I am trying to implement a GroupBy
method with these parameters
function GroupBy(keySelector, elementSelector, comparer)
{
// keySelector = function(e) { return e.ID }
// elementSelector = function(e) { return e.Name }
// comparer = { Equals: function(a,b) { return a==b }, GetHashCode:... }
}
However I don't know an efficient way to implement it.
I created a jsPerf test with linq.js and a method I created which doesn't use a comparer and only work on flat types. ( Output test here )
Other libraries such as underscore and Lo-Dash doesn't take a comparer
parameter. So their implementations are irrelevant.
My key could be a class, so I need something to determine if TKey
is the same in different instances.
So basically what I am trying to do is copy C# Linq GroupBy
behavior documented here .
Sample input:
var arrComplex =
[
{ N: { Value: 10 }, Name: "Foo" },
{ N: { Value: 10 }, Name: "Bar" },
{ N: { Value: 20 }, Name: "Foo" },
{ N: { Value: 20 }, Name: "Bar" }
];
Sample output (or something like this):
[
{
"Key": {"Value":10},
"Elements":["Foo","Bar"]
},
{
"Key": {"Value":20},
"Elements":["Foo","Bar"]
}
]
Any ideas on how to implement it?
For the bounty I would like you take in consideration that:
Well, I expect a complete answer.
I used your jsperf as a reference, for some of the finer points of script. I really, really liked your 'hash' code, so I totally stole it. Mine uses a different method to generate the string used to make the hash, which seems to be a little faster, which increases the performance, according to the 'browserscope' charts. I include in my test a 'too much recursion' proof of concept to show that it has recursion protection, like JSON.stringify and .toSource().
My jsfiddle shows that the code returns the format you need. My jsperf seems to indicate that it outperforms the posted solution. I also included the linq.js solution, but it performs pretty bad in FireFox for me. It works comparably in Safari, Chrome, an IE, but not faster than mine, except in IE. I even tried it on my phone, and still I have the same performance difference. I have personally tested it in the latest versions of all browsers in a side-by-side to the posted solution, and mine out performs by around 40% across each of them. What are everyone's thoughts?
Here is my code:
var arr = [
{ N: 10, Name: "Foo" },
{ N: 10, Name: "Bar" },
{ N: 20, Name: "Foo" },
{ N: 20, Name: "Bar" }
];
var poc = { name:'blah', obj:{} };
poc.obj = poc;
var arrComplex = [
{ N: { Value: 10, TooMuchRecursionProofPOC:poc }, Name: "Foo" },
{ N: { Value: 10, TooMuchRecursionProofPOC:poc }, Name: "Bar" },
{ N: { Value: 20, TooMuchRecursionProofPOC:poc }, Name: "Foo" },
{ N: { Value: 20, TooMuchRecursionProofPOC:poc }, Name: "Bar" }
];
var eArr = Enumerable.From(arr);
var eArrComplex = Enumerable.From(arrComplex);
function setup_hashers() {
// recursion protection idea
var rp = '_rp'+(Math.random()*10000000);
function tstr() {
var out = '', i = '';
if (this[rp]) { this[rp] = undefined; return out; }
for (i in this)
if (i != rp && this.hasOwnProperty(i))
out += this[i] instanceof Object
? ((this[rp] = true) && this[i] != this && !this[i][rp] ? tstr.call(this[i]) : '')
: (this[i].toString || tstr).call(this[i]);
return out;
};
Number.prototype.GetHashCode = function() {
return this.valueOf();
};
Object.prototype.GetHashCode = function() {
var s = (this instanceof Object ? tstr : this.toString || tstr).call(this),
h = 0;
if (s.length)
for (var i = 0; i < s.length; i++)
h = ((h << 5) - h) + s.charCodeAt(i);
return h;
};
}
function group_by(a, keyFunc, valFunc, comp, as_array) {
if (!a.length) return as_array ? [] : {};
var keyFunc = keyFunc || function (e) { return e; },
valFunc = valFunc || function (e) { return e; };
var comp = comp || {
Equals: function (a, b) { return a == b; },
Hash: function (e) { return e.GetHashCode(); }
};
var hashs = {}, key = '', hash = '';
for (var i = 0; i < a.length; i++) {
key = keyFunc(a[i]);
hash = comp.Hash(key);
if (typeof hashs[hash] != 'undefined')
hash = comp.Equals(key, hashs[hash].Key)
? hash
: hash + '-' + i;
hashs[hash] = hashs[hash] || { Key: key, Elements: [] };
hashs[hash].Elements.push(valFunc(a[i]));
}
if (as_array) {
var out = [], j = '', keys = Object.keys(hashs);
for (var j = 0; j < keys.length; j++)
out.push(hashs[keys[j]]);
return out;
}
return hashs;
};
function group_by_control(a, keyFunc, valFunc) {
if (!a.length) return as_array ? [] : {};
var keyFunc = keyFunc || function (e) { return e; },
valFunc = valFunc || function (e) { return e; };
var hashs = {}, key = '', hash = '';
for (var i = 0; i < a.length; i++) {
key = keyFunc(a[i]);
hashs[key] = hashs[key] || { Key: key, Elements: [] };
hashs[key].Elements.push(valFunc(a[i]));
}
var out = [], j = '', keys = Object.keys(hashs);
for (var j = 0; j < keys.length; j++)
out.push(hashs[keys[j]]);
return out;
};
setup_hashers();
console.log(group_by_control(
arr,
function(e) { return e.N },
function(e) { return e.Name }
));
console.log(group_by(
arrComplex, function(e) { return e.N; },
function(e) { return e.Name; },
{
Equals: function(a, b) { return a.Value == b.Value },
Hash: function(e) { return e.GetHashCode(); }
}
));
console.log(group_by(
arrComplex, function(e) { return e.N; },
function(e) { return e.Name; },
{
Equals: function(a, b) { return a.Value == b.Value },
Hash: function(e) { return e.GetHashCode(); }
},
true
));
I managed to implement this way:
I need to get the hashcode from objects.
Object.prototype.GetHashCode = function () {
var s = this instanceof Object ? stringify(this) : this.toString();
var hash = 0;
if (s.length === 0) return hash;
for (var i = 0; i < s.length; ++i) {
hash = ((hash << 5) - hash) + s.charCodeAt(i);
}
return hash;
};
Number.prototype.GetHashCode = function () { return this.valueOf(); };
As JSON.stringify
will fail at circular references, I created another method to stringify it in a way I can get the most of an object as a string and compute the hashcode over it, as follows:
function isPlainObject(obj)
{
if ((typeof (obj) !== "object" || obj.nodeType || (obj instanceof Window))
|| (obj.constructor && !({}).hasOwnProperty.call(obj.constructor.prototype, "isPrototypeOf"))
)
{
return false;
}
return true;
}
function stringify(obj, s)
{
s = s || "";
for (var i in obj)
{
var o = obj[i];
if (o && (o instanceof Array || isPlainObject(o)))
{
s += i + ":" + JSON.stringify(o);
}
else if (o && typeof o === "object")
{
s += i + ":" + "$ref#" + o;
}
else
{
s += i + ":" + o;
}
}
return s;
}
There isn't much impact on performance. For large object it is the same and for small objects it loses, but is still pretty fast and safe. Performance test here .
Name op/s
---------------------------------
JSON.stringify large 62
stringify large 62
JSON.stringify small 1,690,183
stringify small 1,062,452
My GroupBy method
function GroupBy(a, keySelector, elementSelector, comparer)
{
// set default values for opitinal parameters
elementSelector = elementSelector || function(e) { return e; };
comparer = comparer ||
{
Equals: function(a,b) { return a==b },
GetHashCode: function(e) { return e.GetHashCode(); }
};
var key, hashKey, reHashKey;
// keep groups separated by hash
var hashs = {};
for (var i = 0, n = a.length; i < n; ++i)
{
// in case of same hash, but Equals returns false
reHashKey = undefined;
// grabs the key
key = keySelector(a[i]);
// grabs the hashcode
hashKey = comparer.GetHashCode(key);
// if a hash exists in the list
// compare values with Equals
// in case it return false, generate a unique hash
if (typeof hashs[hashKey] !== "undefined")
reHashKey = comparer.Equals(key, hashs[hashKey].Key) ? hashKey : hashKey + " " + i;
// if a new hash has been generated, update
if (typeof reHashKey !== "undefined" && reHashKey !== hashKey)
hashKey = reHashKey;
// get/create a new group and add the current element to the list
hashs[hashKey] = hashs[hashKey] || { Key: key, Elements: [] };
hashs[hashKey].Elements.push(a[i]);
}
return hashs;
}
To test
var arrComplex =
[
{ N: { Value: 10 }, Name: "Foo" },
{ N: { Value: 10 }, Name: "Bar" },
{ N: { Value: 20 }, Name: "Foo" },
{ N: { Value: 20 }, Name: "Bar" }
];
//
var x = GroupBy(arrComplex
, function(e) { return e.N; }
, function(e) { return e.Name; }
, {
Equals: function(a,b) { return a.Value == b.Value },
GetHashCode: function(e) { return e.GetHashCode(); }
}
);
//
console.log(x);
Example on jsFiddle , now with Jedi.
But, accordingly to my tests , my implementation of GroupBy
is slower than linq.js's GroupBy
. It is only faster when I convert ToArray()
. Maybe linq.js only really executes when I convert to array, that's why the difference, I am not sure on this part.
Test results
Name op/s
---------------------------------
GroupBy 163,261
GroupByToArray 152,382
linq.js groupBy 243,547
linq.js groupBy toArray 26,309
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