Kencoder Tavern

2014-03-17

Calculate the distance between two geo locations

We are developing a location based app and we encounter a problem about the geo location yesterday. Which is "how to get the distance between two points in the map". It seem to be easy if using our usual XY coordination system (the Cartesian coordinate). However, the inputs are the latitude and longitude, not the unit of meter;

Formula:

acos(
cos(a.lat) x cos(a.long) x cos(b.lat) x cos(b.long)
+ cos(a.lat) x sin(a.long) x cos(b.lat) x sin(b.long)
+ sin(a.lat)*sin(b.lat)
) x earth.radius

Where:

a.lat, a.long = latitude and longitude of the 1st point ( in radians)
b.lat, b.long = latitude and longitude of the 2nd point ( in radians)
earth.radius = the radius of the earth; (which is: 3443.9 nautical miles or 6378 km)

Source Code (in java):

public static double distanceBetween(double lat1, double long1, double lat2, double long2)
{
 double rLat1 = Math.toRadians(lat1);
 double rLong1 = Math.toRadians(long1);
 double rLat2 = Math.toRadians(lat2);
 double rLong2 = Math.toRadians(long2);
  
 double part1 = Math.cos(rLat1) * Math.cos(rLong1) * Math.cos(rLat2) * Math.cos(rLong2);
 double part2 = Math.cos(rLat1) * Math.sin(rLong1) * Math.cos(rLat2) * Math.sin(rLong2);
 double part3 = Math.sin(rLat1) * Math.sin(rLat2);
 
 return Math.acos(part1 + part2 + part3) * RADIUS;
}

Reference:

There are more documentation talking about the distance calculation, also google provide API to do so.

2014-03-16

Abnormal java.net.UnknownHostException on resin

Recently, I encountered a problem while I am calling to Google API in my java code running on Resin. The problem is that: The http call raise an UnknownHostException;

It doesn't caused by Java itself because the UnitTest show it is correct.
It doesn't caused by Resin because it work in my development platform.

So what go wrong? Finally, I has discovered it is wrong in the staging server environment;
The /etc/hosts isn't configured as the resin want to. (... maybe it is the problem of resin too ..)

To fix it:
- type "hostname" to find your hostname . e.g "my.pc"
- edit /etc/hosts and add the line "127.0.0.1 my.pc"
- restart the server to make it effective

Notes:
- This problem may not be happened in all environment. My macbook doesn't have the problem.

Related Links:

2014-03-13

Make java connection pool auto reconnect

Recently we encountered a problem when using java db connection pool. The problem is that the connection is being closed by the DB server since it is already timeout. To due with this, need to configure two more parameters.

They are:
- testOnBorrow ; define it as "true"
- validationQuery ; define it as "SELECT 1 FROM dual" if you are also using MySQL

Yes, It is easy to config it. But how to test it? (Many sites may not give the answer).

To verify the configuration is work, we can make a simple Unit Test;

Step 1: Make a failure case

Make an unit test base on the following code;

Properties p = new Properties();
prop.put("driver", "org.gjt.mm.mysql.Driver");
prop.put("url", "jdbc://localhost:3306/db");
prop.put("username", "user");
prop.put("password", "123456");
p.setProperties("testOnBorrow", "false");
p.setProperties("validationQuery", "");

BasicDataSource ds = BasicDataSourceFactory.createDataSource(prop);
ds.setMaxActive(1);
ds.setMaxIdle(1);
ds.setMinIdle(0);

Connection conn = ds.getConnection();
System.out.println("First connection");

System.out.println("Sleep start");
Thread.sleep(10000);
System.out.println("Sleep end");

Connection conn = ds.getConnection();
System.out.println("Second connection");

To make the connection fail at the second time, try to kill the connection in DB side while "Thread.sleep";

Step 2: Fix the failure

Simply change "testOnBorrow=false" and "validationQuery=" to
"testOnBorrow=true" and "validationQuery=SELECT 1 FROM dual"

Then, run the test again. we will see "Second connection" show up instead of Exception message.

2014-01-27

Cocos2d-x Error: "Skipping selector 'onPopupYesPressed' since no CCBSelectorResolver is present."

Today, I failed to bind a button and the log said "Skipping selector 'onPopupYesPressed' since no CCBSelectorResolver is present.";

The reason is that simply because I forget to define the "Custom Class" name in CocosBuilder !! =.=

2014-01-23

制作游戏的难点

最近和友人談談各自製作遊戲的情況，發現真有點難;

難一：難在找到志同道合及能力互補的團隊！

難二：難在找出好的賣點！

難三：難在製作出好的體驗！

難四：難在把抽象的事情具體化！

難五：難在把設計實現出來！

難六：難在放下主觀想法，拿起玩家的想法！

游戏开发是梦幻的工业，这是事实，但是令一个事情，就做出好的游戏，是困难重重的，需要热情和能力兼备；

真正製作的遊戲開發者，就是在不斷克服困難，不斷改进的勇者們！

2014-01-20

以恩典為年歲的冠冕@2013

剛過了的2013, 神給我和我家很多的恩典：

醫治母親的大病
幫助天藍媽媽能有力量照顧天藍
天藍健康快樂地成長
我們更明白神的話和祂的恩典
為我們預備了裝修的費用和母親的醫藥費
轉了新工作，可以多些時間照顧天藍和太太
賜予我們聰明智慧，處理家事和公事
爸媽租到一間又大又好的屋
我們可以搬去大一點的屋