I want a query to get all possible paths that start from a given vertex.
For exa. as in below image,
I want to find all paths that start from "Covid/12109" with Query.
So it returns like this
{ "_from":"Covid/12109","_to":"Covid/12110" }
{ "_from":"Covid/12110","_to":"Covid/12111" }
{ "_from":"Covid/12110","_to":"Covid/12115" }
{ "_from":"Covid/12110","_to":"Covid/12114" }
{ "_from":"Covid/12111","_to":"Covid/12115" }
{ "_from":"Covid/12111","_to":"Covid/12114" }
{ "_from":"Covid/12112","_to":"Covid/12110" }
{ "_from":"Covid/12112","_to":"Covid/12113" }
{ "_from":"Covid/12112","_to":"Covid/12114" }
And if i want to start from "Covid/12110" then it should return like this
{ "_from":"Covid/12110","_to":"Covid/12111" }
{ "_from":"Covid/12110","_to":"Covid/12115" }
{ "_from":"Covid/12110","_to":"Covid/12114" }
{ "_from":"Covid/12111","_to":"Covid/12115" }
{ "_from":"Covid/12111","_to":"Covid/12114" }
{ "_from":"Covid/12112","_to":"Covid/12110" }
{ "_from":"Covid/12112","_to":"Covid/12113" }
{ "_from":"Covid/12112","_to":"Covid/12114" }
And if i want to start from "Covid/12112" then it should return like this
{ "_from":"Covid/12112","_to":"Covid/12110" }
{ "_from":"Covid/12112","_to":"Covid/12113" }
{ "_from":"Covid/12112","_to":"Covid/12114" }
{ "_from":"Covid/12110","_to":"Covid/12111" }
{ "_from":"Covid/12110","_to":"Covid/12115" }
{ "_from":"Covid/12110","_to":"Covid/12114" }
{ "_from":"Covid/12111","_to":"Covid/12115" }
{ "_from":"Covid/12111","_to":"Covid/12114" }
Graph traversal is your friend here. There are several ways to accomplish this, but you might start by :
FOR c IN Covid
FILTER c._key == '12109'
FOR v,e IN 1..9 OUTBOUND c
`has`
OPTIONS { uniqueVertices: true }
RETURN e
The name of your edge collection ('has') is tricky because HAS is an AQL keyword (see the docs about naming things with keywords). I've enclosed this in backticks (the AQL escape char), but you could also create a named graph, which (I believe) is much more flexible.
Looking at the query:
We first find documents in the "Covid" collection that match a key. This is optional, you could also swap the "c" in the graph traversal with a document id like "Covid/12109"
FOR v,e represents "vertices" v and "edges" e to return
1..9 is the number of traversal "jumps" to perform. This can be any number (2) or range (5..27)
OUTBOUND refers to the path direction to traverse. Other options here are OUTBOUND and ANY
{ uniqueVertices: true } tells the engine to keep track of which vertices it's returned and not duplicate them on output. See the docs here
RETURN e will return edge ("has") documents. RETURN v would return vertex ("Covid") documents.
I have a custom JS iterator implementation and code for measuring performance of the latter implementation:
const ITERATION_END = Symbol('ITERATION_END');
const arrayIterator = (array) => {
let index = 0;
return {
hasValue: true,
next() {
if (index >= array.length) {
this.hasValue = false;
return ITERATION_END;
}
return array[index++];
},
};
};
const customIterator = (valueGetter) => {
return {
hasValue: true,
next() {
const nextValue = valueGetter();
if (nextValue === ITERATION_END) {
this.hasValue = false;
return ITERATION_END;
}
return nextValue;
},
};
};
const map = (iterator, selector) => customIterator(() => {
const value = iterator.next();
return value === ITERATION_END ? value : selector(value);
});
const filter = (iterator, predicate) => customIterator(() => {
if (!iterator.hasValue) {
return ITERATION_END;
}
let currentValue = iterator.next();
while (iterator.hasValue && currentValue !== ITERATION_END && !predicate(currentValue)) {
currentValue = iterator.next();
}
return currentValue;
});
const toArray = (iterator) => {
const array = [];
while (iterator.hasValue) {
const value = iterator.next();
if (value !== ITERATION_END) {
array.push(value);
}
}
return array;
};
const test = (fn, iterations) => {
const times = [];
for (let i = 0; i < iterations; i++) {
const start = performance.now();
fn();
times.push(performance.now() - start);
}
console.log(times);
console.log(times.reduce((sum, x) => sum + x, 0) / times.length);
}
const createData = () => Array.from({ length: 9000000 }, (_, i) => i + 1);
const testIterator = (data) => () => toArray(map(filter(arrayIterator(data), x => x % 2 === 0), x => x * 2))
test(testIterator(createData()), 10);
The output of the test function is very weird and unexpected - the first test run is constantly executed two times faster than all the other runs. One of the results, where the array contains all execution times and the number is the mean (I ran it on Node):
[
147.9088459983468,
396.3472499996424,
374.82447600364685,
367.74555300176144,
363.6300039961934,
362.44370299577713,
363.8418449983001,
390.86111199855804,
360.23125199973583,
358.4788999930024
]
348.6312940984964
Similar results can be observed using Deno runtime, however I could not reproduce this behaviour on other JS engines. What can be the reason behind it on the V8?
Environment:
Node v13.8.0, V8 v7.9.317.25-node.28,
Deno v1.3.3, V8 v8.6.334
(V8 developer here.) In short: it's inlining, or lack thereof, as decided by engine heuristics.
For an optimizing compiler, inlining a called function can have significant benefits (e.g.: avoids the call overhead, sometimes makes constant folding possible, or elimination of duplicate computations, sometimes even creates new opportunities for additional inlining), but comes at a cost: it makes the compilation itself slower, and it increases the risk of having to throw away the optimized code ("deoptimize") later due to some assumption that turns out not to hold. Inlining nothing would waste performance, inlining everything would waste performance, inlining exactly the right functions would require being able to predict the future behavior of the program, which is obviously impossible. So compilers use heuristics.
V8's optimizing compiler currently has a heuristic to inline functions only if it was always the same function that was called at a particular place. In this case, that's the case for the first iterations. Subsequent iterations then create new closures as callbacks, which from V8's point of view are new functions, so they don't get inlined. (V8 actually knows some advanced tricks that allow it to de-duplicate function instances coming from the same source in some cases and inline them anyway; but in this case those are not applicable [I'm not sure why]).
So in the first iteration, everything (including x => x % 2 === 0 and x => x * 2) gets inlined into toArray. From the second iteration onwards, that's no longer the case, and instead the generated code performs actual function calls.
That's probably fine; I would guess that in most real applications, the difference is barely measurable. (Reduced test cases tend to make such differences stand out more; but changing the design of a larger app based on observations made on a small test is often not the most impactful way to spend your time, and at worst can make things worse.)
Also, hand-optimizing code for engines/compilers is a difficult balance. I would generally recommend not to do that (because engines improve over time, and it really is their job to make your code fast); on the other hand, there clearly is more efficient code and less efficient code, and for maximum overall efficiency, everyone involved needs to do their part, i.e. you might as well make the engine's job simpler when you can.
If you do want to fine-tune performance of this, you can do so by separating code and data, thereby making sure that always the same functions get called. For example like this modified version of your code:
const ITERATION_END = Symbol('ITERATION_END');
class ArrayIterator {
constructor(array) {
this.array = array;
this.index = 0;
}
next() {
if (this.index >= this.array.length) return ITERATION_END;
return this.array[this.index++];
}
}
function arrayIterator(array) {
return new ArrayIterator(array);
}
class MapIterator {
constructor(source, modifier) {
this.source = source;
this.modifier = modifier;
}
next() {
const value = this.source.next();
return value === ITERATION_END ? value : this.modifier(value);
}
}
function map(iterator, selector) {
return new MapIterator(iterator, selector);
}
class FilterIterator {
constructor(source, predicate) {
this.source = source;
this.predicate = predicate;
}
next() {
let value = this.source.next();
while (value !== ITERATION_END && !this.predicate(value)) {
value = this.source.next();
}
return value;
}
}
function filter(iterator, predicate) {
return new FilterIterator(iterator, predicate);
}
function toArray(iterator) {
const array = [];
let value;
while ((value = iterator.next()) !== ITERATION_END) {
array.push(value);
}
return array;
}
function test(fn, iterations) {
for (let i = 0; i < iterations; i++) {
const start = performance.now();
fn();
console.log(performance.now() - start);
}
}
function createData() {
return Array.from({ length: 9000000 }, (_, i) => i + 1);
};
function even(x) { return x % 2 === 0; }
function double(x) { return x * 2; }
function testIterator(data) {
return function main() {
return toArray(map(filter(arrayIterator(data), even), double));
};
}
test(testIterator(createData()), 10);
Observe how there are no more dynamically created functions on the hot path, and the "public interface" (i.e. the way arrayIterator, map, filter, and toArray compose) is exactly the same as before, only under-the-hood details have changed. A benefit of giving all functions names is that you get more useful profiling output ;-)
Astute readers will notice that this modification only shifts the issue away: if you have several places in your code that call map and filter with different modifiers/predicates, then the inlineability issue will come up again. As I said above: microbenchmarks tend to be misleading, as real apps typically have different behavior...
(FWIW, this is pretty much the same effect as at Why is the execution time of this function call changing? .)
Just to add to this investigation, I compared the OP's original code with the predicate and selector functions declared as separate functions as suggested by jmrk to two other implementations. So, this code has three implementations:
OP's code with predicate and selector functions declared separately as named functions (not inline).
Using standard array.map() and .filter() (which you would think would be slower because of the extra creation of intermediate arrays)
Using a custom iteration that does both filtering and mapping in one iteration
The OP's attempt at saving time and making things faster is actually the slowest (on average). The custom iteration is the fastest.
I guess the lesson here is that it's not necessarily intuitive how you make things faster with the optimizing compiler so if you're tuning performance, you have to measure against the "typical" way of doing things (which may benefit from the most optimizations).
Also, note that in the method #3, the first two iterations are the slowest and then it gets faster - the opposite effect from the original code. Go figure.
The results are here:
[
99.90320014953613,
253.79690098762512,
271.3091011047363,
247.94990015029907,
247.457200050354,
261.9487009048462,
252.95090007781982,
250.8520998954773,
270.42809987068176,
249.340900182724
]
240.59370033740998
[
222.14270091056824,
220.48679995536804,
224.24630093574524,
237.07260012626648,
218.47070002555847,
218.1493010520935,
221.50559997558594,
223.3587999343872,
231.1618001461029,
243.55419993400574
]
226.01488029956818
[
147.81360006332397,
144.57479882240295,
73.13350009918213,
79.41700005531311,
77.38950109481812,
78.40880012512207,
112.31539988517761,
80.87990117073059,
76.7899010181427,
79.79679894447327
]
95.05192012786866
The code is here:
const { performance } = require('perf_hooks');
const ITERATION_END = Symbol('ITERATION_END');
const arrayIterator = (array) => {
let index = 0;
return {
hasValue: true,
next() {
if (index >= array.length) {
this.hasValue = false;
return ITERATION_END;
}
return array[index++];
},
};
};
const customIterator = (valueGetter) => {
return {
hasValue: true,
next() {
const nextValue = valueGetter();
if (nextValue === ITERATION_END) {
this.hasValue = false;
return ITERATION_END;
}
return nextValue;
},
};
};
const map = (iterator, selector) => customIterator(() => {
const value = iterator.next();
return value === ITERATION_END ? value : selector(value);
});
const filter = (iterator, predicate) => customIterator(() => {
if (!iterator.hasValue) {
return ITERATION_END;
}
let currentValue = iterator.next();
while (iterator.hasValue && currentValue !== ITERATION_END && !predicate(currentValue)) {
currentValue = iterator.next();
}
return currentValue;
});
const toArray = (iterator) => {
const array = [];
while (iterator.hasValue) {
const value = iterator.next();
if (value !== ITERATION_END) {
array.push(value);
}
}
return array;
};
const test = (fn, iterations) => {
const times = [];
let result;
for (let i = 0; i < iterations; i++) {
const start = performance.now();
result = fn();
times.push(performance.now() - start);
}
console.log(times);
console.log(times.reduce((sum, x) => sum + x, 0) / times.length);
return result;
}
const createData = () => Array.from({ length: 9000000 }, (_, i) => i + 1);
const cache = createData();
const comp1 = x => x % 2 === 0;
const comp2 = x => x * 2;
const testIterator = (data) => () => toArray(map(filter(arrayIterator(data), comp1), comp2))
// regular array filter and map
const testIterator2 = (data) => () => data.filter(comp1).map(comp2);
// combine filter and map in same operation
const testIterator3 = (data) => () => {
let result = [];
for (let value of data) {
if (comp1(value)) {
result.push(comp2(value));
}
}
return result;
}
const a = test(testIterator(cache), 10);
const b = test(testIterator2(cache), 10);
const c = test(testIterator3(cache), 10);
function compareArrays(a1, a2) {
if (a1.length !== a2.length) return false;
for (let [i, val] of a1.entries()) {
if (a2[i] !== val) return false;
}
return true;
}
console.log(a.length);
console.log(compareArrays(a, b));
console.log(compareArrays(a, c));
I have a problem that I'm importing some data and each new row depends on previous row being added (since each row has its order attribute set based on current maximum order from other objects). The flow is that I first try to find object with the same name, if not found I first check maximum order and create new object with order + 1 from that query.
I tried doing this with Q promises which are available under Waterline. I tried using all method as well as combining queries with then from Q docs:
var result = Q(initialVal);
funcs.forEach(function (f) {
result = result.then(f);
});
return result;
But all objects had the same order, just like they would be executed in parallel, instead of waiting for the first chain to finish.
I finally found a solution with recurrency, but I doubt it's the best way of working with promises. Here's the code that works (+ needs some refactor and cleaning etc.), to show the rough idea:
function findOrCreateGroup(groupsQuery, index, callback) {
var groupName = groupsQuery[index];
Group.findOne({ 'name' : groupName }).then(function(group) {
if (!group) {
return Group.find().limit(1).sort('order DESC').then(function(foundGroups) {
var maxOrder = 0;
if (foundGroups.length > 0) {
maxOrder = foundGroups[0].order;
}
return Group.create({
'name' : groupName,
'order' : (maxOrder + 1)
}).then(function(g) {
dbGroups[g.name] = g;
if (index + 1 < groupsQuery.length) {
findOrCreateGroup(groupsQuery, index + 1, callback);
} else {
callback();
}
return g;
});
});
} else {
dbGroups[group.name] = group;
if (index + 1 < groupsQuery.length) {
findOrCreateGroup(groupsQuery, index + 1, callback);
} else {
callback();
}
return group;
}
});
}
I have an array which is:
[ 4ff023908ed2842c1265d9e4, 4ff0d75c8ed2842c1266099b ]
And I have to find if the following, is inside that array
4ff0d75c8ed2842c1266099b
Here is what I wrote:
Array.prototype.contains = function(k) {
for(p in this)
if(this[p] === k)
return true;
return false;
}
Apparently, it doesn't work properly, or better sometimes it works, but it looks to me blocking. Is there anyone that can check that one?
many thanks
Non-blocking search function
Array.prototype.contains = function(k, callback) {
var self = this;
return (function check(i) {
if (i >= self.length) {
return callback(false);
}
if (self[i] === k) {
return callback(true);
}
return process.nextTick(check.bind(null, i+1));
}(0));
}
Usage:
[1, 2, 3, 4, 5].contains(3, function(found) {
if (found) {
console.log("Found");
} else {
console.log("Not found");
}
});
However, for searching the value in the array it is better to use Javascript built-in array search function, as it will be much faster (so that you probably won't need it to be non-blocking):
if ([1, 2, 3, 4, 5].indexOf(3) >= 0) {
console.log("Found");
} else {
console.log("Not found");
}
Also, consider the underscore library which makes all the stuff cross-platform: http://underscorejs.org/
I have a collection of events that look like
{
_id: BSONID
name: "event_name",
values: {a: 10, b: 1000, c: 50}
}
I'm trying to use mapReduce them using
map = function() {
return emit([this.name, this.values['a']], this.values['b']);
}
reduce = function(key, values) {
// stuff
}
collection.mapReduce(map, reduce, { out: { inline: 1 } }, callback);
However, I would like to be able to dynamically change which values I map against. In essence, I'd like to have
var key = 'a';
var value = 'b';
map = function ()
{
return emit([this.name, this.values[key]], this.values[value]);
}
The problem is that the execution context isn't passed to mongodb. Any solution that doesn't rely on using string for functions?
Yes, you can pass a "scope" variable to MapReduce:
scope = {key : "a", value : "b"};
collection.mapReduce(map, reduce, {scope : scope, out: { inline: 1 } }, callback);