\n",
"break it up like so:\n",
"\n",
" _spn_P ::= [[abs] ii <=] [[<>] [pop !-] ||] &&\n",
"\n",
" _spn_T ::= [ !-] [[++]] [[--]] ifte dip\n",
" _spn_E ::= [pop !-] [--] [++] ifte\n",
"\n",
" spiral_next ::= _spn_P [_spn_E] [_spn_T] branch\n",
"\n",
"This way it's easy to see that the function is a branch with two\n",
"quasi-symmetrical paths.\n",
"\n",
"We then used this function to make a simple generator of coordinate\n",
"pairs, where the next pair in the series can be generated at any time by\n",
"using the `x` combinator on the generator (which is just a quoted\n",
"expression containing a copy of the current pair and the \"stepper\n",
"function\" to generate the next pair from that.)"
]
},
{
"cell_type": "code",
"execution_count": 11,
"metadata": {},
"outputs": [],
"source": [
"define('_spn_P [[abs] ii <=] [[<>] [pop !-] ||] &&')\n",
"define('_spn_T [!-] [[++]] [[--]] ifte dip')\n",
"define('_spn_E [pop !-] [--] [++] ifte')\n",
"define('spiral_next _spn_P [_spn_E] [_spn_T] branch')"
]
},
{
"cell_type": "code",
"execution_count": 12,
"metadata": {
"scrolled": true
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
" . 23 18 spiral_next\n",
" 23 . 18 spiral_next\n",
" 23 18 . spiral_next\n",
" 23 18 . _spn_P [_spn_E] [_spn_T] branch\n",
" 23 18 . [[abs] ii <=] [[<>] [pop !-] ||] && [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] . [[<>] [pop !-] ||] && [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] [[<>] [pop !-] ||] . && [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] [[<>] [pop !-] ||] . [nullary] cons [nullary [0]] dip branch [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] [[<>] [pop !-] ||] [nullary] . cons [nullary [0]] dip branch [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] [[[<>] [pop !-] ||] nullary] . [nullary [0]] dip branch [_spn_E] [_spn_T] branch\n",
"23 18 [[abs] ii <=] [[[<>] [pop !-] ||] nullary] [nullary [0]] . dip branch [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] . nullary [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] . [stack] dinfrirst [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] [stack] . dinfrirst [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [[abs] ii <=] [stack] . dip infra first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 . stack [[abs] ii <=] infra first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [18 23] . [[abs] ii <=] infra first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [18 23] [[abs] ii <=] . infra first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 . [abs] ii <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [abs] . ii <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [abs] . [dip] dupdip i <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [abs] [dip] . dupdip i <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [abs] . dip [abs] i <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 . abs 18 [abs] i <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 . 18 [abs] i <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 . [abs] i <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [abs] . i <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 . abs <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 . <= [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" False . [18 23] swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" False [18 23] . swaack first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 [False] . first [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 False . [0] [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 False [0] . [[[<>] [pop !-] ||] nullary] branch [_spn_E] [_spn_T] branch\n",
" 23 18 False [0] [[[<>] [pop !-] ||] nullary] . branch [_spn_E] [_spn_T] branch\n",
" 23 18 . 0 [_spn_E] [_spn_T] branch\n",
" 23 18 0 . [_spn_E] [_spn_T] branch\n",
" 23 18 0 [_spn_E] . [_spn_T] branch\n",
" 23 18 0 [_spn_E] [_spn_T] . branch\n",
" 23 18 . _spn_E\n",
" 23 18 . [pop !-] [--] [++] ifte\n",
" 23 18 [pop !-] . [--] [++] ifte\n",
" 23 18 [pop !-] [--] . [++] ifte\n",
" 23 18 [pop !-] [--] [++] . ifte\n",
" 23 18 [pop !-] [--] [++] . [nullary not] dipd branch\n",
" 23 18 [pop !-] [--] [++] [nullary not] . dipd branch\n",
" 23 18 [pop !-] . nullary not [--] [++] branch\n",
" 23 18 [pop !-] . [stack] dinfrirst not [--] [++] branch\n",
" 23 18 [pop !-] [stack] . dinfrirst not [--] [++] branch\n",
" 23 18 [pop !-] [stack] . dip infra first not [--] [++] branch\n",
" 23 18 . stack [pop !-] infra first not [--] [++] branch\n",
" 23 18 [18 23] . [pop !-] infra first not [--] [++] branch\n",
" 23 18 [18 23] [pop !-] . infra first not [--] [++] branch\n",
" 23 18 . pop !- [18 23] swaack first not [--] [++] branch\n",
" 23 . !- [18 23] swaack first not [--] [++] branch\n",
" 23 . 0 >= [18 23] swaack first not [--] [++] branch\n",
" 23 0 . >= [18 23] swaack first not [--] [++] branch\n",
" True . [18 23] swaack first not [--] [++] branch\n",
" True [18 23] . swaack first not [--] [++] branch\n",
" 23 18 [True] . first not [--] [++] branch\n",
" 23 18 True . not [--] [++] branch\n",
" 23 18 False . [--] [++] branch\n",
" 23 18 False [--] . [++] branch\n",
" 23 18 False [--] [++] . branch\n",
" 23 18 . --\n",
" 23 17 . \n"
]
}
],
"source": [
"V('23 18 spiral_next')"
]
}
],
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