Vjeux

Working on the World of Raids Recruitment Tool we wanted automatic saving while editing. There are basically two ways of handling the problem.

• Send update everytime something changes. The main advantage of this technique is that you are always up to date. However, it is spamming the server on fields that are constantly being updated (text fields for example).
• Send update every X seconds. There is no longer query spike but there are two main disadvantages.

  The delay the data are sent after a modification can vary from nothing to the timer length for no apparent reason. This is a really bad feeling for the user. He will think that the application is buggy.

  The application is going to send queries all the time, even if there are no changes. We can safely assume that people are interacting a lot less than viewing so this is a real waste of resources.

We are interested in the first one, we want instant update but removing the ability to send too much data at once. What we want is to send data right after the user is done typing instead of everytime a character is typed. There is an easy way to do it in Javascript.

When you type the first character, it will start a timer of let say 100ms. Then everytime you type another character it resets the timer to 100ms. When it triggers, it will send data to the server.

This way, data will only be sent when the user interacts and with no risk of spamming the server.

var Object = {
  /* Object Code ... */
 
  timer: null,
  sendChanges: function() {
    if (this.timer) {
      clearTimeout(timer);
      this.timer = null;
    }
 
    this.timer = setTimeout(
      function () { /* Send Changes */ },
      200 /* Timeout in ms */
    );
  }
};

It has been one month that the third year (called Ing1) of EPITA started and this is a recap of the interesting projects that I developed.

malloc (1 week)
The goal of this mini-project is to improve our knowledge of memory management by implementing the C standard library memory allocator (malloc, free, realloc). Implementation using the Binary Buddy technique:

• List of free blocks of size 2^k.
• Split and Merge using adjacent block of the same size called buddy.
• Insertion and Deletion in O(log n).
• 32 & 64 bit ready.

find (1 week)
find is a very well known UNIX command used to satisfy research upon files and directories.

• File listing and options parsing with error handling.
• -name: Filtering using globbing.
• -newer, -atime, -size, -inum, -type, -links: Filtering off information sources. Handles -/+ for "less/more than".
• -perm: Filtering based on permission expressed in octal (0777) or symbolic (u+rwx,go=r) notation.

fnmatch (10 hours)
fnmatch is a function that implements globbing. It returns whether the string matches the pattern. This is mainly used to filter out files (eg: *.txt) or text search in databases (eg: LIKE '%Text%').

• *: any character 0 or more times.
• ?: any character 1 time.
• [abcA-Z]: any character from the class.
• \*: escaping.

Libstream (10 hours)
I/O functions are slow hence they should be called few time as possible. This is why they are typically buffered. The project is to recode all the I/O buffered functions of the C standard library as they are defined by the Single UNIX Specifications V3.

• fopen, fclose, fgetc, fputc, fflush: Core functions to handle buffered I/O at a character level.
• fseek, ftell, frewind: File positioning functions.
• fread, fwrite, fgets, fputs, fgetdelim: Helper functions to use buffered I/O at a string level.
• tinyPrintf: Basic string formatting (%d, %u, %s, %c, %o, %x without modifiers).

Comparison

In Javascript there are 3 types we are often comparing: String, Number and Boolean. After digging through the ECMA-262 specifications, here is the behaviour of the == operator (11.9.3) on these types:

• Number == String
  Typecasted as follow: Number == Number(String)
• Number == Boolean
  Typecasted as follow: Number == Number(Boolean)
• String == Boolean
  Typecasted as follow: Number(String) == Number(Boolean)

This means that when comparing data of two different types, they will first be converted to Number.

Note: The order of the equality is not important: A == B is the same as B == A (except the order of evalution of A and B).

You can force comparison of a and b with the type you want:

• String: "" + a == "" + b
• Number: +a == +b
• Integer: a | 0 == b | 0
• Boolean: !!a == !!b

Addition / Concatenation

The binary + operator follows a simple rule (11.6.1):

• if one of the operands is a String, both operands are converted to String and the + is a concatenation.
• Else, both operands are converted to Number and the + is an addition.

Note: Some Objects are considered as Strings like Arrays. See 8.6.2 for more informations.

Note: The operator is binary so 1 + 1 + 'a' will be executed as (1 + 1) + 'a' and therefore be equal to "2a" and not "11a".

Javascript is a very flexible language, I made a compilation of some edge cases that you may have encountered while programming. The main goal is to point out some interesting specific behaviors.

1] var result = [10] + 1;

2] var result = ['a', 'b', 'c'] + "";

3] var result = 'a' + 5;

4] var result = 3.75 | 0;

5] var result = 65 / 'a';

6] var ob = {"10": 1};
ob[10] = 2;
ob[[1, 0]] = 3;
var result = ob["10"] + ob[10] + ob[[1, 0]];

7] var $ = {"": String};
var result = !!$[([])]();

8] var result = (' \t\r\n ' == 0);

9] var a = new String("123");
var b = "123";
var result = (a === b);

10] var a = {key: 1};
var b = {key: 1};
var result = (a == b);

While on holidays, I played that old (bought in the 80's) board game called Fief. After several games I wanted to point at the good and bad things about this game in order to find some general rule.

Fief - Board Strategy Game

Some assets of the game are innovative and worth mentioning.

The resources are really limited. It is common to have limited number of soldiers or houses, but the limitations are often relative to a player. In this game, there is a limited amount of money. After several rounds, there's no money left in the bank and people cannot earn their income. This opens the possibility to organize bankruptcy.

Movement design is clever. The only way to move your army from town to town are Lords. They are special soldiers that can walk 2 towns a round. While moving, they are able to carry any amount of soldiers with them. Once they are well positioned, the Lords allow you to move around the map really fast.

However, that's not a perfect game, the main problem could be summarized with the statement "10 pages rules book". Here are some consequences of that fact.

• Many rules leads to contradictions and unclear statements
• The learning curve is disastrous
• More time spent reading and discussing the rules than playing the game

In order to find a solution, we are going to analyze some facts and get general rules off them.

Could we play without this rule? Yes. Drop it.

In the game, there are 2 types of building that gives resources: Mill [cost 300, give 200] and Press [cost 200, give 100]. Basically, you first buy all the Mills and when there are no left you start taking Presses. These two buildings are just about the same, do not give any kind of reflexion but have to be learned by the players. Removing one building would not affect the decision range of the user and would make the game easier to play.

Ten things doing slightly different things? Merge them!

There are lots of cards but you can never use them. There are offensive cards and their respective defensive cards. Offensive cards are targeted against a player and aren't that common. No need to say that you never have the defensive card when needed. That's about the same for cards that gives money, you either have to be Clergy, have a Fief or be King to use them. Since most of the time you are nothing, these cards are trash. Being given useless card over useless card is a really bad feeling.

A solution would be to transform these useless cards would be to make them generic. What about having a rare card that can counter any offensive attack. As for the money card, it would give money based on the highest rank you have. By changing how to attribute the cards, we could keep the same usage rate. They would be seen like a great reward and as a side effect simplify the game.

Simple rules, great flexibility.

The methodology adopted by the game is: "There's a problem? Add a special rule for this exact problem". What if we let the user the possibility to do it anyway? It will give flexibility and make the game easier to understand! For further thoughts about this philosophy in the web development, i would suggest you to read the free book Getting Real.