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Point Doubling (P -> 2P) The Base Point of the Secp256k1 Elliptic Curve

as a learning exercise, I am trying to code the first point doubling (Base point P -> 2P) for the Secp256k1 Elliptic Curve. I am using Javascript, and the ethers package for BigNumber. Frustratingly, I am running into a problem where the result I am getting for 2P doesn't appear to lie on the curve. Can someone please help me determine where I am making a mistake?
The coordinates I'm getting as a result are:
X: 0xf1b9e9c77c87bf0ac622382b581826898cfc9232e025d86d904bfd33375faf1a
Y: 0x8162c7b446b54638e9181b71770b2d718e6953a360625a02392097c7db09c608
Which returns false from my isPointOnCurve() method. As a sanity check, I checked the base point in the isPointOnCurve() method, and that returns true (thankfully).
Please see my code below:
const { ethers, BigNumber } = require('ethers');
//variable initialization found from https://en.bitcoin.it/wiki/Secp256k1
bigZero = BigNumber.from(0);
bigTwo = BigNumber.from(2);
bigThree = BigNumber.from(3);
ellipticCurveB = BigNumber.from(7);
generatorPrime = BigNumber.from("0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F");
order = BigNumber.from("0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141");
baseXCoord = BigNumber.from("0x79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798");
baseYCoord = BigNumber.from("0x483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8");
// slope = ( (3*x^2) * (2*y)^-1 ) mod order
// 2Px = slope^2 - 2*baseXCoord
// 2Py = slope * ( 2Px - baseXCoord ) - baseYCoord
m = (bigThree.mul(baseXCoord.pow(bigTwo)).mul(modinv(bigTwo.mul(baseYCoord), order))).mod(order);
TwoPx = (m.pow(bigTwo).sub(bigTwo.mul(baseXCoord))).mod(order);
TwoPy = ((m.mul(baseXCoord.sub(TwoPx))).sub(baseYCoord)).mod(order);
console.log(TwoPx);
console.log(TwoPy);
console.log(isPointOnCurve(TwoPx, TwoPy));
// Helper Functions:
// Check if point is on Curve, Calculate extended GCD, modular inverse
function isPointOnCurve(x,y){
b = ellipticCurveB;
p = generatorPrime;
rem = (y.pow(bigTwo).sub(x.pow(bigThree)).sub(b)).mod(p);
return rem.eq(bigZero);
}
function egcd(a, b) {
var s = BigNumber.from(0), t = BigNumber.from(1), u = BigNumber.from(1), v = BigNumber.from(0);
while (!a.eq(BigNumber.from(0))) {
var q = b.div(a) | BigNumber.from(0), r = b.mod(a);
var m = s.sub(u.mul(q)), n = t.sub(v.mul(q));
b = a;
a = r;
s = u;
t = v;
u = m;
v = n;
}
return [b, s, t];
}
function mod(x, y) {
return (x.mod(y).add(y)).mod(y);
}
function modinv(x, y) {
var tuple = egcd(x.mod(y), y);
if (!tuple[0].eq(BigNumber.from(1))) {
return null;
}
return mod(tuple[1], y);
}

As kelalaka pointed out in a comment on the original post, I was confusing the the order of the group and the finite field Fp. I was getting values modulo the Group Order, when I should've been using the values modulo prime p used to define the finite field.
The new and correct result I get is:
X: 0xc6047f9441ed7d6d3045406e95c07cd85c778e4b8cef3ca7abac09b95c709ee5
Y: 0x1ae168fea63dc339a3c58419466ceaeef7f632653266d0e1236431a950cfe52a
If anyone would like to use this code, I've updated it to be correct, and cleaned it up to make it a little more readable:
bigZero = BigNumber.from(0);
bigTwo = BigNumber.from(2);
bigThree = BigNumber.from(3);
ellipticCurveB = BigNumber.from(7);
generatorPrime = BigNumber.from("0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F");
baseXCoord = BigNumber.from("0x79BE667EF9DCBBAC55A06295CE870B07029BFCDB2DCE28D959F2815B16F81798");
baseYCoord = BigNumber.from("0x483ADA7726A3C4655DA4FBFC0E1108A8FD17B448A68554199C47D08FFB10D4B8");
// slope = ( (3*x^2) * (2*y)^-1 ) mod order
threeXSquared = bigThree.mul(baseXCoord.pow(bigTwo));
modInv2y = modinv(bigTwo.mul(baseYCoord), generatorPrime);
m = threeXSquared.mul(modInv2y).mod(generatorPrime);
// 2Px = slope^2 - 2*baseXCoord
mSquared = m.pow(bigTwo);
twoXbase = bigTwo.mul(baseXCoord);
TwoPx = (mSquared.sub(twoXbase)).mod(generatorPrime);
// 2Py = slope * ( 2Px - baseXCoord ) - baseYCoord
pointSlopeX = m.mul(baseXCoord.sub(TwoPx));
TwoPy = (pointSlopeX).sub(baseYCoord).mod(generatorPrime);
console.log(TwoPx);
console.log(TwoPy);
console.log(isPointOnCurve(TwoPx, TwoPy));
// Helper Functions:
// Check if point is on Curve, Calculate extended GCD, modular inverse
function isPointOnCurve(x,y){
b = ellipticCurveB;
p = generatorPrime;
rem = (y.pow(bigTwo).sub(x.pow(bigThree)).sub(b)).mod(p);
return rem.eq(bigZero);
}
function egcd(a, b) {
var s = BigNumber.from(0), t = BigNumber.from(1), u = BigNumber.from(1), v = BigNumber.from(0);
while (!a.eq(BigNumber.from(0))) {
var q = b.div(a) | BigNumber.from(0), r = b.mod(a);
var m = s.sub(u.mul(q)), n = t.sub(v.mul(q));
b = a;
a = r;
s = u;
t = v;
u = m;
v = n;
}
return [b, s, t];
}
function modulus(x, y) {
return (x.mod(y).add(y)).mod(y);
}
function modinv(x, y) {
var tuple = egcd(x.mod(y), y);
if (!tuple[0].eq(BigNumber.from(1))) {
return null;
}
return modulus(tuple[1], y);
}

JavaScript how to transform String into a function

I am doing a calculator for definite integrals, where I can add the function to integrate by using this interface.Calculator interface
The point is that once I call the function to integrate it recognizes it as a String and i cannot calculate over it. But if I add the function hardcoding it, it works perfectly fine.
prueba() {
//**** this.cadena is the function I add by using the interface
var sal = this.cadena;
console.log(sal);
var f = function (x) {
//**** The comented return works perfecly fine
//return 1/x;
return parseFloat(sal);
};
var integrada = this.integ(sal, 1, 2);
this.cadena = integrada;
this.imprime();
}
integ(f, a, b) {
var area = 0;
var dx = 0.001;
for (let x = a; x < b; x += dx) {
area += dx * f(x);
}
return area;
}

Can you try below line
var integrada = this.integ(parseFloat(sal), 1, 2);

In javascript, after i pass a non null object to a funtion it says the object is null

i was trying to implement the A* algorithm and followed the wikipedia pseudo code to make this.
when i pass a predefined object pixel to the a funtion getG() it says that the object is null. I'm sorry if i am not pointing to a specific problem but i am not even sure how to really specify the problem by name. i have tried commenting out the code to increase readability.
git repository link of the whole project - https://github.com/NirobNabil/WhirlWind
(things are a little messy here because i didn't use github at first and i uploaded it just a little ago for posting the problem)
[ i'm actually making this to use a* to find path for my bot which is powered by arduino. thats why i'm using involt. ]
here goes the code,
$(function() {
// define the height, width and bot size in centemeter
total_width = 200;
total_height = 200;
bot_size = 20;
total_box = (total_height / bot_size) * (total_width / bot_size);
box_in_x = total_width / bot_size;
box_in_y = total_height / bot_size;
//populating the pixels array
populate(total_width / bot_size, total_height / bot_size, "UND");
pathfind(pixels, pixels[13], pixels[pixels.length - 1]);
})
var pixels = []; //an array to hold all the objects(the grid)
var obstacles = []; //array to hold the obstacles
function pixel(X, Y, obs) {
this.X_co_ordinate = X;
this.Y_co_ordinate = Y;
this.state = obs; //availale states OPN, UND, OBS, DIS, NULL
this.g = 0;
this.h = 0;
this.f = 0;
this.last = null;
} //every block in the grid is a pixel
//01719372596
function populate(height, width, obs_val = "UND") {
pixels[0] = new pixel(0, 10, obs_val);
for (h = height, i = 0; h >= 0; h--) {
for (w = 0; w < width; w++, i++) {
var temp_obs = new pixel(w, h, obs_val);
temp_obs.last = pixels[0];
pixels[i] = temp_obs; //saving temp_pixel object to pixels array
}
}
} //populating the grid AKA pixels with pixel objects or blocks
// this funtion is where the problem shows up
function getG(current, start) {
let g = 1;
while (current != start && current.last != start && current) {
current = current.last;
g++;
}
return g;
} //get the g val(cost to come to this pixel from the start) of the current pixel
function getH(current, end) {
let I = Math.abs(current.X_co_ordinate - end.X_co_ordinate) + Math.abs(current.Y_co_ordinate - end.Y_co_ordinate);
return I;
} //get the h val(heuristic) of the current pixel
function getF(start, current, end) {
let G = getG(current, start);
let H = getH(current, end);
return G + H;
} //get the f val(total) of the current pixel
function lowFinArray(arr, start, end) {
// here arr is the grid/pixel
let current_low = arr[0];
for (let i = 0; i < arr.length; i++) {
let getF1 = getF(start, current_low, end);
let getF2 = getF(start, arr[i], end);
if (getF1 < getF2) {
current_low = arr[i];
}
}
console.log("current low");
console.log(current_low);
return current_low;
}
function getneighbours(grid, current) {
let neightbours = [];
neightbours.push(grid[getIndex(current.X_co_ordinate - 1, current.Y_co_ordinate)]);
neightbours.push(grid[getIndex(current.X_co_ordinate + 1, current.Y_co_ordinate)]);
neightbours.push(grid[getIndex(current.X_co_ordinate, current.Y_co_ordinate - 1)]);
neightbours.push(grid[getIndex(current.X_co_ordinate, current.Y_co_ordinate + 1)]);
/*
for(i=0; i<neightbours.length; i++){
neightbours[i].last = current;
}*/
console.log("neightbours");
console.log(neightbours);
return neightbours;
} //get the neighbour pixels of the current pixel
//main algo
function pathfind(grid, start, end) {
let closedSet = [];
let openSet = [];
openSet.push(start);
let current = start;
//trying to debug
console.log("low F in arr");
console.log(lowFinArray(grid, start, end));
console.log(start);
console.log(current);
console.log(end);
console.log(grid);
let x = 0;
while (openSet.length > 0) {
//trying to debug
console.log("executed " + (x++));
console.log("openset");
console.log(openSet);
current = lowFinArray(grid, start, end); //assigning the pixel with lowest f val to current
console.log("current");
console.log(current);
if (current == end) {
console.log(getPath(current));
}
let neighbours = getneighbours(grid, current);
for (let i = 0; i < neighbours.length; i++) {
let neighbour = neighbours[i];
if (closedSet.includes(neighbour)) {
continue;
}
if (!openSet.includes(neighbours)) {
openSet.push(neighbours);
}
//console.log(current);
let getg = getG(current, start);
let geth = getH(current, end);
//console.log(getg);
let tGscore = getg + geth; //here getH is being used as a distance funtion
if (tGscore >= getg) {
continue;
}
neighbour.last = current;
neighbour.g = tGscore;
neighbour.f = getF(neighbour);
}
if (x > 10) {
return 0;
}; //the loop was running forever so i tried this to stop the loop after 10 iterations
}
}
function getPath(current) {
let path = [current];
while (current.last != null) {
path.push(current.last);
}
return path;
}
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
and here is what the console says,
Uncaught TypeError: Cannot read property 'last' of null
at getG (app.js:226)
at getF (app.js:241)
at lowFinArray (app.js:249)
at pathfind (app.js:292)
at HTMLDocument.<anonymous> (app.js:92)
at mightThrow (jquery-3.1.1.js:3570)
at process (jquery-3.1.1.js:3638)

You're doing your checks in the wrong order:
while (current != start && current.last != start && current) {
There's no point in using && current after you've already used current.last.
Perhaps changing the order would solve the problem. It will at least get rid of your current error:
while (current && current != start && current.last != start) {
Regarding the title of this question:
In javascript, after i pass a non null object to a funtion it says the object is null
It may very well be non-null 100% of the time you pass it into the function, but you are repeatedly overwriting its parameter within the function, so all bets are off.

Canvas drawing using query/angular

Here's what I'm trying to achieve. Draw a circle(first circle) on the screen on a mouse click. Then draw successive circles on successive mouse clicks and connect each to the first circle.
I've managed to get till here.
Now the task is if any of the circles have the same y-coordinate as the first one, the connection is a straight line, else it should be a s-curve/inverted s-curve depending on whether the next circle is above or below the first one based on its y-axis.
It may be assumed that all successive circle will be on the right of the first circle.
Here's my code
var app = angular.module('plunker', []);
app.controller('MainController', function($scope) {
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
function reOffset(){
var BB=canvas.getBoundingClientRect();
offsetX=BB.left;
offsetY=BB.top;
}
var offsetX,offsetY;
reOffset();
window.onscroll=function(e){ reOffset(); }
var isDown=false;
var startX,startY;
var radius=10;
var lastX,lastY;
ctx.fillStyle='red';
$("#canvas").mousedown(function(e){handleMouseDown(e);});
function drawCircle(cx,cy){
if(lastX){
ctx.globalCompositeOperation='destination-over';
ctx.beginPath();
ctx.moveTo(lastX,lastY);
ctx.lineTo(cx,cy);
ctx.stroke();
ctx.globalCompositeOperation='source-over';
}else{
lastX=cx;
lastY=cy;
}
ctx.beginPath();
ctx.arc(cx,cy,radius,0,Math.PI*2);
ctx.closePath();
ctx.fill();
}
function handleMouseDown(e){
// tell the browser we're handling this event
e.preventDefault();
e.stopPropagation();
mx=parseInt(e.clientX-offsetX);
my=parseInt(e.clientY-offsetY);
drawCircle(mx,my);
}
});
Here's a link to the plunk that will demonstrate the behavior
http://plnkr.co/edit/rYVLgB14IutNh1F4MN6T?p=preview
Any help appreciated.

I don't know exactly which kind of s-curve are you interested in. As I understand it will be always only two points to connect, the first point and the rest, and you are looking for some sort of quadratic curve to do so. Under this situation you can build a s-curve by joining two ctx.quadraticCurveTo calls.
ctx.beginPath();
ctx.moveTo(lastX,lastY);
ctx.quadraticCurveTo(
(lastX+cx)/2, lastY,
(lastX+cx)/2, (lastY+cy)/2
);
ctx.quadraticCurveTo(
(lastX+cx)/2, cy,
cx, cy
);
ctx.lineWidth = 3;
http://plnkr.co/edit/t10cMPcUtX5ifkWi2LBF?p=preview

To make each of your connectors avoid existing circles, you must use a pathfinding algorithm (A* for example).
Pathfinding algorithms will give you a set of points from Circle1 to Circle2 that avoid all other circles.
Then you can use that set of points to build a connector between those circles using a Spline. See this very good answer by Stackoverflow's Ken Fyrstenberg on how to draw a spline. Make sure you keep the tension on the spline tight (closer to zero) so that your spline connector doesn't stray too far from the unobstructed path:
how to draw smooth curve through N points using javascript HTML5 canvas?
This is a nice script implementing the A* algorithm by Brian Grinstead:
https://github.com/bgrins/javascript-astar/
And here's a Demo of Brian Grinstead's A* script:
http://www.briangrinstead.com/files/astar/
To avoid a link-only answer, I'm attaching Brian's script from GitHub below...
But seriously...if GitHub disappears many of us subscribers are in trouble!
// javascript-astar 0.4.0
// http://github.com/bgrins/javascript-astar
// Freely distributable under the MIT License.
// Implements the astar search algorithm in javascript using a Binary Heap.
// Includes Binary Heap (with modifications) from Marijn Haverbeke.
// http://eloquentjavascript.net/appendix2.html
(function(definition) {
/* global module, define */
if(typeof module === 'object' && typeof module.exports === 'object') {
module.exports = definition();
} else if(typeof define === 'function' && define.amd) {
define([], definition);
} else {
var exports = definition();
window.astar = exports.astar;
window.Graph = exports.Graph;
}
})(function() {
function pathTo(node){
var curr = node,
path = [];
while(curr.parent) {
path.push(curr);
curr = curr.parent;
}
return path.reverse();
}
function getHeap() {
return new BinaryHeap(function(node) {
return node.f;
});
}
var astar = {
/**
* Perform an A* Search on a graph given a start and end node.
* #param {Graph} graph
* #param {GridNode} start
* #param {GridNode} end
* #param {Object} [options]
* #param {bool} [options.closest] Specifies whether to return the
path to the closest node if the target is unreachable.
* #param {Function} [options.heuristic] Heuristic function (see
* astar.heuristics).
*/
search: function(graph, start, end, options) {
graph.cleanDirty();
options = options || {};
var heuristic = options.heuristic || astar.heuristics.manhattan,
closest = options.closest || false;
var openHeap = getHeap(),
closestNode = start; // set the start node to be the closest if required
start.h = heuristic(start, end);
openHeap.push(start);
while(openHeap.size() > 0) {
// Grab the lowest f(x) to process next. Heap keeps this sorted for us.
var currentNode = openHeap.pop();
// End case -- result has been found, return the traced path.
if(currentNode === end) {
return pathTo(currentNode);
}
// Normal case -- move currentNode from open to closed, process each of its neighbors.
currentNode.closed = true;
// Find all neighbors for the current node.
var neighbors = graph.neighbors(currentNode);
for (var i = 0, il = neighbors.length; i < il; ++i) {
var neighbor = neighbors[i];
if (neighbor.closed || neighbor.isWall()) {
// Not a valid node to process, skip to next neighbor.
continue;
}
// The g score is the shortest distance from start to current node.
// We need to check if the path we have arrived at this neighbor is the shortest one we have seen yet.
var gScore = currentNode.g + neighbor.getCost(currentNode),
beenVisited = neighbor.visited;
if (!beenVisited || gScore < neighbor.g) {
// Found an optimal (so far) path to this node. Take score for node to see how good it is.
neighbor.visited = true;
neighbor.parent = currentNode;
neighbor.h = neighbor.h || heuristic(neighbor, end);
neighbor.g = gScore;
neighbor.f = neighbor.g + neighbor.h;
graph.markDirty(neighbor);
if (closest) {
// If the neighbour is closer than the current closestNode or if it's equally close but has
// a cheaper path than the current closest node then it becomes the closest node
if (neighbor.h < closestNode.h || (neighbor.h === closestNode.h && neighbor.g < closestNode.g)) {
closestNode = neighbor;
}
}
if (!beenVisited) {
// Pushing to heap will put it in proper place based on the 'f' value.
openHeap.push(neighbor);
}
else {
// Already seen the node, but since it has been rescored we need to reorder it in the heap
openHeap.rescoreElement(neighbor);
}
}
}
}
if (closest) {
return pathTo(closestNode);
}
// No result was found - empty array signifies failure to find path.
return [];
},
// See list of heuristics: http://theory.stanford.edu/~amitp/GameProgramming/Heuristics.html
heuristics: {
manhattan: function(pos0, pos1) {
var d1 = Math.abs(pos1.x - pos0.x);
var d2 = Math.abs(pos1.y - pos0.y);
return d1 + d2;
},
diagonal: function(pos0, pos1) {
var D = 1;
var D2 = Math.sqrt(2);
var d1 = Math.abs(pos1.x - pos0.x);
var d2 = Math.abs(pos1.y - pos0.y);
return (D * (d1 + d2)) + ((D2 - (2 * D)) * Math.min(d1, d2));
}
},
cleanNode:function(node){
node.f = 0;
node.g = 0;
node.h = 0;
node.visited = false;
node.closed = false;
node.parent = null;
}
};
/**
* A graph memory structure
* #param {Array} gridIn 2D array of input weights
* #param {Object} [options]
* #param {bool} [options.diagonal] Specifies whether diagonal moves are allowed
*/
function Graph(gridIn, options) {
options = options || {};
this.nodes = [];
this.diagonal = !!options.diagonal;
this.grid = [];
for (var x = 0; x < gridIn.length; x++) {
this.grid[x] = [];
for (var y = 0, row = gridIn[x]; y < row.length; y++) {
var node = new GridNode(x, y, row[y]);
this.grid[x][y] = node;
this.nodes.push(node);
}
}
this.init();
}
Graph.prototype.init = function() {
this.dirtyNodes = [];
for (var i = 0; i < this.nodes.length; i++) {
astar.cleanNode(this.nodes[i]);
}
};
Graph.prototype.cleanDirty = function() {
for (var i = 0; i < this.dirtyNodes.length; i++) {
astar.cleanNode(this.dirtyNodes[i]);
}
this.dirtyNodes = [];
};
Graph.prototype.markDirty = function(node) {
this.dirtyNodes.push(node);
};
Graph.prototype.neighbors = function(node) {
var ret = [],
x = node.x,
y = node.y,
grid = this.grid;
// West
if(grid[x-1] && grid[x-1][y]) {
ret.push(grid[x-1][y]);
}
// East
if(grid[x+1] && grid[x+1][y]) {
ret.push(grid[x+1][y]);
}
// South
if(grid[x] && grid[x][y-1]) {
ret.push(grid[x][y-1]);
}
// North
if(grid[x] && grid[x][y+1]) {
ret.push(grid[x][y+1]);
}
if (this.diagonal) {
// Southwest
if(grid[x-1] && grid[x-1][y-1]) {
ret.push(grid[x-1][y-1]);
}
// Southeast
if(grid[x+1] && grid[x+1][y-1]) {
ret.push(grid[x+1][y-1]);
}
// Northwest
if(grid[x-1] && grid[x-1][y+1]) {
ret.push(grid[x-1][y+1]);
}
// Northeast
if(grid[x+1] && grid[x+1][y+1]) {
ret.push(grid[x+1][y+1]);
}
}
return ret;
};
Graph.prototype.toString = function() {
var graphString = [],
nodes = this.grid, // when using grid
rowDebug, row, y, l;
for (var x = 0, len = nodes.length; x < len; x++) {
rowDebug = [];
row = nodes[x];
for (y = 0, l = row.length; y < l; y++) {
rowDebug.push(row[y].weight);
}
graphString.push(rowDebug.join(" "));
}
return graphString.join("\n");
};
function GridNode(x, y, weight) {
this.x = x;
this.y = y;
this.weight = weight;
}
GridNode.prototype.toString = function() {
return "[" + this.x + " " + this.y + "]";
};
GridNode.prototype.getCost = function(fromNeighbor) {
// Take diagonal weight into consideration.
if (fromNeighbor && fromNeighbor.x != this.x && fromNeighbor.y != this.y) {
return this.weight * 1.41421;
}
return this.weight;
};
GridNode.prototype.isWall = function() {
return this.weight === 0;
};
function BinaryHeap(scoreFunction){
this.content = [];
this.scoreFunction = scoreFunction;
}
BinaryHeap.prototype = {
push: function(element) {
// Add the new element to the end of the array.
this.content.push(element);
// Allow it to sink down.
this.sinkDown(this.content.length - 1);
},
pop: function() {
// Store the first element so we can return it later.
var result = this.content[0];
// Get the element at the end of the array.
var end = this.content.pop();
// If there are any elements left, put the end element at the
// start, and let it bubble up.
if (this.content.length > 0) {
this.content[0] = end;
this.bubbleUp(0);
}
return result;
},
remove: function(node) {
var i = this.content.indexOf(node);
// When it is found, the process seen in 'pop' is repeated
// to fill up the hole.
var end = this.content.pop();
if (i !== this.content.length - 1) {
this.content[i] = end;
if (this.scoreFunction(end) < this.scoreFunction(node)) {
this.sinkDown(i);
}
else {
this.bubbleUp(i);
}
}
},
size: function() {
return this.content.length;
},
rescoreElement: function(node) {
this.sinkDown(this.content.indexOf(node));
},
sinkDown: function(n) {
// Fetch the element that has to be sunk.
var element = this.content[n];
// When at 0, an element can not sink any further.
while (n > 0) {
// Compute the parent element's index, and fetch it.
var parentN = ((n + 1) >> 1) - 1,
parent = this.content[parentN];
// Swap the elements if the parent is greater.
if (this.scoreFunction(element) < this.scoreFunction(parent)) {
this.content[parentN] = element;
this.content[n] = parent;
// Update 'n' to continue at the new position.
n = parentN;
}
// Found a parent that is less, no need to sink any further.
else {
break;
}
}
},
bubbleUp: function(n) {
// Look up the target element and its score.
var length = this.content.length,
element = this.content[n],
elemScore = this.scoreFunction(element);
while(true) {
// Compute the indices of the child elements.
var child2N = (n + 1) << 1,
child1N = child2N - 1;
// This is used to store the new position of the element, if any.
var swap = null,
child1Score;
// If the first child exists (is inside the array)...
if (child1N < length) {
// Look it up and compute its score.
var child1 = this.content[child1N];
child1Score = this.scoreFunction(child1);
// If the score is less than our element's, we need to swap.
if (child1Score < elemScore){
swap = child1N;
}
}
// Do the same checks for the other child.
if (child2N < length) {
var child2 = this.content[child2N],
child2Score = this.scoreFunction(child2);
if (child2Score < (swap === null ? elemScore : child1Score)) {
swap = child2N;
}
}
// If the element needs to be moved, swap it, and continue.
if (swap !== null) {
this.content[n] = this.content[swap];
this.content[swap] = element;
n = swap;
}
// Otherwise, we are done.
else {
break;
}
}
}
};
return {
astar: astar,
Graph: Graph
};
});

How to parse pure functions

Let's say you have the following function
var action = (function () {
var a = 42;
var b = 2;
function action(c) {
return a + 4 * b + c;
}
return action;
}());
// how would you parse action into it's serialized LISP / AST format?
var parsed = parse(action);
Is it possible to have a function that takes a reference to the function action and outputs say the LISP format (lambda (c) (plus (plus 42 (multiply 4 2)) c))
We're allowed to put some restrictions on what action can be.
the body should only be a single expression
it should be a pure function
any free variables are constants
The main question is given a function you can invoke with a range of inputs and it's source code can you discover the correct value to substitute the free variables with?
For the above example you know that a and b are constant and you could intellectually plot the output for a few values and see the pattern and then just know what the constants are.
Question:
How would you write a function that takes a function reference and it's source code and produces some form of AST for the function with any free variables substituted for their run-time values.
An example of an AST format would be the LISP equivalent of the code.
I basically want to serialize and deserialize the function and have it behave the same
It should be noted that the problem becomes trivial if you pass { a: a, b: b } to the analysis function. That would be cheating.
Use-case:
I want to generate a language agnostic form of a pure JavaScript function so I can effectively pass it to C++ without requiring the user of my library to use a DSL to create this function
Let's imagine you had a database driver
var cursor = db.table("my-table").map(function (row) {
return ["foo", row.foo]
})
You want to determine at run-time what the function is and convert it into an AST format so that you can use your efficient query builder to convert it into SQL or whatever query engine your database has.
This means you don't have to write:
var cursor = db.table("my-table").map(function (rowQueryObject) {
return db.createArray(db.StringConstant("foo"), rowQueryObject.getProperty("foo"))
})
Which is a function the DB library can execute with a query object and have you build the query object transformation without verbose methods.

Here is a full solution (using catalog of variables which is accessible by the parse function):
var CONSTANTS = {
a: 42,
b: 2,
c: 4
};
function test() {
return a + 4 * b + c;
}
function getReturnStatement(func) {
var funcStr = func.toString();
return (/return\s+(.*?);/g).exec(funcStr)[1];
}
function replaceVariables(expr) {
var current = '';
for (var i = 0; i < expr.length; i += 1) {
while (/[a-zA-Z_$]/.test(expr[i]) && i < expr.length) {
current += expr[i];
i += 1;
}
if (isNumber(CONSTANTS[current])) {
expr = expr.replace(current, CONSTANTS[current]);
}
current = '';
}
return expr;
}
function isNumber(arg) {
return !isNaN(parseInt(arg, 10));
}
function tokenize(expr) {
var tokens = [];
for (var i = 0; i < expr.length; i += 1) {
if (isWhitespace(expr[i])) {
continue;
} else if (isOperator(expr[i])) {
tokens.push({
type: 'operator',
value: expr[i]
});
} else if (isParentheses(expr[i])) {
tokens.push({
type: 'parant',
value: expr[i]
});
} else {
var num = '';
while (isNumber(expr[i]) && i < expr.length) {
num += expr[i];
i += 1;
}
i -= 1;
tokens.push({
type: 'number',
value: parseInt(num, 10)
});
}
}
return tokens;
}
function toPrefix(tokens) {
var operandStack = [],
operatorStack = [],
current,
top = function (stack) {
if (stack) {
return stack[stack.length - 1];
}
return undefined;
};
while (tokens.length) {
current = tokens.pop();
if (current.type === 'number') {
operandStack.push(current);
} else if (current.value === '(' ||
!operatorStack.length ||
(getPrecendence(current.value) >
getPrecendence(top(operatorStack).value))) {
operatorStack.push(current);
} else if (current.value === ')') {
while (top(operatorStack).value !== '(') {
var tempOperator = operatorStack.pop(),
right = operandStack.pop(),
left = operandStack.pop();
operandStack.push(tempOperator, left, right);
}
operatorStack.pop();
} else if (getPrecendence(current.value) <=
getPrecendence(top(operatorStack).value)) {
while (operatorStack.length &&
getPrecendence(current.value) <=
getPrecendence(top(operatorStack).value)) {
tempOperator = operatorStack.pop();
right = operandStack.pop();
left = operandStack.pop();
operandStack.push(tempOperator, left, right);
}
}
}
while (operatorStack.length) {
tempOperator = operatorStack.pop();
right = operandStack.pop();
left = operandStack.pop();
operandStack.push(tempOperator, left, right);
}
return operandStack;
}
function isWhitespace(arg) {
return (/^\s$/).test(arg);
}
function isOperator(arg) {
return (/^[*+\/-]$/).test(arg);
}
function isParentheses(arg) {
return (/^[)(]$/).test(arg);
}
function getPrecendence(operator) {
console.log(operator);
switch (operator) {
case '*':
return 4;
case '/':
return 4;
case '+':
return 2;
case '-':
return 2;
default:
return undefined;
}
}
function getLispString(tokens) {
var result = '';
tokens.forEach(function (e) {
if (e)
switch (e.type) {
case 'number':
result += e.value;
break;
case 'parant':
result += e.value;
break;
case 'operator':
result += getOperator(e.value);
break;
default:
break;
}
result += ' ';
});
return result;
}
function getOperator(operator) {
switch (operator) {
case '+':
return 'plus';
case '*':
return 'multiplicate';
case '-':
return 'minus';
case '\\':
return 'divide';
default:
return undefined;
}
}
var res = getReturnStatement(test);
console.log(res);
res = replaceVariables(res);
console.log(res);
var tokens = tokenize(res);
console.log(tokens);
var prefix = toPrefix(tokens);
console.log(prefix);
console.log(getLispString(prefix));
I just wrote it so there might be some problems in the style but I think that the idea is clear.
You can get the function body by using the .toString method. After that you can use regular expression to match the return statement
(/return\s+(.*?);/g).exec(funcStr)[1];
Note that here you must use semicolons for successful match! In the next step all variables are transformed to number values using the CONSTANTS object (I see that you have some parameters left so you may need little modifications here). After that the string is being tokenized, for easier parsing. In next step the infix expression is transformed into a prefix one. At the last step I build a string which will make the output looks like what you need (+ - plus, - - minus and so on).

Since I'm not sure you're able to get the method's body after having invoked it, here is an alternative solution:
var a = 42;
var b = 2;
function action(c) {
return a + 4 * b + c;
}
/**
* get the given func body
* after having replaced any available var from the given scope
* by its *real* value
*/
function getFunctionBody(func, scope) {
// get the method body
var body = func.toString().replace(/^.*?{\s*((.|[\r\n])*?)\s*}.*?$/igm, "$1");
var matches = body.match(/[a-z][a-z0-9]*/igm);
// for each potential var
for(var i=0; i<matches.length; i++) {
var potentialVar = matches[i];
var scopedValue = scope[potentialVar];
// if the given scope has the var defined
if(typeof scopedValue !== "undefined") {
// add "..." for strings
if(typeof scopedValue === "string") {
scopedValue = '"' + scopedValue + '"';
}
// replace the var by its scoped value
var regex = new RegExp("([^a-z0-9]+|^)" + potentialVar + "([^a-z0-9]+|$)", "igm");
var replacement = "$1" + scopedValue + "$2";
body = body.replace(regex, replacement);
}
}
return body;
}
// calling
var actionBody = getFunctionBody(action, this);
// log
alert(actionBody);
Prints:
return 42 + 4 * 2 + c;
DEMO
You would then have to implement your own function toLISP(body) or any function else you may need.
Note that it won't work for complex scoped variables such as var a = {foo: "bar"}.