Compress javascript a.js into a.min.js

Compress javascript a.js into a.min.js can be done on this site: http://jscompress.com/

Perl - extend lib path

See: http://www.perlhowto.com/extending_the_library_path

This is useful when you want to use personal lib for perl modules, which is not on perl path (@INC) and cannot be found.

3 methods are mentioned.

1) The non-invasive method is to use -I switch:

perl -I /home/path/lib -I /usr/another/lib script.pl

2) add path to environment:

# unix, bourne shell
PERL5LIB=/home/path/lib:/usr/another/path/lib; export PERL5LIB

3) add path to perl code:

#!/usr/bin/perl
use lib "/home/path/lib";
use lib "/usr/another/lib";

use MyCustomModule;

General Game Playing

General Game Playing is a course given by Stanford on the application of AI in computer games.

The course material is available here: http://logic.stanford.edu/ggp/chapters/

Some readings today

Here are links to some readings today:

- Java equals()和hashCode()

   Q: 怎样实现正确的equals()方法
   A: 首先，我们需要遵守Java API文档中equals()方法的约定，如下：
    自反性：对于任何非空引用值 x，x.equals(x) 都应返回 true。
    对称性：对于任何非空引用值 x 和 y，当且仅当 y.equals(x) 返回 true 时，x.equals(y) 才应返回 true。
    传递性：对于任何非空引用值 x、y 和 z，如果 x.equals(y) 返回 true，并且 y.equals(z) 返回 true，那么 x.equals(z) 应返回 true。
    一致性：对于任何非空引用值 x 和 y，多次调用 x.equals(y) 始终返回 true 或始终返回 false，前提是对象上 equals 比较中所用的信息没有被修改。

    对于任何非空引用值 x，x.equals(null) 都应返回 false。
    其次，当我们重写equals()方法时 , 不同类型的属性比较方式不同,如下:
    属性是Object类型, 包括集合: 使用equals()方法。
    属性是类型安全的枚举: 使用equals()方法或==运算符(在这种情况下，它们是相同的)。
    属性是可能为空的Object类型: 使用==运算符和equals()方法。
    属性是数组类型: 使用Arrays.equals()方法。
    属性是除float和double之外的基本类型: 使用==运算符。
    属性是float: 使用Float.floatToIntBits方法转化成int,然后使用 ==运算符。
    属性是double: 使用Double.doubleToLongBits方法转化成long , 然后使用==运算符。

  Q: 重写equals()方法为什么一定要重写hashCode()方法
  A: 如果不这样做就会违反Java API中Object类的hashCode()方法的约定，从而导致该类无法很好的用于基于散列的数据结构(HashSet、HashMap、Hashtable、LinkedHashSet、LinkedHashMap等等)。i.e., when search in hashtable based data structure, it'll first find the bucket based on hashCode(), then do compare by equals() to find exact match. Object.hashCode() by default is different for every object. This can't be used when, say, 2 strings "aaa" and "aaa", are different objects so have different hashCode, then you cannot find "aaa" in a container that contains "aaa".


- 揭秘阿里服务互联网金融的关系数据库——OceanBase
  This lacks details though.

-  JS前台加密,java后台解密实现
   This is not hard to decode though since JS is open.

用 Python 绘制音乐图谱

用 Python 绘制音乐图谱
Original post is from http://www.christianpeccei.com/
And Articles by: PyPer

Very nice. Might worth the time to try to reproduce.

Play midi in browser by Javascript only

Midi files usually have very small size, but rich sound effects. HTML5 Audio tag supports wav, mp3 and ogg formats so far.  Here are methods to play midi music in web browser using Javascript only, with no plugin such as QuickTime.

I wrote the one at [7].

Features

• Can specify these in constructor parameter list: midi, target, loop, maxLoop, end_callback.
  - midi: MIDI file path.
  - target: Target html element that this MIDI player is attached to.
  - loop: Optinoal. Whether loop the play. Value is true/false, default is false.
  - maxLoop: Optional. max number of loops to play when loop is true. Negative or 0 means infinite. Default is 1.
  - end_callback: Optional. Callback function when MIDI ends.
    e.g., use this to reset target button value from "stop" back to "play".
• Can specify a debug div, to display debug message: setDebugDiv(debug_div_id).
• Start/stop MIDI by: start(), stop().
• If a MIDI started play, call start() again will stop and then restart from beginning.

This depends on other 5 javascript files (audio.js, midifile.js, replayer.js, stream.js, synth.js) from [2][3], which is a demo of [1]. This is related to [4], which is a powerful tool to play MIDI in browser.

The disadvantage of [2][3] is that it does not have control over how a MIDI file is played: when clicking on the link the file will be started multiple times and sounds chaotic; and there is no loop feature. Both are well handled by MidiPlayer.js here.

Another midi player javascript is in [5], but it cannot play multiple MIDI files at the same time, cannot play a MIDI file automatically after loading the page, and has no loop feature. All are handled by MidiPlayer.js here.

It can be a good idea to add MIDI support to HTML5 Audio tag, because MIDI files have much smaller size than wav/mp3, and the sound effects are very rich.

[1] http://matt.west.co.tt/music/jasmid-midi-synthesis-with-javascript-and-html5-audio/
[2] http://jsspeccy.zxdemo.org/jasmid/
[3] https://github.com/gasman/jasmid
[4] MIDI.js - Sequencing in Javascript.
[5] MIDI.js - The 100% JavaScript MIDI Player using W3C Web Audio
[6] Dynamically generating MIDI in JavaScript
[7]  The MidiPlayer javascript class

Javascript drag/drop event listener

It is a cool feature that file processing is triggered when you drag a file and drop it onto a web page.

For example, the midi player demo page index.html from [1] has this in header javascript:

    if (FileReader){
        function cancelEvent(e){
            e.stopPropagation();
            e.preventDefault();
        }
        document.addEventListener('dragenter', cancelEvent, false);
        document.addEventListener('dragover', cancelEvent, false);
        document.addEventListener('drop', function(e){
            cancelEvent(e);
            for(var i=0;i            var
                file = e.dataTransfer.files[i]
            ;
            if(file.type != 'audio/midi' && file.type != 'audio/mid'){
                continue;
            }
            var
                reader = new FileReader()
            ;
            reader.onload = function(e){
                midiFile = MidiFile(e.target.result);
                synth = Synth(44100);
                replayer = Replayer(midiFile, synth);
                audio = AudioPlayer(replayer);
            };
            reader.readAsBinaryString(file);
            }
        }, false);
    }

What this does is, when you drag and drop a midi file on the page, it'll be read and played.

HTML5 has native support for drag/drop event. See [2].

[1] https://github.com/gasman/jasmid
[2] Native HTML5 Drag and Drop